Guyton and Hall Physiology Chapter 41 - Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids

Oxygen Transport and Tissue Po2

• Hemoglobin Buffering: Hemoglobin acts as a buffer, stabilizing tissue Po2.
• Tissue Po2 Range: The normal interstitial fluid Po2 in tissues ranges from 5 to 40 mmHg, averaging around 23 mmHg. Even this low Po2 is sufficient for cellular functions.
• Hemoglobin Saturation and O2 Delivery:
  • During normal conditions, 25% of oxygenated hemoglobin releases its oxygen to tissues.
  • Hemoglobin is 97% saturated with oxygen in systemic arterial blood.
• Alveolar Po2 Changes:
  • When alveolar Po2 decreases to 60 mmHg, arterial hemoglobin remains 89% saturated, indicating hemoglobin's buffering effect.
  • Even with alveolar Po2 as high as 500 mmHg, hemoglobin saturation reaches a maximum of 100%, only a 3% increase from normal.
• Increased Cardiac Output and O2 Delivery: Trained marathon runners can increase cardiac output 6- to 7-fold, and O2 transport per blood volume 3-fold, resulting in a 20-fold increase in total O2 delivery to tissues.

CO2 Diffusion

• CO2 Diffusion Gradient:
  • CO2 diffuses from tissues to capillaries due to a higher Pco2 in tissues (45 mmHg) compared to arterial blood (40 mmHg).
  • CO2 diffuses from pulmonary capillaries to alveoli due to a higher Pco2 in the capillaries (45 mmHg) compared to alveolar air (40 mmHg).
• Rapid CO2 Diffusion: CO2 diffusion occurs rapidly, with pulmonary capillary blood achieving near-equilibrium with alveolar Pco2 within one-third of its passage through the capillaries.

Blood Flow and Tissue Po2

• Increased Blood Flow, Increased Tissue Po2: Increasing blood flow delivers more oxygen to tissues, leading to higher tissue Po2.
• Tissue Metabolism and Po2: Increased tissue metabolism consumes more oxygen, lowering interstitial fluid Po2.
• Venous Admixture: Blood flow through the bronchial circulation which bypasses gas exchange, results in a lower Po2 in systemic arterial blood (approximately 95 mmHg).

Hemoglobin Saturation

• Hemoglobin Saturation Curve: The relationship between blood Po2 and hemoglobin oxygen saturation is represented by the hemoglobin saturation curve.
• Normal Hemoglobin Saturation: Normal hemoglobin saturation in systemic arterial blood averages 97% due to the Po2 of 95 mmHg.

Oxygen Delivery

• Oxygen delivery to the tissues is enhanced by increases in blood CO2 and H+ levels
• This shift is called the Bohr effect
• The Bohr effect occurs when the oxygen-hemoglobin dissociation curve moves to the right, enhancing the release of O2 from the blood, and allowing for more oxygen to be picked up from the alveoli.

Carbon Dioxide Transport

• CO2 diffuses from tissue cells in the dissolved molecular CO2 form
• CO2 is transported in three primary forms: dissolved CO2, carbaminohemoglobin, and bicarbonate
• Bicarbonate is the most important form, accounting for 70% of CO2 transported from the tissues to the lungs

Carbon Dioxide Dissociation Curve

• The CO2 dissociation curve demonstrates the relationship between total blood CO2 and Pco2
• The normal blood Pco2 falls within a narrow range of 40mmHg in arterial blood and 45mmHg in venous blood
• Only 4 volume percent of the total blood CO2 is exchanged during transport from the tissues to the lungs
• The normal concentration of CO2 in the blood is about 50 volume percent
• The CO2 concentration increases to 52 volume percent in the tissues and falls to 48 volume percent in the lungs

Haldane Effect

• The Haldane effect explains the shift in the CO2 dissociation curve based on changes in blood oxygen levels
• When PO2 increases (e.g., in the lungs), the CO2 dissociation curve shifts to the left, leading to greater CO2 unloading from the blood
• When PO2 decreases (e.g., in the tissues), the CO2 dissociation curve shifts to the right, allowing for more CO2 to be bound to hemoglobin, enhancing its transport
• The Haldane effect significantly enhances the transport of CO2 from tissues to the lungs

Hemoglobin Saturation and Blood Flow

• Hemoglobin saturation refers to the percentage of hemoglobin molecules bound to oxygen.
• The percent saturation of hemoglobin progressively increases as the partial pressure of oxygen (Po2) in the blood rises.
• Systemic arterial blood, which leaves the lungs and enters the systemic arteries, typically has a Po2 of around 95 mm Hg.
• The usual oxygen saturation of systemic arterial blood averages 97%.

Oxygen Delivery to Tissues

• Each 100 ml of blood delivers about 15 ml of oxygen to the tissues, with three times the normal amount delivered during exercise.
• The cardiac output can increase to six to seven times the normal rate in trained individuals due to increased oxygen delivery to tissues during exercise.
• The utilization coefficient, which is the percentage of blood that releases its oxygen as it passes through tissue capillaries, is about 25%.

Influence of Atmospheric Oxygen Levels

• The normal Po2 in the alveoli is around 104 mm Hg.
• Changes in atmospheric oxygen concentration, whether due to altitude or compressed air environments, have a minimal impact on tissue Po2.
• When alveolar Po2 decreases to 60 mm Hg, arterial hemoglobin remains 89% saturated with oxygen.

The Bohr Effect

• The Bohr effect refers to the rightward shift of the oxygen-hemoglobin dissociation curve in response to increased blood CO2 and H+ levels.
• This shift enhances the release of oxygen from blood in tissues and promotes oxygenation of the blood in the lungs.
• The Bohr effect is facilitated by the diffusion of CO2 from tissue cells into the blood, which increases blood Pco2 and H+ concentration.

Factors Affecting Oxygen Delivery

• 2,3-biphosphoglycerate (BPG) is a metabolically important phosphate compound found in blood.
• BPG can shift the oxygen-hemoglobin dissociation curve to the right, increasing oxygen delivery to tissues.
• Increased temperature, often seen in muscles during exercise, can further enhance oxygen delivery by shifting the curve to the right.

Intracellular Oxygen Usage

• The rate of oxygen usage in cells is primarily limited by the concentration of adenosine diphosphate (ADP) rather than intracellular Po2.
• When intracellular Po2 is above 1 mm Hg, oxygen availability does not limit the rate of chemical reactions.

Carbon Dioxide Transport

• Carbon dioxide is transported in the blood in three main forms: dissolved CO2 (7%), bound to hemoglobin (23%), and as bicarbonate ions (70%).
• Carbonic anhydrase, an enzyme found in red blood cells, catalyzes the reaction between CO2 and water, rapidly forming carbonic acid.
• This reaction is crucial for transporting CO2 in the form of bicarbonate ions, which accounts for the majority of CO2 transport.

Diffusion of Oxygen from Peripheral Capillaries to Tissue Cells

• The intracellular Po2 in peripheral tissues is lower than the Po2 in peripheral capillaries.
• The normal intracellular Po2 ranges from as low as 5 mm Hg to as high as 40 mm Hg, averaging 23 mm Hg.
• Only 1 to 3 mm Hg of O2 pressure is normally required for full support of the chemical processes that use oxygen in the cell.

Diffusion of CO2 from Peripheral Tissue Cells into Capillaries and Lungs

• CO2 diffuses out of tissue cells in the dissolved molecular CO2 form.
• CO2 is transported from tissue cells to the lungs by the reaction of CO2 with water and hemoglobin.
• The tissue capillary blood comes almost exactly to equilibrium with the interstitial Pco2.
• Only a 5 mm Hg pressure difference causes all the required CO2 diffusion out of the pulmonary capillaries into the alveoli.

Chemical Forms in Which CO2 is Transported

• 70% of the CO2 transported is transported in the form of bicarbonate ions, produced by the reaction of CO2 with water.
• A small portion of the CO2 is transported in the dissolved state to the lungs.
• CO2 reacts directly with amine radicals of the hemoglobin molecule to form the compound carbaminohemoglobin.

Effect of BPG to Cause Rightward Shift of the Oxygen-Hemoglobin Dissociation Curve

• The normal BPG in the blood always keeps the O2-hemoglobin dissociation curve shifted slightly to the right.
• This shift allows for greater O2 transport to the tissues.

Transport of CO2 in Combination With Hemoglobin and Plasma Proteins—Carbaminohemoglobin

• The quantity of CO2 that can be carried from the peripheral tissues to the lungs by carbamino combination with hemoglobin and plasma proteins is about 30% of the total quantity transported.
• Because this reaction is much slower than the reaction of CO2 with water inside the red blood cells, it is doubtful that under normal conditions this carbamino mechanism transports a significant amount of CO2.

The Haldane Effect

• The combination of O2 with hemoglobin in the lungs causes the hemoglobin to become a stronger acid.
• The increased acidity of the hemoglobin displaces CO2 from the blood into the alveoli.
• This is accomplished by both decreasing the tendency of hemoglobin to combine with CO2 and releasing H+ ions, which bind with HCO3− to form carbonic acid, which then dissociates into water and CO2.

Respiratory Exchange Ratio

• Under normal resting conditions, only about 82% as much CO2 is expired from the lungs as O2 is taken up by the lungs.
• The ratio of CO2 output to O2 uptake is called the respiratory exchange ratio (R), which is typically 0.825.