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Reproduced from Tortora and Derrickson (2009).

Mean arterial pressure

The MAP indicates the average pressure of blood throughout the pulse cycle and thus is a reliable indication of perfusion (Woodrow 2004a). Mathematically, the MAP is derived from the diastolic pressure and the pulse pressure (which is the difference between systolic and diastolic blood pressure). The equation is:

Therefore, a patient with a blood pressure of 123/90 mmHg has a MAP of 101 mmHg. An adequate MAP is usually deemed to be between 65 mmHg (Hinds and Watson 2009) and 70 mmHg (Woodrow 2004a).

Resistance

Resistance is effectively opposition to blood flow (Marieb and Hoehn 2010) and is created by friction between the walls of blood vessels and the blood itself (Patton and Thiobodeau 2009). It is termed peripheral or systemic resistance because most of the resistance occurs in the vessels away from the heart (Marieb and Hoehn 2010). Peripheral resistance varies depending on the degree of vasoconstriction or vasodilation, and the viscosity of blood and the length of the vessels, although the latter two factors generally remain relatively static (Marieb and Hoehn 2010). Arterioles can dilate or contract; when contracted, peripheral resistance increases and blood flow to the tissues decreases, increasing the arterial blood pressure (Patton and Thiobodeau 2009). This can occur systemically, when there is total peripheral resistance, or locally (Patton and Thiobodeau 2009). Blood pressure in the vessels of the cardiovascular system can be seen in Figure 12.10.

Figure 12.10 Blood pressure in various parts of the cardiovascular system. The dashed line is the mean (average) blood pressure in the aorta, arteries and arterioles.

Reproduced from Tortora and Derrickson (2009).

Blood pressure control

There are many interrelated physiological mechanisms which control blood pressure.

Hormonal control

Many hormones help to regulate blood pressure, including adrenaline and noradrenaline which are released from the adrenal medullae in response to a drop in blood pressure; these hormones increase cardiac contractility and thus cardiac output (Foxall 2009). Atrial natriuretic peptide (ANP) is a hormone which is produced from the atria of the heart in response to hypertension. It works by inhibiting the reninangiotensin system, raising the glomerular filtration rate by causing specific vasodilation, inhibiting sodium reabsorption and causing fluid transfer into the institial space (Levick 2010).

Neural control

When blood pressure increases, the baroreceptors, or stretch receptors, are stimulated and in turn stimulate the cardiac inhibitory centre, reducing sympathetic nerve impulses and increasing parasympathetic nerve impulses (Foxall 2009). This causes a vasodilation and a decrease in cardiac output, thereby reducing the blood pressure (Foxall 2009). Similarly, when blood pressure is low the opposite occurs. This response is termed a reflex arch, continually maintaining homoeostasis (Marieb and Hoehn 2010). Baroreceptors are located in the aortic arch, carotid sinuses and in the walls of most of the large arteries in the thorax and neck (Marieb and Hoehn 2010). Close to these are chemoreceptors which are stimulated when the pH of the blood drops or when the carbon dioxide rises, and when oxygen levels drop significantly, these cause an increase in cardiac output and vasoconstriction, leading to an increase in blood pressure (Marieb and Hoehn 2010).

Renal control

When there is a decrease in blood pressure, adrenaline, increased sympathetic activity and a reduction in the stimulation of stretch receptors stimulate the juxtaglomerular cells within the kidneys to release renin (Levick 2010). This causes angiotensin I to be produced, leading to the production of angiotensinconverting enzyme which converts angiotensin I to angiotensin II (Patton and Thiobodeau 2009). Angiotensin II has a potent effect on blood pressure, causing cardiac output and vasoconstriction