Gas Transport of O2 and CO2

Gas Transport: O2 and CO2

• Oxygen and carbon dioxide are transported between the lungs and tissues through the blood, but through distinct mechanisms.
• Only 2% of oxygen in the blood is dissolved, with the remaining 98% bound to hemoglobin.
• The majority of carbon dioxide in the blood is transported as bicarbonate, with the rest dissolved or bound to hemoglobin.

O2 Transport

• The amount of dissolved oxygen in the plasma affects the percentage of hemoglobin saturation.
• The hemoglobin saturation curve has a "plateau region" where large changes in the partial pressure of oxygen (PO2) result in small changes in hemoglobin saturation. This is important because oxygen, which decreases at high altitudes, has a small effect on hemoglobin saturation.
• In the steep part of the curve, small changes in PO2 result in large changes in hemoglobin saturation. This is important during exercise, where tissue PO2 may be low.

CO2 Transport

• Carbon dioxide is transported in the blood in five forms, but only three are significant:
  • Dissolved (7%, in plasma and hemoglobin)
  • Bound to Hemoglobin (23%, forming carbaminohemoglobin)
  • Bicarbonate ions (70%)
• In red blood cells, carbonic anhydrase converts CO2 and water into carbonic acid, which dissociates into bicarbonate and hydrogen ions.
• Bicarbonate ions quickly diffuse from red blood cells into the plasma.
• Chloride ions move from the plasma into red blood cells to maintain electrical neutrality, a process called the chloride shift.

Internal Respiration (at tissues)

• Oxygen released from oxyhemoglobin diffuses into tissues for cellular respiration.
• Hemoglobin binds with hydrogen ions forming HHb.
• Carbon dioxide diffuses from tissues into the plasma and red blood cells.
• In erythrocytes, the enzyme carbonic anhydrase rapidly converts CO2 and water into carbonic acid (H2CO3), which quickly splits into bicarbonate (HCO3-) and hydrogen (H+).
• Bicarbonate (HCO3-) moves from red blood cells into plasma.
• To maintain neutrality, chloride ions (Cl-) move from plasma into red blood cells, known as the "chloride shift".

External Respiration (at lungs)

• Oxygen diffuses from the alveoli into capillaries and erythrocytes, where it binds with HHb to form oxyhemoglobin (HbO2) and releases H+.
• Bicarbonate ions move into the red blood cells and bind with H+ to form carbonic acid.
• Cl- diffuses out of the cell into plasma (reverse chloride shift).
• Carbonic acid is split by carbonic anhydrase, releasing CO2, which then diffuses from the blood to the alveoli for expiration.

Bohr Effect

• The Bohr Effect is the influence of CO2 and acid on the release of oxygen.
• CO2 and H+ reversibly bind with hemoglobin, changing its structure. This reduces the affinity for oxygen, meaning more oxygen is released in acidic environments.

Haldane Effect

• The Haldane Effect states that an increase in oxygen decreases hemoglobin’s CO2 binding capacity.
• Deoxyhemoglobin (or reduced Hb) has a greater affinity for H+ than hemoglobin.

Bohr and Haldane Effects

• The Bohr and Haldane effects work together to facilitate efficient gas exchange.
• Increased CO2 and H+ cause increased oxygen release because of the Bohr effect.
• Increased oxygen release increases the uptake of CO2 and H+ by Hb (Haldane effect).
• Reduced Hb carries CO2 and H+ back to the lungs to be expelled.

% Hb Saturation ≠ Amount of O2 Transported

• Hemoglobin saturation refers only to the extent of Hb combined with oxygen, not other gases.
• The amount of oxygen transported in the blood depends on both the amount of hemoglobin and its saturation level.
• An anemic patient with low hemoglobin can still have 100% Hb saturation, but their total oxygen carrying capacity is reduced.

Blood pH Buffering

• The normal physiological pH of blood is between 7.35 and 7.45.
• The CO2/HCO3- buffering reaction is the most crucial physiological system for regulating pH.
• Production of CO2 increases H+ ions, making the blood more acidic.
• Removal of CO2 decreases H+ ions, making the blood more basic.

Acidosis and Alkalosis

• Respiratory acidosis is a decrease in pH due to an increase in pCO2, caused by reduced alveolar ventilation, decreased diffusion capacity, or ventilation-perfusion mismatch.
• Respiratory alkalosis results from hyperventilation, which lowers pCO2 and raises pH.
• Metabolic causes of acid-base imbalance also exist and are typically compensated for by the kidneys.

Gas Transport: O2 and CO2

• Oxygen (O2) and Carbon Dioxide (CO2) are transported between the lungs and tissues via blood using different mechanisms.
• Only 2% of O2 in blood is dissolved; the remaining 98% is bound to haemoglobin (Hb).
• The majority of CO2 is transported in blood as bicarbonate.

O2 Transport

• Dissolved O2 determines Partial Pressure of Oxygen (PO2).
• PO2 affects the percentage of haemoglobin saturation.
• The oxygen dissociation curve shows the relationship between PO2 and haemoglobin saturation.
• The plateau region of the curve indicates that large changes in PO2 result in small changes in Hb saturation, which is important for maintaining O2 supply at high altitudes.
• The steep region of the curve shows that small changes in PO2 result in large changes in Hb saturation, which is important for delivering O2 to tissues during exercise.

CO2 Transport

• CO2 is transported in blood in several forms, primarily as:
  • Dissolved CO2 (7%)
  • Carbaminohaemoglobin (23%)
  • Bicarbonate ions (HCO3-) (70%)
• Most CO2 is converted to HCO3- within red blood cells.

Internal Respiration (at Tissues)

• O2 is released from oxyhaemoglobin and diffuses into tissues for cellular respiration.
• CO2 from tissues diffuses into plasma and red blood cells.
• In red blood cells, carbonic anhydrase converts CO2 and water into carbonic acid (H2CO3).
• Carbonic acid dissociates into bicarbonate (HCO3-) and hydrogen ions (H+).
• Bicarbonate diffuses from red blood cells into plasma.
• Chloride ions (Cl-) move from plasma into red blood cells (chloride shift) to maintain electrical neutrality.

External Respiration (at Lungs)

• The processes of internal respiration are reversed at the lungs.
• Inhaled oxygen diffuses from alveoli into capillaries and red blood cells, forming oxyhaemoglobin (HbO2) and H+.
• Bicarbonate ions move into red blood cells and combine with H+ to form carbonic acid.
• Chloride ions (Cl-) move from red blood cells into plasma (reverse chloride shift).
• Carbonic acid is split by carbonic anhydrase to release CO2.
• CO2 diffuses from blood to alveoli for expiration.
• The Bohr effect describes the influence of CO2 and acid on the release of O2.
• CO2 and H+ bind reversibly with Hb, changing its structure and reducing its affinity for O2.
• This helps offload more O2 to tissues with high CO2 or low pH.

Haldane Effect

• Increased O2 decreases Hb's CO2 binding capacity.
• Deoxyhaemoglobin has a greater affinity for H+ than oxyhaemoglobin.
• Therefore, unloading O2 facilitates Hb uptake of CO2-generated H+.
• This helps maintain blood pH, as Hb mops up most of the H+ generated at the tissues.
• The Bohr and Haldane effects work in synchrony:
  • Increased CO2 and H+ cause increased O2 release (Bohr effect).
  • Increased O2 release from Hb causes increased CO2 and H+ uptake by Hb (Haldane effect).

% Hb Saturation ≠ Amount of O2 Transported

• Normal blood contains 15 g Hb per 100 ml blood.
• O2 carrying capacity is 20 ml O2 per 100 ml blood.
• O2 content of arterial blood is 20 ml O2 per 100 ml blood (Hb saturation with O2 is 100% at a PO2 of 100 mmHg).
• O2 content of venous blood is 15 ml O2 per 100 ml blood (Hb saturation with O2 is 75% at a PO2 of 40 mmHg).

Blood pH Buffering

• Normal physiological blood pH is 7.35 to 7.45.
• The CO2/HCO3- buffering system is the most important physiological system regulating pH.
• Production of CO2 leads to an increase in H+ (more acid).
• Removal of CO2 leads to a decrease in H+ (more basic).

Acidosis - Alkalosis

• Respiratory acidosis is a decrease in pH due to increased pCO2, caused by decreased alveolar ventilation, decreased diffusion capacity, or ventilation-perfusion mismatch.
• Respiratory alkalosis is an increase in pH due to decreased pCO2, caused by increased alveolar ventilation or hyperventilation.
• Note that there are also metabolic causes of acid-base imbalance.
• Kidney compensation usually occurs to correct acid-base imbalances.