Gas Exchange - Animals

BIO 151: General Biology II

Lecture Overview

• Topic: Gas exchange in animals

• Instructor: Dr. Rountos

• Text Reference: Ed.11: Pg 937-947

Definition of Gas Exchange

• Gas exchange occurs across specialized respiratory surfaces.

• Function: Supplies $O_2$ for cellular respiration and disposes of $CO_2$.

Mechanisms of Gas Exchange

Basic Concepts

• How gases are exchanged in animals:
    - Partial Pressure: The pressure exerted by a particular gas in a mixture. Applicable to gases in liquids like water.
    - Net Diffusion: Gases diffuse from higher partial pressure to lower partial pressure.

Dalton's Law of Partial Pressures

• Statement: The total pressure in a gas mixture equals the sum of the partial pressures of each individual gas.
    - $P_{total} = P_{gas ext{ }a} + P_{gas ext{ }b}$
    - Example given:
      - Total pressure of 4 atm consists of gas a (1 atm) + gas b (3 atm).

Gas Exchange in Different Media

Respiratory Media

• Animals can utilize either air or water as the $O_2$ source.

• Comparison: Water contains less $O_2$ than air per given volume, requiring more efficient mechanisms for aquatic organisms.

Table 42.1: Comparing Air and Water as Respiratory Media

• Air (Sea Level):
    - $O_2$ Partial Pressure: 160 mm
    - $O_2$ Concentration: 210 ml/L
    - Density: 0.0013 kg/L
    - Viscosity: 0.02 cP

• Water (20°C):
    - $O_2$ Partial Pressure: 160 mm
    - $O_2$ Concentration: 7 ml/L
    - Density: 1 kg/L
    - Viscosity: 1 cP

• Ratios:
    - Air to Water Ratio: $30:1$ for $O_2$ Concentration
    - Density Ratio: $1:770$
    - Viscosity Ratio: $1:50$

Respiratory Surfaces

• Requirement for large, moist respiratory surfaces for gas exchange with cells and respiratory medium.

• Variations include skin, gills, trachea, and lungs.

Gills in Aquatic Animals

• Function of Gills: Outfoldings that facilitate gas exchange with a large surface area.

• Ventilation: Movement of water over gills either by swimming or active water pumping.

Example Structures in Aquatic Animals

• Marine Worm: Parapodium functionality as gill.

• Crayfish: Possesses gills.

• Sea Star: Uses tube feet for gas exchange.

Countercurrent Exchange in Fish Gills

• Blood flows in the opposite direction to water flow over gills, maximizing $O_2$ absorption.

• Over 80% of $O_2$ in water is extracted.

Diagram of Gill Structure

• Overview of components: Gill arch, filaments, blood vessels, directions of water and blood flow (unidirectional).

Tracheal Systems in Insects

• Structure includes a network of branching tubes (tracheae).

• O2 supplied directly to cells, independent of the circulatory system.

• Larger insects require ventilation of the tracheal system for increased O2 demand.

Lungs and Lung Function

• Lungs as infoldings serving as gas exchange sites, with blood facilitating gas transport.

• Complexity: Correlates with an organism's metabolic rate.

Mammalian Respiratory System

• Air pathway: From nostrils through pharynx, larynx, trachea, bronchi, bronchioles, to alveoli.

• Importance of cilia and mucus in trapping particles and cleaning airways (the “mucus escalator”).

Gas Exchange in Alveoli

• Oxygen diffuses through moistened epithelium into capillaries, and $CO_2$ diffuses the opposite way.

Alveoli Characteristics

• Lack cilia, susceptible to contamination.

• Surfactants reduce surface tension; premature infants may lack these, leading to respiratory distress syndrome (RDS).

Breathing Mechanisms

Breathing and Ventilation

• Breathing: Alternate inhalation and exhalation of air; not to be confused with respiration.

Amphibian Breathing

• Example: Frogs use positive pressure breathing to ventilate their lungs by forcing air down the trachea.

Bird Breathing

• Unique features: Eight/nine air sacs aiding in unidirectional airflow, allowing efficient gas exchange—requires two ventilatory cycles.

Mammalian Breathing

• Mechanism: Negative pressure breathing pulls air into lungs, facilitated by diaphragm contraction and rib muscle movements.
    - Diagram showing inhalation and exhalation processes with diaphragm and rib cage actions.
    - Tidal Volume: Volume of air inhaled per breath (approximately 500 mL at rest).
    - Vital Capacity: Maximum tidal volume, ranges from 3.4 to 4.8 L.

Control of Breathing in Humans

Regulation Mechanisms

• Controlled involuntarily by the medulla oblongata in response to changes in blood pH due to $CO_2$ levels.

• Sensors in blood vessels monitor $O_2$ and $CO_2$ concentrations.

Feedback Mechanism

• As $CO_2$ levels rise, and pH drops, breathing rate and depth increase via medulla signaling.

Impact of Ocean Acidification

• Overview: Oceans have absorbed 50% of atmospheric $CO_2$, leading to increased acidity affecting marine life significantly (e.g., corals, plankton, larval fish).

Importance of Adaptations for Gas Exchange

• Respiratory Pigments: Essential proteins (e.g., hemoglobin) increase $O_2$ transport in blood.

• Hemocyanin and hemoglobin are prominent examples; hemoglobin contains iron in heme groups to carry $O_2$.

• The hemoglobin dissociation curve illustrates delivery variations with changing partial pressures.

Coordination of Circulation and Gas Exchange

• Explains the diffusion process of $O_2$ into blood and $CO_2$ out to the alveoli.

• Blood returning from the body is low in $O_2$ and high in $CO_2$, facilitating the necessary exchange.

Diving Mammals

• Adaptations include high blood-to-body volume ratios, myoglobin storage in muscles, reduced blood supply to non-essential muscles, and a slow rate of oxygen depletion.

Lecture Conclusion

• Review Questions: Detail differences in gas exchange structures and functions among fish, amphibians, birds, and mammals; discuss respiratory pigments and their significance.

• Reading Assignment: Prepare for next class using Ed.11: Pg 920-935.