CBSE Class 11 Biology Chapter 21: Excretory Products and Their Elimination NCERT Solutions

Solution: Glomerular Filtration Rate (GFR) is defined as the volume of fluid that is filtered from the glomerular capillaries into Bowman's capsule per unit of time. It represents the rate at which blood is filtered by the kidneys. In a healthy individual, the GFR is approximately 125 mL per minute. The glomerular filtrate contains essential substances like glucose, amino acids, and ions, along with waste products such as urea and uric acid, and a large volume of water.

Solution: The kidney possesses an intrinsic autoregulatory mechanism to maintain a relatively constant Glomerular Filtration Rate (GFR) despite fluctuations in systemic blood pressure. This regulation is primarily managed by the juxtaglomerular apparatus (JGA), a specialized structure located at the vascular pole of the renal corpuscle where the afferent arteriole and distal convoluted tubule meet. If there is a decrease in GFR, the juxtaglomerular cells within the JGA are stimulated to release renin. Renin initiates the renin-angiotensin-aldosterone system, which leads to vasoconstriction or dilation of the afferent arteriole, thereby regulating glomerular blood flow and bringing the GFR back towards its normal level.

Solution:

1. (a) True. Micturition, the process of expelling urine, is a reflex action controlled by the nervous system.
2. (b) False. ADH (Antidiuretic Hormone) promotes water reabsorption, making the urine more concentrated (hypertonic), not hypotonic. It reduces water elimination.
3. (c) True. During glomerular filtration, a protein-free fluid is forced from the blood plasma in the glomerulus into the Bowman's capsule. Large proteins and blood cells are retained in the blood.
4. (d) True. The Loop of Henle is crucial for establishing the osmotic gradient in the renal medulla, which is essential for the counter-current mechanism and the subsequent concentration of urine.
5. (e) True. Glucose is almost completely reabsorbed from the filtrate back into the blood in the proximal convoluted tubule through active transport mechanisms.

Solution: The counter-current mechanism is a vital physiological process in the kidneys that facilitates the conservation of water and the concentration of urine. This mechanism operates through two main components within the renal medulla: the Loop of Henle and the vasa recta.

The Loop of Henle is a U-shaped tubule extending from the proximal convoluted tubule into the medulla. It has two limbs, a descending limb and an ascending limb, through which tubular fluid flows in opposite directions. This arrangement creates a counter-current flow.

The vasa recta are capillaries that run parallel to the Loop of Henle. Blood flows in opposite directions within the descending and ascending limbs of the vasa recta, also establishing a counter-current flow. The descending limb of the vasa recta is permeable to water and solutes, while the ascending limb is less permeable. As blood flows down the descending limb into the hypertonic medulla, water moves out. As it flows up the ascending limb, it encounters the less hypertonic cortex, and solutes move out, while water re-enters.

The interaction between the Loop of Henle and the vasa recta maintains a steep osmotic gradient in the renal medulla, increasing from approximately 300 mOsmol/L in the cortex to as high as 1200 mOsmol/L in the deep inner medulla. This gradient is established and maintained by the differential transport of solutes (like NaCl and urea) and water. This medullary osmotic gradient is crucial for enabling the collecting ducts to reabsorb water under the influence of ADH, thereby producing concentrated urine and conserving body water.

The osmolarity increases from 300 mOsmolL^-1 in the cortex to 1200 mOsmolL^-1 in the inner medulla by counter current mechanism. It helps in maintaining the concentration gradient, which in turn helps in easy movement of water from collecting tubules. The gradient is a result of the movement of NaCl and urea.