Definitive Study Notes on Gas Transport, Carbon Monoxide Poisoning, and Acid-Base Physiology

Oxygen Transport in Blood

  • Oxygen is transported in the blood in two main ways:

    • Most oxygen attaches to hemoglobin molecules inside red blood cells (RBCs) to form oxyhemoglobin.

    • A very small amount of oxygen is carried dissolved in the plasma.

  • Carbon Dioxide (CO2) transport breakdown:

    • 70% as bicarbonate ion (HCO3-) dissolved in plasma.

    • 23% bound to hemoglobin (known as Carbaminohemoglobin).

    • 7% as CO2 dissolved in plasma.

  • Gas Transport in Blood Overview:

    • Oxygen: 99% is bound to hemoglobin and only 1% is dissolved as O2 in plasma.

Carbon Monoxide Poisoning

  • Definition: A special type of hypoxia caused by carbon monoxide (CO).

    • Characteristics of CO:

      • Odorless and colorless gas.

      • Binds to hemoglobin at the same binding sites as oxygen.

      • Competes vigorously with oxygen for these binding sites.

  • Dangers of Carbon Monoxide Poisoning:

    • It silently kills without producing the classic signs of hypoxia (cyanosis and respiratory distress).

  • Management:

    • In uncomplicated intoxications, monitoring venous HbCO levels and providing oxygen therapy are generally sufficient.

    • If persistent impairment occurs after 4 hours of normobaric oxygen therapy, transfer to a hyperbaric center is necessary.

Gas Exchange and Transport

  • External Respiration (Alveolar):

    • Refers to the entire sequence of events involved in the exchange of O2 and CO2 between the external environment and the cells of the body.

  • Ventilation:

    • The process of air exchange between the environment and lung air sacs (alveoli).

  • Internal Respiration:

    • Refers to the intracellular metabolic processes that use O2, produce CO2, and derive energy from nutrient molecules.

  • Diffusion of gases occurs due to differences in partial pressures:

    • Systemic Capillary:

      • PCO2 = 40 mm Hg

      • PO2 = 100 mm Hg

    • Pulmonary Capillary:

      • PCO2 = 45 mm Hg

      • PO2 = 40 mm Hg

    • Additional data for diffusion:

      • PCO2 = 40 mm Hg, PO2 = 100 mm Hg

      • PCO2 = 45 mm Hg, PO2 = 40 mm Hg

Bicarbonate Buffer System

  • Chemical Reaction:

    • ext{H}2 ext{O} + ext{CO}2
      ightleftharpoons ext{H}2 ext{CO}3
      ightleftharpoons ext{HCO}_3^- + ext{H}^+

  • In this system, CO2 diffuses into the red blood cells. The enzyme carbonic anhydrase within the red blood cells quickly converts CO2 into carbonic acid (H2CO3).

    • Carbonic acid is unstable and immediately dissociates into bicarbonate ions (HCO3−) and hydrogen ions (H+).

  • Acid-Base Physiology:

    • The arterial pCO2 is normally maintained at about 40 mmHg.

CO2 Excretion and Acid-Base Balance

  • CO2 is excreted by the lungs, and the arterial pCO2 remains constant.

  • An increased production of CO2 can lead to respiratory acidosis if ventilation remains constant.

  • The system controlling arterial pCO2 is very efficient:

    • Any increase in pCO2 results in increased ventilation rapidly.

    • Consequently, increased CO2 production almost never results in respiratory acidosis.

  • An adult at rest produces approximately 200 mL of CO2 per minute.

Respiratory System Physiology

  • Hydrogen ions [H+] are key determinants of pH.

  • Normal blood pH is around 7.4.

  • Respiratory Regulation:

    • Increased ATP production leads to carbon dioxide and water interaction:
      ext{CO}2 + ext{H}2 ext{O}
      ightleftharpoons ext{H}2 ext{CO}3 = ext{HCO}_3^-

  • Effects of Changes in CO2 Concentration:

    • Increasing [CO2] raises [H+], resulting in increased respiratory rate to expel CO2 and H+.

    • Decreased [CO2] equates to decreased [H+], prompting a reduction in respiratory rate to raise CO2 and H+ levels.

    • A rise in respiratory rate is also associated with an increase in heart rate.

Respiratory Acidosis

  • Respiratory acidosis is diagnosed by an increase in arterial pCO2 (greater than 45 mmHg) which can be caused by:

    • Hypercapnia - Increased levels of CO2 in the blood.

    • Decreased alveolar ventilation - Known as Hypoventilation.

    • Increased production of CO2 by the body.

  • Chronic Respiratory Acidosis:

    • This condition stabilizes over time with the kidneys increasing HCO3- to help restore the body's acid-base balance.

  • Acute Respiratory Acidosis:

    • CO2 builds up rapidly when the lungs cannot remove all produced CO2, leading to body fluids becoming too acidic.

Respiratory Alkalosis

  • Respiratory alkalosis is characterized by a primary acid-base disorder where arterial pCO2 level drops below 35 mmHg (referred to as Hypocapnia).

  • It is always caused by increased alveolar ventilation (known as Hyperventilation).

  • Typically, the low arterial pCO2 level is detected by central and peripheral chemoreceptors, which inhibit further hyperventilation.