As the muscles are less stretched the force of contraction decreases which reduces the stroke volume. Decreased stroke volume results in decreased cardiac output and hence reduces blood pressure initially. This is also called orthostatic hypotension (reduced blood supply to brain due to reduced cardiac output causing fainting). However after sometimes, baroreceptor respond to this decreased blood volume and stimulates the cardio-accelerator center in Rostral Ventrolateral Medulla which cause noradrenergic discharge. Nor adrenaline then acts on Beta-2 adrenergic receptors on myocardium to increase the heart rate and force of contraction to increase the blood pressure to normal and thus maintaining homeostasis (Williams et al, 2004).
Q2. Analyzing the blood pressure it becomes evident that although there was a drop in mean systolic blood pressure (114mm Hg sitting versus 111 mm Hg standing, but it was not statistically significant as p value was > 0.05), even mean diastolic blood pressure decreased (76.3 mm Hg sitting versus 73 mm Hg standing but again it was not statistically significant as p value was > 0.05). However the mean pulse pressure ( difference between systolic and diastolic) increased(84.6 mm Hg sitting versus 87 mm Hg, but this was also not statistically significant as p value was > 0.05). This means that out of 100 observations more than 5 observations has happened due to chance factors of random sampling and change in posture has not statistically altered their blood pressure. Though statistically insignificant it is clearly seen that clinically or physiologically there is reduction of blood pressure from sitting and standing postures as discussed in question 1. The increased pulse pressure was due to the fact to compensate the reduction in cardiac output and increase the peripheral circulation (Williams et al, 2004) (Blair et al, 1980).
t tests are conducted to test the significance of difference between ...Show more