In fish, which body part is the primary mode of respiration?

Appeared in: DSSSB 13 August 2024

Explanation

Fish are aquatic animals and use gills as their primary respiratory organs.
Gills are composed of feathery structures called filaments, which are rich in blood vessels, providing a large surface area for gas exchange.
They are highly efficient at extracting the small amount of dissolved oxygen from water through a process called countercurrent exchange, where water flows over the gills in the opposite direction to the blood flow.

Option A: Tracheae are a system of small tubes that carry air directly to the cells for gas exchange. This is the characteristic respiratory system of insects, not fish.
Option C: Lungs are organs found in most air-breathing vertebrates, including mammals, birds, and reptiles. They are adapted for taking in oxygen from the air, not water.
Option D: Cutaneous respiration (breathing through the skin) is used by some animals like amphibians (e.g., frogs) and earthworms, but it is not the primary method of respiration for fish.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Modes of Respiration in the Animal Kingdom as background academic context rather than a clinical decision trigger.
Understanding comparative anatomy and physiology provides a crucial foundation for grasping human physiology. Seeing how different species solve the problem of respiration highlights the specific adaptations of the human lungs.
This knowledge is foundational in the nursing curriculum, helping students understand why environmental factors (like water for fish, air for humans) dictate biological structures and functions.
What if the question asked about a whale? Although it lives in water, a whale is a mammal and uses lungs to breathe air, so it must surface regularly. This highlights the difference between being an aquatic animal and being a fish.

How to Approach the Question

First, identify the organism specified in the question, which is 'fish'.
Next, recall the natural habitat of this organism. Fish are aquatic and live in water.
Consider the challenge of breathing in this environment. Oxygen is not freely available as a gas but is dissolved in the water.
Evaluate the given options to see which organ is specifically adapted for extracting dissolved oxygen from water.
Gills are the specialized organs for aquatic respiration. Lungs and tracheae are adapted for breathing air. Skin is a primary respiratory surface for other animal groups, not fish.

Concept Tested & Keywords

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E6 Nursing Vital Signs p. 62-64

E6 Physiology Guyton 4SAE Part 2A p. 38-39

Practise the full DSSSB 13 August 2024

Attempt every question from this paper in a timed mock, then review the full solution for each one.