What you'll learn
Circulation and the heart covers the mammalian transport system: the structure of the heart, how the cardiac cycle is generated and controlled, the adaptations of the blood vessels, and the exchange of materials at the capillaries. At CAPE level the detail extends to the myogenic origin of the heartbeat, the conducting system, the interpretation of electrocardiograms and pressure curves, and the formation of tissue fluid and lymph. By the end of this topic you should be able to describe the heart's structure in relation to its function, explain the cardiac cycle and the events controlling valve opening, describe how heart rate is initiated and modified, relate the structure of arteries, veins and capillaries to their functions, and explain the formation of tissue fluid.
Key terms and definitions
Double circulation — a system in which blood passes through the heart twice in one complete circuit
Systole — the contraction phase of the cardiac cycle
Diastole — the relaxation phase of the cardiac cycle
Myogenic — describing cardiac muscle, which contracts of its own accord without nervous stimulation
Sinoatrial node — the region of the right atrium that initiates each heartbeat, the pacemaker
Atrioventricular node — the node that delays and relays the impulse to the ventricles
Purkyne tissue — the conducting fibres carrying the impulse through the ventricle walls
Cardiac output — the volume of blood pumped by one ventricle per minute, equal to stroke volume multiplied by heart rate
Stroke volume — the volume of blood ejected by one ventricle in one contraction
Tissue fluid — the fluid surrounding cells, formed by filtration from capillaries
Hydrostatic pressure — the pressure exerted by a fluid, driving filtration out of capillaries
Oncotic pressure — the osmotic effect of plasma proteins, drawing water back into capillaries
Lymph — tissue fluid that has drained into the lymphatic vessels
Core concepts
Double circulation
Mammals have a double circulation: blood passes through the heart twice on each complete circuit.
The pulmonary circulation carries deoxygenated blood from the heart to the lungs and returns oxygenated blood. The systemic circulation carries oxygenated blood to the body and returns deoxygenated blood.
The advantage is that blood can be repressurised after passing through the lungs. The capillary beds of the lungs offer considerable resistance and reduce the pressure substantially, so blood leaving them travels slowly. Returning it to the heart allows it to be pumped to the body at high pressure, so delivery of oxygen and nutrients is rapid, which supports the high metabolic rate of a mammal.
A single circulation, as in fish, passes blood through the gills and then directly to the body at reduced pressure, so circulation is slower.
Heart structure
The heart has four chambers. The two atria have thin walls, since they pump blood only into the ventricles immediately below.
The right ventricle has a moderately thick muscular wall, pumping blood to the lungs, which are close by and whose delicate capillaries would be damaged by high pressure.
The left ventricle has a much thicker wall, typically two to three times that of the right. It must generate sufficient pressure to drive blood through the entire systemic circulation and overcome the resistance of every capillary bed in the body. This difference in wall thickness is the most frequently asked structural question on the heart, and the answer must refer to the distance and resistance, not merely to the body being larger.
The atrioventricular valves lie between atria and ventricles: the tricuspid on the right, the bicuspid or mitral on the left. Tendinous cords attach them to papillary muscles in the ventricle walls, preventing the valves turning inside out under the high pressure of ventricular systole.
The semilunar valves lie at the exits of the ventricles, in the pulmonary artery and the aorta.
The coronary arteries branch from the aorta immediately above the semilunar valve and supply the cardiac muscle itself with oxygenated blood. Their blockage causes myocardial infarction, since the cardiac muscle beyond the blockage is deprived of oxygen and dies.
The septum separates the two sides completely, preventing the mixing of oxygenated and deoxygenated blood.
The cardiac cycle
The cycle has three stages, and each is driven by pressure differences. Valves open and close passively: a valve opens when the pressure behind it exceeds the pressure in front, and closes when the reverse holds. Stating this principle is what turns a description into an explanation.
In atrial systole, the atria contract. Atrial pressure rises above ventricular pressure, so the atrioventricular valves are open and blood is forced into the ventricles, completing their filling. The semilunar valves remain closed because arterial pressure exceeds ventricular pressure.
In ventricular systole, the ventricles contract. Ventricular pressure rises rapidly. As soon as it exceeds atrial pressure the atrioventricular valves close, producing the first heart sound. When ventricular pressure exceeds arterial pressure, the semilunar valves open and blood is ejected into the pulmonary artery and aorta.
In diastole, all chambers relax. Ventricular pressure falls below arterial pressure, so the semilunar valves close, producing the second heart sound. As atrial pressure exceeds the falling ventricular pressure, the atrioventricular valves open and the ventricles begin to fill passively. Most ventricular filling occurs during diastole rather than during atrial systole.
Interpreting a pressure–time graph is a standard question. The point at which two curves cross marks a valve opening or closing, and identifying which valve requires only asking which two chambers or vessels the crossing curves represent.
Control of the heartbeat
Cardiac muscle is myogenic, meaning it contracts of its own accord without nervous stimulation. Nervous input modifies the rate but does not initiate it.
The sinoatrial node in the wall of the right atrium acts as the pacemaker, initiating a wave of electrical excitation at regular intervals.
The wave spreads across both atria, causing them to contract together. A layer of non-conducting tissue between atria and ventricles prevents it passing directly downwards.
The wave reaches the atrioventricular node, which delays it briefly. This delay is functionally essential: it allows the atria to finish emptying before the ventricles contract, and questions about its purpose expect exactly that answer.
The impulse then travels down the bundle of His, through the septum to the apex of the heart, and spreads upwards through the ventricle walls in the Purkyne tissue. Contraction therefore begins at the apex and moves upwards, which forces blood upwards towards the arteries rather than downwards into a closed base.
Heart rate is modified by the cardiovascular centre in the medulla oblongata. Sympathetic stimulation through the accelerator nerve increases rate; parasympathetic stimulation through the vagus nerve decreases it. Adrenaline also increases rate and stroke volume.
Receptors provide the input. Baroreceptors in the aorta and carotid arteries detect blood pressure, and chemoreceptors detect the concentration of carbon dioxide, and therefore the pH, of the blood. A rise in carbon dioxide concentration lowers blood pH, chemoreceptors detect this, and the cardiovascular centre increases heart rate so that carbon dioxide is removed more rapidly at the lungs.
Cardiac output is stroke volume multiplied by heart rate, and both increase during exercise.
Electrocardiograms
An electrocardiogram records the electrical activity of the heart at the body surface.
The P wave corresponds to atrial depolarisation and therefore atrial systole. The QRS complex corresponds to ventricular depolarisation and therefore ventricular systole, and is the largest deflection because the ventricles have much more muscle. The T wave corresponds to ventricular repolarisation during diastole.
Heart rate is calculated by counting the interval between successive R peaks and dividing sixty by that interval in seconds.
Abnormalities include tachycardia, a resting rate above about 100 beats per minute; bradycardia, a rate below about 60; fibrillation, in which the trace is irregular and uncoordinated so that the chambers do not pump effectively; and ectopic beats.
Blood vessels
Arteries carry blood away from the heart at high pressure. They have a thick wall with a large proportion of elastic tissue and smooth muscle, and a relatively narrow lumen. The elastic tissue stretches during ventricular systole and recoils during diastole, which maintains pressure and smooths the flow into a more continuous stream. The narrow lumen helps maintain pressure. The muscle allows vasoconstriction and vasodilation to redirect blood between organs.
Arterioles have proportionally more smooth muscle and less elastic tissue, and their constriction and dilation is the main means of controlling the distribution of blood to different tissues.
Capillaries have walls one endothelial cell thick, giving an extremely short diffusion distance. Their lumen is only just wide enough for a red blood cell to pass, which forces the cells against the wall and reduces the diffusion distance further, and slows the flow, allowing more time for exchange. The capillary network is extensive, giving an enormous total surface area, and many capillaries have small gaps between the endothelial cells allowing fluid to pass.
Veins carry blood back to the heart at low pressure. Their walls are thin with little elastic or muscular tissue, and the lumen is wide, which reduces resistance to flow. Semilunar valves prevent backflow, and blood is moved largely by the contraction of surrounding skeletal muscles, which squeeze the veins, and by pressure changes during breathing.
Tissue fluid and lymph
Tissue fluid forms at the arterial end of a capillary bed and is largely reabsorbed at the venous end, and the mechanism depends on the balance of two opposing pressures.
At the arterial end, the hydrostatic pressure of the blood is high, having been generated by ventricular contraction. This forces water and small dissolved solutes out through the gaps between endothelial cells, a process called ultrafiltration. Plasma proteins are too large to pass and remain in the capillary.
The retained plasma proteins give the blood a more negative water potential than the tissue fluid, creating an oncotic pressure that tends to draw water back in. At the arterial end the hydrostatic pressure exceeds this, so there is net outward movement.
Along the capillary, hydrostatic pressure falls, partly because fluid has left and partly because of the resistance of the narrow vessel. Meanwhile the plasma proteins have become more concentrated, so the water potential of the blood is even more negative.
At the venous end, the oncotic effect therefore exceeds the reduced hydrostatic pressure, and water moves back into the capillary by osmosis.
About ninety per cent of the fluid is reabsorbed this way. The remainder drains into the lymphatic vessels as lymph, which is returned to the blood in veins near the heart. Lymph nodes along the vessels contain lymphocytes and filter pathogens.
Tissue fluid differs from blood plasma in containing no red blood cells, no platelets and very little protein, and differs from lymph mainly in position, lymph having entered the lymphatic vessels.
Oedema is the accumulation of tissue fluid, and its causes follow directly from the mechanism. Low plasma protein concentration, resulting from malnutrition or liver disease, reduces the oncotic pressure so less fluid returns. High blood pressure increases hydrostatic pressure so more is forced out. Blockage of lymphatic vessels, as in filariasis, prevents drainage of the excess.
Worked examples
Example 1: Explaining valve action from pressure data (5 marks)
At one point in the cardiac cycle, left ventricular pressure is 15 kilopascals and aortic pressure is 12 kilopascals. Later, left ventricular pressure is 5 kilopascals and aortic pressure is 11 kilopascals. Describe the state of the semilunar valve at each point and explain.
A valve opens when the pressure behind it exceeds the pressure in front of it, and closes when the pressure in front is greater.
At the first point, ventricular pressure of 15 kilopascals exceeds aortic pressure of 12 kilopascals, so blood is pushed against the valve from the ventricular side and the semilunar valve is open. Blood is being ejected into the aorta during ventricular systole.
At the second point, aortic pressure of 11 kilopascals exceeds ventricular pressure of 5 kilopascals, so blood tends to flow back towards the ventricle and forces the valve shut. The semilunar valve is therefore closed, preventing backflow into the relaxing ventricle during diastole.
Example 2: Explaining the atrioventricular node delay (4 marks)
Explain the functional importance of the delay at the atrioventricular node.
The wave of excitation from the sinoatrial node spreads across the atria, causing them to contract and push blood into the ventricles.
If the excitation passed immediately to the ventricles, they would begin to contract while the atria were still emptying. Blood would not have finished entering the ventricles, so the volume ejected with each beat would be reduced.
The delay at the atrioventricular node allows time for atrial systole to be completed and the ventricles to fill fully before ventricular contraction begins. This maximises stroke volume and therefore cardiac output.
The delay also ensures that contraction of the atria and ventricles is coordinated rather than simultaneous, which is necessary for blood to be moved in one direction through the heart.
Example 3: Explaining oedema (5 marks)
Explain why a person with a very low concentration of plasma protein develops swollen tissues.
At the arterial end of a capillary, high hydrostatic pressure forces water and small solutes out into the tissue fluid, while plasma proteins remain in the capillary because they are too large to pass through the gaps.
These retained proteins lower the water potential of the blood plasma relative to the tissue fluid, so that at the venous end, where hydrostatic pressure has fallen, water returns to the capillary by osmosis.
If the plasma protein concentration is very low, the water potential of the plasma is less negative than normal. The osmotic gradient drawing water back into the capillary is therefore reduced.
Less fluid is reabsorbed at the venous end, while filtration at the arterial end continues largely unchanged. Fluid therefore accumulates in the tissues faster than the lymphatic system can drain it, causing the swelling known as oedema.
Common mistakes and how to avoid them
The most frequent error is stating that the left ventricle is thicker because the body is bigger than the lungs. The reason is the greater distance and the higher resistance of the systemic circulation, requiring a higher pressure.
Students often say that valves are opened and closed by muscles. Valves operate passively according to pressure differences; the tendinous cords merely prevent inversion.
Another common slip is describing the heart as nerve-initiated. Cardiac muscle is myogenic; nerves modify the rate only.
Many candidates place most ventricular filling during atrial systole. Most filling is passive during diastole, with atrial systole completing it.
In tissue fluid questions, answers frequently omit that plasma proteins remain in the capillary, which is the whole basis of reabsorption.
Finally, candidates often confuse the QRS complex with atrial contraction. The P wave is atrial, the QRS ventricular.
Exam technique for "Circulation and the heart"
For any valve question, state the pressure relationship first and then the consequence. The rule about pressure behind and in front answers every such question.
When interpreting a pressure graph, identify which curves are crossing at the point in question, since that identifies the valve and the stage.
Give numerical support wherever a graph or table is supplied, quoting pressures with units rather than describing them as high or low.
For blood vessel questions, pair each structural feature with the functional reason, and note that arteries and veins differ in the proportion of elastic tissue as well as in wall thickness.
In tissue fluid explanations, name both hydrostatic and oncotic pressure and state which dominates at each end of the capillary.
Quick revision summary
Double circulation allows blood to be repressurised after the lungs, giving rapid delivery to the body. The left ventricle wall is thickest because it must overcome the resistance of the whole systemic circulation; atrioventricular valves are held by tendinous cords and semilunar valves guard the arterial exits. Valves open and close passively according to pressure differences, and the cycle runs atrial systole, ventricular systole and diastole, with most ventricular filling passive. The heart is myogenic, with the sinoatrial node initiating excitation, the atrioventricular node delaying it so the atria empty fully, and the bundle of His and Purkyne tissue carrying it to the apex so contraction proceeds upwards. Rate is modified by the cardiovascular centre via sympathetic and vagus nerves, responding to baroreceptors and chemoreceptors, and cardiac output is stroke volume times heart rate. On an electrocardiogram the P wave is atrial depolarisation, the QRS ventricular depolarisation and the T wave repolarisation. Arteries have thick elastic walls and narrow lumens, capillaries are one cell thick for short diffusion distance, and veins have wide lumens and valves. Tissue fluid forms by ultrafiltration where hydrostatic pressure exceeds oncotic pressure, and returns at the venous end where the reverse holds, with the excess drained as lymph.