Gas Exchange and Circulation

To Produce ATP
- Animals must obtain O2 and eliminate CO2.
- Gas exchange organs maximize the rate of diffusion.

Fick's Law of Diffusion: Oxygen (O2) and carbon dioxide (CO2) diffuse more effectively when:
- Surface area for gas exchange is large.
- Respiratory surface is extremely thin.
- There is a large concentration gradient of the gas across the surface.

Diffusion Constant: Depends on gas solubility and temperature.
Diffusion Rate Equation:
- Rate of diffusion = k * A * (P2 - P1) / D
  - Where:
    - k = diffusion constant
    - A = area for gas exchange
    - P2 - P1 = difference in partial pressures
    - D = distance (thickness of the barrier to diffusion)

Some animals have specialized organs for gas exchange (i.e., gills), while others rely on direct diffusion across their body surfaces.

External Gills:
- Direct contact with water, increasing exposure to O2.
Internal Gills:
- Require water to be brought to them for gas exchange.
Countercurrent Flow Mechanism:
- Water flows over gill membranes in the opposite direction to the blood flow, maximizing O2 diffusion.
- Example Chart: Water oxygen saturation decreases from 100% to 15% while blood oxygen saturation increases correspondingly.

Airways Structure:
- Trachea, bronchi, bronchioles, and alveoli.
- Alveoli allow for gas exchange with deoxygenated blood from the lungs.
Inhalation and Exhalation Mechanics:
- Inhalation: Diaphragm contracts, increasing volume and decreasing pressure inside the lungs, drawing air in.
- Exhalation: Diaphragm relaxes, decreasing volume and increasing pressure, forcing air out.
One-way airflow in Avian System:
- Air sacs enable a continuous flow of air through the lungs, enhancing gas exchange efficiency.

The Bohr Effect:
- O2 unloading from hemoglobin is driven by the difference in O2 partial pressure between blood and tissues.
- Hemoglobin’s cooperative binding enhances oxygen delivery.
- A change in pH and temperature also influences hemoglobin’s ability to release O2.

O2 and CO2 transport through the blood:
- CO2 is mainly transported as bicarbonate, bound to amino groups or dissolved in plasma.
Role of Erythropoietin (EPO):
- Stimulates red blood cell production in response to low oxygen levels.

Circulatory systems carry transport fluids (blood) to facilitate movement and exchange between cells.

Open Circulatory System:
- Hemolymph is pumped through the body but not confined to blood vessels, resulting in lower overall pressure.
Closed Circulatory System:
- Blood is contained in vessels and pumped through the body under higher pressure.
Variants include 2-chambered (fish), 3-chambered (frogs), and 4-chambered hearts (birds and mammals).

Arteries and Veins:
- Arteries have thicker walls due to higher pressures.
- Veins contain valves to prevent backflow and rely on muscle action to assist blood return to the heart.
Capillaries:
- Site for gas and nutrient exchange; they have very thin walls for efficient diffusion.

Pulmonary Circulation:
1. Blood returns from the body to the right atrium.
2. Enters the right ventricle.
3. Pumped to the lungs for oxygenation.
Systemic Circulation:
1. Oxygenated blood returns to the left atrium from the lungs.
2. Enters the left ventricle.
3. Pumped to the body.

Systolic Pressure: Measured during ventricular contraction (ejecting blood).
Diastolic Pressure: Measured when the heart relaxes.
Blood Pressure Regulation:
- Baroreceptors in arteries detect pressure changes, initiating compensatory mechanisms (heart rate and vessel diameter adjustments).
Cardiac Output: Influenced by heart rate and stroke volume adjustment.

Responses to falling blood pressure include an increase in heart rate and stroke volume to restore normal levels.
Active regulation keeps CO2 and O2 levels within optimal ranges, influenced by chemoreceptor feedback.
Overall, the interplay between gas exchange and circulation is critical for maintaining cellular respiration and energy production in animals.