CVS adjustments during postural changes

The slowed pulse rate in the horizontal posture as compared to a quicker rate in standing posture depends on different mechanisms of blood pressure regulation in the body. The sitting-lying difference is due to hydrostatic influence acting mainly through the carotid sinus reflex. Carotid sinus is the sensor of how strong is the effect of gravity on the body. The body then will try to compensate the effect of gravity that sensed be carotid sinus by changing [heart rate]. The two main changes in postural from vertical to horizontal position, and from horizontal to ventricle position will be discussed.

Sudden stand after sleep

The main event that takes place immediately after standing is shifting of large volume of blood to lower extremities’ veins. This happens because the gravity increases the pressure force in all parts of the body below the heart, but it decreases the pressure force in organs above the heart level. A dramatic decrease of blood volume returning to heart (venous return) because the gravity helps blood to go to legs but it restricts to flow from leg to heart. By decreasing the preload the end diastolic volume decreases so decrease stroke volume. This will lead to reduction in cardiac output and reduction in blood pressure - usually at least 20 mm Hg - that leads to insufficient supply of oxygen and nutrition to different body tissues especially organs higher than the heart, mainly the brain. Baroreceptors’ stimulation decreased by the reduction in blood pressure. Cardiovascular integration center in the medulla increases sympathetic activity and decreases parasympathetic activity. The heart rate will be increased by increasing the frequency of action potential (pacemaker potential) of sinoatrial node, reduction in the delay time in the atrioventricular node, and increase contractility of cardiac myocytes. The stroke volume will be increased by increase vasoconstriction, increase reabsorption of water by release of antidiuretic hormone, and afferent arteriolar constriction. These mechanisms will help restoration of blood pressure by increasing heart rate and stroke volume. If any or both mechanisms are not sufficient, the body will tend to fall down to be in horizontal position (fainting mechanism) to overcome the effect of gravity on blood pressure.

Lie down after being standing

A change in posture from upright to supine causes an increase in cardiac filling and stroke volume. An increase in cardiac output with posture change from upright to supine is associated with an increase in blood pressure result in stimulation of baroreceptors. The cardiovascular center in the medulla activates the parasympathetic activity and deactivates the sympathetic system. The heart rate will be decreased be making the pacemaker potential slower. Stroke volume will be decreased by decreasing the stimulation of to efferent vasodilatation nerve endings, and increase in afferent arteriolar dilation. These mechanisms result in decrease blood pressure.

Fainting is a symptom of an inadequate supply of oxygen and other nutrients to the brain, usually caused by a temporary decrease in blood flow. Blood flow to the brain can decrease whenever the body cannot quickly compensate for a fall in blood pressure. The body fall when blood pressure fall in case the body mechanisms is insufficient to raise blood pressure. Fainting is a mechanism in which body end the effect of gravity on the body during standing.