(a) Describe the structure and function of the basic filtering unit of kidney. (b) List two factors on which reabsorption of water from urine depends?

(a) Structure and Function of the Nephron (Basic Filtering Unit of Kidney)

#### Structure of Nephron:

1. Bowman's Capsule and Glomerulus: Each nephron has a cup-shaped bag at its upper end called Bowman's capsule, which encloses a cluster of very thin-walled blood capillaries called the glomerulus. The afferent arteriole brings blood into the glomerulus, and the efferent arteriole carries it away.
2. Tubular Part: The Bowman's capsule leads into a long, highly coiled tubular structure (consisting of proximal tubule, loop of Henle, and distal tubule). This tubular part is closely wrapped by a network of blood capillaries.
3. Collecting Duct: The tubular parts of several nephrons open into a common collecting duct, which carries the urine to the ureter.

#### Function of Nephron:

1. Ultrafiltration: Blood flows through the glomerulus under high pressure. Small molecules like water, salts, glucose, amino acids, and nitrogenous wastes (urea) are filtered out of the blood into the Bowman's capsule. Large molecules like proteins and blood cells remain in the capillaries.
2. Selective Reabsorption: As the filtrate flows along the tubular part of the nephron, useful substances such as glucose, amino acids, salts, and a major portion of water are selectively reabsorbed back into the surrounding blood capillaries.
3. Excretion: The remaining fluid containing concentrated nitrogenous waste (urea) forms urine, which flows into the collecting duct and is eventually excreted.

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(b) Two Factors on which Reabsorption of Water Depends:

The amount of water reabsorbed in the tubular part of the nephron depends on:

1. The amount of excess water present in the body: If the body is well-hydrated, less water is reabsorbed, leading to dilute urine. If the body is dehydrated, more water is reabsorbed to conserve water, leading to concentrated urine.
2. The amount of dissolved waste to be excreted: If there is a high concentration of dissolved nitrogenous wastes (like urea and salts) in the blood, more water is required to flush them out, which decreases the net reabsorption of water.