Female (left) and male (right) adult human bodies photographed in ventral (above) and dorsal (below) perspectives. Naturally-occurringpubic,body, andfacial hair have beendeliberately removed to show anatomy.
Thehuman body is the entire structure of ahuman being. It is composed of many different types ofcells that together createtissues and subsequentlyorgans and thenorgan systems.
The study of the human body includesanatomy,physiology,histology andembryology. The bodyvaries anatomically in known ways. Physiology focuses on the systems and organs of the human body and their functions. Many systems and mechanisms interact in order to maintainhomeostasis, with safe levels of substances such assugar,iron, andoxygen in the blood.
The body is studied byhealth professionals, physiologists, anatomists, and artists to assist them in their work.
The adult male body is about 60%total body water content of some 42 litres (9.2 imp gal; 11 US gal). This is made up of about 19 litres (4.2 imp gal; 5.0 US gal) ofextracellular fluid including about 3.2 litres (0.70 imp gal; 0.85 US gal) ofblood plasma and about 8.4 litres (1.8 imp gal; 2.2 US gal) ofinterstitial fluid, and about 23 litres (5.1 imp gal; 6.1 US gal) of fluid inside cells.[1] The content,acidity and composition of the water inside and outside cells is carefully maintained. The mainelectrolytes in body water outside cells aresodium andchloride, whereas within cells it ispotassium and otherphosphates.[2]
The body contains trillions ofcells, the fundamental unit of life. At maturity, there are roughly 30 trillion cells, and 38 trillion bacteria in the body,[3][4] an estimate arrived at by totaling the cell numbers of all theorgans of the body andcell types. The skin of the body is also host to billions of commensal organisms as well as immune cells.[5] Not all parts of the body are made from cells. Cells sit in anextracellular matrix that consists ofproteins such ascollagen, surrounded by extracellular fluids.
Each of the cells of the human body experiences, on average, tens of thousands ofDNA damages per day.[6] These damages can blockgenome replication or genome transcription, and if they are notrepaired or are repaired incorrectly, they may lead tomutations, or other genome alterations that threaten cell viability.[6]
Cells in the body function because ofDNA. DNA sits within thenucleus of a cell. Here, parts of DNA arecopied and sent to the body of the cell viaRNA.[7] The RNA is then used tocreateproteins, which form the basis for cells, their activity, and their products. Proteins dictate cell function and gene expression, a cell is able to self-regulate by the amount of proteins produced.[8] However, not all cells have DNA; some cells such as maturered blood cells lose their nucleus as they mature.
Cells that line surfaces exposed to the outside world or gastrointestinal tract (epithelia) or internal cavities (endothelium) come in numerous shapes and forms – fromsingle layers of flat cells, to cells with small beating hair-likecilia in the lungs, to column-like cells that line thestomach. Endothelial cells are cells that line internal cavities including blood vessels and glands. Lining cells regulate what can and cannot pass through them, protect internal structures, and function as sensory surfaces.[10]
1905 diagram of the internal organs of the human body
Organs, structured collections ofcells with a specific function,[11] mostly sit within the body, with the exception ofskin. Examples include theheart,lungs andliver. Many organs reside withincavities within the body. These cavities include theabdomen (which contains the stomach, for example) andpleura, which contains the lungs.
The heart is composed oftwo atria andtwo ventricles. The primary purpose of the atria is to allow uninterrupted venous blood flow to the heart duringventricular systole. This allows enough blood to get into the ventricles duringatrial systole. Consequently, the atria allows acardiac output roughly 75% greater than would be possible without them.[13] The purpose of the ventricles is topump blood to the lungs through the right ventricle and tothe rest of the body through the left ventricle.[14]
The heart has anelectrical conduction system to control the contraction and relaxation of the muscles. It starts in thesinoatrial node traveling through the atria causing them topump blood into the ventricles. It then travels to theatrioventricular node, which makes the signal slow down slightly allowing the ventricles to fill with blood before pumping it out and starting the cycle over again.[15]
Gallstones is a common disease in which one or more stones form in the gallbladder orbiliary tract. Most people are asymptomatic but if a stone blocks the biliary tract, it causes agallbladder attack; symptoms may include sudden pain in the upper right abdomen or center of the abdomen. Nausea and vomiting may also occur. Typical treatment is removal of the gallbladder through a procedure called acholecystectomy.[22][23] Having gallstones is a risk factor forgallbladder cancer, which, although quite uncommon, is rapidly fatal if not diagnosed early.[24]
Thecirculatory system consists of theheart andblood vessels (arteries,veins andcapillaries). The heart propels the circulation of the blood, which serves as a "transportation system" to transferoxygen, fuel, nutrients, waste products, immune cells and signaling molecules (i.e.hormones) from one part of the body to another. Paths of blood circulation within the human body can be divided into two circuits: thepulmonary circuit, which pumps blood to the lungs to receiveoxygen and leavecarbon dioxide, and the systemic circuit, which carries blood from the heart off to the rest of the body. The blood consists of fluid that carriescells in the circulation, including some that move from tissue to blood vessels and back, as well as thespleen andbone marrow.[25][26][27]
Digestion begins in themouth, whichchews food into smaller pieces for easier digestion. Then it isswallowed, and moves through theesophagus to thestomach. In the stomach, food is mixed withgastric acids to allow the extraction ofnutrients. What is left is calledchyme; this then moves into thesmall intestine, which absorbs the nutrients and water from the chyme. What remains passes on to thelarge intestine, where it is dried to formfeces; these are then stored in therectum until they are expelled through theanus.[29]
Theintegumentary system consists of the covering of the body (theskin), including hair andnails as well as other functionally important structures such as thesweat glands andsebaceous glands. The skin provides containment, structure, and protection for other organs, and serves as a major sensory interface with the outside world.[32][33]
Thelymphatic system extracts, transports and metabolizes lymph, the fluid found in between cells. The lymphatic system is similar to the circulatory system in terms of both its structure and its most basic function, to carry a body fluid.[34]
Female puberty generally occurs between the ages of 9 and 13 and is characterized byovulation andmenstruation; the growth of secondary sex characteristics, such as growth ofpubic andunderarm hair,breast,uterine andvaginal growth, widening hips and increased height and weight, also occur during puberty.[40] Male puberty sees the further development of theHuman penis andtesticles.[41]
Therespiratory system consists of the nose,nasopharynx,trachea, andlungs. It brings oxygen from the air and excretescarbon dioxide and water back into the air. First,air is pulled through the trachea into the lungs by thediaphragm pushing down, which creates avacuum. Air is briefly stored inside small sacs known asalveoli (sing.: alveolus) before being expelled from the lungs when the diaphragm contracts again. Each alveolus is surrounded bycapillaries carrying deoxygenated blood, which absorbs oxygen out of the air and into thebloodstream.[50][51]
For the respiratory system to function properly, there need to be as few impediments as possible to the movement of air within the lungs.Inflammation of the lungs and excessmucus are common sources of breathing difficulties.[51] Inasthma, the respiratory system is persistently inflamed, causingwheezing orshortness of breath.Pneumonia occurs throughinfection of the alveoli, and may be caused bytuberculosis.Emphysema, commonly a result ofsmoking, is caused by damage to connections between the alveoli.[52]
Theurinary system consists of the twokidneys, twoureters,bladder, andurethra. It removes waste materials from the blood through urine, which carries a variety of waste molecules and excessions and water out of the body.
First, thekidneys filter the blood through their respectivenephrons, removingwaste products likeurea,creatinine and maintaining theproper balance ofelectrolytes and turning the waste products intourine by combining them with water from the blood.[53] The kidneys filter about 150 quarts (170 liters) of blood daily, but most of it is returned to the blood stream with only 1-2 quarts (1-2 liters) ending up as urine,[54] which passes from the kidneys through theureters into thebladder.
Thesmooth muscles lining the ureter walls continuously tighten and relax through a process calledperistalsis, forcing small amounts of urine into the bladder every 10–15 seconds.
The bladder is a hollow balloon shaped organ located in thepelvis. It stores urine until the brain signals it to relax theurinary sphincter and release the urine into the urethra startingurination.[55] A normal bladder can hold up to 16 ounces (half a liter) for 3–5 hours comfortably.
Humananatomy is the study of the shape and form of the human body. The human body has fourlimbs (two arms and two legs), a head and aneck, which connect to thetorso. The body's shape is determined by a strongskeleton made ofbone andcartilage, surrounded by fat (adipose tissue), muscle,connective tissue, organs, and other structures. Thespine at the back of the skeleton contains the flexiblevertebral column, which surrounds thespinal cord, which is a collection of nerve fibres connecting thebrain to the rest of the body.Nerves connect the spinal cord and brain to the rest of the body. All major bones, muscles, and nerves in the body are named, with the exception ofanatomical variations such assesamoid bones andaccessory muscles.
Blood vessels carry blood throughout the body, which moves because of the beating of theheart.Venules andveins collect blood low in oxygen from tissues throughout the body. These collect in progressively larger veins until they reach the body's two largest veins, thesuperior andinferior vena cava, which drain blood into the right side of the heart. From here, the blood is pumped into thelungs where it receives oxygen and drains back into the left side of the heart. From here, it is pumped into the body's largestartery, theaorta, and then progressively smaller arteries andarterioles until it reaches tissue. Here, blood passes from small arteries intocapillaries, then small veins and the process begins again. Blood carriesoxygen, waste products, andhormones from one place in the body to another. Blood is filtered at thekidneys andliver.
Height, weight,shape and otherbody proportions vary individually and with age and sex. Body shape is influenced by the distribution of bones,muscle andfat tissue.[58]
Humanphysiology is the study of how the human body functions. This includes the mechanical, physical,bioelectrical, andbiochemical functions of humans in good health, fromorgans to thecells of which they are composed. The human body consists of many interacting systems of organs. These interact to maintainhomeostasis, keeping the body in a stable state with safe levels of substances such as sugar and oxygen in the blood.[59]
Each system contributes to homeostasis, of itself, other systems, and the entire body. Some combined systems are referred to by joint names. For example, the nervous system and the endocrine system operate together as theneuroendocrine system. The nervous system receives information from the body, and transmits this to the brain vianerve impulses andneurotransmitters. At the same time, theendocrine system releases hormones, such as to help regulateblood pressure and volume. Together, these systems regulate the internal environment of the body, maintaining blood flow, posture, energy supply, temperature, and acid balance (pH).[59]
Development of the human body is the process of growth to maturity. The process begins with fertilisation, where an egg released from theovary of a female is penetrated bysperm. The egg then lodges in theuterus, where anembryo and laterfetus develop untilbirth. Growth and development occur after birth, and include both physical and psychological development, influenced by genetic, hormonal, environmental and other factors. Development and growth continue throughout life, throughchildhood,adolescence, and throughadulthood toold age, and are referred to as the process ofaging.
Health professionals learn about the human body from illustrations, models, and demonstrations. Medical and dental students in addition gain practical experience, for example bydissection of cadavers. Human anatomy,physiology, andbiochemistry are basic medical sciences, generally taught to medical students in their first year at medical school.[60][61][62]
In Western societies, the contexts for depictions of the human body includeinformation,art andpornography. Information includes both science and education, such as anatomical drawings. Any ambiguous image not easily fitting into one of these categories may be misinterpreted, leading to disputes.[63] The most contentious disputes are between fine art and erotic images, which define the legal distinction of which images are permitted or prohibited.
Boitano, Scott; Brooks, Heddwen L.; Barman, Susan M.;Barrett, Kim E. (2016).Ganong's Review of Medical Physiology. McGraw-Hill Education.ISBN978-0-07-182510-8.
Susan Standring, ed. (2008).Gray's anatomy: the anatomical basis of clinical practice (40th ed.). London: Churchill Livingstone.ISBN978-0-8089-2371-8.
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