Human Physiology Exam 3 Ch 18

The Fick Equation and Oxygen Consumption

• Mass Flow and Mass Balance: The Fick equation relates oxygen consumption (Q_O2) to cardiac output (CO) by substituting the mass flow of oxygen into the mass balance equation.

• The Equation: CO x (Arterial [O₂] - Venous [O₂]) = Q_O2

• Physiological Relationship: It uses cardiac output and the difference in oxygen concentration between arterial and venous blood to estimate total cellular oxygen consumption (V_O2)

Role of Hemoglobin (Hb) in Oxygen Transport: Molecular Level

One hemoglobin molecule can bind up to four oxygen molecules to form oxyhemoglobin (HbO₂)

Role of Hemoglobin (Hb) in Oxygen Transport: Systemic Level

Hemoglobin is essential because oxygen has low solubility in plasma; Hb allows blood to carry much more oxygen than plasma alone

Role of Hemoglobin (Hb) in Oxygen Transport: Oxygen Distribution

In arterial blood, over 98% of oxygen is bound to hemoglobin inside red blood cells, while less than 2% is dissolved in plasma

Plasma P_O2 and Oxygen Transport

• Primary Determinant: Plasma P_O2 determines the percent saturation of hemoglobin.

• Diffusion Gradient: Gases entering capillaries first dissolve in plasma; as red blood cells pick up O₂, they maintain a continued gradient for more O₂ to enter the blood.

• Equilibrium: Oxygen dissolves in plasma until the P_O2 in the blood equals the P_O2 in the alveoli (normally 100 mm Hg in arterial blood).

Oxyhemoglobin Saturation Curve

Curve Shape: An S-shaped (sigmoidal) curve that relates $P_O2 to Hb saturation.

Physiological Significance: The steepest part of the curve (between 20–60 mm Hg) occurs at the P_O2 levels typically found in tissues, allowing for significant oxygen offloading where it is needed most.

• Rightward Shifts (Decreased Affinity/Increased Offloading):

  • pH: Decreased pH (more acidic/Bohr effect).

  • Temperature: Increased temperature.

  • P_CO2: Increased P_CO2 .

  • 2,3-BPG: Increased levels of the metabolic compound 2,3-BPG.

Conversion of CO₂ to HCO₃^-

The Reaction: CO₂+ H₂O = H₂CO₃ = H⁺ + HCO₃^-

Catalyzing Enzyme: Carbonic anhydrase (CA), which is located inside red blood cells.

Carbon Dioxide Transport Forms

Transport Forms:

• 70% as Bicarbonate (HCO₃^-) in plasma.

• 23% bound to hemoglobin as carbaminohemoglobin (HbCO₂)

• 7% dissolved in plasma.

Carbon Dioxide Transport Map: Systemic v Alveoli

• At the Cells (Systemic): CO₂ diffuses out of cells into the blood; CA converts it to HCO₃^- and H⁺; HCO₃^- enters plasma via the chloride shift (exchanging for Cl^-).

• At the Lungs (Alveoli): The reaction reverses; HCO₃^- is pulled back into the RBC, converted back to CO₂, and diffuses into the alveoli to be exhaled.

Chemoreceptor Mechanisms: Peripheral Chemoreceptors (Carotid and Aortic bodies)

• Monitor: Arterial P_O2, P_CO2, and pH.

• Trigger: Activated by significantly low P_O2 (< 60 mm Hg), low pH, or high P_CO2.

• Action: Glomus cells close K⁺ channels, depolarize, and release neurotransmitters to increase ventilation.

Chemoreceptor Mechanisms: Central Chemoreceptors (Medulla oblongata)

• Monitor: Cerebrospinal fluid (CSF) pH.

• Mechanism: CO₂ crosses the blood-brain barrier and is converted to H⁺ and HCO₃^- in the CSF; the resulting change in pH stimulates the receptors.