Within the embryo, the essential body parts are: Everything in the body derives from one of these six embryonic parts. The mesoderm constitutes a considerable pad of tissue on each side of the embryo, extending all the way from the back to the front sides of the body wall.
It is hollow, for a cleftlike space appears in it on each side. These are 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 two pleural cavities, which house the lungs; the peritoneal cavity, which contains the abdominal organs; and the pericardial cavity, which encloses the heart.
The dorsal part of the mesoderm becomes separated from the ventral mesoderm and divides itself into serial parts like a row of blocks, 31 on each side. These mesodermal segments grow in all directions toward the epidermal membrane. They form bones, muscles, and the deeper, leathery part of the skin. Dorsally they form bony arches protecting the spinal cord, and ventrally the ribs protecting the alimentary canal and heart. Thus they form the body wall and the limbs—much the weightier part of the body.
They give the segmental character to the body wall in neck and trunk, and, following their lead, the spinal cord becomes correspondingly segmented.
Vital organs
The ventral mesoderm is not so extensive; it remains near the alimentary tube and becomes the continuous muscle layer of the stomach and intestine. It also forms the lining of the body cavities, the smooth, shining, slippery pleura and peritoneum. The mesenchyme forms blood and lymph vessels, the heart, and the loose cells of connective tissues. The neural tube itself is formed from the ectoderm at a very early stage. It is not in immediate contact with the epidermis, for the dorsal mesoderm grows up around it and around the roots of the cranial nerves as a covering, separating the brain from the epidermis.
Posteriorly the neural tube terminates in the adult opposite the first lumbar vertebra. If the cylindrical body wall is followed headward, it is found to terminate ventrally as the tongue , dorsally in the skull around the brain, ears, and eyes. There is a considerable interval between eyes and tongue. This is occupied partly by a deep depression of the epidermis between them, which dips in to join the alimentary tube lining of the mouth.
Posteriorly the ventral body wall joins the dorsal at the tailbone coccyx , thus terminating the body cavities. Headward, the alimentary tube extends up in front of the notochord and projects above the upper part of the body wall tongue and in front of and below the brain to join the epidermal depression. From the epidermal depression are formed the teeth and most of the mouth lining; from the upper end of the alimentary canal are formed the pharynx, larynx , trachea , and lungs.
The alimentary canal at its tail end splits longitudinally into two tubes—an anterior and a posterior. The anterior tube becomes the bladder , urethra, and, in the female, the lining of the vagina , where it joins a depression of the ectoderm. The posterior dorsal tube becomes the rectum and ends just in front of the coccyx by joining another ectodermal depression the anus.
As the human body ages it undergoes various changes, which are experienced at different times and at varying rates among individuals. The skin is one of the most accurate registers of aging. It becomes thin and dry and loses elasticity.
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Patches of darker pigmentation appear, commonly called liver spots, though they have no relation to that organ. Hair grays and thins. Wounds take longer to heal; some reparations take five times as long at 60 as at 10 years of age. Sensory fibres in spinal nerves become fewer; the ganglion cells become pigmented and some of them die.
Human body
In the auditory apparatus some nerve cells and fibres are lost, and the ability to hear high notes diminishes. In the eye the lens loses its elasticity. Organs such as the liver and kidneys lose mass with age and decline in efficiency. The brain is somewhat smaller after the age of 40 and shrinks markedly after age 75, especially in the frontal and occipital lobes. This shrinkage is not, however, correlated with declines in mental capacity. Intellectual declines in the elderly are the consequence of underlying disease conditions, such as Alzheimer disease or cerebrovascular disease.
The bones become lighter and more brittle because of a loss of calcium. This loss in bone mass is greater in women than men after the fifth decade. In joints the cartilage covering the ends of bone becomes thinner and sometimes disappears in spots, so bone meets bone directly and the old joints creak. Compression of the spinal column can lead to a loss of height. Muscular strength decreases but with marked individual variability.
The arteries become fibrous and sclerosed. Because of decreasing elasticity, they tend to become rigid tubes. Fatty spots, which appear in their lining even in youth, are always present in old age. Thus, the potential longevity of the human body—about years—seems to be encoded within the very cells of the body. Although the basic form of the human body was established in human anthropoid ancestors, evolutionary adaptations to different environments are apparent among various human populations.
For example, physical adaptations in humans are seen in response to extreme cold , humid heat, and high altitudes. Extreme cold favours short, round persons with short arms and legs, flat faces with fat pads over the sinuses, narrow noses, and a heavier than average layer of body fat. These adaptations provide minimum surface area in relation to body mass for minimum heat loss, minimum heat loss in the extremities which allows manual dexterity during exposure to cold and guards against frostbite , and protection of the lungs and base of the brain against cold air in the nasal passages.
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In hot climates the problem is not in maintaining body heat but in dissipating it. Ordinarily the body rids itself of excess heat by sweating. In conditions of humid heat, however, the humidity of the surrounding air prevents the evaporation of perspiration to some extent, and overheating may result. Hence, the heat-adapted person in humid climates is characteristically tall and thin, so that there is maximum surface area for heat radiation.
The person living in hot climates has little body fat; often a wide nose, since warming of the air in the nasal passages is not desirable; and, usually, dark skin, which provides a shield from harmful solar radiation.
Human body systems
High altitudes demand a degree of cold adaptation , as well as adaptation for low air pressure and the consequent low oxygen. This adaptation is accomplished by an increase in lung tissue generally. Despite the fact that the general shape and size of the body and its parts are determined by heredity , the body can undergo some modifications in response to present conditions. Thus, a person who moves from a home at sea level to one at mountain altitudes will experience an increase in the number of red blood cells; this increase helps compensate for the lower oxygen levels of the new environment.
Similarly, a light-skinned individual who moves to a hot tropical region will develop increased pigmentation in the skin.
In such situations, the resultant form is seldom perfect for the new conditions, but it is adapted to present needs well enough to maintain life with the least waste of energy. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind. Your contribution may be further edited by our staff, and its publication is subject to our final approval. Unfortunately, our editorial approach may not be able to accommodate all contributions.
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Organization of the body
Learn More in these related Britannica articles: Blood , fluid that transports oxygen and nutrients to the cells and carries away carbon dioxide and other waste products. Technically, blood is a transport liquid pumped by the heart or an equivalent structure to all parts of the body, after which it is returned to the heart to repeat the….
The human body, like that of most animals, develops from a single cell produced by the union of a male and a female gamete or sex cell. This union marks the beginning of the prenatal period, which in humans encompasses three distinct stages: More About Human body 12 references found in Britannica articles Assorted References application of property law In property law: The human body measurement In anthropometry anatomy and physiology genetics In heredity: It consists mainly of the trachea, the diaphragm and the lungs. The urinary system helps eliminate a waste product called urea from the body, which is produced when certain foods are broken down.
The whole system includes two kidneys, two ureters, the bladder, two sphincter muscles and the urethra. Urine produced by the kidneys travels down the ureters to the bladder, and exits the body through the urethra. The skin, or integumentary system , is the body's largest organ.
It protects us from the outside world, and is our first defense against bacteria, viruses and other pathogens. Our skin also helps regulate body temperature and eliminate waste through perspiration.
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In addition to skin, the integumentary system includes hair and nails. Humans have five vital organs that are essential for survival. These are the brain, heart, kidneys, liver and lungs. The human brain is the body's control center, receiving and sending signals to other organs through the nervous system and through secreted hormones.
It is responsible for our thoughts, feelings, memory storage and general perception of the world. The human heart is a responsible for pumping blood throughout our body. The job of the kidneys is to remove waste and extra fluid from the blood. The kidneys take urea out of the blood and combine it with water and other substances to make urine.
The liver has many functions, including detoxifying of harmful chemicals, breakdown of drugs, filtering of blood, secretion of bile and production of blood-clotting proteins. The lungs are responsible for removing oxygen from the air we breathe and transferring it to our blood where it can be sent to our cells.
The lungs also remove carbon dioxide, which we exhale. You use your eyes to see, your ears to hear and your muscles to do the heavy lifting. In fact, most body parts are far more complicated than that, while some seem to have no business being inside there at all. Start the Quiz 0 of 10 questions complete Ready for Med School? Test Your Body Smarts You use your eyes to see, your ears to hear and your muscles to do the heavy lifting. Rachael has been with Live Science since Live Science Health Reference: Ready for Med School? Test Your Body Smarts. Start Over More Quizzes.
Rachael Rettner, Senior Writer on.