All living beings must excrete, or remove, waste substances from their bodies. In humans, the kidneys are the main organs for performing this task. Kidneys filter excess water & waste chemicals from the blood. These are disposed of in the form of urine.
In humans the kidneys are a pair of bean-shaped, reddish-brown organs. They are located on either side of the backbone, against the rear wall of the abdomen level with the lowest ribs.
Each kidney is about 4.5 inches (11.4 centimeters) long. It has an outer layer, the cortex, & an inner layer, the medulla. The medulla is arranged in 10 to 15 fan-shaped groups called pyramids. The inner curve of the kidney has a dent in the middle called the hilus. The hilus is the point at which blood vessels enter & leave the kidney. These blood vessels include a renal artery & a renal vein. The word renal is Latin for kidney. Blood enters the kidney through the renal artery & leaves through the renal vein.
A funnel-shaped structure called the renal pelvis is located at the hilus. The renal pelvis is a urine-collecting chamber. It is connected to a tube called the ureter that leads out of the kidney. The ureter is connected to the urinary bladder. The urinary bladder is a sac, or bag, in the lower abdomen. From the bladder the urine leaves the body through another tube, the urethra.
Every few minutes the entire blood supply of the body runs through the kidneys. Although the kidneys are very small, they have an efficient system of tiny blood vessels for circulating blood. Each kidney contains arterioles & venules that are small branches of the renal artery & vein. The arterioles divide into very small or thin blood vessels called capillaries.
The capillaries lead to tiny units called nephrons that produce urine. Each kidney has at least a million nephrons. Each nephron is a long, extremely fine tubule with a cuplike structure at one end. This cuplike structure is called Bowman's capsule. The capsule encloses a cluster of capillaries called the glomerulus. The nephron's tubules are U-shaped, like a hairpin. This section of the tubule looks like a loop & is called the loop of Henle.
The formation of urine involves three processes: filtration, reabsorption, & secretion. These processes occur mainly in the nephrons. When blood enters the glomerulus, its capillaries act as a filter. The glomerulus removes the water & some small molecules from the blood. Some of these molecules are waste & must be removed. However, some are of value to the body & must be retained. These molecules are later reabsorbed by the capillaries in the loop of Henle.
The fluid that remains in the nephrons' tubules is the urine. Drop-by-drop, the urine passes into the renal pelvis for transport to the urinary bladder. The bladder stores the urine until it is expelled through the urethra.
Common disorders of the kidneys are nephritis, a disorder of the glomerulus; nephrosis, the breakdown of kidney tissue; & kidney stones, which are hard lumps formed when minerals get deposited in the kidney tissue. Such stones may become so large that the kidneys stop working.
Sometimes the kidneys cannot perform their function of waste removal. This inability of the kidneys to do their work is called kidney failure or renal failure. Patients with kidney failure have to be kept alive by a medical technique called dialysis. Dialysis is the process of purifying blood & returning it to the patient's bloodstream. A machine called the hemodialyzer does this. Patients whose kidneys have failed may receive a healthy kidney from another person. This surgical procedure is called a kidney transplant.
