How are the heart and blood vessels structured to pump blood and meet the body's needs?
The structure and function of the heart and the cardiac cycle, the control of heart rate by the autonomic nervous system and the SAN, blood pressure and its measurement, and the structure and function of arteries, capillaries and veins.
An SQA Higher Human Biology answer on the cardiovascular system, covering the structure of the heart and the cardiac cycle, control of heart rate by the SAN and the autonomic nervous system, the measurement of blood pressure, and the structure and function of arteries, capillaries and veins.
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What this dot point is asking
The SQA wants you to describe the structure of the heart and the cardiac cycle, explain how heart rate is controlled by the SAN and the autonomic nervous system, describe how blood pressure is measured, and describe how the structure of arteries, capillaries and veins suits their functions.
The heart and the cardiac cycle
The heart is a muscular pump with four chambers: two atria that receive blood and two ventricles that pump it out. During the cycle, the atria contract first and push blood into the ventricles; the ventricles then contract and push blood out into the arteries, while the atria relax and refill. Valves between the atria and ventricles, and at the exits of the ventricles, ensure blood flows in one direction only and does not flow backwards.
Control of heart rate
Two opposing nerves fine-tune the rate:
- The sympathetic nerve releases noradrenaline, which increases the heart rate, for example during exercise or stress.
- The parasympathetic nerve (the vagus nerve) releases acetylcholine, which decreases the heart rate, for example at rest.
By balancing these two inputs, the medulla matches the heart rate to the body's needs.
Blood pressure
Blood pressure is highest in the arteries and is described by two numbers: the systolic pressure (when the ventricles contract) over the diastolic pressure (when the ventricles relax). It is measured using a sphygmomanometer. Persistently high blood pressure (hypertension) damages the artery walls over time and is a major risk factor for cardiovascular disease, which links this key area to CVD.
Arteries, capillaries and veins
Each type of blood vessel has a structure matched to its function:
- Arteries carry blood away from the heart at high pressure, so they have a thick, muscular and elastic wall that withstands and smooths the pressure surges, and a narrow lumen.
- Capillaries are the site of exchange. Their walls are only one cell thick, so oxygen, nutrients and wastes can pass easily between the blood and the tissues, and they form vast networks that bring blood close to every cell.
- Veins carry blood back to the heart at low pressure, so they have a thinner wall and a wide lumen. They contain valves that prevent the backflow of blood, helping return it against gravity.
Examples in context
Example 1. Heart rate during exercise. As you start to exercise, the medulla increases sympathetic activity, releasing noradrenaline that speeds the heart. The higher heart rate raises cardiac output, delivering more oxygenated blood to the working muscles.
Example 2. Why veins have valves. Blood in the veins of the legs must travel upward back to the heart against gravity at low pressure. The valves stop it falling back between contractions of the surrounding muscles, ensuring a steady return to the heart.
Try this
Q1. Name the region of the heart that acts as the pacemaker. [1 mark]
- Cue. The sinoatrial node (SAN).
Q2. Explain why capillary walls are only one cell thick. [1 mark]
- Cue. So that oxygen, nutrients and wastes can be exchanged quickly between the blood and the surrounding tissues.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20194 marksDescribe how the heart rate is controlled by the autonomic nervous system, including the role of the SAN.Show worked answer →
A 4-mark answer needs the pacemaker and the two opposing nerve effects.
The heartbeat is started by the sinoatrial node (SAN), a region of the right atrium that acts as the pacemaker. It sends out electrical impulses that set the basic rate at which the heart contracts.
The rate is then adjusted by the autonomic nervous system, controlled from the medulla. The sympathetic nerve releases noradrenaline, which speeds the heart rate up, for example during exercise. The parasympathetic nerve (the vagus nerve) releases acetylcholine, which slows the heart rate down, for example at rest.
Award (1) the SAN is the pacemaker that sets the rate, (2) control comes from the medulla via the autonomic nervous system, (3) sympathetic nerve and noradrenaline speed it up, and (4) parasympathetic nerve and acetylcholine slow it down.
SQA Higher 20213 marksCompare the structure of an artery and a vein, and relate each feature to its function.Show worked answer →
This is a 3-mark compare-and-relate question.
An artery carries blood away from the heart at high pressure, so it has a thick, muscular and elastic wall that withstands and smooths the pressure surges from the heartbeat, and a relatively narrow lumen.
A vein carries blood back to the heart at low pressure, so it has a thinner wall and a wider lumen. Veins contain valves that prevent the backflow of blood, helping return it to the heart against gravity.
Capillaries, by contrast, have walls only one cell thick to allow exchange of materials. Markers reward (1) artery thick muscular and elastic wall for high pressure, (2) vein thinner wall and wide lumen for low pressure, and (3) valves in veins to prevent backflow.
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