Problem 137 - Objective Biology for NEET

Question

Match the excretory functions of section I with the parts of the excretory system in section II. Choose the correct combinations from among the answers given. Section I (i) Ultrafiltration (ii) Concentration of urine (iii) Transport of urine (iv) Storage of urine Section II (a) Henle's loop (b) Ureter (c) Urinary bladder (d) Malpighian corpuscles (e) Proximal convoluted tubules (a) (i) -(d), (ii)-(a), (iii) -(b), (iv)-(c) (b) (i) - (d), (ii)-(c), (iii) -(b), (iv)-(a) (c) (i)-(e), (ii) - (d), (iii) -(a), (iv)-(c) (d) (i) -(e), (ii)-(d), (iii)-(a), (iv)-(b)

Step-by-Step Solution

Answer

The correct combinations are (a) (i) - Malpighian corpuscles, (ii) - Henle's loop, (iii) - Ureter, (iv) - Urinary bladder.

Step 1: Understand the function of each part of the excretory system

Each part of the excretory system carries out specific functions. Malpighian corpuscles are involved in ultrafiltration; Henle's loop is involved in the concentration of urine; the ureter is responsible for the transport of urine from kidneys to the bladder, and the urinary bladder is used for the storage of urine before it is excreted.

Step 2: Match excretory system parts with their functions

Now, we match the function in section I with the parts of the excretory system in section II based on their characteristics. (i) Ultrafiltration corresponds with (d) Malpighian corpuscles, (ii) Concentration of urine corresponds with (a) Henle's loop, (iii) Transport of urine corresponds with (b) Ureter, and (iv) Storage of urine corresponds with (c) Urinary bladder.

Step 3: Find the correct combinations

Based on the matching we performed in step 2, we can now find the correct combinations from the options provided in section II. Therefore, the correct combination is (a) (i) -(d), (ii)-(a), (iii) -(b), (iv)-(c).

Key Concepts

Ultrafiltration

Ultrafiltration is the initial step in the excretion process within the kidneys. It involves the filtration of blood through the tiny structures called the Malpighian corpuscles, or more specifically through the glomerulus nestled inside Bowman's capsule. Here, the blood pressure forces water and small molecules like glucose, salts, and urea out of the blood and into the capsule, forming a substance called the glomerular filtrate. Larger molecules, like proteins and blood cells, are too large to pass through the fine mesh of capillaries and remain in the bloodstream.

The process of ultrafiltration is essential because it segregates the waste substances from the blood. This filtration, driven purely by blood pressure, lays the foundation for the formation of urine by first creating a filtrate that is free of cells and proteins.

Concentration of Urine

After ultrafiltration, the next significant function is the concentration of urine, which primarily takes place in the Henle's loop within the kidney's nephron. Henle's loop is a U-shaped tube that extends into the kidney's medulla and plays a crucial role in the counter-current exchange mechanism. This system allows for the reabsorption of water and salts back into the body, which is vital for maintaining the body's fluid and electrolyte balance.

Counter-Current Mechanism

Inside Henle's loop, a descending limb allows water to pass out and an ascending limb pumps salts out. This creates a high osmolarity in the medulla, which in turn pulls water from the collecting duct, effectively concentrating the urine. Precise regulation of this process ensures that the body conserves water efficiently, producing urine that is up to four times more concentrated than the initial filtrate.

Transport of Urine

Once the urine is concentrated, it needs to be transported from the kidneys to the urinary bladder for storage until excretion. The tubes responsible for this vital function are the ureters. Each kidney is connected to a ureter, a muscular duct that uses peristaltic waves to propel urine downwards, preventing backflow and ensuring unidirectional movement towards the bladder.

Peristalsis in Ureters

Peristalsis is a series of wave-like muscle contractions that move the urine efficiently despite gravity, especially when one is lying down or standing. It's an automatic process that ensures continuous movement of urine, highlighting the ureter's essential role in the excretory system. This continual transport is important to prevent urine stagnation that could lead to infections or kidney stones.

Storage of Urine

Finally, the urinary bladder serves as a temporary reservoir for the storage of urine. This hollow, muscular organ is quite flexible and can expand significantly to store urine that accumulates between voiding. The process of urination, or micturition, is controlled by internal and external sphincters at the base of the bladder.

Bladder Control and Micturition Reflex

The micturition reflex is an involuntary spinal reflex that ensures the bladder can hold urine until it's socially acceptable to release it. However, conscious control over the external sphincter allows for voluntary determination of the timing of urine excretion. The ability of the bladder to store urine helps us maintain our daily activities without frequent interruptions for urination, highlighting the bladder's critical function in the excretory system.