Human physiology is the study of how the organs and systems of the human body work. The body keeps its internal conditions steady, a process called homeostasis. The normal human body temperature is about 37°C (98.6°F). The skin is the largest organ of the human body. The adult human skeleton has 206 bones.
The heart is the organ that pumps blood throughout the body. The sinoatrial (SA) node is the natural pacemaker of the heart. Its intrinsic firing rate is about 100 beats per minute, but at rest the vagus nerve (vagal tone) slows it down to about 60 to 80 beats per minute. The normal resting blood pressure of a healthy young adult is about 120/80 mmHg.
Blood carries oxygen with the help of hemoglobin, the pigment found in red blood cells. One hemoglobin molecule can carry up to four oxygen molecules when fully saturated, one on each of its four heme groups. According to the Bohr effect, a fall in blood pH (more acidic blood, for example from more CO2) decreases the affinity of hemoglobin for oxygen. This helps hemoglobin release more oxygen to active tissues that need it.
In the respiratory system, the exchange of gases between air and blood happens across the alveoli, tiny air sacs in the lungs, and the thin walls of the surrounding capillaries. The tidal volume is the amount of air moved in or out of the lungs in one normal breath, and in a healthy resting adult it is about 500 mL. Most carbon dioxide (about 70 percent) is carried in the blood as bicarbonate ions. The enzyme carbonic anhydrase, found in red blood cells, speeds up the conversion of carbon dioxide and water into carbonic acid, which then splits into bicarbonate and hydrogen ions.
The nervous system controls and coordinates the body. The neuron is its structural and functional unit. The cerebellum is the part of the brain that controls balance and coordination.
The endocrine system uses hormones to regulate the body. The pituitary gland is known as the “master gland” because it controls many other glands. The thyroid gland needs the trace element iodine to make thyroxine. The pancreas produces insulin, which lowers blood sugar, and glucagon, which raises blood sugar. Insulin is made by the beta cells, and glucagon is made by the alpha cells of the pancreas. The hormone adrenaline (epinephrine), released during stress, is secreted by the adrenal medulla, the inner part of the adrenal gland.
The kidneys filter the blood and produce urine. The nephron is the structural and functional unit of the kidney. In the nephron, the initial filtration of blood plasma takes place at the glomerulus, together with Bowman’s capsule, where water, salts, and small molecules are filtered out of the blood.