Introduction
- If you exercise on a hot day, you are likely to lose a lot of water in sweat. Then, for the next several hours, you may notice that you do not pass urine as often as normal and that your urine is darker than usual.
- Do you know why this happens? Your body is low on water and trying to reduce the amount of water lost in urine. The amount of water lost in urine is controlled by the kidneys, the main organs of the excretory system.
Excretion
- Excretion is the process of removing wastes and excess water from the body. It is one of the major ways the body maintains homeostasis.
- Although the kidneys are the main organs of excretion, several other organs also excrete wastes.
- They include the large intestine, liver, skin, and lungs. All of these organs of excretion, along with the kidneys, make up the excretory system.
- The roles of the other excretory organs are summarized below:
- The large intestine eliminates solid wastes that remain after the digestion of food.
- The liver breaks down excess amino acids and toxins in the blood.
- The skin eliminates excess water and salts in sweat.
- The lungs exhale water vapor and carbon dioxide.
Urinary System
- The kidneys are part of the urinary system, which is shown in Figure below.
- The main function of the urinary system is to filter waste products and excess water from the blood and excrete them from the body.
Kidneys and Nephrons
- The kidneys are a pair of bean-shaped organs just above the waist. A cross-section of a kidney is shown in Figure below.
- The function of the kidney is to filter blood and form urine.
- Urine is the liquid waste product of the body that is excreted by the urinary system. Nephrons are the structural and functional units of the kidneys.
- A single kidney may have more than a million nephrons!
Filtering Blood and Forming Urine
- As shown in Figure below, each nephron is like a tiny filtering plant. It filters blood and forms urine in the following steps:
- Blood enters the kidney through the renal artery, which branches into capillaries. When blood passes through capillaries of the glomerulus of a nephron, blood pressure forces some of the water and dissolved substances in the blood to cross the capillary walls into Bowman’s capsule.
- The filtered substances pass to the renal tubule of the nephron. In the renal tubule, some of the filtered substances are reabsorbed and returned to the bloodstream. Other substances are secreted into the fluid.
- The fluid passes to a collecting duct, which reabsorbs some of the water and returns it to the bloodstream. The fluid that remains in the collecting duct is urine.
Excretion of Urine
- From the collecting ducts of the kidneys, urine enters the ureters, two muscular tubes that move the urine by peristalsis to the bladder (see Figure above). The bladder is a hollow, sac-like organ that stores urine.
- When the bladder is about half full, it sends a nerve impulse to a sphincter to relax and let urine flow out of the bladder and into the urethra.
- The urethra is a muscular tube that carries urine out of the body. Urine leaves the body through another sphincter in the process of urination.
- This sphincter and the process of urination are normally under conscious control.
Kidneys and Homeostasis
- The kidneys play many vital roles in homeostasis. They filter all the blood in the body many times each day and produce a total of about 1.5 liters of urine.
- The kidneys control the amount of water, ions, and other substances in the blood by excreting more or less of them in urine.
- The kidneys also secrete hormones that help maintain homeostasis. Erythropoietin, for example, is a kidney hormone that stimulates bone marrow to produce red blood cells when more are needed.
- The kidneys themselves are also regulated by hormones. For example, antidiuretic hormone from the hypothalamus stimulates the kidneys to produce more concentrated urine when the body is low on water.
Kidney Disease and Dialysis
- A person can live a normal, healthy life with just one kidney. However, at least one kidney must function properly to maintain life.
- Diseases that threaten the health and functioning of the kidneys include kidney stones, infections, and diabetes.
- Kidney stones are mineral crystals that form in urine inside the kidney. They may be extremely painful. If they block a ureter, they must be removed so urine can leave the kidney and be excreted.
- Bacterial infections of the urinary tract, especially the bladder, are very common. Bladder infections can be treated with antibiotics prescribed by a doctor. If untreated, they may lead to kidney damage.
- Uncontrolled diabetes may damage capillaries of nephrons. As a result, the kidneys lose much of their ability to filter blood. This is called kidney failure.
- The only cure for kidney failure is a kidney transplant, but it can be treated with dialysis. Dialysis is a medical procedure in which blood is filtered through a machine.