CIRCULATION AND RESPIRATION IN FISHES

CIRCULATION AND RESPIRATION IN FISHES (CARDIOVASCULAR & RESPIRATORY PHYSIOLOGY)

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B.Sc. Fish & Fisheries 1st Year / Semester-I DSPMU Ranchi Syllabus

CIRCULATION & RESPIRATION IN FISHES (CARDIORESPIRATORY PHYSIOLOGY)

Branchial Gills, Counter-Current Gas Exchange, Venous Heart Anatomy & Accessory Respiratory Organs (AROs)

1. Introduction: The Aquatic Respiratory & Circulatory Challenge

Water is an exceptionally demanding physical medium for respiration. It is 800 times denser and 50 times more viscous than air, yet contains less than 1% oxygen by volume (typically 5 to 9 mg/L, compared to 210 ml/L in atmospheric air).

Core Physiological Axioms:
  • Single-Circuit Venous Heart: The fish heart is a strictly venous muscular pump receiving exclusively deoxygenated blood and driving it through a single circuit: Heart → Gills → Body Tissues → Heart.
  • Counter-Current Exchange: Water flowing across the secondary gill lamellae moves in the exact opposite direction to blood flow in the capillaries, achieving an extraordinary 80% to 85% oxygen extraction efficiency.

2. Branchial Respiration & Counter-Current Gas Exchange

Teleosts possess 4 pairs of functional gill arches (Holobranchs) covered by a bony operculum. Each gill arch bears primary filaments, which are packed with microscopic, leaf-like Secondary Lamellae supported by contractile Pillar Cells.

Figure 1: Counter-Current vs. Concurrent Exchange Mechanics

CONCURRENT FLOW (Inefficient - Hypothetical) Water: 100% → 70% → 50% Blood: 0% → 30% → 50% Equilibrium reached at 50%! Diffusion halts; max yield only 50% COUNTER-CURRENT FLOW (Actual Fish Gills) Water: 100% → 70% → 40% → 15% Blood: 85% ← 55% ← 30% ← 5% Continuous Diffusion Gradient! High Efficiency: Extracts 80% to 85% O2
The Dual-Pumping Ventilation Mechanism:
Teleosts irrigate their gills using a coordinated two-phase pressure-and-suction pump:
  • Phase 1 (Inhalation / Expansion): Buccal and opercular cavities expand, creating negative pressure (partial vacuum). Opercular valve closes; mouth opens; water rushes into the buccal chamber.
  • Phase 2 (Exhalation / Compression): Mouth closes; buccal floor elevates, raising internal pressure above ambient levels. Opercular valve opens; water is forced smoothly over the secondary lamellae and discharged.
  • Ram Ventilation: Fast pelagic fishes (Tunas, Mackerels, Pelagic Sharks) swim with mouths open, driving water over the gills purely via forward body velocity.

3. The Fish Heart: Anatomy & Single Circulation

The fish heart is located in the ventral pericardial cavity and consists of four chambers arranged in a linear series:

Figure 2: The Four-Chambered Teleost Heart (Sagittal Flow Plan)

SINUS VENOSUS ATRIUM (Muscular Chamber) VENTRICLE (Thick Pump) BULBUS ARTERIOSUS VENTRAL AORTA (Forward to Gills)
Feature Bulbus Arteriosus (Teleostei, e.g., Labeo rohita) Conus Arteriosus (Elasmobranchii, e.g., Scoliodon)
Origin Dilated anterior expansion of the root of the ventral aorta. True 4th embryonic cardiac chamber derived from the heart tube.
Muscular Nature Composed of elastic fibers and smooth muscle; no cardiac muscle. Composed of true, striated contractile cardiac muscle.
Internal Valves No internal valves (devoid of semilunar valves). Possesses 2 to 7 transverse rows of semilunar valves.
Function Acts as a Windkessel organ: dampens high ventricular systolic pressure pulses into a continuous smooth capillary flow. Contracts actively and rhythmically in sequence with the ventricle to pump blood.

4. Accessory Respiratory Organs (AROs) in Air-Breathing Fishes

In tropical freshwater wetlands and swamps (such as the derelict water bodies of Jharkhand), water temperatures rise in summer and organic matter decomposes, depleting dissolved oxygen to near zero. Indigenous air-breathing fishes survive by utilizing specialized vascular chambers:

Figure 3: Primary Accessory Respiratory Organs of Indian Fishes

LABYRINTH ORGAN Anabas testudineus (कवई) Folded 1st Epibranchial Bone ARBORESCENT ORGAN Clarias magur (मांगुर) Dendritic 2nd & 4th Epibranchials TUBULAR AIR SAC Heteropneustes fossilis (सिंघी) Pneumatic Tubes in Tail Muscle

5. Representative Indian Commercial Air-Breathing Fishes

Anabas testudineus - Day
Figure 4: Anabas testudineus (Bloch) — The Climbing Perch (कवई), equipped with folded labyrinthine suprabranchial plates.
Clarias magur - Day
Figure 5: Clarias magur (Hamilton) — The Indian Walking Catfish (मांगुर), featuring dendritic arborescent organs.
Heteropneustes fossilis - Day
Figure 6: Heteropneustes fossilis (Bloch) — The Stinging Catfish (सिंघी), with paired pneumatic air sacs extending into the tail musculature.
Labeo rohita - Day
Figure 7: Labeo rohita (Hamilton) — Rohu, an obligate water-breather requiring dissolved oxygen levels > 5.0 mg/L.

6. Hematology: The Bohr Effect vs. The Root Effect

Key Physiological Distinctions:
  • Bohr Effect (बोहर प्रभाव): A decrease in blood pH (or elevation in CO2) reduces hemoglobin's affinity for oxygen, shifting the dissociation curve to the right. This facilitates oxygen unloading to actively respiring, acid-producing tissues.
  • Root Effect (रूट प्रभाव - केवल मछलियों में): A specialized teleost adaptation wherein an extreme decrease in pH dramatically reduces the maximum oxygen-carrying capacity of hemoglobin, even under high partial pressures of oxygen. This allows teleosts to pump oxygen into the swim bladder against extreme hydrostatic pressures via the Rete Mirabile.

7. University Examination Practice Questions (With Explanations)

Q1. The maximum oxygen extraction efficiency achieved by fish gills under counter-current exchange is approximately:
A) 30% to 40%      B) 50%      C) 80% to 85%      D) 100%
✔ Correct Answer: C (80% to 85%)

Explanation: Counter-current flow maintains a continuous partial pressure gradient across the entire secondary lamella, yielding up to 85% extraction.
Q2. In teleost fishes (*Labeo*, *Catla*), the outflow chamber of the heart is the:
A) Conus arteriosus      B) Bulbus arteriosus      C) Sinus venosus      D) Ductus Cuvier
✔ Correct Answer: B (Bulbus arteriosus)

Explanation: Teleosts possess an elastic, non-contractile bulbus arteriosus; elasmobranchs (sharks) possess a muscular conus arteriosus.
Q3. Which of the following fishes breathes atmospheric air using paired tubular air sacs extending into the tail musculature?
A) Anabas testudineus      B) Clarias magur
C) Heteropneustes fossilis      D) Labeo rohita
✔ Correct Answer: C (Heteropneustes fossilis)

Explanation: Singhi (*Heteropneustes*) possesses long hollow pneumatic tubes running through the lateral body musculature.
Q4. The specialized teleost phenomenon where low pH causes an irreversible drop in maximum oxygen-carrying capacity is termed:
A) Bohr Effect      B) Root Effect      C) Windkessel Effect      D) Haldane Effect
✔ Correct Answer: B (Root Effect)

Explanation: The Root Effect depresses maximum saturation capacity at low pH, enabling gas secretion into the swim bladder.

⚡ Quick Revision Checklist for University Exams:

  • Venous Heart Flow: Sinus venosus → Atrium → Ventricle → Bulbus arteriosus → Ventral Aorta.
  • Bulbus vs. Conus: Bulbus (Teleosts; elastic, no cardiac muscle, no valves); Conus (Sharks; cardiac muscle, semilunar valves).
  • Secondary Lamellae: Functional gas exchange units supported internally by contractile Pillar Cells.
  • AROs in Indian Taxa: Anabas (Labyrinth organ), Clarias (Arborescent organ), Heteropneustes (Tubular air sac).
  • DO Requirements: IMC require dissolved oxygen > 5.0 mg/L; Air-breathers thrive at < 1.0 mg/L.

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