Many people are aware that kidneys filter blood, but they often don't know why blood needs to be filtered in the first place. Likewise, they are not aware of the many roles the kidneys play in keeping the body functioning normally. Kidney disease has a huge impact on a person's well-being, because the kidney contributes, directly or indirectly, to just about every physiological process in the body.
Filtering Blood
Just as there are waste products from digestion, there are waste products from muscular activity. During the course of a day, your body produces creatinine, urea, and other nitrogenous wastes. If the levels of these wastes build up, they rapidly become toxic. The kidneys constantly filter these toxins out of the blood. These waste products and extra water are diverted into the bladder where they are excreted as urine.
Maintaining Blood Pressure
The kidneys also play an important role in maintaining blood pressure. As the kidneys filter blood, they are uniquely attuned to the blood pressure in the body, because there must be sufficient pressure for them to perform their filtration functions. If blood pressure is too high, the kidneys secrete excess sodium. However, if the kidneys believe blood pressure is too low, they secrete renin into the bloodstream. Renin in turn activates the antiotensin-aldosterone system, which raises blood pressure by constricting blood vessels.
This system works well in health people, but it fails people with kidney disease. According to the Merck Manual, people with kidney disease are more likely to have high blood pressure. This is because as the kidneys become progressively scarred, they filter less blood. The lower blood volumes are incorrectly construed as a drop in blood pressure, when it really is caused by a drop in the filtering capacity of the kidneys. The kidneys respond by putting out more renin, which is turn raises blood pressure, which causes greater scarring.
Red Blood Cell Maturation
Kidneys are also responsible for the maturation of red blood cells. Newly formed red blood cells are called reticulocytes. These reticulocytes are incapable of carrying oxygen until the cells are exposed to erythropoietin, or EPO for short. EPO is a hormone secreted by the kidneys.
As kidneys get progressively scarred by disease, they become less capable of secreting EPO. According to the National Institute of Diabetes and Digestive and Kidney Diseases, anemia may appear when a person reaches 20 to 50 percent of his normal kidney function. This is why patients with kidney disease are often fatigued. Fortunately, this anemia can be treated with injections of genetically engineered EPO.
Acid-Base Balance
The kidneys help regulate the balance between acids and bases in the body. When the physiological pH gets too high, cells reabsorb more bicarbonate. The reverse is true if the pH gets too low. This function is often overlooked, but it is very important because many enzymes only function in a narrow pH range.
