Respiration and Gas Exchange: Mechanisms and Regulation

Respiration and Gas Exchange

Partial Pressure and Gas Movement

Gas exchange in animals relies on the concept of partial pressure, which determines the direction of movement for oxygen (O2) and carbon dioxide (CO2) between the lungs, blood, and tissues.

• Partial Pressure: The pressure exerted by a single gas in a mixture of gases. Gases diffuse from regions of higher partial pressure to regions of lower partial pressure.

• Oxygen Movement: O2 moves from the alveoli (high partial pressure) into the blood (lower partial pressure), and from blood into tissues where O2 is lowest.

• Carbon Dioxide Movement: CO2 moves from tissues (high partial pressure) into the blood, and from blood into the alveoli to be exhaled.

• Equation for Partial Pressure: $P_{gas} = (\text{Fraction of gas}) \times (\text{Total pressure})$

• Example: At sea level, atmospheric pressure is 760 mmHg. If O2 is 21% of air, its partial pressure is $0.21 \times 760 = 160$ mmHg.

Transport of Oxygen and Carbon Dioxide in Blood

O2 and CO2 are transported in the blood in several forms, ensuring efficient delivery and removal from tissues.

• Oxygen Transport:

  • Most O2 is carried bound to hemoglobin (Hb) in red blood cells.

  • A small amount is dissolved directly in plasma.

• Hemoglobin Saturation Percentage: The percentage of hemoglobin molecules carrying O2. This depends on the partial pressure of O2 (PO2).

• Carbon Dioxide Transport:

  • About 7% dissolved in plasma.

  • About 23% bound to hemoglobin (as carbaminohemoglobin).

  • About 70% transported as bicarbonate (HCO3-) ions in plasma.

• Key Reaction (CO2 to Bicarbonate): $\mathrm{CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-}$

Effects of Exercise and pH on Hemoglobin Saturation and Oxygen Exchange

Physical activity and changes in blood pH influence how hemoglobin binds and releases oxygen.

• Exercise: Increases tissue demand for O2 and production of CO2, lowering blood pH.

• Bohr Effect: Lower pH (more acidic) and higher CO2 reduce hemoglobin's affinity for O2, promoting O2 release to tissues.

• Hemoglobin Saturation Curve: Shifts to the right with lower pH or higher temperature, meaning more O2 is released at a given PO2.

• Equation (Bohr Effect): $\mathrm{HbO_2 + H^+ \rightarrow HHb + O_2}$

• Example: During intense exercise, muscle cells receive more O2 due to increased unloading from hemoglobin.

Regulation of Breathing Rate: Role of CO2, pH, and O2

The brain regulates breathing rate in response to changes in blood CO2, pH, and O2 levels to maintain homeostasis.

• Medulla Oblongata: The primary respiratory control center in the brainstem. It detects changes in blood pH (which reflects CO2 levels).

• CO2 and pH: Increased CO2 lowers blood pH, stimulating the medulla to increase breathing rate (hyperventilation) to expel more CO2.

• O2 Levels: Significant drops in O2 are detected by chemoreceptors in the aorta and carotid arteries, which also signal the brain to increase breathing rate.

• Hyperventilation: Rapid breathing that decreases CO2 levels and increases blood pH.

• Example: Holding your breath increases CO2 and lowers pH, eventually triggering the urge to breathe.

Key Terms and Definitions

• Partial Pressure: The pressure exerted by a single gas in a mixture.

• Oxygen (O2): Essential for cellular respiration; transported mainly by hemoglobin.

• Carbon Dioxide (CO2): Waste product of metabolism; transported dissolved, bound to hemoglobin, or as bicarbonate.

• Bicarbonate (HCO3-): Main form of CO2 transport in blood.

• Hemoglobin (Hb): Protein in red blood cells that binds O2 and CO2.

• Hemoglobin Saturation Percentage: Proportion of hemoglobin molecules carrying O2.

• pH: Measure of hydrogen ion concentration; affects hemoglobin's affinity for O2.

• Hyperventilation: Increased breathing rate that lowers CO2 and raises blood pH.

• Medulla: Brain region controlling involuntary breathing.

• Lungs: Organs where gas exchange occurs between air and blood.