Human Body Part 4

ENDOCRINE SYSTEM

Hormones are chemical messengers that influence virtually every body cell and function. Often working in concert with each other, as well as with the nervous system, hormones control growth, metabolism, digestion, blood pressure, reproduction and response to stress, among many other functions. Understandably, hormonal imbalances can have profound effects throughout the body.

Although scientists know that hormones are key to almost every body process, much remains to be learned about how they work. For example, we still do not understand how the thymus and pineal glands and their hormones work. And from time to time, yet another hormone is discovered.

In addition to being produced by various endocrine glands, hormones are secreted by other organs, including the lungs, intestines, heart and kidneys. Regardless of their origin, however, they all travel through the bloodstream in very small amounts, seeking out target organs or cells, which they then stimulate to perform a particular function. Some hormones, such as insulin are too large to actually enter a cell; instead, they attach themselves to a preprogrammed receptor that triggers the desired response. Other hormones, such as the steroids produced by the adrenal glands, are small enough to penetrate target cells and elicit the desired response from its genetic material.

REPRODUCTIVE SYSTEM

Reproduction remains one of life’s most profound wonders. Just the notion that two barely visible cells can merge and form a new human being in just nine months is nothing short of a miracle. Of course, many things can go awry along the way, but most babies are healthy at birth with all the organs needed to grow into a normal adult.

Sex hormones – principally testosterone in men and estrogen in women – directly control reproduction. But many factors influence both the male and female reproductive system including overall health, nutrition and stress. Genetics are also instrumental. Both the mother and father contribute half of the genes needed to make a new human being and it is this genetic material that determines many of the off-spring’s characteristics, such as eye and hair color, height, body build and blood type.

Sex is also determined at the moment of conception and depends upon which sex chromosome is donated by the father. Female cells have two X chromosomes; thus, when an egg divides, it must have an X chromosome. In contrast, males have an X and a Y chromosome and a sperm can carry either one. So if an egg is fertilized by an X sperm, the baby will be a girl with two female X chromosomes; if the father contributes a Y sperm, the offspring will be a boy with the characteristic male XY chromosomes.

THE IMMUNE SYSTEM

The human body is constantly bombarbed by millions of viruses, bacteria and other disease – causing microorganisms, or pathogens. Fortunately, most of these are thwarted by the body’s own protective physical and chemical barriers, such as the skin, saliva, tears, mucus and stomach acid. The millions of bacteria that live on the skin and the body’s mucus membranes also help protect against certain invaders. When a pathogen does manage to evade these defences and enter the body, it is attacked almost immediately by one or more components of the immune system.

The immune system uses extremely sensitive chemicals sensors to recognise a foreign organism or tissue, especially one that can cause disease. Sometimes it over reacts to a harmless substance, such as pollen or a certain food or medication; this can set the stage for allergic reaction. In other cases, the immune system mistakenly attacks normal body tissue as if it were foreign, resulting in an autoimmune disease such as lupus or rheumatoid arthritis. Most of the time, however, the immune system holds fast as our first line of defense against a host of potentially deadly diseases.

End of Human Body Part 4/4.

Human Body Part 3

RENAL SYSTEM

The body’s excretory system is made up of a pair of kidneys and ureters, urinary bladder and urethra. Kidneys do most of the work; the other structures transport or store urine.

The kidneys are bean-shaped organs, about four inches long and weighing only five ounces. They function as extraordinarily efficient chemical treatment plants, cleansing the blood of urea and other toxic wastes while maintaining the proper balance of fluid, salts and other blood components. They are also instrumental in maintaining blood pressure.

The renal arteries branch off the abdominal aorta and carry a prodigious amount of blood. Each day, up to 500 quarts of fluid circulate through the kidneys. After it is cleansed, most of this fluid is returned to the bloodstream; only two to four pints are excreted as urine. This waste material collects in the central portion of the kidney – the renal pelvis – and from there it passes into the ureter, a long, narrow tube that carries the urine to the bladder. A normal adult bladder can hold about one pint of liquid, but when it is about half full, it begins to send nerve signals of an urge to urinate. Voluntary muscles in the pelvic floor control bladder function; when these muscles drop, the sphincter that controls the bladder opening relaxes and urine flows into the urethra. The female urethra is about 1.5 inches long and carries only urine; the 8 inch male urethra transport both semen and urine.

RESPIRATORY SYSTEM

Of all the substances needed to sustain life, oxygen – an odourless, colorless and tasteless gas – is perhaps the most critical because it is essential for all stages of metabolism, the various biochemical functions that maintain the body. Without oxygen, cells begin to die within minutes.

With each breath, oxygen is taken into the lungs and carbon dioxide and other wastes are expelled. Although you can deliberately hold your breath for a short period, breathing actually is an automatic process controlled by the brain’s respiratory center. When performing quiet activities, a person takes about 14 breaths a minute, but the respiration rate may be slower during sleep or mediation and higher during exercise or other activities that demand extra oxygen.

Air is inhaled through the nose or mouth and passes through the larynx, or voice box, into the trachea, or the windpipe and then to the bronchi and bronchioles, air tubes that branch off the trachea. These tubes are lined with millions of cilia, hairlike strands that beat rhythmically to keep dust, germs and other airborne particles out of the lungs. The cilia also help clear the lungs of mucus produced by the mucous cells lining the bronchial tubes.

The bronchioles terminate in clusters of alveoli, tiny, balloon-like air sacs that are responsible for ensuring that the blood has a steady supply of fresh oxygen. Oxygen exchange takes place on the surface of the lungs 700 million or so alveoli, which, if flattened out, would almost cover a tennis courts. The air sacs are elastic, expanding during inhalation and deflating partially as air is exhaled. If alveoli lose their elasticity, as is the case in emphysema, stale air becomes trapped in the sacs and the body becomes starved for oxygen.

SENSES

Virtually everything that we perceive about our surroundings comes through information collected by the five basic senses – sight, hearing, taste, smell and touch. Of these, sight and hearing are generally considered the most vital; in fact, however all work in concert to provide a total picture. This cooperative process is especially apparent when you eat – odor is critical in distinguishing between foods that have a similar taste and texture. This is the reason that food seems to lack taste when you have a cold. However, when you are deprived of one particular sense, others can help compensate; for examples, you can use touch and sound to find your way in the dark.

All sensory organs are complex extensions of the central nervous system (refer Human Body Part 2), with a direct pathway to the brain, which allows instantaneous processing of information. (The eye’s optic nerve is actually an extension of the brain). The moment you touch an object, you know whether it is soft or hard, hot or cold, smooth or rough. This is because information is processed so fast, we give little thought to the complexity of what is involved. Sounds entering the ear or light coming into the eye are immediately broken down and transformed into electrical impulses that are decoded and reassembled in the brain. A similar electrical transformation takes place in identifying an odor, interpreting a touch and a recognizing a taste.

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Human Body Part 2

CARDIOVASCULAR SYSTEM

The adult body has some 60,000 miles of blood vessels that supply oxygen and other nutrients to every cell and carry away carbon dioxide and other wastes. The heart, one of nature’s most durable pumps, constantly circulates 8 to 10 pints of blood through this vast network. On a typical day, the heart beats more than 100,000 times, pumping out 2,600 gallons of blood. This adds up to more than 2.5 billion heartbeats over an average lifetime, with never more than a fraction of a second’s rest between each beat.

Although the heart is designed to last a lifetime, cardiovascular disease remains our leading cause of death, claiming more than 900,000 lives a year. Most of these deaths are due to heart attacks, often in the prime of life. The American Heart Association estimates that 56 million Americans suffer from a cardiovascular disorder, with high blood pressure and coronary artery disease the most prevalent.

These disorders are epidemic worldwide, concentrated mostly in developed nations. They are a relatively modern phenomenon that experts attribute to a combination of lifestyle factors (for example, eating a high-fat diet, smoking, not exercising) and heredity. Increasingly, however, researchers are showing that heart attacks, strokes and other cardiovascular events can be prevented by adopting a prudent, heart-healthy lifestyle. 

BRAIN AND NERVOUS SYSTEM

All our movements, thoughts, sensations and bodily functions are controlled by the brain and nervous system, the most highly evolved among all living creatures and the least understood. Neuroscientists are only beginning to unravel the myriad mysteries of the human brain and many predict we will never fully understand so many of the things we take for granted: memory, language, creativity and so forth.

Taken as a whole, the nervous system is actually a complex branching network of systems with many overlapping parts and functions, all controlled by the brain and its spinal cord extension. Such automatic or involuntary, functions as breathing, circulation and digestion are directed largely by the automatic nervous system, which is divided into the sympathetic and parasympathetic components. In simplified terms, these two systems act as switches to turn organs on and off, thus maintaining a state of balance.

Superficial sensory nerves receive messages from the outside world and transmit them to the brain, where they are interpreted and sent back through the body via the cranial or spinal nerves. All this takes only a split second and often requires little or no thought. However, when something goes awry with the brain or other components of the nervous system, manifestations can be disastrous, ranging from trivial movement disorders to paralysis and dementia.

DIGESTIVE SYSTEM 

Digestion is a complex chemical and mechanical process that begins when food is chewed and mixed with saliva, which adds moisture and also begins breaking down starches. Swallowing forces a bolus of food into the esophagus, a 10-inch muscular tube that transports it to the stomach. 

Contractions of this muscular organ further pulverize food and mix it with hydrochloric acid and other powerful gastric juices. Little by little, the partially digested food passes from the stomach to the duodenum, the site of even more chemical action. Pancreatic enzymes and juices flow into this uppermost segment of the small intestine, where they break down proteins and carbohydrates. To make fats more soluble, the liver produces bile, which exerts an emulsifying action that transforms globules of fat into minute droplets.

Peristalsis, rhythmic contractions of the intestinal muscles, propels the digested food onward through the small intestine, which is lined with villi, tiny hairlike structures. Molecules can pass through these tiny projections and are then absorbed by the underlying network of blood and lymph vessels. Finally, material that cannot be absorbed from the small intestine passes into the colon. Here, fluid is extracted and returned to the circulation and the remaining fecal material is passed in a bowel movement. The total time required to fully digest a meal varies, but on average, it takes 24 to 36 hours.

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Human Body Part 1

 OVERVIEW OF THE HUMAN BODY

Philosophers and scientists alike have observed that human beings and other forms of animal life are made up of the same elements found in ordinary soil and water. But when these 20 or so basic elements combine in thousands of different ways to form a human body, the result is one of the most complex organisms on the planet and a never-ending source of both wonderment and mystery.

About 75 to 80 percent of an adult’s body consists of slightly salty water; the rest is made up of chemical compounds, many of them unique to human beings. These various compounds are arranged to form hundreds of different kinds of cell, the body’s smallest, most basic units.

All human life begins with the fusing of two cells and the subsequent division and multiplication of cells to form a complete body follows the same general blueprint even though no two people are exactly alike. The average body contains 80 to 100 trillion cells, each programmed to grow, carry out a specific function and even replicate itself. But with the exception of blood cells, none function independently; instead, similar cells join together to from specific types of tissue – muscle, nerve, bone and so forth. Each body organ is made up of a collection of related tissues. Finally, organs are organized into the different body systems illustrated above and on the below article.

BONES AND JOINTS

The human skeleton is an engineering marvel with numerous functions. Not only does it give the body its needed support and a protective framework for vital internal organs, but it also serves as a storehouse for calcium and other essential minerals and is critical in making new blood cells. Although we tend to regard bones as being inert, in reality they are in a constant state of flux and also change dramatically over a lifetime. At birth, a baby has about 350 bones, a number of which are soft and pliable. As the child grows, the bones harden and many, such as those in the skull, fuse together.

The typical adult skeleton has 206 bones and weighs only about 20 pounds. Ounce for ounce, however, compact bone tissue is one of nature’s strongest materials. A cubic inch of bone can bear 19,000 pounds, making it four times stronger than reinforced concrete. Bones derive their incredible strength from their honeycombed structure and composition of calcium, phosphorus and other mineral salts held together by collagen fibers. Nerves and blood vessels permeate the honeycombed structure and calcium and other minerals constantly move in and out of bone tissue. New blood cells are continually being made in the marrow, the spongy interior.

Cartilage, a tough, slippery material, covers the ends of bones, cushioning the joints and reducing friction. Ligaments act as bindings to keep bones in place and tendons attach muscles to the bones. To permit movement, bones act as levers, the joints are fulcrums and muscles contract to provide the necessary force (see below article)

Age takes its toll on bones and joints. The knees and hips are especially vulnerable to degenerative arthritis. And with advancing age, bones begin to lose some of their calcium, making them porous and weak, a condition called osteoporosis.

MUSCLES AND SKIN

The body contains three types of muscles: cardiac, found only in the heart; involuntary smooth muscles, which are part of various organs; and the skeletal muscles, which are attached to bones and make voluntary movement possible.

Skeletal muscles – the body’s most abundant tissue – are made up of bundles of long fibers bound together by connective tissue. Each fiber is surrounded by tiny capillaries, which deliver a steady supply of oxygen needed for the muscles to function. The fibers in the particular muscle remain constant in number throughout life, but they enlarge when exercised frequently and shrink, or atrophy with disuse and age.

Every set of muscles is served by one or more nerves. Movement occurs when nerve signals set off specific chemical reactions that cause certain muscles to contract. Most muscle disorders are actually due to nerve problems. The muscle weakness of multiple sclerosis is one example.

 

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