Oxygen Delivery and Tissue Oxygenation Quiz

Hemoglobin and Oxygen Binding

• Combination of the first heme in the Hb molecule with O2 increases the affinity of the second heme for O2, and so on, resulting in a sigmoid curve.
• Three important conditions affect the oxygen-hemoglobin dissociation curve: pH, temperature, and concentration of 2,3-biphosphoglycerate (BPG).

Myoglobin

• Myoglobin is an iron-containing pigment found in skeletal muscle.
• It resembles hemoglobin but binds 1 rather than 4 mol of O2 per mole.
• Its dissociation curve is a rectangular hyperbola rather than a sigmoid curve.

Acid-Base Balance and Gas Transport

• Acid and base shifts in the blood are largely controlled by three main buffers in blood: proteins, hemoglobin, and the carbonic acid-bicarbonate system.
• The pH of the arterial plasma is normally 7.40 and that of venous plasma slightly lower.
• A decrease in pH below the norm (acidosis) is technically present whenever the arterial pH is below 7.40, and an increase in pH (alkalosis) is technically present whenever pH is above 7.40.

Acid-Base Disorders

• Acid-base disorders are split into four categories: respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis.
• Respiratory acidosis: short-term rise in arterial PCO2 (above 40 mm Hg) due to decreased ventilation.
• Respiratory alkalosis: short-term increase in ventilation that lowers PCO2 below what is needed for proper CO2 exchange (below 35 mm Hg).
• Metabolic acidosis: occurs when strong acids are added to blood, lowering the available Hb-, Prot-, and HCO3- buffers.
• Metabolic alkalosis: results when the free [H+] level falls due to addition of alkali, or removal of large amounts of acid.

Respiratory and Renal Compensation

• The respiratory system compensates for metabolic acidosis or alkalosis by altering ventilation, and consequently, the PCO2, which can directly change blood pH.
• Respiratory mechanisms tend to be fast.
• In response to metabolic acidosis, ventilation is increased, resulting in a decrease of PCO2 and a subsequent increase in pH toward normal.
• In response to metabolic alkalosis, ventilation is decreased, PCO2 is increased, and a subsequent decrease in pH occurs.

Hypoxia

• Hypoxia is O2 deficiency at the tissue level.
• Traditionally, hypoxia has been divided into four types: hypoxic hypoxia, anemic hypoxia, stagnant or ischemic hypoxia, and histotoxic hypoxia.
• Hypoxic hypoxia: reduced arterial PO2.
• Anemic hypoxia: reduced amount of hemoglobin available to carry O2.
• Stagnant or ischemic hypoxia: inadequate blood flow to a tissue.
• Histotoxic (Cytotoxic) hypoxia: adequate O2 delivery to a tissue, but the tissue cells cannot make use of the O2 supplied due to a toxic agent.

Hypoxic Hypoxia

• Hypoxic hypoxia is a condition of reduced arterial PO2.
• Common causes include high altitude, pneumonia, and other diseases of the respiratory system.

Diseases Causing Hypoxic Hypoxia

• Diseases can be divided into three categories: those in which the gas exchange apparatus fails, those in which large amounts of blood are shunted from the venous to the arterial side of the circulation, and those in which the respiratory pump fails.

Anemic Hypoxia

• Anemic hypoxia is not severe at rest unless the hemoglobin deficiency is marked, because red blood cell 2,3-BPG increases.
• However, anemic patients may have considerable difficulty during exercise because of limited ability to increase O2 delivery to the active tissues.

Carbon Monoxide Poisoning

• Carbon monoxide poisoning is often listed as a form of anemic hypoxia because the amount of hemoglobin that can carry O2 is reduced, but the total hemoglobin content of the blood is unaffected by CO.
• The affinity of hemoglobin for CO is 210 times its affinity for O2, and COHb liberates CO very slowly.

Hypoperfusion Hypoxia

• Hypoperfusion hypoxia, or stagnant hypoxia, is due to slow circulation and is a problem in organs such as the kidneys and heart during shock.
• The liver and possibly the brain are damaged by hypoperfusion hypoxia in congestive heart failure.

Histotoxic Hypoxia

• Hypoxia due to inhibition of tissue oxidative processes is most commonly the result of cyanide poisoning.
• Cyanide inhibits cytochrome oxidase and possibly other enzymes.

Hypercapnia

• Retention of CO2 in the body (hypercapnia) initially stimulates respiration.