Learning objectives

• Understand the Effect of low oxygen pressure on the body. 

• Understand the Effect of high partial pressure of individual gases on the body

• Predict how the presence of abnormally low and high V/Q ratios in a person's lungs will affect arterial PO2 and PCO2.

• Define right-to-left shunts, anatomic and physiological shunts, and physiologic dead space (wasted ventilation). Describe the consequences of each for pulmonary gas exchange.

• Understand Hypoxemia and Hypoxia:Be able to calculate the alveolar to arterial PO2

difference, (A-a)DO2. Describe the normal value for (A-a) DO2 and the significance of an elevated (A-a) DO 2.

initially

• At high altitude, there is a permanent depression in alveolar and systemic arterial PO2.

• The low PO2 stimulates the peripheral chemoreceptors, inducing a hyperventilation and a decrease in alveolar and systemic arterial PCO2

• The loss of CO2produces a respiratory alkalosis. To compensate, the kidney Loses bicarbonate to return arterial pH close to normal.

• Acutely, arterial oxygen content is depressed because of reduced hemoglobin saturation.

• Acclimatization returns oxygen content toward normal because of an increase in hemoglobin concentration

Acute Changes and Long-Term Adaptations (Acclimatization)

• Increased Diffusing Capacity After Acclimatization

• Increased tissue capillarity (or angiogenesis)• Cellular Acclimatization -cell mitochondria and

cellular oxidative enzyme systems are slightly more plentiful

Which of the following sets of changes best describes a Himalayan native living in the Himalayas, compared to a sea-level native living at sea level?

Hematocrit Arterial Po2 Arterial O2 contentA) Decreased Decreased DecreasedB) Decreased Decreased No differenceC) Decreased Increased DecreasedD) Decreased Increased No differenceE) Increased Decreased DecreasedF) Increased Increased DecreasedG) Increased Increased No differenceH) Increased Decreased No difference

High-Pressure Environment

• The partial pressure of O2 and N2 will increase in the alveoli and systemic arterial blood.

• The adverse effect of a high PO2can be oxygen toxicity. The high PN2can cause nitrogen narcosis, but, more important, it can lead to the bends (caisson disease/decompression sickness).

• Bends , Chokes and strokes

Hypoxemia and Hypoxia

• Hypoxemia is defined as a decrease in arterial PO2.

• Hypoxia is defined as a decrease in O2 delivery to, or utilization by, the tissues. Hypoxemia is one of the causes of tissue hypoxia.

Hypoxemia

NORMAL STATE

HYPO VENTILATION

• There is no increase in the A–a oxygen gradient.

• Supplemental oxygen can relieve the hypoxemia.

• End-tidal air still reflects the systemic arterial compartment.

• The problem is not within the lung itself.

DIFFUSION IMPAIRMENT

Perfusion limited situation

• Increase in A–a oxygen gradient• Supplemental oxygen ineffective at returning

arterial PO2to normal• End-tidal air does not reflect the arterial

values

Hypoxia

HYPERCAPNIA

• Hypercapnia means excess carbon dioxide in body fluids.

• Hypercapnia is not seen in hypoxia caused by hypoxemia, anemia or poisoning of the oxidative enzymes.

• Hypercapnia occurs in association with hypoxia caused by hypoventilation.

• At PCO2 of 80-100mmHg, the person is lethargic and at 120-150mmHg, death supervenes.