Excretion is the process of removal of toxic wastes from the body of an organism. The major wastes produced by us are: Carbon dioxide and Urea. Carbon dioxide is produced by the process of respiration and urea is produced by the decomposition of unused proteins in the liver. It is necessary to remove these wastes from our body as their accumulation is poisonous and harm us. There are different organs which removes waste from the body. These are lungs and kidneys. Our lungs excrete carbon dioxide and kidneys urea. So, the kidney is the main excretory organs of the human body.
First we will see how carbon dioxide is removed through lungs: Carbon dioxide is produces as a waste product in the body by the oxidation of food during respiration. This carbon dioxide enters from the body tissues in to the blood stream by diffusion. Blood carries this carbon dioxide to the lungs. When we breathe out, then the lungs excrete carbon dioxide which goes in to the air through nostrils.
The excretory system of human beings collects the liquid wastes of the body and helps it get rid of them. It consists of the following main organs: Two kidneys, two ureters, Bladder and Urethra. The kidneys are bean shaped organs towards the back of our body just above the waist. Everyone has two kidneys. Blood constantly passes through our kidneys. The renal artery or kidney artery brings in the dirty blood containing waste substances in to the kidneys. So, the function of kidney is to remove the poisonous substance urea, other waste salts and excess water from the blood and excrete them in the form of yellowish liquid called urine. The cleaned blood is carried away from the kidneys by the renal vein or kidney vein.
The ureter, one from each kidney, opens in to urinary bladder. Ureters are the tubes which carry urine from the kidneys to the bladder. Here urine is stored. The bladder is big it store urine till the time we go to toilet. A tube called urethra which is connected with the bladder, urine is passed from the body through it.
Now, we will discuss about kidney and its function in detail
In the structure of kidney it is shown that each kidney is made up of a large number of excretory units called nephrons. The nephron has a cup shaped bag at its upper end which is called Bowman’s capsule. The lower end of Bowman’s capsule tube shaped called tubule. And these together form nephron. One end of a tube is connected to the Bowman’s capsule and other is connected to a urine- collecting duct of the kidney. The Bowman’s capsule contains a bundle of blood capillaries which is called glomerulus. One end of glomerulus is attached to the renal artery which brings the dirty blood containing the urea waste in to it and the other end is joined with renal vein for putting urea clean blood in to it.
The function of glomerulus is to filter the blood passing through it. Only the small molecules of substances present in blood like glucose, amino acids, salts, urea and water etc. passes through it and collects as filtrate in Bowman’s capsule. The large molecules like proteins and blood cells cannot pass through glomerulus capillaries and remain behind in the blood. The nephron tubule allow the selective reabsorption of the useful substances like glucose, amino acids, salts and water in to blood capillaries. But urea remains behind the tubule and does not get reabsorbed in to blood capillaries.
Working of the excretory system
As discussed above the dirty blood containing waste like urea enters the glomerulus and blood is filtered here. During filtration substances like glucose, amino acids, salts, water, urea etc. present in blood pass in to Bowman’s capsule and then enters the tubule of nephron. Here
useful substances are reabsorbed in to the blood through blood capillaries surrounding the tubule. The liquid left behind the tubule of nephron is urine. The nephron carries this urine in to the collecting duct of the kidney from where it is carried to the ureter. And from here urine passes to the urinary bladder. And after some time urine is passed out of the body through urethra.
What is Kidney failure or Renal failure?
Due to an infection in the kidney, an injury to kidney or restricted blood flow to the kidney’s, may stop working and complete failure of kidney’s allows urea and other waste products to build up in the blood. Even the amount of water in the body is not regulated which may cause death if the patient is not given proper treatment. The best solution is kidney transplant. In this the damaged kidney is removed and from healthy person matching kidney is transplanted by a surgical operation. If, this is not possible then periodically patient is kept on a kidney machine by a procedure called dialysis. The kidney machine is also known as artificial kidney which removes waste products from the blood through dialysis.
So, dialysis is the procedure used for cleaning the blood of a person by separating the waste substance urea from it. The blood from an artery in the patient’s arm is made to flow in to the dialyser of a dialysis machine made of long tubes of selectively permeable membrane which are coiled in a tank containing dialysis solution. The dialysis solution contains water, glucose and salts in similar concentrations to those in normal blood. As the patient’s blood passes through the dialyzing solution, most of the wastes like urea present in it pass through the selectively permeable membrane cellulose tubes in to the dialyzing solution. The cleaned blood is pumped back in to a vein of the patient’s arm.
Facts about excretion in animals:
Image courtesy: www.estrellamountain.edu