Function of circulation
The primary aim of circulation is to transport oxygen and nutrients to the tissues, while removing carbon dioxide and the waste products of metabolism. The rate of flow is largely determined by organ demand.
Circulatory system
There are two distinct circulations: the pulmonary system from the right side of the heart (around 8% of blood), and the systemic system from the left side of the heart (around 85% of blood, 65% of which is in veins). The heart itself contains the remaining 7%.
Structure of the heart
The heart is a muscle on a fibrous skeleton, with pumps on the left and right. Each side has an atrium and a ventricle. Blood enters through the atria and is ejected from the ventricles. Coordinated pumping of all four chambers is essential. The right side is a low-pressure pulmonary pump (going to the lungs), and the left side is a high-pressure systemic pump (going to the rest of the body).
Cardiac muscle is formed of myocytes, specialised involuntary striated muscle that is highly fatigue-resistant. There are large numbers of mitochondria present, giving the heart a good blood supply. Myocytes are interconnected by gap junctions, permitting rapid transmission of impulses from cell to cell.
Blood vessels
- Arteries carry oxygenated blood away from the heart (except the pulmonary artery). They are muscular with elastic recoil, and act as resistance vessels.
- Veins carry deoxygenated blood to the heart (except the pulmonary vein). They have no muscular layer, low resistance, and rely on skeletal muscle pumps with valves to maintain flow.
- Microcirculation (arterioles, venules and capillaries) is one cell thick and autoregulated, allowing bidirectional flow of gases and metabolites across the cell wall.
Blood pressure
Blood pressure is generated by cardiac contraction, a result of systolic squeeze and diastolic relaxation. The cardiac cycle is one-third systolic and two-thirds diastolic, regulated by the renin-angiotensin system. When the volume of blood is low, the kidneys secrete renin, which stimulates the production of angiotensin, causing blood vessels to constrict and pressure to rise. Mean blood pressure typically increases with age.
Electrical impulses
The signal is initiated in the SA node (sino-atrial), located in the wall of the right atrium, generating sinus rhythm. The atria are electrically isolated from the ventricles, with the AV node (atrio-ventricular) being the only conductive point. Purkinje fibres carry the signal rapidly from the AV node to the ventricles.
Autonomic nervous system
The ANS is made up of the sympathetic nervous system (fight or flight, adrenaline and noradrenaline) and the parasympathetic nervous system (rest and digest, acetylcholine). Adrenaline and noradrenaline increase the opening of calcium and sodium channels, causing an increase in SA node firing. Acetylcholine increases the opening of potassium channels, slowing it. Baroreceptors compensate for a fall in blood pressure (for example moving from lying to standing) by signalling to the medulla oblongata.
Oxygen delivery
The quantity of oxygen delivered to the tissues is determined by oxygen content multiplied by cardiac output. Central factors (effective gas exchange, adequate cardiovascular reserve, haemoglobin) and peripheral factors (regional blood flow, microcirculation, oxygen-haemoglobin affinity, tissue oxygen extraction) all influence this.