Abstract
The cardiovascular system distributes blood around the body. It is divided into the pulmonary circulation and the systemic circulation. The systemic circulation is a ‘pressure-constant, flow-variable’ system.
The cardiovascular system distributes blood around the body. It is divided into the pulmonary circulation and the systemic circulation. The systemic circulation is a ‘pressure-constant, flow-variable’ system.
What are the functions of the circulation?
Functions include:
Transport of O2 from the lungs to the tissues;
Transport of CO2 from the tissues to the lungs;
Transport of metabolic waste products from the tissues to the liver and kidneys for excretion;
Distribution of nutrients from the sites of absorption (gut) or production (liver) to the tissues;
Distribution of body water and electrolytes between intracellular, interstitial and intravascular body compartments;
Transport of immunologically active substances (antibodies, leukocytes, complement);
Transport of hormones from their site of production (e.g. the parathyroid gland) to their target site (e.g. the kidney);
Production of hormones (e.g. atrial natriuretic peptide);
Assisting in thermoregulation by redistributing blood flow between the core organs and the skin.
The transport and distributive properties of the circulation are utilised by anaesthetists to distribute a range of substances: drugs, fluids, nutrition and heat.
What are the constituent parts of the systemic circulation?
The systemic circulation is composed of:
A ‘pump’ – the left ventricle (LV) – which drives blood through the vessels;
The arterial system, consisting of arteries and arterioles;
Capillaries;
The venous system, consisting of venules and veins.
Only 15% of circulating blood volume is found within the arterial system. Most of the circulating blood (65%) is found within the venous system due to its greater compliance. The veins thus act as an important reservoir for blood, with venous tone responsible for maintaining venous return to the right atrium. The remainder of the circulating volume is found within the pulmonary circulation (10%), the cardiac chambers (5%) and the capillaries (5%).
What are the main differences between the systemic and pulmonary circulations?
The primary function of the pulmonary circulation is the transport of blood from the right ventricle (RV) to the lungs for participation in gas exchange (see Chapter 23). The RV therefore acts as a flow generator around a low-resistance pulmonary circulation. This is in contrast to the role of the systemic circulation, where the LV generates pressure in the arterial system. This pressure is then used as the energy gradient to perfuse tissues (create flow) according to the demand. The transport of blood from the LV to the rest of the body is thus determined by local tissue metabolism or by stereotyped responses (e.g. an increase in sympathetic outflow). The main differences between the two circulations stem from this pressure difference (Table 31.1).
Table 31.1 Differences between the systemic and pulmonary circulations (PVR = pulmonary vascular resistance; SVR = systemic vascular resistance).
| Systemic circulation | Pulmonary circulation | |
|---|---|---|
| Arteries | ||
| Typical pressure (systolic/diastolic) | 140/80 mmHg | 25/8 mmHg |
| Typical mean pressure | 100 mmHg | 15 mmHg |
| Vessel wall | Thick walled, elastic | Thin walled, distensible |
| Resting vasoconstrictor tone | Highly constricted at rest | No vasoconstrictor tone at rest |
| Arterioles | ||
| Vessel wall | Thick walls, small lumen, muscular | Thin walls, large lumen |
| Vessel resistance | High resistance, typical SVR 1600 dyn.s.cm–5 | Low resistance, typical PVR 160 dyn.s.cm–5 |
| Response to hypoxia | Vasodilatation | Vasoconstriction |
| Capillaries | ||
| Wall thickness | Thin, to allow exchange of O2, CO2 and nutrients | Extremely thin, to allow efficient gas exchange |
| Blood flow | Owing to high resistance in arterioles, flow is continuous | Low resistance in arteries and arterioles means blood flow is still pulsatile at the capillaries |
| Distensibility/compressibility | Little change in radius, as little change in external/internal pressures | Compressible with increased alveolar pressure; distensible with increased pulmonary venous pressure |
| Veins | ||
| Typical mean pressure | 2 mmHg | 5 mmHg |
| Venous reservoir | High venous capacitance, holding >1000 mL, which can be released back into the circulation if required | Only holds around 500 mL of blood, which can be released back into the circulation if required |
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