Answer and Explanation:
The Frank-Starling law of the heart explains the influence of venous return on stroke volume. According to this law, the heart's stroke volume (the amount of blood pumped out by the ventricle with each contraction) is directly proportional to the volume of blood returning to the heart (venous return).
When venous return increases, it stretches the myocardial fibers in the ventricles, leading to an increase in the force of ventricular contraction. This increased force of contraction results in a greater stroke volume. On the other hand, when venous return decreases, the myocardial fibers are less stretched, leading to a decrease in the force of ventricular contraction and a decrease in stroke volume.
In the case of the middle-aged woman with left ventricular failure resulting from a myocardial infarction, the failure of the left ventricle can cause the signs and symptoms mentioned. Here's how:
1. Pale and cold skin: The reduced ability of the left ventricle to pump blood effectively can lead to inadequate perfusion of the peripheral tissues. This reduced blood flow to the skin can result in pale and cold skin, as there is less oxygen and nutrient delivery to the tissues.
2. Moist sounds over lower regions of both lungs: Left ventricular failure can lead to fluid accumulation in the lungs, a condition known as pulmonary edema. When the left ventricle fails to adequately pump blood forward, blood can back up into the lungs, causing fluid to leak into the lung tissue. This fluid accumulation leads to the development of moist sounds, such as crackles or rales, heard during auscultation of the lungs.
In summary, left ventricular failure resulting from a myocardial infarction can cause pale and cold skin due to reduced peripheral tissue perfusion and moist sounds over the lower regions of both lungs due to pulmonary edema. These signs and symptoms are a result of the impaired pumping ability of the left ventricle, leading to inadequate circulation and fluid accumulation in the lungs.