EXCRETORY SYSTEM
The system that helps to eliminate waste materials from the body is called excretory system and the process of removing the waste products from the body is known as excretion.
In order to maintain homeostasis, excretory system removes non solid wastes through sweat and urine. Carbon dioxide is expelled out of the body with the help of lungs.
The main organs of excretory system are the skin, lungs, kidneys, ureters, urinary bladder and urethra, as shown in fig 1.6.
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Fig.1.6:The Excretory System |
The kidneys are organs that removes wastes by filtering and cleaning the blood to produce urine. The urine moves through ureters, is collected in the bladder and excreted by the urethra.
Ureter is a tube that take the urine from kidneys to bladder. The urinary bladder is sac like organ. In adults, it can store up to half of a liter (over 2 cups) of urine at a time, the urine is released through a single tube the urethra, to the outside.
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Structure of the Kidneys
Kidney are pair of bean shaped organs. The size of a kidney is 10 cm in length and 5 cm in width. They are located on either side of lower back. Each kidney have weight about 27 grams. Human body has two kidneys. However, if one is damaged or be removed, one can still live comfortably with only one kidney.
The main parts of the kidney are illustrated in fig 1.7.
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Fig.1.7: Structure Of Kidney |
Each kidney has the inner layer called, the medulla and the outer layer, called the cortey.
The cortex is filled with nephrons, which extend through the cortex and partly to the medulla.
Each of your kidney contains about one million nephrons. A large volume of blood continuously enters the kidney through the renal artery and exits through the renal vein. The functions of the kidneys are largely controlled by how much water, salts, and other materials are concentrated in the blood.
Kidneys and Homeostasis
Kidneys have three main functions in maintaining homeostasis.
- They remove waste products from the blood, such as those produced from lingestion and cellular respiration.
- They help to maintain water level in the body.
- They release hormones that help keep bones healthy, make red blood cells and regulate blood pressure.
The kidneys are very important part of the homeostasis system of our body. Failure in the function of kidney can cause many severe problems. Waste products build up rapidly in the bloodstream, causing severe disruption in homeostasis in many organ systems.
For example, imbalances in electrolytes such as sodium and potassium could disrupt the rhythm of the heart causing this organ to fail. A build up of toxic substances such as ammonium salts into the blood can impair the functioning of neurons in the brain.
Structure of Nephron
Each kidney is composed of large number of tiny tubes, the urinary tubules or nephrons.Each nephron consists of long thin excrétory tubule, a cluster of capillaries, known as glomerulus and a network of capillaries which surrounds the tubule. The excretory tubule is closed at one end which enlarges into a cup shaped called Bowman's capsule.
The Bowman's capsule surrounds the capillaries of glomerulus and it is collectively called the "renal corpuscle". The tubular part of nephron is much longer. It originates from the Bowman's capsule and its wall is single celled in thickness.
Its first portion is twisted upon itself in a very complex way and the other portion is in the form of U- shaped structure called Loop of Henle.Last portion of the tubule is again convoluted.
It joins with last portions of several other nephrons and ultimately opens into large duct called collecting duct. The structure of nephron is shown in the Fig. 1.8.
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Fig. 1.8: Structure Of Nephron |
Function of Nephron
Nephrons clean the blood in a three step which are filtration, re-absorption and excretion.First water then other materials move out of the capillaries and into the nephron. Then some of these materials are reabsorbed and returned to the blood.Eventually the rest of waste products are excreted in the urine.
Filtration
As shown in figure 1.8, cach nephron is supplied with blood through an arteriole and a venule. When blood enters the kidneys, it flows into the arteries and then into the glomerulus of each nephron.
Because the blood is under pressure, small molecules such as water, amino acids, salts, glucose and urea are pushed out of the capillaries into Bowman's capsule. Anything too large to move out of the capillaries such as blood cells, plasma proteins or platelets remain stays in the blood.
Re-absorption of Materials
The contents of a Bowman's capsule are called the filtrate. The nephrons process about 180 liters (48gallon) of filtrate every day, yet only about one percent is excreted as urine. Most of the filtrate is reabsorbed into the capillaries and return to the blood.
This process ensures that nutrients such as water, amino acid, glucose and sodium ions (Na+) are made available to the body. The re-absorption of water and (Na) helps it to maintain body fluid balance.
For example, if too much water is drunk, the nephrons will reabsorb less of the fluid and more urine will be produced.
