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Difficulty: Very hardCirculatory and Transport Systems in Animals

A physiological comparison between the circulatory systems of teleost fishes and mammals reveals a major constraint on the rate of oxygen delivery to metabolically active systemic tissues in fishes. Which of the following statements correctly explains the anatomical and hydrostatic basis for this difference?

  1. In teleost fishes, blood passes through two capillary beds in series (gill capillaries followed by systemic capillaries) during a single circuit, resulting in a substantial drop in hydrostatic pressure before reaching systemic tissues, whereas mammals re-pressurize oxygenated blood using a four-chambered double circulation.Answer
  2. B
    In teleost fishes, the two-chambered heart pumps deoxygenated blood directly into the systemic aorta prior to reaching the gills, causing venous and arterial blood to mix completely before tissue oxygenation.
  3. C
    Mammals achieve higher systemic pressure because their three-chambered heart permits partial mixing of pulmonary and systemic blood inside a single ventricle, boosting total stroke volume.
  4. D
    Teleost fishes utilize an open circulatory system where blood leaves vessels into a hemocoel to bathed organs directly, whereas mammals possess a closed double circulatory system.

Answer

In teleost fishes, blood passes through two capillary beds in series (gill capillaries followed by systemic capillaries) during a single circuit, resulting in a substantial drop in hydrostatic pressure before reaching systemic tissues, whereas mammals re-pressurize oxygenated blood using a four-chambered double circulation.
The statement explaining serial capillary beds accurately describes the fundamental physiological constraint of single circulation in fishes. Blood pumped by the single ventricle must pass through the high-resistance gill capillaries where oxygen is absorbed. This causes a steep drop in hydrostatic pressure. Consequently, oxygenated blood flows relatively slowly and under low pressure to systemic organs. Mammals avoid this constraint because their complete cardiac septum creates a separate pulmonary circuit and systemic circuit powered by two independent ventricular pumps.

Step-by-Step Solution

1
Analyze the circulatory pattern of teleost fishes (single circulation).
Fish have a 2-chambered heart (one atrium, one ventricle) that pumps blood through a single circuit: Heart →\rightarrow Gill Capillaries →\rightarrow Systemic Capillaries →\rightarrow Heart.
Tracing the physical pathway of blood flow helps identify pressure drop locations across vascular beds.
2
Evaluate the hydrodynamic consequence of passing through gill capillaries.
As blood flows through the narrow capillary network of the gills to pick up oxygen, high vascular resistance causes a major reduction in blood pressure.
Fluid dynamics dictates that passing through a high-resistance capillary bed lowers pressure significantly.
3
Compare systemic pressure dynamics between fish single circulation and mammalian double circulation.
Because blood in fishes goes directly from gill capillaries to systemic organs without returning to the heart, systemic blood flow is slow and under low pressure. Mammals have a 4-chambered heart providing double circulation, returning oxygenated blood from lungs to the left side of the heart to be repressurized before systemic distribution.
Re-pressurization via a separate ventricular pump (double circulation) is essential for maintaining high systemic blood pressure and rapid metabolic delivery.

Key Concept

Single versus double circulatory pathways and comparative vertebrate heart anatomy
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Topics

Circulatory and Transport Systems in Animals

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