Updated: 09.01.2020
Human Body
The human body is the structure of a human being. It is composed of many different types of cells that together create tissues and organ systems. They ensure homeostasis and the viability of the human body. Many systems and mechanisms interact in order to maintain homeostasis, with safe levels of substances such as sugar and oxygen in the blood.
It body is composed of elements including hydrogen (9,5%), oxygen (65%), carbon (18,5%), calcium (1,5%) and phosphorus (1%). The main electrolytes outside cells are sodium and chlorides, whereas within cells it is potassium and other phosphates. Trace elements are less than 1% combined. The adult male body is about 60% water, a total water content of about 42 litres, which is made up of about 19 litres of extracellular fluid, including about 3,2 litres of blood plasma.
The human body comprises of a
head, neck, trunk, which includes the thorax and abdomen, and
limbs, arms and hands, and legs and feet. The shape of a human body is determined mainly by the skeleton, but also by the distribution of muscle and fat. The study of the human body involves anatomy, physiology, histology and embryology. The body is studied by health professionals, physiologists, anatomists, but also by artists.
The whole body is covered by the skin. There are five main senses, sight, hearing, smell, taste and touch. There are four main cavities, cranial, spinal, thoracic and abdominopelvic, divided into abdominal and pelvic cavities.
The
head is the seat of the brain and several sense organs and it is supported by seven cervical vertebrae. It consists of the skull which encloses the brain and is covered by the scalp with hair and the face. The skull supports the structures of the face and provides a protective cavity for the brain. It consists of the cranium and the mandible, the jawbone. The face is made up of the forehead and the temples, eyes, upper and lower lids and eyebrows, cheeks, nose, mouth with two lips, the oral cavity with the tongue and teeth, chin and ears. The face is crucial for human identity and can express many emotions.
The
neck joins the head and the trunk, it supports the weight of the head, protects the spine and allows movement. It contains cervical vertebrae, the trachea, the pharynx, the larynx, the thyroid gland and important blood vessels and nerves. The muscles of the neck bound two major neck triangles, anterior and posterior. The thyroid cartilage of the larynx forms a bulge in the middle of the neck called Adam's apple, which is more prominent in men.
The
trunk, also called torso, is the central part of the body and houses many critical organs. The diaphragm divides the torso
into the thoracic cavity and the abdominal cavity. The main parts of the thorax are lungs within the pleural cavity, the heart, the trachea, and the esophagus. The upper chest is protected by the rib cage. On the chest there are nipples and, in women, breasts. Inside the abdominal cavity we can find the stomach, the liver, the gallbladder, the pancreas, the spleen, the kidneys, the urinary bladder, the small and large intestines, and the internal reproductive organs within the pelvic region. The back is the large area rising from the top of the buttocks to the back of the neck and the shoulders and the vertebral column, the spine, runs through its length.
There are two upper and two lower
extremities, also called limbs. The upper limb, the arm, is divided into the shoulder, the upper arm, the elbow, the forearm, the wrist and the hand, which is composed of the back and the palm, four fingers and a thumb. The leg is attached to the pelvic girdle. It is constituted of the hip, the thigh, the knee, the lower leg with the shin in the front and a calf at the back, the ankle and the foot with the instep, the sole, the arches and five toes.
Cells
The body contains trillions of cells (roughly 35 trillion), the fundamental unit of life. It is also host to about the same number of non-human cells and multicellular organisms which reside in the gastrointestinal tract and on the skin. Not all parts of the body are made from cells, they sit in an extracellular matrix that consists of bone and connective tissue, surrounded by extracellular fluids.
Deoxyribonucleic acid (DNA) is a molecule composed of two chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, growth, and reproduction of all known organisms and many viruses.
Ribonucleic acid (RNA) is a polymeric molecule essential in coding, decoding, regulation and expression of genes. DNA and RNA, along with lipids, proteins and carbohydrates, constitute major macromolecules essential for all known life forms. They are assembled as a chain of nucleotides.
Cells in the body function because of DNA, which sits within the nucleus of a cell, where its parts are copied and sent to the body of the cell via RNA to create proteins. However, not all cells have DNA, some cells, such as mature red blood cells, lose their nucleus as they mature.
Stem cells are cells that can differentiate into other types of cells and can also divide in self-renewal to produce more stem cells. There are two broad types, embryotic stem cells and adult stem cells. Adult stem cells are frequently used in various therapies, for example bone marrow transplantation.
Progenitor cells have a tendency to differentiate into specific types of cells, but are already more specific, they have target cells. They can divide only a limited number of times. In adult organisms, stem cells and progenitor cells act as a repair system.
Tissues
The body consists of many different types of tissue, defined as cells that act with a specialized function. There are four main types of tissues, lining cells (epithelia), connective tissue, nervous tissue and muscle tissue. Endothelial cells line internal cavities including blood vessels and glands.
Epithelial tissue, lining cells cover surfaces, regulate what can or cannot pass through them, protect internal structures and function as sensory surfaces.
Muscle tissue form the active contractile tissue, they function to produce force and cause motion. They have three distinct categories, smooth muscle (visceral) in the inner linings of organs, skeletal muscle typically attached to bones and cardiac muscle of the heart.
Nervous cells, neural tissue, are cells comprising the central and peripheral nervous system. They form the brain sn spinal cord and cranial and spinal nerves.
Connective cells are fibrous tissues made up of cells separated by an extracellulal matrix. They can be liquid or rigid and they give shape to organs and hold them in place. They are divided into three types, fibrous, skeletal and fluid.
Organs and Systems
Organs are structures collections of cells with a specific function. Many organs reside within cavities, such as the abdomen and pleura, which contains the lungs.
There is no universally standard definition of what constitutes and organ. Therefore, the number of organs varies depending on various definitions.
An
organ system is a group of organs that work together as a biological system to perform one or more functions. Each organ does a particular job and is made up of distinct tissues.
There are
eleven distinct organ systems in human beings, respiratory, digestive, cardiovascular, urinary, integumentary, skeletal and muscular, endocrine, lymphatic, nervous and reproductive. There are other systems in the body, but not organ systems. The immune system is not an organ system. On the other hand, some organs are in more than one system.
Circulatory (Cardiovascular) System
 |
| red blood cell, platelet and white blood cell |
This organ system permits blood to circulate and transport nutrients, oxygen and carbon dioxide, hormones and blood cells from and to the cells to provide nourishment, stabilize temperature and pH and maintain homeostasis. The system comprises blood, heart and blood vessels and it is connected to the lymphatic system. It is a
closed system which has two components, the systemic circulation and the pulmonary circulation.
Blood is a fluid consisting of plasma, red blood cells, white blood cells and platelets that is circulated by the heart through the system. The movement of blood around the body is called circulation, it takes less than a minute to pump blood to every cell in the body. An average adult contains roughly 4.7 to 5.7 litres of blood, which is about 7% of their weight.
Plasma is about 55% of blood, it is a fluid that functions as the liquid medium for blood. It is straw-yellow and contains mostly water, but also proteins, glucose, clotting factors, electrolytes, hormones, carbon dioxide and oxygen. It is separated from the blood by spinning a tube of fresh blood containing an anticoaticoagulant in a centrifuge until the blood cells fall to the bottom of the tube.
Red blood cells, erythrocytes, occupy about 45% of blood, they contain hemoglobin and distribute oxygen. They are marked by glycoproteins that define the different blood types, mainly the ABO and Rh antigen systems.
White blood cells, leukocytes, are the cells of the immune system that are involved in protecting the body against infectious disease and foreign invaders.
Platelets, thrombocytes, ale clotting cells, their function is to react to bleeding by clumping, they also contribute to homeostasis.
Blood pressure is the pressure of circulating blood on the walls of blood vessels and it is one of the vital signs. Systolic pressure is the maximum during one heartbeat and diastolic pressure is the minimum in between two heartbeats. Normal resting blood pressure in an adult is 120/80mmHg. Traditionally, it was measured using auscultation with a mercury-tube sphygmomanometer, but today, semi-automated methods became common.
Heart rate is the speed of the heartbeat measured by the number of contractions per minute. It can vary according to the body's physical need. The average heart rate is 60-100bpm, usually 70bpm.
Pulmonary circulation leads oxygen-poor blood under low pressure from the heart through the lungs where it is oxygenated.
Systemic circulation is a loop through the rest of the body to provide oxygenated blood.
Arteries are blood vessels that deliver oxygen-rich blood from the heart to the tissues of the body. Each artery is a muscular tube lined with smooth tissue. The largest artery is the
aorta, a massive, thick-walled and elastic artery which delivers blood and helps to maintain blood pressure. It branches into smaller arteries, arterioles and
capillaries, small passages that merge to bring blood into the veins.
Veins carry deoxygenated blood from the tissues towards the heart. They are less muscular and often closer to the skin. They are equipped with valves to prevent blood flowing in the reverse direction.
The
heart lies between the lungs, a little to the left of the middle of the chest. It is a hollow muscular
organ which pumps oxygenated blood to the body and deoxygenated blood into the lungs. It consists of four chambers, separated by a septum into left and right halves, left and right atrium and left and right ventricle. There are valves between the atria and ventricles that prevent the backflow of blood. The right atrium is the upper chamber of the right side, deoxygenated blood is returned into it and passed into the right ventricle to be pumped through the pulmonary artery to the lungs for reoxygenation and removal of carbon dioxide. The left atrium receives newly oxygenated blood from the lungs and passes in to the strong left ventricle to be pumped through the aorta back into the body.
Lungs receive oxygen-depleted blood from the heart and gas exchange occurs. The carbon dioxide is released from the blood and oxygen is absorbed instead.
Lymphatic System
This organ system returns filtered blood plasma as lymph. The system comprises of lymph, lymph nodes and lymph vessels. It is a
opened system providing an accessory route for excess interstitial fluid to be returned to the blood. Its other function is in the adaptive immune system and absorption of fatty acids and fats.
Lymph is a clear fluid, essentially recycled excess blood plasma. It is very similar to blood plasma and it contains
lymphocytes, a subtype of white blood cells. The passage of lymph takes much longer than that of blood.
Immune System
The immune system is a host defense system that protects against disease. To function properly, it must detect a wide variety of
pathogens.
The
innate immune system recruits immune cells to sites of infection, identifies and removes foreign substances and activates the adaptive immune system. It acts as a physical and chemical barrier to infectious agents, such as skin.
The
adaptive (acquired) immune system is composed of highly specialised systemic cells and processes to eliminate pathogens. Acquired immunity creates immunological memory after an initial response to a specific pathogen, which leads to enhanced response in the future. This is the basis of
vaccination.
Antigens are substances that elicit the acquired immune response and
antibodies travel through the blood stream and bind to the foreign antigen causing it to inactivate.
The
immune system includes primary lymphoid organs, secondary lymphatic tissues and other. The key organs are thymus, bone marrow, spleen, tonsils, lymph vessels and nodes, adenoids, skin and liver.
White blood cells act like independent organisms. They identify and eliminate pathogens. There are five different types, lymphocytes are the main type of cell found in lymph.
The
tonsils are a set of lymphoid organs which play an important role in the immune system. They are the first line of defense against ingested or inhaled foreign pathogens. They are set in the aerodigestive tract and consist of palatine, lingual, tubal and adenoid tonsils.
Bone marrow is found in the interior of bones and it produces red blood cells and lymphocytes.
The
thymus is a lymphoid gland between the lungs. Within it, the T cells, which are critical to the adaptive immune system, mature.
Digestive and Excretory System
The function of the digestive system is digestion, absorption and metabolism.
Peristalsis is the rhythmic contraction of muscles that begins in the esophagus and continues in the gastrointestinal tract.
The
digestion process involves the breakdown of food into smaller components and it takes 12 to 48 hours. There are two types of digestion, mechanical and chemical. Mechanical digestion changes large pieces into small pieces by chewing and solid food into liquid form. Chemical digestion begins in the mouth with the addition of the enzyme ptyalin to the food, also, saliva contains digestive enzymes called amylase and lingual lipase. Digestion continues with swallowing through the esophagus and then the additional digestive processes in the stomach. Food is broken down by mixing with gastric acid and digestion is complete when nutrients have been reduced ti their simplest form.
Digested food is able to pass through the gastrointestinal tract.
Absorption of most of the nutrients occurs in small intestine, the majority in the jejunum. Absorption of water happens in the large intestine and the remaining waste material is defecated.
Metabolism is the chemical process by which the body uses nutrients for energy and the building of body tissue. Two processes are involved in metabolism, anabolism and catabolism. Anabolism is the building, repairing and maintaining of tissue, it is a constructive process used to synthetize complex molecules from precursors. On the other hand, catabolism is the breaking down of tissues and excretion of the byproducts, large molecules are broken down to provide energy.
The digestive system is composed of the
gastrointestinal tract and accessory organs. The gastrointestinal tract is made of the mouth, oropharynx, esophagus, stomach, small and large intestines, and the anus. The accessory organs are the liver, gallbladder and pancreas, their secretion flow into the small intestine and bring chemicals in food.
Some parts of the digestive system are also part of the
excretory system, including the large intestine. The excretory system is a passive biological system that removes excess, unnecessary material from the body to help maintain homeostasis and prevent damage. Its function is to eliminate waste and drain the body of used components.
The
mouth is the first part of the upper gastrointestinal tract and it begins digestion. The oral cavity is lined with a mucous membrane which secrets mucus which contains the glycoprotein mucin. Within the mouth, there are the tongue, teeth and three pairs of salivary glands which produce saliva to moisten and soften food. The tongue is a muscular sensory organ with taste buds and teeth are complex structures made of bone-like material called dentin covered by enamel. The roof of the mouth is called the palate and it separates the oral and the nasal cavities. The front part is called hard palate and the backpart is the soft palate, made of muscle and connective tissue which allows swallowing. There are also the uvula, the tonsils, the palatine arch and the epiglottis, a flap elastic cartilage attached to the entrance of the larynx to prevent food from going into the trachea.
The
pharynx is the common pathway of food and air, it is the part of the throat behind the nasal cavity. It is made of three parts, nasopharynx, oropharynx and laryngopharynx. When its muscular tissue contracts during swallowing, food and fluid move into the esephagus.
The
esophagus is a muscular tube about 25cm long and its peristalsis moves food into the stomach. At rest it is closed at both ends, the upper esophageal sphincter is triggered by the swallowing reflex.
The
diaphragm is a muscular separation of the thoracic cavity and the abdominal cavity. It anchors the liver and is attached to the duodenum.
The
stomach is a hollow, J-shaped organ composed of three parts, the fundus, the body and the pylorus. Stomach muscles contraction mix food with gastric secretion to form chyme after an hour or two. The stomach also produces hormones gastrin and enterogastrone and gastric juices that help in digestion. Gastric acid mainly contains hydrochloric acid and sodium chloride and the enzyme pepsin begins the digestion of proteins, the walls of the stomach are protected by a mucus layer. The stomach can expand to hold about one lite of food.
The lower gastrointestinal tracts begins with the small intestine and ends at the anus. The
small intestine is divided into the duodenum, jejunum and ileum and most food is digested here. Its glands secrete digestive enzymes and hormones. In the duodenum, the acidic chyme is neutralised by an alkaline fluid mix and absorption begins. The jejunum is about 2,5m long and it specialises in absorption of nutrients and magnesium. The ileum is from 2 to 4m long and it absorbs vitamin B12, bile salts and other products of digestion.
The
cecum is a pouch marking the division between the small and the large intestine and it is also the part where the appendix is attached. The
appendix is a blind-ended tube, it helps to maintain gut flora and it is an important part of the immune and lymphatic system.
The
large intestine, also called large bowel, is about 1,5m long and it begins at the cecum and consists of the colon and rectum. The passage of the digesting food is slower at the colon, it takes from 12 to 5 hours until it is removed by defecation. It serves to absorb water and fermentate digestible matter by the gut flora which it turned into feces. The rectum is the last section of the large intestine, it holds the formed feces awaiting elimination through the anus. The typical brown colour of mammal waste is due to bilirubin, a breakdown product of normal heme catabolism.
The
spleen is the largest lymphoid organ in the body, its function is to break down red and white blood cells. The product of this digestion is bilirubin, which is sent to the liver and secreted in the bile.
The
liver is the second largest organ on the right side under the diaphragm. It has many functions, it destroys toxic substances, helps with digesting carbohydrates and fats, synthetises proteins and cholesterol, stores glycogen, vitamins A, D, E, K, B12, iron, cholesterol and produces bile. Bile is a yellowish-green fluid that assembles and stores in the gallbladder, it helps in the absorption of vitamin K.
The
gallbladder is a hollow part of the biliary tract just beneath the liver. Bile, also called gall, is stored in this small organ before it is released into the small intestine to help break down fats.
The
pancreas lies on the left side under the stomach. It produces secretions and enzymes which change starches, fats and proteins into nutrients. It secrets insulin when the blood sugar becomes high, it moves glucose into the muscles for use as energy.
Endocrine System
The
endocrine system is a chemical messenger system of hormones released by internal glands into the circulatory system. The system operates via feedback loops. The major glands include the pineal gland, pituitary gland, pancreas, ovaries or testes, thyroid gland and parathyroid gland, hypothalamus and adrenal glands.
Hormones are signaling molecules produced by glands to target distant organs. Their function is to regulate physiology and behaviour, as a form of communication within the body. They bind to specific receptor proteins in the target cells resulting in a change in cell function.
The
thyroid gland is located at the front of the neck and it si shaped like a butterfly. The thyroid hormones increase the rate of cellular metabolism which can affect growth and development in children. The thyroid hormones can also affect the appetite, heartbeat, breathing, sexual function, sleep, blood flow and body temperature.
There are also 4 to 6 parathyroid glands, found at the back of the thyroid glands. Their hormones cause an increase in blood calcium levels.
The
adrenal glands are located above the kidneys and produce a variety of hormones, including adrenaline and various steroids. It controls the behaviour during crisis and emotional situations, stimulates the heart and metabolic processes.
The
pancreas is located in the abdomen, below and behind the stomach, it has both an endocrine and an exocrine function. It releases insulin, glucagon and other hormones into the blood, which influences sugar levels. Also, it secrets pancreatic juice into the duodenum.
The
pineal gland, also called conarium, is located in the brain and it is shaped like a pine cone. It primarily releases melatonin which influences daily rhythms and modulates sleep patterns.
The
hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions. It synthetizes and secretes certain neurohormones, such as dopamine, oxytocin and vasopressin, that stimulate or inhibit the secretion of hormones from the pituitary gland. It controls hunger and thirst, body temperature, attachment behaviour, fatigue and sleep.
The
pituitary gland, also called hypophysis, is a pea-sized gland located in the brain. It secretes hormones which help to control growth, blood pressure, body temperature, pain control, energy management, sex organs, thyroid glands and metabolism.
The
gonad, also called sex glands, are mixed glands that produce sex cells gametes and sex hormones. Female reproductive cells are the egg cells produced by the ovaries and they produce estrogens and progesterone to stimulate maturation and establish the menstrual cycle. Male reproductive cells are called sperm and produced by the testes which produce testosterone.
Integumentary and Exocrine System
The
integumentary system comprises of the skin and it includes skin, hair and nails. Its function is to protect the body from various kinds of damage, changes of temperature and dehydration, and it generates vitamin D through exposure to ultraviolet light.
The
skin is the larges organ of the body, it accounts for about 12 to 15% of total body weight and covers from 1,5 to 2m² of surface area. It is composed of at least two major layers of tissue, the epidermis and dermis.
The
epidermis is the outermost layer, providing the initial barrier to the external environment. It gives colour to the skin, it does not contain blood vessels and it consists of keratinized stratified squamous epithelium. The protein keratin also stiffens epidermal tissue to form nails which grow from the nail matrix at an average of 1mm per week. The deepest level of epidermis also contains nerve endings, it is separated from the dermis by the basement membrane.
The
dermis is the middle layer of skin, it comprises two sections, the papillary and reticular layers. It contains connective tissues, vessels, glands, follicles, hair roots. sensory nerve endings, and muscular tissue. Its function is to give elasticity to the integument while resisting distortions. The deepest layer, the hypodermis, is primarily made up of adipose tissue. Substantial collagen bundles anchor the dermis to the hypodermis in a way that permits most areas of the skin to move freely over the deeper tissue layers.
Hair is a protein filament that grows from follicles found in the dermis and only mammals have it. Each strand of hair is made up of the medulla, cortex and cuticle. The medulla is not always present, it is an open, unstructured region. The cortex is the highly structural and organized source of mechanical strength and water uptake, it contains melanin. The shape of hair depends on the shape of the follicle and the cortex and the colour depends on the type of melanin, eumelanin or pheomelanin. The diameter of human hair varies from 0,017 to 0,18mm.
The
hypodermis, also called the subcutaneous layer, is the layer beneath the skin. It is attached to the skin by collagen and elastin fibres and it is composed of adipocytes specialized in accumulating and storing fats. It acts as an energy reserve, the fats can be put back into circulation during intense effort or when a lack of energy occurs. It also participates in thermoregulation.
The
exocrine system is a system of hormones and glands, where the glands secrete straight to a target site. It includes the salivary glands, sweat glands, ciliary, ceruminous lacrimal, meibomian, sebaceous, mucous and mammary glands. These glands work independently responding to the needs of the body.
Exocrine glands contain a glandular portion and a duct portion the structures of which can be used to classify the gland. They are named based on how their products are secreted, by merocrine, apocrine or holocrine secretion.
Sweat glands, also known as sudoriferous or sudoriparous, are small tubular structures of the skin that produce sweat. Eccrine sweat glands are distributed almost all over the body, most densely in palms and soles, and they function as a cooling mechanism with a water-based secretion. Apocrine sweat glands are mostly limited to the armpits and perineal area. Modified appocrine glands form ceruminous, mammary and ciliary glands.
Ceruminous glands are located in the external auditory canal where they produce cerumen, simply called earwax, by mixing their secretion with sebum and dead epidermal cells.
Mammary glands produce milk to feed young offspring, they are arranged in organs such as breasts. Lactation is the secretion of milk and is stimulated by hormones, with humans only females can lactate. The female breast consists of subcutaneous fat which covers and envelops a network of ducts that converge on the nipple.
Ciliary glands, also called Moll's glands, are found on the margin of the eyelid and they secrete sebum that keeps lashes supple.
Lacrimal glands are paired, almond-shaped glands located in the eyes. They secrete the aqueous layer of the tear film and produce tears.
Sebaceous glands are located in the skin that opens into a hair follicle. They secrete an oily or waxy matter, called sebum, which lubricates the hair and skin. In humans, they occur mainly on the face and scalp, but there are no glands in the palms and soles.
Meibomian glands, also called tarsal glands, are a type of sebaceous glands in the eyes. They produce meibum, an oily substance that prevents tears spilling.
Mucous glands, called muciparous glands, are found in several parts of the body. They produce mucus, a polymer, a slippery aqueous secretion that serves to protect epithelial cells. Most of the mucus is produced in the gastrointestinal tract.
Salivary glands are three paired major glands that produce saliva. This benefits the oral cavity and health in general because it provides protection, helps digestion and dissolves solid particles which can then enter the taste buds.
Musculoskeletal System
The
muscular system is an organ system consisting of skeletal, smooth and cardiac muscles. It permits movement of the body, maintains posture and circulates blood throughout the body, it provides strength, balance, and heat. Some muscles are controlled by the nervous system and some are completely autonomous. Striated muscle tissue is a tissue that features repeating functional units called sarcomes, they are the basic building block. There are two types of striated tissue, cardiac and skeletal muscle tissue.
Skeletal muscles are composed of muscle fibers called myocytes, which are composed of myofibrils, which are composed of sarcomes. Upon stimulation by an action potential, skeletal muscles perform a coordinated contraction by shortening each sarcome. Within the sarcome, actin and myosin fibres overlap in a contractile motion towards each other. This process consumes large amounts of adenosine triphosphate ATP, the energy source of the sell. When ATP is used, it becomes adenosine diphosphate ADP. Muscle tissue also contains a stored supply of a fast acting recharge chemical, creatine phosphate. Calcium ions are also required for each cycle of sarcome. There are approximately 639 skeletal muscles in the human body.
At rest, the body produces the majority of its ATP
aerobically in the mitochondria without producing lactic acid or other fatiguing byproducts. During exercise, the method changes to
anaerobic pathways, such as the use of creatine phosphate and the phosphagen system of anaerobic glycolysis. This produces ATP much faster, but produces significant amounts of lactic acid which renders high-intensity exercise unsustainable for a longer time.
Cardiac muscles, heart muscle tissue, have muscle fibres laterally connected to each other. Their movement is involuntary and they are controlled by the autonomic nervous system. The myocardium forms a thick middle layer between the outer layer of the heart wall, the epicardium, and the inner layer, the endocardium. It is composed by cells joined together by intercalated discs, encased by collagen fibres and other substances that form the extracellular matrix.
Smooth muscles are controlled directly by the autonomic nervous system and are involuntary. They are divided into two groups, the unitary, single-unit, and multiunit smooth muscle. Within single-unit cells, the whole bundle or sheet contracts as a syncytium. Smooth muscles are found in the walls of hollow organs and in the walls of the passageways. Smooth muscle cells have a fusiform shape and can tense and relax, they demonstrate greater elasticity and function within a larger length-tension curve than striated muscle.
Fascia are fibrous connective tissue, primarily collagen fibres oriented in a wavy pattern parallel to the direction of pull. They connect or separate, stabilize and enclose muscles to other muscles or other internal organs.
The
skeletal system is the internal framework of the human body. It is composed of around 270 bones at birth, which decreases to around 206 bones by adulthood, after some bones are fused together. The bone mass in the skeleton reaches maximum density around age 21. The human skeleton can be divided into the axial skeleton and the appendicular skeleton. It performs six major functions, support, movement, protection, production of blood cells, storage of minerals and endocrine regulation. It is not as sexually dimorphic as that of many other primate species, but subtle differences between the sexes exist. Unlike most primates, human males do not have penile bones.
The skeleton is the site of
haematopoiesis, the development of blood cells that takes place in the bone marrow. The
bone marrow is a semi-solid tissue composed of hematopoietic cells, marrow adipose tissue and supportive stromal cells. It produces approximately 500 billion blood cells every day in the heads of long bones.
The
axial skeleton consists of 80 bones and is formed by the vertebral column 32-34 bones), the rib cage (12 pair bones and the sternum), the skull and other (22 bones and 7 associated bones).
The
skull consists of the cranium and the facial bones. The cranial vault holds and protects the brain, it is formed from eight plate-shaped bones which fit together. There are 14 facial bones, including the jaw, called mandible, the upper jaw, called the maxilla, and the cheek and nasal bones. The only bone that remains separate from the rest is the mandible.
The
rib cage is composed of 12 pairs of ribs and the sternum, a total of 25 bones. It protects vital organs, such as the heart and lungs. The upper seven pairs are true ribs, they attach to the sternum. The eighth to tenth ribs have non-costal cartilage which connects them to the ribs above. The last two are called floating ribs, because they are not attached.
The
vertebral column consists of five parts, 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 sacral vertebrae and 4 coccygeal vertebrae .At birth the majority of humans have 33 separate vertebrae. However, during normal development, several vertebrae fuse together, leaving a total of 24 vertebrae, the sacrum and the coccyx in most cases. The two lowest verterbrae, the sacrum and the coccyx, are single bones made up of several smaller bones fused together, they are simply called the sacral bone.
The
appendicular skeleton, which is attached to the axial skeleton, consists of 126 bones divided into six major regions, as it is formed by the shoulder girdle, the pelvic girdle and the bones of the upper and lower limbs. Their function is to make locomotion possible, they form from cartilage by the process of endochondral ossification. These bones are not fused and allow a much greater range of motion.
The
shoulder girdles are four bones, they consist of the left and right clavicle and scapula.
The
arms and forearms consist of sic bones, left and right humerus of the arm, the ulna and radius of the forearm.
The
hands consist of 54 bones, 16 left and right carpals (wrists), 10 metacarpals, 10 proximal phalanges, 8 intermediate phalanges and 10 distal phalanges.
The
pelvis consists of two bones, the left and right hip bone.
The
legs consist of eight bones, left and right femur of the thigh, two patella (knees), left and right tibia and fibula of the leg.
The
feet and ankles consist of 52 bones, 14 left and right tarsals (ankles), 10 metatersals, 10 phalanges, 8 intermediate phalanges and 10 distal phalanges.
Joints, also called articulations, are the connection made between bones, they allow different degrees and types of movement. Some joints are self-lubricating and almost frictionless, like the knee or elbow. Other joints permit very little movement. The connection between tooth and the jawbone is also called a joint, a fibrous joint known as a gomphosis.
Ligaments are fibrous connect tissue that connect bones to form joints. They are a band of dense regular connective tissue bundles made of collagenous fibres. They are viscoelastic, they gradually strain under tension and then return to their original shape.
Tendons, also called sinew, are a tough band of fibrous connective tissue that connect muscle to bone and is capable of withstanding tension. They are made of collagen fibres parallel to each other.
Cartilage is a resilient and smooth elastic tissue, a rubber-like padding that covers and protects the ends of long bones at the joints. It is a structural component of the rib cage, the ear and nose and many other. The matrix of cartilage is made up of glycosaminoglycans, proteoglycans, collagen fibres and sometimes elastin.
Nervous System
The
nervous system is a highly complex part of the human body that coordinated its actions and sensory information by transmitting signals to and from its different parts. It detects environmental changes and works with other systems to respond to such events. It consists of the central nervous system (CNS) and the peripheral nervous system (PNS).
Nerves that transmit signals from the brain are called motor or efferent, while those that transmit information from the body to the CNS are called sensory or afferent. The nerve cell is called a neuron, they have special structures that allow them to send signals rapidly and precisely to other cells. They send these signals in the form of electrochemical waves travelling along thin fibers called axons, which cause chemicals called neurotransmitters to be released at junctions called synapses. The connections between neurons can form neural pathways, neural circuits and larger networks that generate the perception of the world and determine behaviour.
Gray matter consists of neuronal cell bodies, neuropil (dendrites and myelinated and unmyelinated axons), glial cells, synapses and capillaries.
White matter consists of axons and oligodendrocytes, it contains relatively few cell bodies, it is composed chiefly of long-range myelinated axons called tracts. The colour difference arises mainly from the whiteness of myelin.
Glial cells, simply called glia, are specialized cells that provide structural and metabolic support. The white matter contains more glia.
The
CNS consists of the brain and spinal cord. Nerves that exit from the cranium are called cranial nerves while those exiting the spinal cord are called spinal nerves. The CNS is composed of white and gray matter.
The
brain is encased in the skull and protected by the cranium. It consists of a cortex, composed of neuron-bodies constituting gray matter. There is also the subcortical gray matter making up a large number of different nuclei. The brain is the centre of the nervous system and the most complex organ which contains 14-16 billion neurons in the cerebral cortex and 55-70 billion neurons n the cerebellum.
The
spinal cord is continuous with the brain and lies caudally to the brain, it is protected by the vertebrae. It is a long, thin, tubular structure made up of nervous tissue, it extends from the brainstem to the lumbar region of the spine. It encloses the central canal of the spinal cord which contains cerebrospinal fluid. It is made of 31 segments, is around 44cm long and can be 6,4mm wide in the thoracic area. It is a center for coordinating many reflexes and contains reflex arcs that can independently control reflexes.
The
PNS consists mainly of nerves which are enclosed bundles of long fibres and axons that connect the CNS to every other part of the body. It is divided into three separate sybsystems, somatic, automatic and enteric. Somatic nerves mediate voluntary movement. The autonomic nervous system is further divided into the sympathetic and parasympathetic systems. The sympathetic system is activated in cases of emergencies to mobilize energy, while the parasympathetic system is activated in a relaxed state. The enteric system functions to control the gastrointestinal system. Both autonomic and enteric systems function involuntarily.
Reproductive System
The
reproductive system, genital system, is a system of sex organs which work together for the purpose of sexual reproduction. The sexes of differentiated species have often significant differences between males and females. Humans have a high level of sexual differentiation, in addition to differences in nearly every reproductive organ, there are numerous differences in typical secondary sex characteristics. These allow for the combination of genetic material between two individuals, which allows for the possibility of greater genetic fitness of the offspring.
A
gamete is a haploid cells that fuses with another haploid cell during fertilization in organisms that sexually reproduce. The female gamete is a larger cell called ovum and the male gamete is a smaller cell called sperm.
Human reproduction involves internal fertilization by sexual intercourse. In this process, the male inserts his penis into the female's vagina and ejaculates semen, which contains sperm. A small portion of the sperm pass through the cervix into the uterus and then into the Fallopian tubes for fertilization of the ovum. Only one sperm is required to fertilize the ovum, which then turns into a diploid zygote, travels out of the Fallopian tube and into the uterus, where it implants in the uterine wall.
Pregnancy, also called gestation, is the time during which one or more offspring develops inside a woman. It continues for around nine months, about 38 weeks from fertilization, as the fetus develops. It is divided into three trimesters. At a certain point, pregnancy is concluded with childbirth, involving labor. During labor, the muscles of the uterus contract and the cervix dilates over the course of hours, and the baby passes out of the vagina.
Human infants are nearly helpless and require high levels of parental care. They rely rely on their parents for comfort, cleanliness and food, which can be provided by
breastfeeding, also called nursing, the feeding of a baby with milk from a woman's breast. The breast is prepared for lactation during pregnancy.
The
male reproductive system is a series of organs primarily located outside the body that have one function, to produce and deposit sperm. The main male sex organs are the penis and the testicles, the internal parts are the epididymis, sperm ducts and three glands, for example the prostate.
Testosterone is the primary male sex hormone and anabolic steroid. It plays a key role in the development of male reproductive tissues as well as promoting secondary sexual characteristics.
Semen, also called seminal fluid, contains spermatozoa and other enzymes and fructose.
Sperm is the male reproductive cell, it is able to move independently and it is much smaller than the female reproductive cell. On its own, it has a limited life span, but after fusion with an egg cell during fertilization, a new organism begins developing. It is haploid, so that its 23 chromosomes can join the 23 chromozomes of the egg to form a diploid cell.
The
penis is the male intromittent organ. It has a long shaft and an enlarged bulbous-shaped tip called the glans penis, which supports and is protected by the foreskin. When the male becomes sexually aroused, the penis becomes erect and ready for sexual activity. Erection occurs because sinuses within the erectile tissue of the penis become filled with blood. The arteries of the penis are dilated while the veins are compressed so that blood flows into the erectile cartilage under pressure. Ejaculation is the ejecting of semen from the penis, it is usually accompanied by orgasm. A series of muscular contractions delivers semen and usually, a refractory period succeeds the ejaculation. The penis also serves as the urinal duct.
The
scrotum is a pouch-like structure that hangs behind the penis, it holds and protects the
testicles. Testicles or testis are a male reproductive gland which produces sperm by meiotic division within the seminiferous tubules, and to synthetise and secrete androgens, primarily testosterone, that regulate the male reproductive functions.
The
epididymis is a long whitish mass of tightly coiled tube. It connects a testicle to a vas deferens, where the sperm undergo maturation and are concentrated.
The vas deferens, also known as
sperm duct, is a thin tube that carries the spermatozoa from the epididymis to ejaculatory duct in anticipation of ejaculation.
Three
accessory glands provide fluids that lubricate the duct system and nourish the sperm cells. These are the seminal vesicles, the prostate gland and the bulbourethal glands. The seminal vesicles, or seminal glands, are a pair of simple tubular glands that secrete fluid that partly composes the semen. The prostate secretes a fluid which contributes to the volume of the semen which is slightly alkaline to help prolonging the lifespan of sperm.
The
female reproductive system is a series of organs primarily located inside the body which have two functions, to produce egg cells and to protect and nourish the fetus until birth. The main female sex organs are the uterus, Fallopian tubes, ovaries and the vagina, the external part is the vulva.
Estrogen is the primary female sex hormone, its three forms are estrone, estradiol and estriol, estetrol is produced during pregnancy. It is responsible for the development and regulation of the female reproductive system and secondary sex characteristics.
Progestogens, also called gestagens, are steroid hormones, the most important is progesterone. Theie purpose is to maintain pregnancy.
Menstruation, also known as period, is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. It stops occurring after menopause and during pregnancy.
The
egg cell, or ovum, is the female reproductive cell, typically not capable of active movement. It is 0,1mm big, the largest cell in the human body. On maturity, the follicle and the ovary's wall rapture, allowing the egg to escape and the Fallopian tube. This is called ovulation, it is periodic and it impacts directly to the length of a menstrual cycle.
The
ovaries are small, paired organs that
produce egg cells and the secretion of hormones. At certain intervals, they release an ovum which passes through the Fallopian tube into the uterus.
The
Fallopian tubes, also called salpinges, are two tubes leading from the ovaries into the uterus.
The
uterus, of womb, accommodates the embryo which develops into a fetus. It provides mechanical protection, nutritional support, and waste removal for the developing baby. Contractions in its muscular wall are important in pushing out the baby. It produces vaginal and uterine secretions which help the transit of sperm to the Fallopian tubes. It contains three suspensory ligaments that help stabilize its position and limit its movement. It is pear-shaped and about 7,6cm long, 4,5cm broad and 3cm thick.
The
cervix is the neck of the uterus, the lower, narrow portion where it joins with the upper part of the vagina. It is cylindrical and conical in shape, usually 2-3cm long, and it protrudes through the upper anterior vaginal wall.
The
vagina is a fibromuscular canal leading from the outside of the body to the womb. It is elastic, it accommodates the penis during sexual intercourse and allows childbirth. The outer vaginal opening is partly covered by a membrane called hymen.
The external sex organ is the
vulva, which includes the mons pubis, labia, clitoris, urinary meatus, Bartholin's and Skene's vestibular glands and the vaginal opening. It protects the entrance to the vagina and it is supported by the pelvic floor muscles.
Respiratory System
The
respiratory system is associated with the exchange of gasses, oxygen and carbon dioxide, between the human body and the environment, and also between the tissue cells and the blood. These functions are achieved by ventilation, which consists of two acts,
inspiration, or taking air into the lungs because of the negative interpleural pressure created by enlarging of the thoracic cavity, and
expiration, or exhaling air out of the lungs during which the muscles of respiration return to their former position.
The structures concerned with ventilation are the upper and lower respiratory muscles, thorax and parts of nervous system.
External respiration involves the exchange of oxygen and carbon dioxide in the capillaries and alveoli of the lungs.
Internal respiration is the exchange of oxygen and carbon dioxide between cells and capillaries throughout the body. The normal rate of respiration varies with age, for an adult it is about 12 to 20 respirations per minute, for small children from 20 to 30.
The
upper respiratory tract is formed by the nose and mouth, nasal cavities and paranasal sinuses, pharynx and larynx.
Air passes through the
nose with two nostrils (nares) into the
nasal cavities which are separated by the nasal septum. The air is warmed and moistened. Several small cavities known as
paranasal sinuses are located in the skull near the nasal cavities, they are a group of four paired spaces, ethmoidal, sphenoidal, maxillary and frontal.
From the nasal cavities the air passes into the
pharynx, or throat, which is separated into three parts, nasopharynx, oropharynx and laryngopharynx. The adenoids are located in the nasopharynx and the tonsils in the oropharynx. These two pairs of lymphoid tissue protect the body against infection. The pharynx provide a common passageway for air entering the larynx and food entering the esophagus. It is also important in vocalization.
The
epiglottis is a leaf-shaped closure, that prevents the entrance of food and fluid into the respiratory tract. It closes during swallowing.
The
larynx, also called the voice box, is a tube-like structure made up of muscles and a series of cartilage rings, the largest is called Adam's apple. It contains the vocal cords and manipulates pitch and volume of phonation. It also prevents foreign objects from entering the lungs by coughing and other reflexive actions.
The
vocal cords are folds of tissue that are key in creating sounds through vocalization. They vibrate, modulating the flow of air being expelled from the lungs during phonation.
The
lower respiratory tract consists of the trachea, bronchi, bronchioles and the alveoli inside two lungs.
The
trachea, also called windpipe, is a cartilaginous tube that connects the larynx and the lungs. Its function is to provide a passage for air to reach the lungs. It is also lined with cilia and the mucous membrane.
The
bronchi conduct air into the lungs. They are situated in the thoracic cavity which is lined with a moist membrane called the pleura. The right bronchus is shorter and more vertical. Each bronchus enters a lung where it branches like a tree to form many smaller tubes called
bronchioles. These are smaller branches of the bronchial airways where the gas exchange occurs.
The microscopic
pulmonary alveolus, an air sac, is located at the end of each bronchiole. Across their membrane, oxygen is diffused and carbon dioxide released from the capillaries.
The
lungs are two large organs covered with visceral pleura and separated by the mediastinum. The right lung is divided into three lobes and the left into two lobes. Together they weigh about 1,3kg, the right one is bigger and heavies, because the left one shares space with the heart.
Urinary (Renal) System
The
urinary system, also known as the renal system or urinary tract, consists of the kidneys, ureters, bladder, and the urethra. It functions to eliminate waste from the body, regulate blood volume and blood pressure, control levels of electrolytes and metabolites, and regulate blood pH. It is the drainage system for the eventual removal of urine. It is regulated by the endocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone, parathrin. Vasopressin regulates the retention of water by increasing water reabsorption in the nephrons.
The female and male urinary system are very similar, differing only in the length of the urethra. The male urethra begins at the internal urethral orifice of the bladder, continues through the external urethral orifice, and then becomes prostatic, membranous, bulbar and penile urethra. The female urethra is much shorter, beginning at the bladder neck and terminating in the vaginal vestibule.
Urination, also called voiding, is the ejection of urine from the urinary bladder to the outside of the body. In healthy humans, the process is under voluntary control.
The
kidneys have an extensive blood supply via the renal vein. Each kidney consists of functional units called nephrons. Its chief function is to regulate the concentration of water and soluble substances like sodium, reabsorbing what is needed and excreting the rest. Urine flows through the nephrons, through a system of converging tubes called collecting ducts, which then form minor calyces and major calyces, that join in the renal pelvis. Following filtration of blood and further processing, wastes in the form of urine exit the kidneys via the ureters. The kidneys filtrate about 180l per day ad reabsorb about 99% of this.
The
ureters are tubes made of smooth muscle fibres that propel urine towards the urinary bladder. They are usually 25-30cm long and around 3-4mm wide.
The
urinary bladder is a hollow muscular organ that stores and subsequently expels urine from the body by urination. A typical bladder will hold 300-500ml of urine before the urge to empty occurs, but it can hold considerably more.
During urination, the urine s passed from the bladder through the
urethra to the outside of the body. A healthy human produces 800-2000ml of urine every day.
