# Excretion

## NCERT Solution

Question 1: Define Glomerular Filtration Rate (GFR)

Answer: Glomerular Filtration Rate (GFR): The amount of the filtrate formed by the kidneys per minute is called glomerular filtration rate (GFR). GFR in a healthy individual is approximately 125 ml/minute, i.e., 180 litres per day. On an average, 1100-1200 ml of blood is filtered by the kidneys per minute which constitute roughly 1/5th of the blood pumped out by each ventricle of the heart in a minute.

Question 2: Explain the autoregulatory mechanism of GFR.

Answer: Regulation of GFR: The kidneys have built-in mechanisms for the regulation of glomerular filtration rate. One such efficient mechanism is carried out by juxta glomerular apparatus (JGA). JGA is a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact. A fall in GFR can activate the JG cells to release renin which can stimulate the glomerular blood flow and thereby the GFR back to normal.

Question 3: Indicate whether the following statements are true or false:

1. Micturition is carried out by a reflex.
2. ADH helps in water elimination, making the urine hypotonic.
3. Protein-free fluid is filtered from blood plasma into the Bowman’s capsule.
4. Henle’s loop plays an important role in concentrating the urine.
5. Glucose is actively reabsorbed in the proximal convoluted tubule.

Answer: (a) True, (b) False, (c) True, (d) True, (e) True

Question 4: Give a brief account of the counter current mechanism.

The Henle’s loop and vasa recta play a significant role in this. The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of vasa recta is also in a counter current pattern. The proximity between the Henle’s loop and vasa recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium, i.e., from 300 mOsmolL–1 in the cortex to about 1200 mOsmolL–1 in the inner medulla. This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta.

Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule.

The above described transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the counter current mechanism. This mechanism helps to maintain a concentration gradient in the medullary interstitium. Presence of such interstitial gradient helps in an easy passage of water from the collecting tubule thereby concentrating the filtrate (urine). Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed.

Question 5: Describe the role of liver, lungs and skin in excretion.

Answer: ROLE OF OTHER ORGANS IN EXCRETION

Lungs: Lungs remove large amounts of CO2 (18 litres/day) and also significant quantities of water every day.

Liver: Liver, the largest gland in our body, secretes bile containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Most of these substances ultimately pass out alongwith digestive wastes.

Skin: The sweat and sebaceous glands in the skin can eliminate certain substances through their secretions. Sweat produced by the sweat glands is a watery fluid containing NaCl, small amounts of urea, lactic acid, etc. Though the primary function of sweat is to facilitate a cooling effect on the body surface, it also helps in the removal of some of the wastes mentioned above. Sebaceous glands eliminate certain substances like sterols, hydrocarbons and waxes through sebum. This secretion provides a protective oily covering for the skin.

Question 6: Explain micturition.

Answer: Micturition: Urine formed by the nephrons is ultimately carried to the urinary bladder where it is stored till a voluntary signal is given by the central nervous system (CNS). This signal is initiated by the stretching of the urinary bladder as it gets filled with urine. In response, the stretch receptors on the walls of the bladder send signals to the CNS. The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter causing the release of urine. The process of release of urine is called micturition and the neural mechanisms causing it is called the micturition reflex.

Question 7: Match the items of column I with those of column II:

Column IColumn II
(a) Ammonotelism(i) Birds
(b) Bowman’s capsule(ii) Water reabsorption
(c) Micturition(iii) Bony fish

Answer: (a) iii, (b) v, (c) iv, (d) i, (e) ii

Question 8: What is meant by the term osmoregulation?

Answer: Osmoregulation is the regulation of blood volume, body fluid volume and ionic concentration. Following is the method of osmoregulation:

• Osmoreceptors in the body are activated by changes in blood volume, body fluid volume and ionic concentration. An excessive loss of fluid from the body can activate these receptors which stimulate the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis. ADH facilitates water reabsorption from latter parts of the tubule, thereby preventing diuresis.
• An increase in body fluid volum can switch off the osmoreceptors and suppress the ADH release to complete the feedback. ADH can also affect the kidney function by its constrictory effects on blood vessels. This causes an increase in blood pressure. An increase in blood pressure can increase the glomerular blood flow and thereby the GFR.

Question 9: Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic, why?

Answer: A lot of water is required to remove ammonia from the body. Water is a scarce resource in terrestrial habitat; unlike aquatic habitat. Hence, terrestrial animals needed to develop adaptations so that water can be conserved. Due to this, terrestrial animals are generally ureotelic or uricotelic.

Question 10: What is the significance of juxta glomerular apparatus (JGA) in kidney function?

Answer: Significance of Juxta Glomerular Apparaus (JGA):

The JGA plays a complex regulatory role. A fall in glomerular blood flow/glomerular blood pressure/GFR can activate the JG cells to release renin which converts angiotensinogen in blood to angiotensin I and further to angiotensin II. Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby GFR. Angiotensin II also activates the adrenal cortex to release Aldosterone. Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule. This also leads to an increase in blood pressure and GFR. This complex mechanism is generally known as the Renin-Angiotensin mechanism.

Question 11: Name the following:

1. A chordate animal having flame cells as excretory structures
2. Cortical portions projecting between the medullary pyramids in the human kidney
3. A loop of capillary running parallel to the Henle’s loop.

Answer: (a) Cephalochordata, (b) Columns of Bertini, (c) Vasa recta

Question 12: Fill in the gaps: