Final answer:
A decrease in venous return causes a reduction in preload, which can decrease cardiac output and blood supply to organs. The body may compensate by increasing heart rate and contraction strength, as signaled by baroreceptors detecting lower blood pressure.
Step-by-step explanation:
Effects of Decreased Venous Return
When venous return decreases, a series of events occur due to the changes in blood pressure and volume. Since blood moves from higher pressure to lower pressure, any reduction in venous pressure will impact how blood returns to the heart. Typically, blood velocity increases as it moves from venules to veins, helping return blood to the heart aided by one-way valves and mechanisms like the skeletal muscle and respiratory pumps.
However, a decrease in venous return can lead to a drop in the preload, which in turn affects cardiac output. The Frank-Starling principle suggests that the heart's stroke volume will increase along with an increase in preload. When the venous return is low, the stroke volume and thus cardiac output will also be reduced. This can result in organs receiving less blood, potentially leading to conditions such as dizziness or even syncope due to insufficient blood flow to the brain if compensatory mechanisms do not intervene.
Furthermore, baroreceptors detect decreases in blood pressure as a result of decreased venous return. These receptors then signal the heart to increase its rate and strength of contractions to compensate for the lowered preload and maintain an adequate cardiac output.