Describe the physiological control of systemic vascular resistance (SVR).
This question invited a detailed discussion of the physiological control mechanisms in health, not pathophysiology nor drug-mediated effects. The central and reflex control mechanisms that regulate SVR over time are distinct from the local determinants of SVR. There was often confusion between dependent and independent variables. Cardiac output is generally depended upon SVR, not vice versa, even though SVR can be mathematically calculated from CO and driving pressures. The question asked about systemic vascular resistance and did not require a discussion of individual organs except for a general understanding that local autoregulation versus central neurogenic control predominates in different tissues. Emotional state, temperature, pain and pulmonary reflexes were frequently omitted. Peripheral and central chemoreceptors and low-pressure baroreceptors were relevant to include along with high pressure baroreceptors.
Often, with SAQs like this, the college examiners insist on some sort of definition to be given first, even if it is not explicitly stated in the question stem. This SAQ looked like one of those. Even though the examiners did not specifically ask for it in their comments, this suggested model answer was furnished with such a definition.
Control mechanisms of systemic vascular resistance consist of systemic and regional mechanisms.
Systemic mechanisms include:
Local/regional mechanisms include:
Clifford, Philip S. "Local control of blood flow." Advances in physiology education 35.1 (2011): 5-15.
Green, Harold D., and John H. Kepchar. "Control of peripheral resistance in major systemic vascular beds." Physiological reviews 39.3 (1959): 617-686.
Skimming, Jeffrey W., Sidney Cassin, and Wilmer W. Nichols. "Calculating vascular resistances." Clinical cardiology 20.9 (1997): 805-808.
Joyce, William, et al. "Weighing the evidence for using vascular conductance, not resistance, in comparative cardiovascular physiology." Journal of Experimental Biology 222.6 (2019): jeb197426.
Lautt, W. Wayne. "Resistance or conductance for expression of arterial vascular tone." Microvascular research 37.2 (1989): 230-236.