Correct Option: C. Nephron Detailed Explanation: The nephron is the fundamental structural and functional unit of the kidney, and it plays a crucial role in the excretory system by filtering blood to form urine. Each kidney contains approximately one million nephrons, which work together to maintain the body's fluid balance, electrolyte levels, and waste removal. Step-by-Step Breakdown of the Nephron's Function:

1. Blood Filtration:
2. The process begins in the glomerulus, a network of tiny blood vessels (capillaries) located within the nephron. Blood enters the glomerulus through the afferent arteriole.
3. The high pressure in the glomerulus forces water, ions, and small molecules (like glucose and urea) out of the blood and into the Bowman's capsule, a cup-like structure that surrounds the glomerulus. This process is called glomerular filtration.
4. Reabsorption:
5. The filtrate (the fluid that has been filtered) then moves into the proximal convoluted tubule (PCT), where a significant amount of water, glucose, and essential ions (like sodium and potassium) are reabsorbed back into the bloodstream. This is crucial for maintaining the body's homeostasis.
6. As the filtrate continues through the nephron, it passes through the loop of Henle, where further reabsorption of water and salts occurs, concentrating the urine.
7. Secretion:
8. In the distal convoluted tubule (DCT) and the collecting duct, additional substances (like hydrogen ions and certain drugs) are secreted into the filtrate, which helps regulate pH and eliminate waste products.
9. Formation of Urine:
10. Finally, the filtrate, now called urine, is collected in the collecting duct and transported to the renal pelvis, from where it moves to the ureter and eventually to the bladder for storage before excretion.

Why Other Options Are Incorrect:

• A. Ureter:
• The ureter is a tube that carries urine from the kidney to the bladder. It does not play a role in the filtration of blood; rather, it is involved in the transport of urine after it has been formed.
• B. Bladder:
• The bladder is a storage organ for urine. It holds urine until it is excreted from the body. Like the ureter, it does not participate in the filtration process.
• D. Urethra:
• The urethra is the duct through which urine is expelled from the bladder to the outside of the body. It is not involved in the filtration of blood or urine formation.

Summary of Key Points:

• The nephron is the primary unit responsible for filtering blood and forming urine in the kidneys.
• It consists of structures like the glomerulus, Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.
• The filtration process involves glomerular filtration, reabsorption of essential substances, and secretion of waste products.
• Other structures like the ureter, bladder, and urethra are involved in urine transport and storage, not in the filtration process.

Revision Summary:

• The nephron is the key structure for blood filtration in the kidneys.
• It consists of several parts that work together to filter blood, reabsorb necessary substances, and secrete waste.
• The ureter, bladder, and urethra are involved in urine transport and storage, not filtration.
• Understanding the nephron's function is crucial for grasping how the excretory system maintains homeostasis.