A 24-year-old healthy college student enrolls into a research study investigating the factors affecting blood oxygen content. The study is performed at sea level, on room air with no additional oxygen. A technique is devised to measure the accurate partial pressure of oxygen in the blood as it flows through the circulation. Blood samples are obtained from the vena cavae, alveolar capillaries, and the aorta. All data are plotted, and the curve obtained is shown in the graph below. Which of the following factors most likely explains the change in partial pressure indicated by the arrow?

Question

A 24-year-old healthy college student enrolls into a research study investigating the factors affecting blood oxygen content.  The study is performed at sea level, on room air with no additional oxygen.  A technique is devised to measure the accurate partial pressure of oxygen in the blood as it flows through the circulation.  Blood samples are obtained from the vena cavae, alveolar capillaries, and the aorta.  All data are plotted, and the curve obtained is shown in the graph below.  Which of the following factors most likely explains the change in partial pressure indicated by the arrow?

Quizplus · Accepted Answer

11ec160b_3ce9_cfa5_9249_296655288c49_MD0001_00 The graph illustrates the pO₂ of blood as it moves through the lungs and into the systemic circulation. Blood in the pulmonary arteries comes from the systemic venous system and is normally deoxygenated. As blood moves through the pulmonary capillaries, it becomes progressively more oxygenated until it equilibrates with the alveolar pO₂ (~104 mm Hg when breathing room air). This is represented by the curve's first upward deflection and subsequent plateau. However, blood in the left atrium and ventricle has a slightly lower pO₂ (~100 mm Hg) than blood in the pulmonary capillaries. This is due to the mixing of deoxygenated blood with oxygenated blood from the pulmonary veins and is represented by the downward deflection identified by the arrow. This deoxygenated blood comes from the following sources: The bronchial arteries carry blood to the bronchi and bronchioles and, together with the pulmonary artery, form the dual blood supply to the lungs. The bronchial veins return only a portion of this blood to the right heart via the azygos and hemi-azygos veins; most of the blood supplied by the bronchial arteries returns to the left heart via the pulmonary veins. Small cardiac (thebesian) veins that drain into the left atrium and ventricle also contribute to normal anatomic shunting. (Choice A) Hypoxia due to a diffusion limitation frequently results from pulmonary fibrosis, acute respiratory distress syndrome, emphysema, or hyaline membrane disease of the infant. In diffusion-limited gas exchange, the blood pO₂ does not equilibrate with the alveolar pO₂ by the end of its traversal through the alveolar capillaries. (Choice B) Dead space ventilation (wasted ventilation) refers to the volume of inspired air that does not participate in gas exchange. Anatomic dead space refers to the volume of air that remains in the conducting parts of the airways, such as the bronchi and bronchioles. Alveolar dead space refers to the volume of air in the alveoli that cannot participate in gas exchange due to poor perfusion. (Choice D) Perfusion-limited gas exchange occurs in the normal physiologic resting state, as equilibration of the blood with the alveolar gas is complete by the time the blood traverses one-third of the total pulmonary capillary length. (Choice E) The hemoglobin dissociation curve shifts to the right in the systemic capillaries to increase oxygen delivery to the body tissues. Increases in pCO₂, temperature, and 2,3-diphosphoglycerate concentration cause right-shifting of the curve; the opposite conditions cause a left shift and favor oxygen loading in the lungs. Educational objective: The pO₂ in the left atrium and ventricle is lower than that in the pulmonary capillaries due to mixing of oxygenated blood from the pulmonary veins with deoxygenated blood from the bronchial circulation and thebesian veins.

A 24-Year-Old Healthy College Student Enrolls into a Research Study