Normal Kidneys and Their Function
The kidneys are a pair of bean-shaped organs that lie on either side of the spine in the lower middle of the back. Each kidney weighs about ΒΌ pound and contains approximately one million filtering units called nephrons. Each nephron is made of a glomerulus and a tubule. The glomerulus is a miniature filtering or sieving device while the tubule is a tiny tube like structure attached to the glomerulus.
The kidneys are connected to the urinary bladder by tubes called ureters. Urine is stored in the urinary bladder until the bladder is emptied by urinating. The bladder is connected to the outside of the body by another tube like structure called the urethra.
- The main function of the kidneys is to remove waste products and excess water from the blood.
- The kidneys allow consumption of a variety of foods, drugs, vitamins and supplements, additives, and excess fluids without worry that toxic by-products will build up to harmful levels.
- The kidney also plays a major role in regulating levels of various minerals such as calcium, sodium, and potassium in the blood.
- As the first step in filtration, blood is delivered into the glomeruli by microscopic leaky blood vessels called capillaries. Here, blood is filtered of waste products and fluid while red blood cells, proteins, and large molecules are retained in the capillaries. In addition to wastes, some useful substances are also filtered out. The filtrate collects in a sac called Bowman's capsule.
- The tubules are the next step in the filtration process. The tubules are lined with highly functional cells which process the filtrate, reabsorbing water and chemicals useful to the body while secreting some additional waste products into the tubule.
The kidneys also produce certain hormones that have important functions in the body, including the following:
- Active form of vitamin D (calcitriol or 1,25 dihydroxy-vitamin D), which regulates absorption of calcium and phosphorus from foods, promoting formation of strong bone.
- Erythropoietin (EPO), which stimulates the bone marrow to produce red blood cells.
- Renin, which regulates blood volume and blood pressure.