The kidneys are two bean-shaped organs that sit just below the rib cage on either side of the spinal cord. Each is about the size of a bar of soap. At any one time 20–25% of the body's blood flows through them, even though they comprise only 0.5% of the body's total weight. At this rate, the kidneys filter the entire blood supply 60 times per day.
Blood flows into the kidney through the renal artery and exits through the renal vein. Within the kidney are many small capillaries that perfuse it with blood, giving the organ its reddish-brown color.
The gross anatomy of the kidney can be divided into four parts:
- Capsule: A thin but tough outer membrane that protects the kidney against infection and trauma.
- Cortex: The outer layer of the kidney's interior, about 1 cm (0.4 inch) thick.
- Medulla: The inner layer of the kidney's interior, which contains triangular structures called renal pyramids. Between pyramids are sections of cortex called renal columns.
- Renal pelvis: A large funnel for collecting urine from all parts of the kidney, connected to the bladder by the ureter.
Each cortex and medulla together contain about a million nephrons, microscopic filtering systems that are the basic unit of each kidney. Each nephron has two main components. The first is a vascular system that includes1) the glomerulus, 2) afferent and efferent arterioles, and3) peritubular capillaries. The second, tubular component contains five main parts: Bowman's capsule, proximal tubule, loop of Henle, distal tubule, and the collecting duct.
Bowman's capsule forms one end of each nephron. It contains a bundle of tiny capillaries called the glomerulus, which receives its bloodflow from the afferent arteriole. The glomerulus filters minerals, nutrients, wastes, and water from the blood that flows through it, and passes them down into the proximal tubule. The glomerulus also returns large plasma proteins and red blood cells to the blood supply through the efferent arteriole. The efferent arteriole is connected to a second capillary bed called the peritubular capillaries. These two successive capillary beds create a pressure difference that forces fluid through the nephron.
Once filtrate enters the proximal tubule, specialized cells reabsorb sodium and other ions, water, glucose, and amino acids back into the blood. The fluid then goes into the loop of Henle, which helps concentrate the waste products to be excreted in urine. After the loop of Henle, fluid then flows into the highly coiled distal tubule, where potassium is secreted and more water and sodium are reabsorbed back into the blood. The fluid then flows into the last part of the nephron, the collecting duct, where final adjustments are made to the urine concentration. The collecting ducts respond to the antidiuretic hormone (ADH), which regulates the amount of water reabsorbed by the blood. The urine then flows through the renal pelvis to the ureter, which delivers urine to the bladder for excretion.
By filtering the blood, the kidneys play a very important role in the body. They adjust the water volume, remove wastes such as urea, ammonia, and drugs, establish acid-base balance, determine the composition of blood, help maintain blood pressure, stimulate the production of red blood cells, and determine calcium levels.
The maintenance of water volume is a particularly important function. When the body sweats on a hot day or during exercise, it needs a way to sense water loss to avoid dehydration. Water volume is monitored by specialized osmoreceptors in the hypothalamus that measure sodium concentration in the blood. A high sodium concentration means there is insufficient water; this signals the hypothalamus to increase ADH secretion, which in turn prevents the kidneys from reabsorbing water from the blood in the collecting ducts. If the sodium concentration is low, there is too much water in the blood, so the hypothalamus reduces ADH secretion, which tells the kidneys to increase the water concentration of the urine.
The kidneys are also crucial in removing waste products such as urea, ammonia, and any chemical compounds such as medications from the blood. For this reason patients with damaged kidneys must be monitored closely when they take medications that are excreted in the urine. If the kidneys are not working properly, drug concentrations in the blood could rise to fatal levels.
In addition, the kidneys play a pivotal role in the body's acid-base balance. The blood's pH is maintained by a fixed ratio of hydrogen-to-bicarbonate ions in the blood. If the number of hydrogen ions increase, then the blood becomes acidic, a condition known as acidosis. Likewise, if the number of sodium bicarbonate ions rise, the blood becomes basic, a condition known as alkalosis. The kidneys help sustain this hydrogen-to-bicarbonate ratio by adjusting the amount of bicarbonate in the blood. If the blood is too basic the proximal and distal tubules of the kidney will decrease bicarbonate reabsorption and more bicarbonate will be excreted into the urine. If the blood is acidic, then the proximal tubule will allow reab-sorption of bicarbonate back into the blood and excrete more hydrogen into the urine.
Another major task the kidneys perform is to help maintain blood pressure. Kidney cells can recognize when a drop in blood pressure occurs, because when this happens, blood flow to the kidney decreases. This means less sodium is present in the kidney cells, a condition that causes the kidney cells to release an enzyme called renin. Renin converts angiotensin I into angiotensin II, which in turn constricts blood vessels and causes sodium retention by the kidneys, thereby raising blood pressure. This is known as the renin-angiotensin system. Angiotensin II also causes the adrenal glands to release the hormone aldosterone, which tells the kidneys to allow more sodium and water to be reabsorbed back into the blood. This
Adrenal gland—Small gland on top of each kidney that produces and releases several different hormones that are involved in maintaining internal fluid and salt levels and also mediates stress responses.
Angiotensin I—Inactive form of angiotensin that circulates in the blood; it is a precursor of angiotensin II.
Angiotensin II—Active form of angiotensin that constricts blood vessels, thus raising blood pressure.
Capillary—Small blood vessel that is the point of connection for blood and veins and where exchanges occur between the blood and tissue.
Hydronephrosis—Distention of the renal pelvis that occurs when urine is trapped in the kidney and blocked from flowing into the bladder.
Ureter—Carries urine from the kidney to the bladder.
increase in water volume in the blood increases blood pressure. Many medications for high blood pressure act by working on the kidneys to decrease blood volume and therefore blood pressure. These blood pressure medications are collectively known as diuretics.
Role in human health
The kidneys play a crucial role in human health because they perform many vital functions. The kidneys work constantly, simultaneously, and influence each other. Individuals are born with two kidneys but can function with one. However, a person with kidney function at 10–15% of capacity will require dialysis or a kidney transplant to sustain life. Individuals with high blood pressure and diabetes have a significant risk of kidney disease.
Diabetic patients cannot process blood glucose properly, and if their disease is untreated or poorly controlled, it can lead to high blood sugar levels. This can damage the nephrons, leading to diabetic neuropathy. This usually means that soft kidney tissue hardens and thickens, a process called sclerosis; this is especially true for the glomerulus. The American Diabetes Association estimates that 35–45% of type 1 diabetics and 20–30% of type 2 diabetics have damaged kidneys. Because the symptoms of nephropathy may not appear until 80% of kidney function is gone, periodic tests of kidney function and strict compliance with diet and treatment regimens are important for patients with diabetes.
High blood pressure
The kidneys use small blood vessels called capillaries to filter blood and to help create a pressure gradient to move fluid through the nephron. Continuous high blood pressure can damage the fragile walls of these vessels. When this happens, blood may not filter properly, allowing waste products and/or drug levels to build up, some times to dangerous or fatal levels. Kidney stones Kidneys stones occur when crystals form in the lumen of the tubules or in the ureters. The stones are most commonly made of calcium and oxalate or phosphate. The basis of stone formation is not clear but certain foods in certain people can cause them to accrete. Kidney stones can be extremely painful, and can also cause hydronephrosis. Patients with kidney stones are encour aged to drink plenty of water in effort to have the stone excreted in the urine. In some cases, kidney stones must be surgically removed.
Polycystic kidney disease
Polycystic kidney disease (PKD) is an inherited disease in which cysts form in the kidney. These fluid-filled cysts can take over a significant amount of space in the kidney, eventually reducing kidney function and causing kidney failure. Most cases of PKD show no symptoms until the patient is well into adulthood. PKD that appears in children is often more virulent, frequently leading to kidney failure and death. Nutrition and dietary modification play a major role in controlling the progression of PKD.
Wilms' tumor, or nephroblastoma, is a cancer of the kidney that appears during childhood. Both sporadic cases and a few rare inherited cases have been linked to mutations in the Wilm's tumor gene (WT1) on chrom-some 11. Many cases of Wilms' tumor are curable if caught early enough.
Cohen, Barbara, and Dena Wood. Structure and Function of the Human Body, 7th ed. Philadelphia, PA: Lippincott Williams and Wilkins, 2000.
Johnson, R.J., and J. Feehally. Comprehensive Clinical Nephrology. London: Harcourt Publishers, 2000.
National Kidney Foundation. 30 East 33rd St. Suite 1100, New York, NY 10016. (800) 622-9010. <http://www.kidney.org>.
Polycystic Kidney Disease Foundation. 4901 Main St. Suite 200, Kansas City, MO 64112. (800) PKD-CURE. <http://www.pkdcure.org>.
Susan M. Mockus, Ph.D.