Physiology Of Blood Flow And Blood Pressure

The purpose of the arterial system is to provide oxygenated blood to the tissues by converting the intermittent cardiac output into a continuous capillary flow and this is achieved by the structural organization of the arterial system.

The blood flow in a vessel is basically determined by two factors:

1. The pressure difference between the two ends of the vessel, which provides the driving force for the flow

2. The impediment to flow, which is essentially the vascular resistance

This can be expressed by the following formula:

where 0 is the flow, AP is the pressure difference, and R is the resistance.

The pressure head in the aorta and the large arteries is provided by the pumping action of the left ventricle ejecting blood with each systole. The arterial pressure peaks in systole and tends to fall during diastole.

Briefly, the peak systolic pressure achieved is determined by (see Chapter 2):

1. The momentum of ejection (the stroke volume, the velocity of ejection, which in turn are related to the contractility of the ventricle and the afterload)

2. The distensibility of the proximal arterial system

3. The timing and amplitude of the reflected pressure wave

When the arterial system is stiff, as in the elderly, for the same amount of stroke output, the peak systolic pressure achieved will be higher. The poor distensibility causes a greater peak pressure. In addition, a stiff arterial system results in faster transmission and reflection ofthe pressure wave, thereby adding to the peak pressure. The narrow and peaked pressure seen in the more peripheral muscular arteries is the effect of such reflection. The level to which the arterial pressure will fall during diastole is primarily dependent on the state of the peripheral resistance, which controls the runoff. Conditions with low peripheral resistance and vasodilatation will cause the diastolic pressure to fall to low levels.

The mean arterial pressure is the average of all the pressures obtained over an entire duration of a cardiac cycle. Since diastole is longer than systole, the mean pressure is estimated as the sum of 60% diastolic pressure and 40% systolic pressure. More accurate measurement will be derived by integrating the area under a recorded pressure curve. The pulse pressure, which is the difference between the systolic and the diastolic pressure, reflects not only the stroke volume but also the state of the peripheral resistance. Conditions associated with a large stroke volume and low peripheral resistance will be expected to give rise to a large pulse pressure, and this will be reflected in the amplitude of the arterial pulse by palpation.

While the control of the cardiac output is usually determined by local tissue flow under physiological states, the control ofthe arterial pressure is independent ofthese and is regulated through a complex system, which involves nervous reflexes and neurohumoral mechanisms for short-term needs (such as "flight," "fright," and "fight" type reactions or in situations like those following acute loss of blood volume) and neuroendocrine, renin-angiotensin-aldosterone system, and renal mechanisms for long-term adaptation. These control systems in the normal as well as their alterations in hypertension and in heart failure are well discussed in standard texts for physiology and medicine. In this chapter our focus will be mainly on measurement of blood pressure by the sphyg-momanometer and its use in special clinical situations.

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Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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