Final answer:
Shifting from a supine to a standing position mainly leads to an increase in SVR, while SV and CO may temporarily decrease. However, the body's compensatory mechanisms, primarily through sympathetic activation, strive to maintain homeostasis by adjusting HR, SV, and SVR to support adequate cerebral perfusion.
Step-by-step explanation:
When transitioning from a supine (lying down) position to standing, the body experiences shifts in blood volume and distribution due to gravity. These changes can affect systemic vascular resistance (SVR), stroke volume (SV), and cardiac output (CO).
Upon standing, blood pools in the lower extremities, which can lead to a decrease in venous return to the heart, reducing the preload and consequently the SV. To counteract this, autonomic reflexes lead to an increase in heart rate (HR) and vasoconstriction, thus increasing SVR to maintain blood pressure. Despite this compensatory mechanism, there might be a transient drop in CO due to the initial decrease in SV.
Normally, the sympathetic nervous system responds to maintain homeostasis by increasing HR and SVR to support CO. However, in cases of orthostatic hypotension, this compensation may be inadequate, leading to dizziness or lightheadedness due to decreased cerebral perfusion. This demonstrates the importance of cardiovascular reflexes in maintaining CO and SV during postural changes.