When the kidneys are completely damaged and do not function properly, the condition is called renal failure. . In the patients suffering from renal failure the amount of toxic nitrogenous wastes (like urea) increases in the blood which is very harmful. In such patients the nitrogenous wastes are removed from the blood by using artificial kidney. The working of artificial kidney is based on the principle of haemodialysis.Haemodialysis is the separation of certain micromolecules from blood by use of selectively permeable membrane.

Artificial kidney possesses a number of fine cellophane tubes suspended in a tank having dialysis solution. The blood of the patient rich in nitrogenous waste materials is passed through the cellophane tubes. When the blood passes through cellophane tubes the nitrogenous waste materials present in the blood pass into the dialyzing solution through the process by diffusion. The blood coming out of the artificial kidney is pure blood which is sent back in the body of patient.

Test Your Understanding and Answer These Questions:

1. Name the procedure used in the working of an artificial kidney.
2. Give the name of the principle on which artificial kidneys work.
3. What do you mean by renal failure? What are the reasons of renal failure?

 The process of removal of excess water and salts from the body is called osmoregulation.

The process of osmoregulation is also controlled by the kidneys. Osmoregulation is related to the habitat of an animal. The fresh water animals like fishes take up a large amount of water and frequently excrete a lot of urine in order to get rid of excess water. But other animals like birds and mammals take up a small quantity of water, thus they need to conserve the water. In these animals, a large quantity of water is reabsorbed in the tubules of the nephrons. In this way, we see that kidneys not only remove the toxic waste materials out of the body but also control the balance of water and salts in the body.

Test Your Understanding and Answer These Questions:

1. Write a note on haemodialysis.

Each human kidney is made up of about ten lakh excretory units called nephrons. A nephron is a long, narrow and twisted tubular structure. The upper end of the nephron is a cup shaped bag and it is called Bowman’s capsule. The lower end of the nephron is tube shaped and is called tubule. The Bowman’s capsule possesses a bundle of blood capillaries called glomerulus. The blood capillaries bring the dirty blood containing urea and excess water from renal artery.

During the passage of dirty blood through glomeruler capillaries, the small molecules of urea, glucose, amino acids, salts and water etc. present in the blood are passed into Bowman’s capsule as filtrate. Thus, the function of glomerulus is to filter the blood passing through it. The filtrate present in the Bowman’s capsule is then passed through the tubular part of the nephron. As the filtrate passes through the tubular part of the nephron, the useful substances such as glucose, amino acids, salts and water are reabsorbed by the blood capillaries surrounding the nephron and only the waste substances such as urea, some salts and excess water remain behind in the tubule. Finally, the waste materials present in the tubule (also called urine) is passed to the urinary bladder through the collecting ducts

Test Your Understanding and Answer These Questions:

1. What is nephron? Explain its structure.
2. What is glomerulus? What is its function?
3. Give differences between nephron and nephridium.
4. Briefly describe the mechanism of urine formation.
5. What happens to glucose which enters the nephron along with the filtrate?
6. Which is the major nitrogenous waste product in a human being? How is it removed from the body?

 In human beings excretion of waste materials take place through a well developed system of organs called urinary system. The urinary system of human beings consists of: two kidneys, two ureters, a urinary bladder and a urethra.

    
Kidneys

In human beings a pair of kidneys is present in the abdomen. The kidneys are dark red, bean shaped organs. Each kidney is 10 – 13 cm. long and 2.5 to 4 cm. wide. The function of kidney is to remove the waste materials present in the blood in the form of yellowish liquid called urine. As shown in Fig. 14ach kidney is covered by a layer of fibrous tissue called renal capsule. The renal capsule protects the kidneys from injuries and infection. Internally the kidney is divisible into two parts: outer renal cortex and inner renal medulla. The renal cortex is dark red while the renal medulla is light in colour. The medulla is divided into a number of conical areas called pyramids.

 Each kidney is made up of about ten lakh excretory units called nephrons. We shall discuss the structure of nephron later on.

Ureters
Ureters are two narrow tubes which carry urine from kidneys to the urinary bladder.

Urinary bladder
The urinary bladder is a hollow, pear shaped bag. The function of urinary bladder is to store urine temporarily.

Urethra
Urethra is a muscular tube, through which the urine collected in the urinary bladder is passes out of the body.

Functions of kidneys

• Kidneys remove the nitrogenous wastes such as urea from the blood.
• Kidneys also remove excess of water from the body.
• Kidneys maintain the proper pH of blood by removing excess of acids and bases from the blood.
• Kidneys maintain the proper balance of concentration of salts in the blood.
• Kidneys also remove other toxic substances from the blood.

Test Your Understanding and Answer These Questions:

1. What is urethra?
2. Name the excretory unit of a kidney.
3. Where is urine carried through the ureters?
4. Explain excretory system of human beings.
5. State the vital functions of the human kidney.
6. How many nephrons are present in each kidney?
7. Draw and label the parts of the human excretory system.
8. Explain internal structure of kidney by drawing a well labeled diagram.
9. How many kidneys are present in human beings? Where are these situated in human body?

In earthworm carbon dioxide and nitrogenous wastes are the main waste materials. The carbon dioxide is excreted out from the body through its moist skin by the process of diffusion. The nitrogenous wastes are excreted out of the body by special excretory organs called nephridia (singular nephridium).

 A Nephridium is tubular coiled structure. It starts from a rounded funnel shaped structure called nephrostome. The nephrostome opens in a coiled tube called nephridial tubule which is lined with cilia. The nephridial tubule opens outside the body through a small opening called nephridiopore.

The body fluid of earthworm rich in nitrogenous wastes enters the nephridium through nephrostome and passes into nephridial tubule. In the nephridial tubule the useful substances present in fluid are absorbed. The remaining fluid containing high percentage of nitrogenous wastes is excreted out of the body through nephridiopore with the help of movements of cilia.

Location of nephridia in earthworm

In earthworms, the nephridia are present on septa and in the skin.The nephridia present on the septa are called septal nephridia. And the nephridia present in the skin are called integumentary nephridia.

Test Your Understanding and Answer These Questions:

1. Explain the process of excretion in earthworm.
2. What is nephrostome?
3. Name the part of the body responsible for excretion in earthworm.
4. What are the types of nephridia found in earthworm? Where are nephridia present in an earthworm?
5. What happens to the useful substances which flow into nephridia in earthworm along with the waste?

 Amoeba is a unicellular organism found in fresh water. In amoeba carbon dioxide and ammonia are the main waster materials. These waste materials are excreted out by the process of diffusion through general body surface.

In amoeba, contractile vacuoles also play some role in the removal of waste materials. The waste materials present in the cytoplasm of amoeba enters the contractile vacuole. Then the contractile vacuole moves close to the plasma membrane, come in contact with plasma membrane and burst to release its contents in the surrounding.

Test Your Understanding and Answer These Questions:

1. Explain the process of excretion in amoeba.
2. Name the part of the body responsible for excretion in Amoeba.
3. From where do waste materials collect into the excretory part of amoeba?