How The Body Works
The body’s breathing apparatus is enclosed in an expansile bony cage, formed by the ribs, the spinal column, and the sternum. The floor of the cage is closed by the muscular diaphragm. Air passes down the windpipe, or trachea, to enter the lungs. During inspiration, or inhaling, the diaphragm moves downward and becomes flatter, and the rib cage expands. This creates a partial vacuum in the lungs and, to equalize the pressure, the air is drawn in. Expiration, or exhaling, is passive. The lungs have elastic tissue in their walls and during inspiration, this is stretched. In expiration its natural recoil causes the lungs to partially deflate.
Inside the Heart
The heart is a muscular pumping organ which beats nonstop to circulate blood around the body. It functions as two halves, each consisting of a holding chamber, or atrium, and a pumping center, or ventricle. After circulating around the body, blood, now deoxygenated, returns to the right atrium through large veins, the superior vena cava, and the inferior vena cava.
When the atrium is full, blood is forced through the tricuspid valve into the right ventricle. It is then pumped to the lungs through the pulmonary valve into the pulmonary artery. The oxygenated blood returns via the pulmonary veins to the left atrium. After flowing through the mitral valve it is pumped out of the left ventricle into the aorta to return to the general circulation.
The Anatomy of the Central Nervous System
The nervous system is a complex network of nerve cells and nerve fibers spread throughout the body. Its function is to interpret, store, and respond to information received from inside and outside. The central nervous system or CNS consists of the brain and spinal cord and is responsible for processing information gathered from the rest of the nerves and transmitting instructions to the body.
Messages passing to and from the CNS are carried by the nerves of the peripheral nervous system. This system includes twelve pairs of cranial nerves and thirty-one pairs of spinal nerves. The cranial nerves and the spinal nerves control voluntary movements and sensations. The autonomic nervous system, consisting of sympathetic and parasympathetic nerve fibers, controls such involuntary body functions as the heartbeat.
The Digestive System
Starting at the mouth, the digestive system helps provide the energy your body needs to perform its many functions. Upon entry into a person’s mouth, the teeth cut, tear, crush and grind food. In the mouth, salivary glands respond to the thought or presence of food by producing a fluid containing mucus and the enzymes amylase and maltase.
The tongue then mixes the food and rolls it into a softball, called the bolus, which is pushed toward the esophagus. Passing through the esophagus the bolus is dropped into the stomach where gastric glands, one secreting digestive enzymes and the other secreting hydrochloric acid, begin to break the food down into smaller pieces.
The stomach wall discharges mucus during this phase to protect itself against the action of the gastric acid. From there the food passes into the small intestine through the pylorus, a sphincter muscle that controls the flow of food. It is in the small intestine where a major part of digestion and absorption occurs. The intestinal wall releases enzymes which digest proteins, fats, and carbohydrates.
Blood and lymph vessels, which supply the small intestine, take away the final products of digestion. The lymphatics transport the fats around the body and finally release them into the bloodstream. Blood takes sugars and amino acids to the liver via the portal vein. The pancreas, like the small intestine, secretes enzymes in an alkaline juice to digest proteins, fats, and carbohydrates.
It also manufactures hormones which regulate the blood sugar level. From the small intestine, the digested food is received by the liver, which manufactures bile for the gall bladder. The gall bladder stores and discharges the bile, which helps to break down fats into minute droplets. Undigested food from the small intestine is then passed into the large intestine.
There blood vessels supplying the large intestine carry away water extracted from the undigested waste. After passing through to the large intestine the ileocecal valve prevents digested food from returning to the small intestine. Undigested food is eliminated from the system through the anus.