Autonomic Changes In Venous Pooling

d) When you stand up, venous pooling occurs due to gravity where about 20% of the blood in the body pools in the lower limbs and does not return to the heart straight away therefore the amount of blood that is pumped around the body when you stand up decreases and the arterial blood pressure drops. Venous compliance is high and veins are able to easily expand with blood therefore most of the shift in blood volume occurs in the veins. Venous volume in the lower limbs becomes very high when standing up. The shift in blood volume decreases thoracic venous blood volume and therefore the central venous pressure decreases. This decreases the filling phase pressure of the right ventricle of the heart leading to a decrease in stroke volume. Left ventricular
A decrease in arterial blood pressure results in decreased baroreceptor firing. The autonomic neurons within the medulla respond to this by increasing sympathetic outflow and decreasing parasympathetic (via the vagal nerve) outflow. Upon standing up, there is disinhibition of the sympathetic activity in the medulla so sympathetic activity coming from the rostral ventrolateral medulla increases. These autonomic changes trigger vasoconstriction, tachycardia and positive inotropy. These changes increase the cardiac output and therefore increases the heart
The change when the subject stood up from a seated position is slightly higher than the change seen when the subject stands up and immediately starts walking. This can be explained by the fact that the effect of venous pooling is counteracted by the skeletal muscle pump and this aids the baroreceptor reflex. Veins that are located in large muscle groups are compressed when the muscles surrounding them contract. This contraction will occur when the subject stands up and starts walking immediately. Due to this, the cycles of relaxation and concentration alter and this means that the veins are alternatively decompressed and compressed. As the muscles contract when the subject starts walking, blood is propelled forward through the open distal valves and impedes blood flow into the muscles in the lower limbs due to the proximal valves closing during contraction therefore preventing venous pooling. This therefore means that the mABP does not drop as much as it would if the subject just stood up and did not walk forward