For an organism to survive, it needs to get rid of metabolic waste materials, as well as maintain proper concentrations of various necessary materials. For example, human food consists of carbohydrates, fats, proteins, various salts, and water. As these materials are metabolized, certain waste products are produced, such as carbon dioxide, water, urea and related nitrogenous compounds, salt, and various minerals. If these waste materials remain in the body, they quickly upset the body’s equilibrium (homeostasis), so they must be excreted rapidly and efficiently.
The kidneys are very important organs within the human body because they are essential to maintaining homeostasis. Humans have two bean-shaped kidneys that are found at the back of the abdominal cavity, one found on each side of the spine. Each kidney is approximately the size of a person’s fist. All the blood in the body must pass through the kidneys. The large amount of blood that is passed through the kidneys allows them to do the following:
- Assist in the regulation of blood pressure
- Stimulate red blood cell production
- Maintain calcium levels in the body
- Regulate the composition of the blood by keeping the pH, concentration of various ions, and the volume of water constant. The kidneys filter wastes (urea, ammonia, salts, drugs, water, and other toxic substances) from the bloodstream in order to keep the blood clean and chemically balanced.
The anatomy of the kidneys provides a greater understanding of the major role they have in maintaining homeostasis within a person’s body (Figure 1).
The renal artery transports blood from the body into the kidney, and the renal vein transports filtered blood back into the body. The light colored outer region is known as the cortex, while the darker inner region is known as the medulla. The pelvis is a flat, funnel-shaped cavity that collects urine in the ureters. Upon closer inspection of these regions in the kidney, thousands of tiny structures, known as nephrons, can be seen. Nephrons, composed of small arteries, are the filtering units of the kidneys. Each kidney contains over one million nephrons, which is a tube closed at one end and open at the other (Figure 2). Nephrons make urine by filtering the blood of its smallest molecules and ions and then reclaiming the needed amounts of useful materials. Surplus or waste molecules and ions are left to flow out as urine.
Filtration begins as blood is carried to a tuft of capillaries known as the glomerulus. The glomerulus filters the blood by hydrostatic blood pressure, forcing the blood through the basement membrane and into the cavity of the glomular capsule, also known as the Bowman’s capsule. Blood cells and proteins too large to be filtered through this membrane remain in the blood. Water, small molecules, and ions filter through the capillary walls into the Bowman’s capsule, which is located at the closed end of the nephron. This fluid is called nephric filtrate. The nephric filtrate collects within the Bowman’s capsule and then flows into the proximal tubule. Here most of the glucose, amino acids, and approximately 60% of the salts are reabsorbed through active transport. As these solutes are removed from the nephric filtrate, a large volume of the water follows through osmosis.
As the remaining filtrate flows into the loop of Henle, it is isotonic to the blood, but while in the loop of Henle, more sodium ions are pumped out into the peritubular capillary network. Since water remains in the loop of Henle, the interstitial fluid in the capillary tubes becomes very hypertonic and the fluid within the loop of Henle becomes hypotonic. In the distal tubes, active transport moves more sodium, but at this point, water also crosses the membrane through osmosis.
After filtrate has passed through the loop of Henle, it enters the collecting tubules, where it then is carried toward a nearby collecting duct. The collecting duct leaves the cortex and descends into the medulla, carrying fluid toward a papillary duct. The filtrate then enters the renal papillae, which are small openings into the renal pelvis. From here, the remaining filtrate, now known as urine, is excreted through the ureters to the bladder, where the urine is stored until it is excreted. The kidneys are able to filter approximately 180 liters of filtrate per day; however, they only excrete 1 to 1.5 liters of urine, conserving the amount of water loss and greatly concentrating the salts and other wastes in the urine.
The function that the kidneys perform is so vital to a human’s survival that total kidney failure can cause a person to die in a very short time. Most kidney diseases attack the nephrons, causing them to lose their filtering capacity. Damage to the nephrons may happen quickly, often as a result of injury or poisoning, or, more commonly, most damage occurs slowly and silently. Years and years may go by before the damage even becomes apparent. The two most common causes of kidney disease are:
- High blood pressure can damage the small blood vessels in the kidney, not allowing for filtration of poisons from the bloodstream.
- Diabetes keeps a person’s body from using sugar as it should. The sugar then stays in the bloodstream instead of breaking down. In turn, it acts like a poison that can damage the nephrons.
Kidney disease cannot be cured. However, it may be possible to make the kidneys last longer if kidney disease is detected in the early stages. Certain precautionary measures that can be taken in these early stages include regularly checking blood pressure, avoiding pain pills that may make kidney disease worse, carefully watching the diet by limiting proteins, salt, and cholesterol, and watching blood sugar levels very closely.
When the kidneys stop working completely, a condition known as uremia becomes apparent. One symptom of uremia is edema, which makes a person’s hands and feet swell. In addition, the body also fills with waste products, which leads to feelings of tiredness and weakness because tissues need clean blood to function properly. If uremia is left untreated, a person may have seizures or enter into a coma that may ultimately result in death. Two methods for treating uremia are dialysis or kidney transplantation.
Fortunately, medical technology has developed a machine, known as dialysis, that can serve as an artificial kidney, filtering out wastes and replenishing the body with “clean” blood. The two major forms of dialysis are hemodialysis and peritoneal dialysis. In hemodialysis, blood filled with waste products is sent through a machine that filters away waste products and the clean blood is returned to the body. Hemodialysis is usually performed at a dialysis center three times per week for 3 to 4 hours. In peritoneal dialysis, a fluid, called dialysate, is put in your abdomen. Dialysate captures the waste products from the blood. After a few hours, the dialysate containing the wastes is drained from the abdomen. Then, a fresh bag of dailysate is dripped into the abdomen. Patients can learn to do this themselves without going to a doctor’s office each time. On average, patients change dialysate four times a day.
With only a few hours of dialysis a week, a person may live for years without functioning kidneys while they wait for a suitable kidney transplant to become available. A donated kidney may come from an anonymous donor who has recently died or from a living person, usually a relative. It is essential that a person receive a kidney that is a good match for their body because then it is less likely that the immune system will reject it. Scientists have also developed special drugs that can trick a person’s immune system to help it accept a transplanted kidney.
DID YOU KNOW?
- The kidneys represent on 0.5% of the total weight of a body, but receive 20-25% of the total arterial blood pumped by the heart.
- The rate of filtration is approximately 125 ml/min. or 45 gallons (180 liters) per day. Considering that you have 7 to 8 liters of blood in your body, this means that your entire blood volume gets filtered approximately 20-25 times each day.
- Each kidney contains over one million nephrons.
- The right kidney is slightly lower than the left.
- Each kidney weighs about 113 - 170 grams and is about 11.4 cm long, 6 cm wide, and 2.5 cm thick.
- The first workable artificial kidney was developed during World War II in 1944 by Dr. William Kolff who was living in Holland.
- According to the National Kidney Foundation, more than 370,000 Americans are being treated with dialysis or kidney transplantation for kidney failure. Nearly 12 million Americans may be at risk for chronic kidney disease.
- The use of a dialysis machine is suggested when a patient’s blood urea nitrogen value exceeds 100 mg/dl (the normal value is 30 mg/dl).
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