Oxygen Hemoglobin Dissociation Curve and Regulation of Ventilation

Oxygen Hemoglobin Dissociation Curve

• Structure of Hemoglobin:

  • Comprised of two alpha and two beta chains.

  • Contains a heme group with iron at its center.

  • Review the structure in the blood chapter if necessary.

• Graph Overview:

  • Y-axis: Percent oxygen saturation of hemoglobin.

  • X-axis: Partial pressure of O$_2$ in the blood, akin to oxygen concentration.

• Points on the Curve:

  • Point A:

  • Hemoglobin is nearly 100% saturated.

  • Fully saturated with four oxygen molecules.

  • No oxygen unloaded to tissues (arterial end of capillary).

  • Point B:

  • At rest, hemoglobin unloads about 25% oxygen.

  • Hemoglobin is 75% saturated moving from capillary to tissues.

  • Point C:

  • During vigorous exercise, hemoglobin unloads most oxygen.

  • Hemoglobin may only be 25% saturated upon returning to venous blood.

• Unloading vs. Loading:

  • The curve is S-shaped due to cooperativity in binding.

  • The binding of one oxygen molecule changes shape, making it easier for subsequent molecules to bind.

  • Without this feature, the curve would appear linear, implying independent binding.

• Effects of Environmental Conditions:

  • Increased body temperature shifts the curve to the right (more oxygen unloading).

  • Increased hydrogen ion concentration (lower pH) shifts the curve to the right (more oxygen unloading).

  • Increased partial pressure of carbon dioxide (PCO$_2$) shifts the curve to the right (more oxygen unloading).

  • Conversely, decreased temperature or lower CO$_2$ and hydrogen levels shift the curve to the left (less oxygen unloading).

Control of Ventilation

• Higher Brain Centers:

  • Cerebral Cortex:

  • Located in the frontal lobes.

  • Controls skeletal muscles involved in breathing (e.g., diaphragm, intercostals).

  • Can exert stimulatory or inhibitory effects on respiratory centers.

• Respiratory Centers:

  • Located in the medulla oblongata and pons.

• Chemoreceptors:

  • Peripheral Chemoreceptors:

  • Located in aortic and carotid sinuses.

  • Respond to low oxygen, high CO$_2$, and high hydrogen ions to stimulate breathing.

  • Central Chemoreceptors:

  • Respond to CO$_2$ and hydrogen ion levels, stimulating breathing when levels are high.

• Receptors in Muscles and Joints:

  • Detect oxygen use during muscle contraction, stimulating increased respiration when active.

• Additional Receptors:

  • Pain and Emotional Stimuli:

  • Can alter breathing patterns through the hypothalamus.

  • Irritant Receptors:

  • Respond to harmful chemicals/fumes, potentially inhibiting breathing.

  • Stretch Receptors:

  • Detect lung expansion, signaling to slow breathing as lungs inflate.

Chemoreceptor Feedback Mechanism

• Increased PCO$_2$ and/or Hydrogen Ion Concentration:

  • Detected by central chemoreceptors.

  • Stimulates respiratory centers to initiate hyperventilation, increasing oxygen release.

• Decreased PCO$_2$ and/or Hydrogen Concentration:

  • Detected by the same chemoreceptors.

  • Stimulates reduced breathing (hypoventilation) to retain CO$_2$ and stabilize levels.

• This mechanism illustrates a negative feedback loop vital for regulating respiration based on blood chemistry.