Problem 1 - Campbell Biology

Question

Unlike an earthworm's metanephridia, a mammalian nephron $$\begin{array}{l}{\text { (A) is intimately associated with a capillary }} \\\ {\text { network. }} \\ {\text { (B) functions in both osmoregulation and }} \\ {\text { excretion. }} \\ {\text { (C) receives filtrate from blood instead of coelomic fluid. }} \\ {\text { (D) has a transport epithelium. }}\end{array} $$

Step-by-Step Solution

Answer

Option C is correct because the mammalian nephron receives filtrate from blood instead of coelomic fluid.

Step 1: Understand the Question

The question asks to identify a characteristic of the mammalian nephron that distinguishes it from the metanephridia of earthworms.

Step 2: Analyze Option A

Both the mammalian nephron and the earthworm's metanephridia are associated with a capillary network for filtration and reabsorption, so this option doesn't distinguish between them.

Step 3: Analyze Option B

Both function in osmoregulation and excretion, making this option also not unique to mammalian nephrons.

Step 4: Analyze Option C

This option is key because metanephridia receive coelomic fluid, whereas mammalian nephrons receive filtrate from blood. This is a distinguishing characteristic.

Step 5: Analyze Option D

Both have transport epithelium, so this option does not distinguish the nephron from metanephridia.

Step 6: Conclude the Correct Answer

After analyzing all options, it is clear that Option C is correct because it identifies the exclusive characteristic of the mammalian nephron.

Key Concepts

Excretory System

The excretory system plays a critical role in maintaining the body's internal environment by removing waste products and regulating essential processes. In mammals, this system includes organs like the kidneys, which house nephrons, the functional units responsible for filtering blood.
Nephrons perform tasks such as:

• Removing waste products like urea
• Balancing electrolytes and fluids
• Regulating blood pressure

Earthworms, on the other hand, use metanephridia for their excretory needs. These structures collect coelomic fluid—body cavity fluid—filter it, and expel waste from the body.
Both systems perform similar roles but achieve them in distinct ways, reflecting their adaptation to different living environments.

Osmoregulation

Osmoregulation ensures the balance of water and salts within an organism's body, vital for its health. Mammalian nephrons play a major role in this by regulating the composition of blood. They achieve this through processes like:

• Filtration in the glomerulus
• Selective reabsorption and secretion in the tubules
• Urine concentration in the collecting ducts

Metanephridia in earthworms also contribute to osmoregulation. They filter coelomic fluid and return essential ions and water back to the body while excreting wastes.
Despite the differences in their structural components, both nephrons and metanephridia maintain the essential balance of fluids and electrolytes, crucial for survival.

Filtration Process

Filtration is the first step in the excretory processes of both mammals and earthworms. However, they differ in the fluids they filter:
In mammals, nephrons filter blood. This starts in the glomerulus where blood pressure forces water and small solutes through a filtration membrane, creating filtrate. This filtrate then undergoes further processing in the nephron's tubules before becoming urine.
In earthworms, metanephridia filter coelomic fluid. The fluid is drawn into the metanephridium where waste products are separated from useful nutrients and ions, which are reabsorbed back into the coelomic cavity.
This step is crucial as it begins the process of waste removal and helps regulate internal conditions.

Coelomic Fluid vs. Blood Filtrate

Understanding the difference between coelomic fluid and blood filtrate is important when comparing mammalian nephrons with earthworm metanephridia.
Mammalian nephrons pull their filtrate from blood. The filtration process occurs at the glomerulus, using pressure to separate wastes and small solutes from blood cells and large proteins. This filtrate then enters the nephron’s tubules for further refinement.
On the other hand, metanephridia in earthworms draw their filtrate from coelomic fluid. This fluid originates from the worm's body cavity and contains waste products dissolved in water. As the fluid passes through the metanephridium, valuable substances are reabsorbed, and waste is expelled.
This distinction highlights how different organisms have evolved specific adaptations to meet their physiological needs.