Understanding Respiratory System

O₂ Transport

• O₂ transport involves 2 stages:
  • Diffusion of O₂ across the blood-gas barrier (plasma and red blood cell interior).
  • Reaction with hemoglobin in red blood cells (erythrocytes).

Forms of O₂ Transport

1. Dissolved O₂ in plasma:

   • Solubility of O₂ is low: 0.003 ml O₂ per 100 ml of blood per 1 mmHg of Po₂.
   • At alveolar Po₂ of 100 mmHg, only 0.3 ml O₂ is carried dissolved per 100 ml of blood.
   • Inadequate for tissue O₂ consumption.

2. O₂ carried by hemoglobin: 97%

   • Hemoglobin (Hb) has 4 subunits, each containing a heme group with an iron atom (Fe²⁺).
   • Over 300 million Hb molecules per RBC; 15 gm Hb/100 ml blood.
   • At rest, ~25% of O₂ is released from Hb to tissues; during exercise, up to 75% can be released.

Oxygen Capacity

• Defined as the maximum amount of O₂ combining with Hb.
• 1 gm of Hb can bind with 1.39 ml of O₂.
• Normal capacity: 20.85 ml O₂/100 ml blood (1.39 ml/gm × 15 gm/100 ml).
• Factors:
  1. Number of RBCs/volume.
  2. Amount of Hb per RBC.

Oxygen-Hemoglobin Binding Factors

• % Saturation of Hb formula: total O₂ combined with Hb divided by O2 capacity of Hb × 100%

• O₂ saturation of 97.5% in fully saturated blood does not guarantee adequate O₂ supply for tissues.

• Example: Anemia with Hb at 7.5 gm/100 ml can still yield a saturation of 97.5%, but the total O₂ that combines is lower (10.43 ml O₂/100 ml blood).

Oxygen-Hemoglobin Dissociation Curve

• Characteristics:

  • Sigmoidal shape facilitating oxygen delivery under varying conditions.
  • Physiological significance:
  • Flat upper portion: Drop from 100 to 80 mmHg has minimal effect on saturation (remains ~97.5%).
  • Steep middle portion: Rapid O₂ release with small decreases in Po₂, important for tissue O₂ extraction.

• Right Shift Factors:

  • Increased temp, Pco₂, 2,3 DPG, and decreased pH cause a release of O₂ due to decreased affinity of Hb for O₂ (known as the Bohr effect).

CO₂ Transport

• Transported in 3 forms:
  1. Dissolved CO₂:
  • Approximately 8-10% of CO₂ is dissolved, roughly 20 times more soluble than O₂.
  1. Bicarbonate Ions (HCO₃⁻):
  • ~60% of CO₂ is converted to bicarbonate in plasma and inside red blood cells via carbonic anhydrase.
  • After conversion, HCO₃⁻ exits RBCs leading to a Chloride Shift, where Cl- enters to maintain electrical neutrality.
  1. Carbamino Compounds:
  • About 30% of CO₂ combines with a terminal amine of blood proteins (no enzyme needed).

Regulation of Respiration

• Sensors for Control:
  1. Central Chemoreceptors:
  • Located in the medulla, respond to changes in [H+] and Pco₂.
  1. Peripheral Chemoreceptors:
  • Found at carotid and aortic bodies responding to low Po₂, acidosis, and high Pco₂.
  1. Lung Receptors:
  • Types:
    • Pulmonary stretch receptors inhibit inspiratory neurons to prevent over-inflation.
    • Irritant receptors activate bronchoconstriction and hyperpnea.
    • Juxta-capillary (J) receptors induce rapid, shallow breathing in response to pulmonary congestion.
  1. Joint and Muscle Receptors:
  • Activate with movement, aiding ventilation during exercise.
  1. Arterial Baroreceptors:
  • Increased arterial pressure may lead to hypoventilation.
  1. Pain and Temperature Responses:
  • Pain can cause apnea followed by hyperventilation; heating may initiate hyperventilation.

Sample Questions

1. Approximately 97% of the O₂ transported in blood is:
   • a) dissolved in plasma
   • b) bound to Fe³⁺ in hemoglobin
   • c) in carbonic acid form
   • d) associated with ferrous (Fe²⁺) ions in erythrocytes (Answer: d)
2. Under normal conditions, roughly ___% of O₂ carried by Hb is released to resting tissue:
   • a) 25
   • b) 50
   • c) 75
   • d) 100 (Answer: a)
3. A right shift of the Oxygen-Hemoglobin Dissociation Curve could be triggered by:
   • a) increased plasma pH
   • b) decreased Pco₂
   • c) decrease in 2,3 DPG
   • d) heavy exercise (Answer: d)