Gas Exchange Notes

CHPTR 15.42 Gas Exchange

Gas Exchange Systems in Animals

Gas exchange systems in animals include:

• Body surface
• Epidermis
• Papula
• Trachea
• Gill
• Alveoli
• Spiracle
• Cutaneous respiration
• Tracheoles

Respiratory Surfaces

All respiratory surfaces must be surrounded by water to remain stable.

Fick's Law of Diffusion

Fick's Law of Diffusion is represented by the formula:

$R \propto A \frac{\Delta p}{d}$

Where:

• $R$ = rate of diffusion between two regions
• $A$ = area over which diffusion will occur
• $\Delta p$ = pressure difference between the two regions
• $d$ = distance across which diffusion must occur (distance between the two regions)

Need for Respiratory Organs

Organisms need respiratory organs or strategies for respiration because:

• Surface area to volume ratio is high in small cells and low in large cells.
• Large organisms do not have enough surface area to take in the oxygen they need, so require special organs or strategies.

Example:

• A cell with dimensions 4 x 4 x 4 has less surface area to volume ratio compared to a cell with dimensions 1 x 1 x 1.

Limits of Cell Size

• A small surface-area-to-volume ratio results in a decreased rate of chemical exchange, and the cell may die.
• To build larger organisms, they must be built up from small cell subunits.

Example:

• Surface area of one large cube = $5400 \mu m^2$

• Total surface area of 27 small cubes = $16,200 \mu m^2$

• For a sphere with radius r = 1 μm:

  • Surface area $(4\pi r^2) = 12.6 \mu m^2$
  • Volume $(4/3 \pi r^3) = 4.2 \mu m^3$
  • Surface/Volume = 3

• For a sphere with radius r = 2 μm:

  • Surface area = $50.3 \mu m^2$
  • Volume = $33.5 \mu m^3$
  • Surface/Volume = 1.5

Evolutionary Strategies to Maximize Gas Diffusion

• Beating cilia over the respiratory surface
• Increasing the surface area of the respiratory surface
• Decreasing the distance between the external environment and blood

Examples:

• Nereis
• Clam

Nereis Evolutionary Strategies

Nereis employs parapodia and cilia as evolutionary strategies.

Tracheal System

A tracheal system includes:

• Tracheae
• Air sacs
• Body cell
• Tracheoles
• Spiracle

Respiratory System - Mollusk

The respiratory system of a mollusk includes:

1. Incurrent siphon
2. Gills
3. Excurrent siphon

Gills in Bony Fish

Gills in bony fish are specialized extensions of tissue that protrude into the surrounding water. They are involved in gas exchange. Typically, the mouth opens while the operculum closes and vice versa.

Operculum in Bony Fish

Moving operculum in bony fish helps in maintaining a water current over gills.

Ram Ventilation

Immobile opercula or absent opercula lead to ram ventilation.

Structure of Gills

The structure of gills includes:

• Gill arch
• Gill filaments
• Lamellae
• Water flow
• Blood flow

Countercurrent Exchange

Countercurrent exchange mechanism:

• Water (100% $O<em>2$ saturation) and Blood (0% $O</em>2$ saturation) initially.
• As water flows, $O<em>2$ saturation decreases, but remains higher than the blood $O</em>2$ saturation.
• Blood $O_2$ saturation increases along the gill lamellae.
• This maintains a concentration gradient, facilitating efficient oxygen uptake.

Concurrent exchange mechanism:

• Water (100% $O<em>2$ saturation) and Blood (0% $O</em>2$ saturation) initially.
• The exchange reaches equilibrium quickly, limiting further net diffusion.

Fish Gills

Fish gills consist of gill filaments and lamellae. The countercurrent exchange mechanism facilitates efficient oxygen transfer from water to blood. The partial pressure of oxygen (Po₂) in water decreases as it flows between lamellae, while the Po₂ in blood increases as it flows through capillaries in the lamellae, maintaining a favorable gradient for oxygen diffusion.

Lungs vs. Gills

Disadvantages of gills in a terrestrial environment:

• Gills do not have any inherent support.
• Desiccation can occur over the increased surface area of gills.

Amphibian Respiration

Amphibian respiration involves positive pressure breathing.
Air is pushed into the lungs using pressure.

Reptile and Mammal Respiration

Respiration in reptiles and mammals involves negative pressure breathing.

Avian vs. Mammalian Respiration

Avian Parabronchi

Like human alveoli, avian parabronchi are covered by a rich supply of capillaries and are the sites for gas exchange. Parabronchi are located throughout the lungs between secondary bronchi.

Bird Respiration

Bird respiration involves a cycle of inhalation and exhalation through anterior and posterior air sacs and lungs (parabronchi).

Mammalian Respiration

Mammalian respiration involves the upper and lower respiratory tracts.

• Upper Respiratory Tract: nose, pharynx, larynx
• Lower Respiratory Tract: trachea, bronchi, lungs

Lower Respiratory Tract

The lower respiratory tract includes the trachea, bronchi, bronchioles, and alveoli. Alveoli are surrounded by a capillary network for gas exchange.

Mechanism of Breathing

Mammalian respiration mechanism: Negative pressure breathing.

• Inhalation: Muscles contract, diaphragm contracts, expanding the thoracic cavity.
• Exhalation: Muscles relax, diaphragm relaxes, reducing the thoracic cavity.

Nervous Control of Breathing

The respiratory center in the medulla oblongata automatically regulates breathing. Intercostal nerves stimulate intercostal muscles, and the phrenic nerve stimulates the diaphragm.

Chemical Control of Breathing

• Peripheral chemoreceptors (carotid and aortic bodies)
• Central chemoreceptors in the medulla oblongata

Gas Exchange in the Body

• External gas exchange occurs in the lungs. $Po<em>2$ = 105 mm Hg, $Pco</em>2$ = 40 mm Hg
• Internal gas exchange occurs in the peripheral tissues. $Po<em>2$ = 40 mm Hg, $Pco</em>2$ = 46 mm Hg

Partial Pressure

• Partial pressure of $O_2$ is higher in alveoli compared to pulmonary blood.
• Partial pressure of $CO_2$ is lower in alveoli compared to pulmonary blood.

Transport of Carbon Dioxide

Transport of carbon dioxide by the blood involves:

1. $CO_2$ combines with Hb (20%)
2. $CO_2$ dissolved in plasma (8%)
3. Carbaminohemoglobin