Jennifer Ellison-Brown: The excretory system
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.
This happens because your body is low on water and trying to reduce the amount of water lost in urine. The amount of water the body loses in urine is controlled by the kidneys, the main organs of the excretory system.
Excretion is the process of removing waste and excess water from the body. It is one of the major ways the body maintains homeostasis (the process of maintaining a stable internal environment).
Although the kidneys are the main organs of excretion, several other organs also excrete waste. 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 focus will be on the role of the kidneys in excretion. The roles of the other excretory organs are summarised below:
- The large intestine eliminates solid waste that remains 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 vapour and carbon dioxide.
The kidneys are part of the urinary system, which is shown above. The main function of the urinary system is to filter waste products and excess water from the blood and excrete them from the body.
The kidneys are a pair of bean-shaped organs just above the waist. A cross section of a kidney is shown in the diagram above. 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. Each kidney is supplied by a renal artery and a renal vein.
As shown in figure above, each nephron is like a tiny filtering plant. It filters blood and forms urine in the following steps:
1. Blood enters the kidney through the renal artery, which branches into capillaries. When blood passes through capillaries of a nephron, blood pressure forces some of the water and dissolved substances in the blood to cross the capillary walls into the Bowman's capsule.
2. 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.
3. 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.
From the collecting ducts of the kidneys, urine enters the ureters, two muscular tubes that move the urine by peristalsis to the bladder. 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 homestasis
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 litres 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.
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. This process is known as osmoregulation.