EXCRETION IN ANIMALS

                          

1. What is excretion? Why is it required?

The process of removal of toxic metabolic wastes from the body of an organism is called excretion.

The biochemical reactions which are taking place in the cells of an organism produce toxic wastes. The accumulation of these toxic substances in the body is harmful, and therefore needs to be removed as and when it is formed.

2. What are the components of the excretory system in man? Describe their structure with a diagram.

The excretory system consists of        i) a pair of kidneys

                                                            ii) a pair of ureters

                                                            iii) Urinary bladder

                                                            iv) Urethra

The kidneys are a pair of reddish brown, bean shaped structures that are present along the posterior wall in the abdominal cavity. They are positioned on either side of the vertebral column, protected by the last two pairs of ribs (floating ribs). On the inner concave side of the kidney is the hilum, where renal artery enters into the kidney, the renal vein and ureter come out.

Pair of ureters originate, one from each kidney and open into the urinary bladder. The ureters transport urine from the kidneys to the urinary bladder.

Urinary bladder is a muscular reservoir of urine, in the pelvic cavity of the abdomen. It has a pair if sphincters around its neck, which help it to remain closed until urination.

Urethra is a membranous tube that arises from the neck of the urinary bladder that extends upto the urethral orifice.

3. What is the physiological unit / functional unit of a kidney?

The physiological unit of kidney is the – Nephron

4. Describe the structure and function of a nephron using a diagram.

Structure of the nephron: Each kidney consists of more than a million nephrons. Each nephron consists of a bowman’s capsule, glomerulus, tubule and urine collecting duct.

A nephron consists of a double layered cup shaped structure lined by a single layer of squamous epithelial cells called the Bowman’s capsule. The bowman’s capsule surrounds a tuft or network of blood capillaries called the glomerulus. These two structures are together called the malpighian capsule. One end of the glomerulus is connected to the renal artery that brings impure blood for purification into the kidney. The other end of the glomerulus comes out of the bowman’s capsule and surrounds the uriniferous tubule of the nephron and finally joins the renal vein. The proximal end of the bowman’s capsule extends into a narrow tube called the uriniferous tubule, which is surrounded by a network of capillaries. The distal end of the tubule is connected to the urine collecting duct. All the ducts further converge to join the ureter.

Function of the nephron: the impure blood that reaches the kidneys (renal artery) enters into the glomerulus, which is a network of capillaries. As the blood passes thro the capillaries, it gets filtered. During filtration, substances like glucose, amino acids, water, salts, urea, etc pass thro as filtrate into the tubule. As the filtrate passes thro the tubule, useful substances such as glucose, aminoacids, some salts, excess water present in the filtrate are reabsorbed by the capillaries that surround the tubule. Only the wastes such as urea, certain salts, with some water remain in the tubule. This liquid left behind in the tubule is urine. The nephron carries this urine into the urine collecting duct of the kidney, where many such ducts together form the ureter. From the ureter, urine passes into the urinary bladder. Urine is stored for sometime here and then passed out of the body thro urethra.

5. How is the amount of urine produced regulated?

The amount of urine produced in the body depends on:

        i)            The amount of excess water present in the body

       ii)            The amount of waste / dissolved salts to be excreted.

6. What could cause renal failure / kidney failure?

Any infection or injury to kidneys or restricted blood flow can cause damage to the kidneys. This leads to accumulation of toxins in the blood, which circulates in the body and may lead to toxemia and death.

The blocking of the renal blood vessels could be due to the formation of oxalate crystals in the kidneys, which does not get dissolved. Inadequate water consumption could result in dehydration in very dry environments – resulting in reduced flushing out of salts from the body.

What is the suggested method for treating renal failure?

Permanent solution for renal failure is the transplant of kidney from a suitable donor to the patient.

In case of non availability of a donor kidney, dialysis is the only procedure to purify the blood at regular intervals to prevent toxicity in blood.

7. What is an artificial kidney?

It is a device used to remove the nitrogenous waste products from the blood in a patient having renal failure / kidney failure, by a process called dialysis.

8. How does the artificial kidney function?

The blood of the person with kidney failure is cleaned regularly using an artificial kidney machine. The procedure is called dialysis. Artificial kidneys contain a number of tubes with semi permeable membranes made of cellulose, suspended in a tank filled with dialyzing fluid. This fluid is an aqueous solution of glucose, and salts very similar in concentration to that of blood plasma. As the patient’s blood is passed through these tubes, the waste products in the blood like urea diffuse into this solution. The purified blood is now pumped back into the vein of the patient. This procedure has to be carried out at regular intervals for a patient with kidney failure.

The only difference between the normal kidney and this machine is that – there is no re absorption of nutrients into the blood.

9. Draw a neat labeled diagram of the human excretory system.

10. Explain the filtration process of blood in the nephron with the help of a diagram.

Refer to diagrams given in the class notes.

11. What are the excretory structures seen in different groups of animals?

In unicellular animals like amoeba – gaseous wastes diffuse out thro the cell membrane. Nitrogenous wastes like ammonia are removed by the contractile vacuoles.

In case of helminthes –worms such as earthworm – moist skin facilitates expulsion of CO2 through moist skin. Nitrogenous wastes are removed to the outside through nephridia, which are paired coiled tubes opening to the exterior thro nephridiopores.

In case of arthropods – the malpighian tubules do the excretory function.

EXCRETION IN PLANTS

What are the wastes products produced in plants that need to be excreted?

The waste products produced by plants during photosynthesis are oxygen and water vapor which gets partially reused by the plant for respiration. The wastes produced during respiration are carbon dioxide and water vapor – which get reused by plants for photosynthesis.

The other metabolic wastes produced by plants are gums, resins, latex etc stored in leaves, bark of trees and buds. These are expelled from the plant body when the plants shed leaves, bark of trees, fruits and flowers. Some of the salts produced by plants in the roots are expelled into the soil by the roots.