Question

Kidneys return water and small solutes to the blood by the process of ________. a. filtration b. reabsorption c. tubular secretion d. dialysis

Short answer

The process is reabsorption.

Step-by-step solution

Understanding the process

To determine which process returns water and small solutes to the blood, we need to understand each option. Filtration is the movement of substances from the blood into the kidney tubules. Reabsorption is the process by which water and solutes are taken back into the blood from the kidney tubules. Tubular secretion involves the transfer of substances from blood into the urine. Dialysis is a medical procedure to remove waste.

Identifying the correct process

From our understanding, filtration and tubular secretion involve movement out of the blood or into urine, not returning to the blood. Dialysis is not a natural kidney process. Reabsorption, however, specifically refers to the process in the kidneys where water and solutes are taken back into the bloodstream from the kidney tubules.

Conclusion

Since reabsorption is the only process that involves returning water and small solutes to the blood, it is the correct answer.

Key concepts

Reabsorption

Reabsorption is a vital process in kidney function where water and small solutes are returned to the blood from the kidney tubules. This process occurs primarily in the nephrons, the functional units of the kidney. Nephrons filter blood, allowing the body to selectively retain the necessary substances while removing waste.
During reabsorption, water, glucose, amino acids, and essential ions such as sodium and potassium are selectively transported back into the bloodstream. This ensures that the body retains vital nutrients and maintains electrolyte and fluid balance.

• Water reabsorption helps stabilize fluid levels in the body, preventing dehydration.
• Solutes like glucose and amino acids are valuable energy sources, and their reabsorption prevents loss in urine.
• Electrolytes balance the body's pH and support nerve and muscle function.

Reabsorption is a highly efficient process, allowing over 99% of the filtrate to be recovered and only a small amount to be excreted as urine. The ability of the kidneys to reabsorb vital substances is crucial for overall health and the principal reason they are known as the body's natural filtration systems.

Nephron

The nephron is the fundamental structural and functional unit of the kidney, playing a crucial role in filtering and purifying the blood. Each kidney contains around one million nephrons that work together to ensure the body's waste removal and fluid balance.
A nephron consists of several parts:

• The Bowman’s capsule, which encases a bundle of capillaries known as the glomerulus, where filtration begins.
• The proximal convoluted tubule, which is the main site for reabsorption of water and solutes back into the blood.
• The loop of Henle, which plays a critical role in concentrating urine.
• The distal convoluted tubule and collecting duct, where further reabsorption and secretion fine-tune the urine output.

The coordination of these components allows nephrons to efficiently manage waste and maintain vital chemical balances. Nephrons’ ability to process and filter blood supports the body's internal environment stability, known as homeostasis.

Filtration

Filtration is the first step in the kidney's ability to remove waste and excess substances from the blood. This process occurs in the glomerulus of each nephron, where the blood pressure forces water and small molecules out of the blood and into the Bowman’s capsule.
Unlike reabsorption, filtration is a non-selective process where molecules are moved based on size. Larger molecules, such as proteins and blood cells, remain in the blood, while water, ions, glucose, and urea pass into the renal tubule.
Filtration plays a crucial role in:

• Initial separation of waste and useful substances based on their molecular size.
• Allowing the kidney to assess the body's hydration and nutrient status, adjusting reabsorption and secretion accordingly.
• Providing a continual process ensuring that cellular waste, toxins, and excess ions are processed efficiently.

The filtration process is vital in maintaining blood composition and pressure, setting the foundation for subsequent kidney functions.