Correct Option: C. Kidney Detailed Explanation: The human excretory system is responsible for removing waste products from the body and regulating water and electrolyte balance. The primary organ involved in filtering blood and producing urine is the kidney. Here’s a step-by-step breakdown of why the kidney is the correct answer:

1. Structure and Function of the Kidney:
2. The kidneys are two bean-shaped organs located on either side of the spine, just below the rib cage. Each kidney contains approximately one million tiny filtering units called nephrons.
3. Each nephron consists of a glomerulus (a network of capillaries) and a renal tubule. The glomerulus filters blood, allowing water, salts, and small molecules to pass through while retaining larger molecules like proteins and blood cells.
4. Filtration Process:
5. Blood enters the kidney through the renal artery, which branches into smaller arterioles and eventually leads to the glomeruli.
6. In the glomerulus, blood pressure forces water, ions, and small molecules out of the blood and into the Bowman's capsule, starting the process of urine formation.
7. The filtrate then passes through the renal tubule, where further reabsorption of water and essential nutrients occurs, and waste products are secreted into the tubule.
8. Urine Production:
9. After filtration and reabsorption, the remaining fluid in the renal tubule is called urine. This urine is collected in the renal pelvis and then transported to the bladder via the ureters.
10. Role in Homeostasis:
11. The kidneys play a crucial role in maintaining homeostasis by regulating the volume and composition of blood, which includes balancing electrolytes and removing metabolic waste products.

Why Other Options Are Incorrect:

• A. Ureter:
• The ureters are tubes that carry urine from the kidneys to the bladder. They do not filter blood or produce urine; their primary function is transportation.
• B. Bladder:
• The bladder is a muscular sac that stores urine until it is excreted from the body. It does not have any role in filtering blood or producing urine.
• D. Urethra:
• The urethra is the duct through which urine is expelled from the bladder to the outside of the body. Like the ureters and bladder, it does not filter blood or produce urine.

Summary of Key Points:

• The kidney is the primary organ responsible for filtering blood and producing urine in the human excretory system.
• Blood filtration occurs in the glomeruli of the nephrons, where waste products are separated from the blood.
• The ureters, bladder, and urethra are involved in the transport and storage of urine but do not participate in the filtration process.
• Understanding the structure and function of the kidneys is essential for grasping how the body maintains fluid and electrolyte balance.

This comprehensive understanding of the kidneys and their role in the excretory system is crucial for anyone studying biology, especially in the context of human physiology.