Wednesday, November 23, 2011
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.
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.
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.
Continue reading on Human Body Part 4..