A multinational research institute conducting experiments on human circulatory physiology enrolls a healthy 30-year-old male volunteer to assess the oxygen consumption rate of various organs. During the study, the blood oxygen content of the aorta and several other vessels is measured at rest. The greatest difference in these measurements will most likely be between the aorta and which of the following blood vessels?

Question

A multinational research institute conducting experiments on human circulatory physiology enrolls a healthy 30-year-old male volunteer to assess the oxygen consumption rate of various organs.  During the study, the blood oxygen content of the aorta and several other vessels is measured at rest.  The greatest difference in these measurements will most likely be between the aorta and which of the following blood vessels?

Quizplus · Accepted Answer

11ec160b_3c92_87f2_9249_c5b327003276_MD0001_00 Arterial blood supply to the cardiac muscle (myocardium) is provided by the right and left coronary arteries arising directly from the aortic root. Most cardiac venous blood drains into the right atrium via the coronary sinus, with the remainder draining directly into the other chambers of the heart. Three specific features distinguish cardiac circulation from blood flow to skeletal muscle and viscera: The left ventricle is perfused only during diastole. Myocardial contraction during systole leads to compression of the coronary vessels and disruption of blood flow. Wall tension is highest near the endocardium, making the subendocardial region the most prone to ischemia. Myocardial oxygen extraction is very high. The heart has a capillary density far exceeding that of skeletal muscle. Oxygen extraction from arterial blood is very effective within the heart as the resting myocardium extracts 60%-75% of oxygen from blood. This amount is higher than that extracted by any other tissue or organ in the body. As a result, the cardiac venous blood in the coronary sinus, before it reaches the right atrium and mixes with blood returning from the systemic circulation, is the most deoxygenated blood in the body (Choices A, C, D, and F). Myocardial oxygen demand and coronary blood flow are tightly coupled. Because oxygen extraction by the resting heart is already very high, there is little capacity to increase myocardial oxygen extraction during periods of increased oxygen demand (eg, during exercise). Therefore, increased oxygen delivery to the heart can be achieved only through increased coronary blood flow. Adenosine and nitric oxide are the most important vasodilators responsible for increasing coronary flow. (Choice E) Deoxygenated systemic blood returning via the vena cava contains more oxygen than coronary sinus venous blood. Because the pulmonary artery contains an admixture of blood returning via coronary sinus and systemic venous circulation, its oxygen content is also higher than pure coronary venous return. Educational objective: Myocardial oxygen extraction exceeds that of any other tissue or organ; therefore, the cardiac venous blood in the coronary sinus is the most deoxygenated blood in the body. Due to the high degree of oxygen extraction, increases in myocardial oxygen demand can only be met by an increase in coronary blood flow. __________ References: Regulation of coronary blood flow during exercise. (http://www.ncbi.nlm.nih.gov/pubmed/18626066)

A Multinational Research Institute Conducting Experiments on Human Circulatory Physiology