Passage
The heart is a muscular organ comprising four chambers, two atria and two ventricles, that function synergistically to pump blood through a vast, closed network of blood vessels. The chambers are separated by membranous muscular barriers known as septa. Oxygenated blood returning from the lungs via the pulmonary veins fills the left atrium and enters the left ventricle, which then pumps the blood into the systemic arteries to supply other organs in the body. By contrast, deoxygenated blood returning from systemic veins first enters the right atrium, then the right ventricle, and is ultimately siphoned into the pulmonary arteries leading to the lungs. The necessary oxygen, nutrient, and waste exchange occurs between the thinnest blood vessels (capillaries) and neighboring tissues of the systemic and pulmonary circuits.Blood flow throughout the circulatory system is dictated by blood (hydrostatic) pressure, vascular resistance (force opposing blood flow through a vessel), and cardiac output (blood volume expelled from the ventricles per unit time). When blood traverses a vessel, it exerts hydrostatic pressure on the vessel walls, which results in the forced movement of fluid out of the vessel and into the interstitial space. The circulating plasma proteins cause the osmotic pressure within the vessel to be higher than that of the interstitial fluid. In turn, osmotic pressure causes fluid to flow from the interstitial space into the blood vessel, opposing hydrostatic pressure.Cardiac output depends in part on heart rate, which is tightly regulated by the sinoatrial (SA) and atrioventricular (AV) nodes, specialized groups of self-depolarizing cells located in the upper right atrial wall and lower interatrial septum, respectively. Action potentials (APs) generated by SA nodal cells stimulate atrial contraction as they travel to the AV node. The AV node delays AP transmission to ventricular cells, ensuring ventricular filling is complete prior to heart contraction.A 63-year-old male patient collapsed while exercising and was hospitalized. The patient presented with an elevated heart rate, low systemic blood pressure, and abnormally low blood oxygen levels. In addition, x-rays revealed excess fluid in his lungs.
-The systemic blood pressure gradient ΔP is equal to the mean pressure of blood exiting the heart through arteries minus the mean pressure of blood returning to the heart through veins. The relationship of ΔP to cardiac output (CO) and vascular resistance (VR) is given by: ΔP=CO×VRGiven this, which of the following physiological changes would cause increased VR within vessels of the circulatory system? (Note: Assume all other physiological factors remain constant.)

Question

Passage
The heart is a muscular organ comprising four chambers, two atria and two ventricles, that function synergistically to pump blood through a vast, closed network of blood vessels.  The chambers are separated by membranous muscular barriers known as septa.  Oxygenated blood returning from the lungs via the pulmonary veins fills the left atrium and enters the left ventricle, which then pumps the blood into the systemic arteries to supply other organs in the body.  By contrast, deoxygenated blood returning from systemic veins first enters the right atrium, then the right ventricle, and is ultimately siphoned into the pulmonary arteries leading to the lungs.  The necessary oxygen, nutrient, and waste exchange occurs between the thinnest blood vessels (capillaries) and neighboring tissues of the systemic and pulmonary circuits.Blood flow throughout the circulatory system is dictated by blood (hydrostatic) pressure, vascular resistance (force opposing blood flow through a vessel), and cardiac output (blood volume expelled from the ventricles per unit time).  When blood traverses a vessel, it exerts hydrostatic pressure on the vessel walls, which results in the forced movement of fluid out of the vessel and into the interstitial space.  The circulating plasma proteins cause the osmotic pressure within the vessel to be higher than that of the interstitial fluid.  In turn, osmotic pressure causes fluid to flow from the interstitial space into the blood vessel, opposing hydrostatic pressure.Cardiac output depends in part on heart rate, which is tightly regulated by the sinoatrial (SA) and atrioventricular (AV) nodes, specialized groups of self-depolarizing cells located in the upper right atrial wall and lower interatrial septum, respectively.  Action potentials (APs) generated by SA nodal cells stimulate atrial contraction as they travel to the AV node.  The AV node delays AP transmission to ventricular cells, ensuring ventricular filling is complete prior to heart contraction.A 63-year-old male patient collapsed while exercising and was hospitalized.  The patient presented with an elevated heart rate, low systemic blood pressure, and abnormally low blood oxygen levels.  In addition, x-rays revealed excess fluid in his lungs.
-The systemic blood pressure gradient ΔP is equal to the mean pressure of blood exiting the heart through arteries minus the mean pressure of blood returning to the heart through veins.  The relationship of ΔP to cardiac output (CO) and vascular resistance (VR) is given by:  ΔP=CO×VRGiven this, which of the following physiological changes would cause increased VR within vessels of the circulatory system?  (Note: Assume all other physiological factors remain constant.)

Quizplus · Accepted Answer

11ecba2d_1036_2098_a3bf_1d4fa4aa647f_MD0008_00 The passage states that blood flow through the circulatory system is dictated by blood (hydrostatic) pressure, vascular resistance (VR; force opposing blood flow through a vessel), and cardiac output (CO; blood volume expelled from the ventricles per unit time). Generally, blood flows from high to low pressure; therefore, a pressure gradient would also affect blood flow. The question states that the systemic blood pressure gradient (ΔP) is calculated by subtracting the mean pressure of blood returning to the heart (venous pressure) from the mean pressure of blood exiting the heart (arterial pressure). Accordingly, the relationship of ΔP to CO and VR is given by: 11ecba2d_1036_2099_a3bf_b1ad4332ca7d_MD0008_00 ΔP=CO×VR Manipulation of this equation gives the relationship of VR to ΔP and CO: 11ecba2d_1036_47aa_a3bf_1139f884f32c_MD0008_00 ΔPCO=VR Consequently, an increase in ΔP or a decrease in CO would increase VR. Specifically, CO is dependent on both heart rate (number of times a person's heart beats per minute) and stroke volume (blood volume expelled per left ventricular contraction). Stroke volume depends on the amount of blood loaded into the left ventricle prior to contraction. Reduced ventricular filling would lead to less blood being expelled from the ventricle with each contraction and decreased CO. Given the relationship of VR to CO, decreased CO (in the absence of other physiological changes) would ultimately increase VR. (Choice A) Vasodilation of arteries will decrease the hydrostatic pressure exerted by blood on arterial walls and ultimately decrease ΔP. Therefore, increased arterial vasodilation would decrease ΔP as well as reduce the interaction (ie, friction) of blood with vessel walls, leading to decreased (not increased) VR. (Choice B) According to the passage, the SA node generates action potentials that induce atrial contraction and travel to the AV node, which transfers the electrical signals to the ventricles, stimulating ventricular contraction. Accordingly, increased SA node activity will increase heart rate, increasing CO and decreasing VR. (Choice D) Because ΔP is calculated by subtracting venous pressure from arterial pressure, a decrease in arterial pressure alone would decrease ΔP. Given the relationship of VR to ΔP, decreased ΔP in the absence of changes to CO would most likely decrease VR. Educational objective: Blood flow through the circulatory system is influenced by blood pressure, vascular resistance (VR; force opposing blood flow through a vessel), and cardiac output (CO; blood volume expelled from the ventricles per unit time). The relationship of these factors in the systemic circulatory system is given by: ΔP = CO × VR.