Problem 7 - Biology: Concepts and Applications

Question

Kidneys return most of the water and small solutes back to blood by way of _____. a. glomerular filtration b. tubular reabsorption c. tubular secretion d. both a and b

Step-by-Step Solution

Answer

The correct answer is b. tubular reabsorption.

Step 1: Understand the Question

The question is about the process by which kidneys return water and small solutes back to the blood. You are asked to choose the correct option among the given processes related to kidney function.

Step 2: Identify the Kidney Processes

There are three main processes: glomerular filtration, tubular reabsorption, and tubular secretion. Glomerular filtration is the first step where blood is filtered in the glomerulus. Tubular reabsorption involves moving substances from the tubular fluid back into the blood. Tubular secretion involves transferring materials from the blood into the tubular fluid.

Step 3: Focus on Return to Blood

The question emphasizes returning substances to the blood. Glomerular filtration is about removing substances from the blood, so it does not fit this aspect. Tubular secretion also does not fit since it moves substances into the tubular fluid, not back to the blood.

Step 4: Select the Correct Option

Tubular reabsorption is the process where the kidneys return water and smaller solutes back to the bloodstream, which matches the requirement of the question. Thus, the correct answer is option b.

Key Concepts

Glomerular Filtration

The process of glomerular filtration is the first step in the kidney's function of filtering blood. It occurs in the glomerulus, which is a network of tiny blood vessels, or capillaries, located in the nephron of the kidney. During this stage:

• Blood pressure forces water and solutes, like small molecules and ions, through the walls of the capillaries.
• This creates a filtrate that contains waste products, nutrients, and water.

The filtrate is collected in a part of the nephron known as Bowman's capsule.
While glomerular filtration removes many substances from the blood, it does not differentiate between needed substances and waste. Thus, it is not directly involved in returning substances to the bloodstream, which is why it doesn't apply to the question's requirement of returning water and solutes to the blood.

Tubular Reabsorption

Tubular reabsorption is crucial for returning essential substances back to the bloodstream after they were filtered out during glomerular filtration. This process primarily occurs in the renal tubules:

• As the filtrate travels through the tubules, cells on the walls actively transport necessary molecules, such as glucose and amino acids, back into the blood.
• Water follows these solutes back into the blood, also through osmosis.

This process ensures that important substances are not wasted but instead reclaimed and reused by the body.
Without tubular reabsorption, the body would lose a significant amount of useful materials and water, which could lead to deficiency and dehydration.
The importance of tubular reabsorption cannot be underestimated; it's the step that matches the exercise's requirement of returning water and small solutes to the blood.

Tubular Secretion

Tubular secretion is a selective process that occurs in the renal tubules and involves removing unnecessary substances from the blood and adding them to the tubular fluid:

• This helps to regulate levels of various ions, such as potassium and hydrogen.
• It also assists in the removal of waste products and some drugs from the body.

As opposite to reabsorption, this process adds substances into the filtrate instead of withdrawing them.
Through tubular secretion, kidneys help maintain the pH balance of the blood and regulate electrolyte levels.
While tubular secretion is crucial for waste management, it does not satisfy the requirement of returning water and solutes back to the bloodstream, which is why it was not the correct answer in the exercise.