Tuesday, November 22, 2011
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.
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.
Continue reading on Human Body Part 3