Diastolic Pr. In Aorta is maintained by (AIPG 2009)
|A||Elastic recoil of aorta|
|C||Resistance in arterioles|
Due to presence of elastic fibres in the Windkessel vessels there is a elastic recoil during the diastole (they are stretched during systole), which results in a forward flow and a aortic pressure wave called as diastolic pressure.
a. Although the mean velocity of the blood in the proximal portion of the aorta is 40 cm/s, the flow is phasic, and velocity ranges from 120 cm/s during systole to a negative value at the time of the transient backflow before the aortic valve closes in diastole. In the distal portions of the aorta and in the large arteries, velocity is also greater in systole than it is in diastole.
b. However, the vessels are elastic, and forward flow is continuous because of the recoil during diastole of the vessel walls that have been stretched during systole. This recoil effect is sometimes called the Windkessel effect, and the vessels are called Windkessel vessels; Windkessel is the German word for an elastic reservoir.
c. Pulsatile flow appears, in some poorly understood way, to maintain optimal function of the tissues.
a. The pressure in the aorta and in the brachial and other large arteries in a young adult human rises to a peak value (systolic pressure) of about 120 mm Hg during each heart cycle and falls to a minimum value (diastolic pressure) of about 70 mm Hg.
b. The arterial pressure is conventionally written as systolic pressure over diastolic pressure—eg, 120/70 mm Hg.
c. The pulse pressure, the difference between the systolic and diastolic pressures, is normally about 50 mm Hg.
d. The mean pressure is the average pressure throughout the cardiac cycle.
e. Because systole is shorter than diastole, the mean pressure is slightly less than the value halfway between systolic and diastolic pressure.
f. It can actually be determined only by integrating the area of the pressure curve , however, as an approximation, mean pressure equals the diastolic pressure plus one-third of the pulse pressure.
g. The pressure falls very slightly in the large and medium-sized arteries because their resistance to flow is small, but it falls rapidly in the small arteries and arterioles, which are the main sites of the peripheral resistance against which the heart pumps.
h. The mean pressure at the end of the arterioles is 30-38 mm Hg.
i. Pulse pressure also declines rapidly to about 5 mm Hg at the ends of the arterioles.
j. The magnitude of the pressure drop along the arterioles varies considerably depending upon whether they are constricted or dilated.