The systemic circulation

The systemic blood vessels are divided into arteries, arterioles, capillaries and veins. Arteries supply blood to the organs at high pressure, whereas arterioles are smaller vessels with muscular walls which allow direct control of flow through each capillary bed. Capillaries consist of a single layer of endothelial cells, and the thin walls allow exchange of nutrients between blood and tissue. Veins return blood from the capillary beds to the heart, and contain 70% of the circulating blood volume, in contrast to the 15% in the arterial system. Veins act a reservoir, and venous tone is important in maintaining the return of blood to the heart, for example in severe haemorrhage, when sympathetic stimulation causes venoconstriction.

Blood flow

The relationship between flow and driving pressure is given by the Hagen-Poisseuille formula. This states that flow rate in a tube is proportional to:

In blood vessels flow is pulsatile rather than continuous, and viscosity varies with flow rate, so the formula is not strictly applicable, but it illustrates an important point; small changes in radius result in large changes in flow rate. In both arterioles and capillaries changes in flow rate are brought about by changes in tone and therefore vessel radius.

Viscosity describes the tendency of a fluid to resist flow. At low flow rates the red blood cells stick together, increasing viscosity, and remain in the centre of the vessel. The blood closest to the vessel wall (which supplies side branches) therefore has a lower haematocrit. This process is known as plasma skimming. Viscosity is reduced in the presence of anaemia, and the resulting increased flow rate helps maintain oxygen delivery to the tissues. 
 
Control of the systemic circulation

Arteriolar tone determines blood flow to the capillary beds. A number of factors influence arteriolar tone, including autonomic control, circulating hormones, endothelium derived factors and the local concentration of metabolites.

Autonomic control is largely by the sympathetic nervous system, which supplies all vessels except capillaries. Sympathetic fibres arise from the thoracic and lumbar segments of the spinal cord. These are under the control of the vasomotor centre in the medulla, which has distinct vasoconstrictor and vasodilator areas.

Although there is a baseline sympathetic discharge to maintain vascular tone, increased stimulation affects some organs more than others (Figure 4). This tends to redistribute blood from skin, muscle and gut to brain, heart and kidney. Increased sympathetic discharge is one of the responses to hypovolaemia, for example in severe blood loss, with the effect of protecting blood supply to the vital organs. The predominant sympathetic influence is vasoconstriction via alpha-adrenergic receptors. However, the sympathetic system also causes vasodilation via beta adrenergic and cholinergic receptor stimulation, but only in skeletal muscle. This increased blood flow to muscle is an important part of the "fight or flight" reaction, when exercise is anticipated.

Circulating hormones such as adrenaline and angiotensin II are potent vasoconstrictors, but they probably have little effect on acute cardiovascular control. In contrast, endothelium derived factors play an important role in controlling local blood flow. These substances are either produced or modified in the vascular endothelium, and include prostacyclin and nitric oxide, both potent vasodilators. An accumulation of metabolites such as CO₂, K⁺, H⁺, adenosine and lactate causes vasodilation. This response is probably an important mechanism of autoregulation, the process whereby blood flow through an organ is controlled locally, and remains constant over a wide range of perfusion pressure. Autoregulation is a particular feature of the cerebral and renal circulations. 
 
Control of arterial pressure

Systemic arterial pressure is controlled closely in order to maintain tissue perfusion. The mean arterial pressure (MAP) takes account of pulsatile blood flow in the arteries, and is the best measure of perfusion pressure to an organ. MAP is defined:

where pulse pressure is the difference between systolic and diastolic arterial pressure.

MAP is the product of cardiac output (CO) and systemic vascular resistance (SVR):

If cardiac output falls, for example when venous return decreases in hypovolaemia, MAP will also fall unless there is a compensatory rise in SVR by vasoconstriction of the arterioles. This response is mediated by baroreceptors, which are specialised sensors of pressure located in the carotid sinus and aortic arch, and connected to the vasomotor centre. A fall in blood pressure causes reduced stimulation of the baroreceptors, and consequent reduced discharge from the baroreceptors to the vasomotor centre. This causes an increase in sympathetic discharge leading to vasoconstriction, increased heart rate and contractility, and secretion of adrenaline. Conversely, rises in blood pressure stimulate the baroreceptors, which leads to increased parasympathetic outflow to the heart via branches of the vagus nerve, causing slowing of the heart. There is also reduced sympathetic stimulation to the peripheral vessels causing vasodilation.

Baroreceptor responses provide immediate control of blood pressure; if hypotension is prolonged, other mechanisms start to operate, such as the release of angiotensin II and aldosterone from the kidneys and adrenal glands, which leads to salt and water being retained in the circulation.

Teaching Point

The valsalva manoeuvre is a simple test of the baroreceptor reflex. The patient tries to breathe out forcefully against a closed larynx - "straining" - resulting in an increased intrathoracic pressure. This causes decreased venous return, cardiac output and a fall in blood pressure leading to reduced baroreceptor discharge to the vasomotor centre. This then causes peripheral vasoconstriction, and an increase in heart rate which is the normal response. This has the effect of maintaining systolic pressure, although the pulse pressure is reduced due to vasoconstriction.

(Continued ...)

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