human body

Physique of the human organism, consisting of living cells and extracellular materials, arranged into tissues, organs, and systems.

Many articles deal with human anatomy and physiology. The following topics are discussed in more detail: human blood; cardiovascular system; digestive system; endocrine system; renal system; skin; human muscle system; nervous system; reproductive system; respiration; sensory reception; skeletal system. See aging; growth; prenatal development; human development for a description of how the body develops from conception to old age.

See protein, carbohydrate, lipid, nucleic acid, vitamin, and hormone for detailed coverage of the body's biochemical components. See cell for information about the structure and function of cells.

The body's major structures are described in many entries. See, for example, the abdominal cavity; the adrenal gland; the aorta; the bone; the brain; the ear; the eye; the heart; the kidney; the large intestine; the lung; the nose; the ovary; the pancreas; the pituitary gland; the small intestine; the spinal cord; the spleen; the stomach; the testis; the thymus; the thyroid gland; the tooth; the uterus; and the verte

As animals, humans fall into the order Primates in the subphylum Vertebrata of the phylum Chordata. All chordates have a bilaterally symmetrical body that is characterized by a rod-like dorsal support (the notochord), gill slits in the region of the pharynx, and a hollow dorsal nerve cord. In humans, the first two embryonic features are present only during the embryonic stage; the notochord is replaced by the vertebral column, and the pharyngeal gill slits disappear entirely. In humans, the dorsal nerve cord is the spinal cord, which remains throughout life.

The human body has a backbone of vertebrae that is characteristic of the vertebrate form. Humans have a mammalian body structure, including hair, mammary glands, and highly developed sense organs.

Despite these similarities, there are some profound differences as well. Humans are the only mammals that have a predominantly two-legged (bipedal) posture, a fact that has changed the general body plan of mammals. A kangaroo, which walks on four legs when standing, hops on two legs when moving rapidly. Furthermore, the human brain, particularly the neocortex, is by far the most highly developed among the animal kingdom. Chimpanzees and dolphins are also intelligent, but none have reached the intellectual level of humans.

Chemical composition of the body

The human body is primarily composed of water and organic compounds, such as lipids, proteins, carbohydrates, and nucleic acids. Extracellular fluids of the body (blood plasma, lymph, and interstitial fluid) and cells themselves contain water. The chemistry of life would not be possible without it. Humans contain 60 percent water by weight.

Major structural components of the human body are lipids, particularly fats and phospholipids. The body's fat provides energy reserves, and fat pads act as insulation and shock absorbers. A cell's membrane is composed primarily of phospholipids and cholesterol, a steroid compound.

Proteins are also important structural components of the body. A cell's membrane is made up of proteins. Additionally, hair and nails are composed of protein. As is collagen, a fibrous, elastic substance found in much of the skin, bones, tendons, and ligaments of the body. The body's proteins also serve a variety of functions. The chemical reactions that sustain life are catalyzed by cellular proteins called enzymes.

The human body uses carbohydrates primarily as fuel, either by circulating simple sugars through the bloodstream or by storing them as glycogen, which is found in the liver and in the muscles. A small amount of carbohydrates also occurs in cell membranes, but humans have relatively little structural carbohydrate in their bodies compared to plants and many invertebrates.

The genetic material of the body is composed of nucleic acids. Deoxyribonucleic acid (DNA) contains the body's genetic code, the instructions for every cell. DNA, passed from parents to children, is what determines an individual's inherited characteristics. RNA, of which there are several types, helps carry out DNA instructions.

The body's constituents include various inorganic minerals in addition to water and organic compounds. Calcium, phosphorus, sodium, magnesium, and iron are among the most important elements. The body's bones are made up of calcium-phosphate crystals, a combination of calcium and phosphorus. The blood and interstitial fluid contain calcium and sodium ions as well. Among the intercellular fluid's constituent ions are phosphorus, potassium, and magnesium. The body's metabolic processes depend on all of these ions. The iron in red blood cells is largely contained in hemoglobin, the pigment that carries oxygen. Minerals present in minute concentrations in the body include cobalt, copper, iodine, manganese, and zinc.

Organization of the body

Cells are the basic units of life in the human body - indeed, in all organisms. Each cell in the human body is capable of growing, metabolizing, responding to stimuli, and, with some exceptions, reproducing. Despite the fact that the body contains 200 different types of cells, these can be grouped into four basic classes. Together, these four types of cells form the four basic tissues of the human body: (1) epithelial tissues, which cover the surface of the body and line internal organs, cavities, and passageways; (2) muscle tissues, which contain muscle fibers and form the body's musculature; (3) nerve tissues, which conduct electrical signals and constitute the nervous system; and (4) connective tissues, which are composed of collagen. Cells with a large amount of intercellular matrix that binds together the various body structures. (Bone and blood are specialized connective tissues, in which the intercellular matrix is hard and liquid, respectively.)

Organs are the next level of body organization. A group of tissues that form a distinct structural and functional unit is an organ. Hearts consist of all four tissues, and their function is to pump blood throughout the body. The heart is part of a larger system that includes blood vessels and blood as well. Therefore, the organ system is the highest level of body organization.

There are nine major organ systems in the body, each consisting of various tissues and organs that function together as an integrated whole. Each system also has its own primary function. (1) The integumentary system, which is made up of the skin and its associated structures, keeps the body protected from harmful microorganisms and chemicals, as well as preventing water loss. (2) The musculoskeletal system (also known as the muscle system and skeletal system) consists of muscles and bones (with about 206 in adults), which move the body and protect its internal organs. (3) The respiratory system, composed of the airways, lungs, and muscles of respiration, gets the oxygen it requires for cellular metabolism from the air, and returns to it the carbon dioxide it produces as a waste product. (1) The circulatory system, which consists of the heart, blood, and blood vessels, circulates a transport fluid throughout the body, providing cells with oxygen and nutrients and removing waste products such as carbon dioxide and nitrogen compounds. The digestive system, consisting of the mouth, esophagus, stomach, and intestines, breaks down food into edible substances (nutrients), which are then absorbed from the blood or lymph; this system also eliminates unusable or excess portions of food as feces. The excretory system, which consists of the kidneys, ureters, urinary bladder, and urethra, removes toxic nitrogen compounds from the blood. (1) The nervous system, which consists of the sensory organs, the brain, the spinal cord, and the nerves, transmits, integrates, and analyzes sensory information, as well as transmitting impulses to effect the appropriate muscular or glandular responses. (8) The endocrine system consists of hormone-secreting glands and tissues that coordinate various body functions through chemical communications. (9) The reproductive system, comprising the male or female sex organs, enables reproduction, ensuring the continuation of the species.

Basic form and development

The human body can be described as a cylinder that encloses two tubes and a rod. In the embryo, the body plan is most evident; at birth, it is only evident in the trunk region, that is, the thorax and abdomen.

Cylinder-shaped bodies are formed by body walls. The alimentary canal is ventrally located (i.e., the digestive tract), and the neural tube is dorsally located (i.e., the spinal cord). There is a rod that lies between the tubes of the embryo - the notochord - which becomes the vertebral column before birth. (The terms dorsal and ventral refer to a creature's back and belly, respectively.)

An embryo's essential body parts include: (1) the outer epidermal membrane (in the embryo called ectoderm); (2) the dorsal neural tube; (3) the notochord; (4) the ventral alimentary tube, which becomes the lining of the stomach and intestines (in the embryo called endoderm); and (6) a rather fluid tissue that fills the interspaces and is derived from the mesoderm (in the embryo called mesenchyme). A part of these embryonic parts gives rise to everything in the body.

There is a considerable amount of mesoderm on each side of the embryo, extending from the back to the front of the body wall. It is hollow, for on each side there is a cleft-like opening. This is the right and left body cavities. In the dorsal part of the body, they are temporary; in the ventral part, they become permanent, forming the pleural cavity, which encloses the lungs; the peritoneal cavity, which encloses the abdominal organs; and the pericardial cavity, which encloses the heart. In the dorsal mesoderm, the ventral mesoderm splits into 31 serial parts like a row of blocks. Toward the epidermal membrane, these mesodermal segments grow in all directions. Bones, muscles, and deeper, leathery parts of the skin are formed by them. Bony arches protect the spinal cord on the dorsal side, and ribs protect the alimentary canal and heart on the ventral side. As a result, they form the body wall and the limbs, which are much heavier. Segmented body walls in the neck and trunk correspond to segmented spinal cords. As it is not so extensive, the ventral mesoderm remains near the alimentary tube and becomes the continuous muscle layer of the stomach and intestine. The lining of body cavities is also formed by it, the smooth, shiny, slippery pleura and peritoneum. Blood and lymph vessels, the heart, and the loose cells of connective tissues are made up of mesenchyme.

Early in development, the neural tube is formed from the ectoderm. On the anterior side (i.e., towards the head), it extends above the open end of the cylinder and is enlarged to form the brain. Dorsal mesoderm surrounds the brain and surrounds the roots of the cranial nerves as a covering, separating the brain from the epidermis. The neural tube terminates posteriorly in the adult opposite the first lumbar vertebra.

The cylindrical body wall terminates ventrally as the tongue and dorsally in the skull around the brain, ears, and eyes. Between the eyes and tongue there is a considerable distance. It is partly occupied by a deep depression of the epidermis between them, where it joins the alimentary tube (lining of the mouth). Upon termination of the body cavities, the ventral and dorsal walls of the body unite at the tailbone (coccyx).

In front of and just below the brain, the alimentary tube extends up to join the epidermal depression in front of the notochord. The epidermal depression gives rise to the teeth and most of the mouth lining; the upper end of the alimentary canal gives rise to the pharynx, larynx, trachea, and lungs. At its tail end, the alimentary canal divides longitudinally into an anterior and a posterior tube. Anterior tubes become bladders, urethras, and, in females, linings of the vagina, where they join an ectodermic depression. By joining another ectodermal depression (the anus), the posterior (dorsal) tube becomes the rectum and ends just in front of the coccyx.

Effects of aging

Individuals experience different aging processes at different times and at varying rates as their bodies age.

One of the best indicators of aging is the skin. The skin becomes thin, dry, and loses its elasticity with age. There are patches of darker pigmentation, commonly called liver spots, even though they have no relation to that organ. The hair thins and grays. Some wounds heal more slowly at 60 than at 10 years of age; some repairs take five times longer at 60. In spinal nerves, sensory fibres become fewer; ganglion cells become pigmented and some of them die. Some nerve cells and fibres in the auditory apparatus are lost, and high notes become more difficult to hear. Your eyes lose their elasticity as well.

A person's liver and kidneys lose mass as they age and become less efficient. The brain gets smaller after age 40 and shrinks significantly after age 75, especially in the frontal and occipital lobes. However, this shrinkage is not linked to declines in mental ability. Degenerative diseases such as Alzheimer's and cerebrovascular disease are associated with cognitive decline in the elderly.

The bones become lighter and brittle due to a loss of calcium. The bone mass loss after the fifth decade is greater in women than in men. When cartilage covering the ends of bones thins and disappears in places, bone meets bone directly, causing creaking. Loss of height may occur when the spinal column is compressed. Individuals may experience varying degrees of muscular weakness.

Sclerosed and fibrous arteries develop. Their elasticity decreases, and they tend to become rigid tubes. Old people always have fatty spots on their skin, which appear in their lining even at a young age.

Experiments in vitro have demonstrated that the body's cells are programmed to undergo a finite number of divisions before they lose their reproductive capacity. The human body's potential longevity appears to be encoded in the very cells of the body-about 100 years.

Change incident to environmental factors

Even though the basic shape of the human body was established by human anthropoid ancestors, different human populations have evolved to fit their environments. Humans, for instance, have physical adaptations in response to extreme cold, humid heat, and high altitudes.

Short, round people with short arms and legs, fat pads over the sinuses, narrow noses, and a heavier layer of body fat are more prone to extreme cold. In order to provide minimum heat loss, minimal heat loss in the extremities (which allows manual dexterity during exposure to cold and prevents frostbite), and protection of the lungs and base of the brain against cold air in the nasal passages, these adaptations reduce the surface area of the body in relation to the mass.

When it's hot outside, the problem isn't maintaining body heat, but dissipating it. The body normally gets rid of excess heat by sweating. Nevertheless, humidity in humid heat can prevent perspiration from evaporating to some extent and result in overheating. In humid climates, the heat-adapted person is therefore tall and thin, allowing maximum surface area to radiate heat. People living in hot climates have little body fat; a wide nose, as the air in the nasal passages warms too rapidly; and dark skin, which shields them from harmful UV rays.

In high altitudes, one must adapt to cold conditions as well as low air pressure and low oxygen levels. Lung tissue increases in response to this adaptation.

Despite the fact that the general shape and size of the body and its parts is determined by heredity, the body can undergo some modifications in response to current conditions. Consequently, someone moving from a sea-level to a mountain-altitude home will experience an increase in red blood cells; this increase compensates for the decrease in oxygen levels in the new environment. In the same way, a light-skinned person moving to a hot tropical region will develop increased pigmentation. The resultant form is rarely perfect for the new conditions, but it is adapted to present needs well enough to maintain life with the least amount of energy waste.