Question

Compare standing and supine heart rates. What factors may account for differences in HR with these postural changes? Think in terms of cardiac output (CO=HRx SV)

Answer 1

In adults, heart rate tends to be slower (about 60-70) when supine, and speeds up (70-90) when we either sit or stand. This is a natural body response to slight drop in blood pressure with upright posture and its purpose seems to be to maintain a relative normal blood flow to our body. Normal heart rate can vary a lot depending upon various factors including age, gender, level of physical activity, mental stress or anxiety, and presence or absence of disease. In an adult, normal heart rate can vary anywhere between 50-100 bpm.     

the normal resting heart rate for adults varies considerably from person to person. Certain medical conditions, certain medications and cardiovascular fitness status can all affect .

person who is lying down and then suddenly stands up. When the person is lying down (supine position), gravitational forces are similar on the thorax, abdomen and legs because these compartments lie in the same horizontal plane. In this position, venous blood volumes and pressures are distributed evenly throughout the body. When the person suddenly stands upright, gravity acts on the vascular volume causing blood to accumulate in the lower extremities. (Compare the size of veins in the top of your feet while lying down and standing.) Because venous complianceis high and the veins readily expand with blood, most of the blood volume shift occurs in the veins. Therefore, venous volume (Vol) and pressure (VP) become very high in the feet and lower limbs when standing. This shift in blood volume decreases thoracic venous blood volume (CV Vol) and therefore central venous pressure (CVP) decreases. This decreases right ventricular filling pressure (preload), leading to a decline in stroke volume by the Frank-Starling mechanism. Left ventricular stroke volume also falls because of reduced pulmonary venous return (decreased left ventricular preload). This causes cardiac output (CO) and mean arterial pressure (MAP) to fall. If arterial pressure falls appreciably upon standing, this is termed orthostatic or postural hypotension.This fall in arterial pressure can reduce cerebral blood flow to the point where a person might experience syncope (fainting).