gas exchange

Importance of Gas Exchange and Circulation

• Cells must continuously:

  • Obtain oxygen.

  • Expel carbon dioxide.

  • Support ATP production by mitochondria.

• Oxygen and carbon dioxide must:

  • Be continuously exchanged with the environment.

  • Be transported throughout the body along with wastes, nutrients, and other molecules.

Lecture Goals on Gas Exchange & Circulation

• Understand that:

  • Animals need to take in oxygen and expel carbon dioxide for cellular respiration.

  • Different species face unique challenges with gas exchange.

• Key principles:

  • Gas-exchange organs increase diffusion rates by:

  1. Presenting a large, thin surface area.

  2. Maintaining a steep partial-pressure gradient for O2 entry and CO2 elimination.

  • Blood's role:

  • Transports gases, nutrients, and wastes.

  • Hemoglobin efficiently carries oxygen from lungs to tissues.

  • Circulatory systems:

  • Utilize pressure from one or more hearts to transport substances.

Steps of Gas Exchange

1. Ventilation: Movement of air or water through gas-exchange organs (e.g., lungs, gills).

2. Gas Exchange: Diffusion of CO2 and O2 between air/water and blood at the ventilatory surface.

3. Circulation: Transportation of dissolved O2 and CO2 throughout the body.

4. Tissue Gas Exchange: Diffusion of O2 and CO2 between blood and cells, driven by cellular respiration.

Dependence of Gas Exchange on Diffusion

• Oxygen concentration: High in the environment and low in tissues.

• Carbon dioxide concentration: High in tissues and low in the environment.

• Diffusion leads to:

  • Movement of oxygen and carbon dioxide along their partial-pressure gradients in air and water.

Understanding Gas Movement by Diffusion

• Think in terms of partial pressures:

  • Partial pressure = Pressure of a specific gas in a mixture.

  • Calculation: Fractional composition of gas × Total pressure of mixture.

Factors Affecting Gas Solubility in Water

1. Solubility of the gas in water.

2. Temperature of the water.

3. Presence of other solutes.

4. Partial pressure of the gas in contact with water.

Fick’s Law of Diffusion

• Fick’s law states that gases diffuse most effectively when three conditions are met:

  1. Large surface area for gas exchange.

  2. Thin respiratory surface.

  3. Steep partial pressure gradient across the surface.

Expression of Fick’s Law

• Rate of diffusion expressed as: $D = k imes A imes \frac{(P<em>2 - P</em>1)}{D}$

  • Where:

  • D = Rate of diffusion

  • k = Diffusion constant (depends on solubility and temperature)

  • A = Area for gas exchange

  • P2 - P1 = Difference in partial pressure

  • D = Distance (thickness of barrier to diffusion)

O2 Availability in Aquatic Habitats

• Water's density and lower oxygen content increase the energy required for aquatic respiration compared to air breathing.

• Aquatic animals must process approximately 30 times more water for equivalent oxygen amounts due to lower availability in water.

• Habitats with dense photosynthesis are usually oxygen-rich, while organic material reliant habitats are oxygen-poor.

• Surface area significantly impacts oxygen diffusion into water.

Structure of Fish Gills

• Gills as solutions to the challenges of aquatic breathing:

  • They provide large surface area across a thin epithelium.

  • Composed of gill filaments with numerous gill lamellae containing capillary beds.

Fick’s Law in Fish Gills

• Water flow through gills goes against blood flow (countercurrent exchange).

• This maintains a large partial pressure difference for effective diffusion of gases.

Overview of the Human Lung

• Composed of:

  • Airways: Trachea, bronchi, bronchioles leading to alveoli.

• Approximately 150 million alveoli per lung, facilitating gas exchange.

• Average breath: 450 mL; only two-thirds involved in gas exchange due to dead space.

Ventilation of the Human Lung

• Ventilation is driven by muscular contractions.

• Inhalation occurs via negative pressure ventilation:

  • Achieved by lowering chest cavity pressure (diaphragm motion).

• Exhalation is passive, resulting from elastic recoil of lungs and chest wall.

Homeostatic Control of Ventilation

• At rest, breathing rate is regulated by the medullary respiratory center in the brain.

• During exercise, demand for oxygen increases, and CO2 levels rise, leading to:

  1. Rapid response to changes in blood gas levels (using H2CO3 as a buffer).

  2. Increase in breathing rate to restore balances.

• Chemical reaction:
  $CO2 + H2O ⇌ H2CO3 ⇌ H^+ + HCO3^-$

Hemoglobin Structure and Function

• Composed of four polypeptide chains, each binding to a heme group with an Fe2+ that binds oxygen.

• Each hemoglobin can transport up to four oxygen molecules.

Cooperative Binding of Hemoglobin

• Each oxygen binding induces a conformational change, increasing affinity for subsequent bindings (Cooperative binding).

• This ensures sensitivity to tissue PO2 changes, facilitating oxygen release.

Oxygen-Hemoglobin Equilibrium Curve

• Plots percentage saturation of hemoglobin against PO2 levels in blood, demonstrating higher saturation in lungs compared to tissues, promoting oxygen unloading.

Response of Hemoglobin to pH and Temperature Changes

• Hemoglobin's conformation is influenced by:

  • Decreases in pH and increases in temperature facilitate oxygen release during high CO2 conditions (Bohr shift).

Fetal Hemoglobin

• Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin, ensuring efficient oxygen transfer from mother to fetus.

CO2 Transport and Blood pH Regulation

• Greatest amount of CO2 in blood is transported as bicarbonate ion (HCO3^-).

• Carbonic anhydrase in red blood cells facilitates:

  1. Producing protons influencing hemoglobin's oxygen release.

  2. Maintaining strong CO2 gradients through bicarbonate conversion.

Function of CO2 in the Lungs

• H+ ions from carbonic acid dissociation are buffered by hemoglobin, reducing pH changes.

• CO2 diffuses from the blood to the alveoli through a partial-pressure gradient:

  • Hemoglobin contributes H+ to form carbonic acid, dissociating into CO2 for exhalation.

Circulatory Systems in Large Animals

• Large animals utilize circulatory systems for effective diffusion, transporting blood to all cells.

• Types of circulatory systems:

  • Open circulatory systems: Hemolymph in direct tissue contact (less pressure); suitable for sedentary organisms.

  • Closed circulatory systems: Blood in vessels, allowing higher pressure and precise flow control (e.g., vertebrates).

Blood Vessel Types

• Arteries: Thick-walled vessels that transport blood away from the heart, withstand high pressure.

• Veins: Return blood to the heart; thinner walls and larger diameters than arteries; contain valves.

• Capillaries: Smallest vessels; one-cell thick walls enable nutrient and gas exchange in capillary beds.

Structure of Blood Vessels

• Capillaries enable diffusion between blood and tissues due to their thin walls.

• Arteries differ structurally by being thicker and having elastic walls to withstand pressure surges.

Blood Pressure Dynamics

• Blood pressure is the force exerted on vessel walls and decreases as it moves through capillaries due to increased total cross-sectional area.

• The drop allows sufficient time for gas and nutrient exchange between blood and tissues.

Homeostatic Control of Blood Pressure

• Control mechanisms include:

  • Baroreceptors detecting pressure changes.

  • Responses include increased heart rate, constriction of noncritical arterioles, and blood volume shifts to maintain pressure.

Electrical Activity and Cardiac Cycle

• Electrical activation starts at the SA node, causing atrial contraction, delayed at the AV node, with a subsequent signal leading to ventricular contraction.

Important Points to Ponder

• How surface area for diffusion is addressed in large animals.

• Effects of exercise on blood pressure and gas exchange dynamics.

Summary of Blood Pressure and Flow

• The regulation of blood flow and pressure is crucial for maintaining homeostasis during physiological changes.