How do the heart, blood vessels and lungs respond to exercise and supply working muscles with oxygen?
The structure and function of the cardiovascular and respiratory systems, cardiac output and ventilation, the redistribution of blood, and the regulation of heart rate and breathing during exercise.
A focused WJEC A-Level PE answer on the cardiovascular and respiratory systems, covering cardiac output, stroke volume, heart rate, the vascular shunt, Starling's law, ventilation and gaseous exchange during exercise.
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What this dot point is asking
WJEC wants you to describe the structure and function of the cardiovascular and respiratory systems and explain how they respond to exercise. You need cardiac output as heart rate times stroke volume, the way each variable changes with intensity, Starling's law, the vascular shunt, and how breathing and gaseous exchange increase to supply oxygen and remove carbon dioxide.
The cardiovascular system and cardiac output
The heart is a double pump. The right side sends deoxygenated blood to the lungs (the pulmonary circuit) and the left side sends oxygenated blood to the body (the systemic circuit). The left ventricle has the thickest wall because it generates the highest pressure.
During exercise, cardiac output rises sharply. Heart rate increases roughly in line with intensity, from about 70 beats per minute at rest up to a maximum estimated as . Stroke volume also rises, because more forceful contractions empty the ventricle more completely, but it plateaus at around 40 to 60 per cent of maximal intensity. From that point onward, further increases in cardiac output come almost entirely from a rising heart rate.
Redistribution of blood: the vascular shunt
At rest, only about 15 to 20 per cent of cardiac output goes to skeletal muscle. During hard exercise this can exceed 80 per cent. The vascular shunt mechanism achieves this redistribution. The vasomotor centre in the medulla oblongata adjusts the diameter of arterioles and the pre-capillary sphincters:
- Vasodilation widens arterioles supplying active muscle, increasing blood flow there.
- Vasoconstriction narrows arterioles supplying inactive areas such as the gut and kidneys, reducing flow there.
This delivers more oxygen and glucose to working muscle, clears carbon dioxide and heat faster, and keeps aerobic respiration going.
The respiratory system and ventilation
Air travels down the trachea, bronchi and bronchioles to the alveoli, where gaseous exchange occurs. Inspiration at rest uses the diaphragm and external intercostal muscles; during exercise the sternocleidomastoid and pectoralis minor help force a deeper breath, and expiration becomes active using the internal intercostals and abdominals.
Gaseous exchange happens by diffusion down partial-pressure gradients. Oxygen diffuses from the alveoli (high partial pressure of oxygen) into the blood, and carbon dioxide diffuses the other way. At the muscles the gradients reverse: oxygen leaves the blood for the cells, and carbon dioxide enters the blood. The large alveolar surface area, thin walls and dense capillary network speed this exchange.
Examples in context
Example 1. Anticipatory rise. Just before a sprint, heart rate increases above resting levels because the sympathetic nervous system and the hormone adrenaline act on the sino-atrial node. This anticipatory rise primes the cardiovascular system before the muscles even contract, a favourite WJEC application of nervous and hormonal control.
Example 2. Bradycardia in endurance athletes. A trained marathon runner may have a resting heart rate below 50 beats per minute (bradycardia) because cardiac hypertrophy has increased stroke volume. The same cardiac output at rest is achieved with fewer, more powerful beats, illustrating long-term adaptation.
Try this
Q1. Define cardiac output and give the equation used to calculate it. [2 marks]
- Cue. The volume of blood pumped by the heart per minute; cardiac output equals heart rate multiplied by stroke volume.
Q2. Explain why stroke volume increases during exercise using Starling's law. [3 marks]
- Cue. Increased venous return stretches the ventricle walls more, increasing the force of contraction and so the volume ejected per beat.
Q3. Describe how gaseous exchange occurs at the alveoli during exercise. [4 marks]
- Cue. Oxygen diffuses from high partial pressure in the alveoli to lower in the blood; carbon dioxide diffuses the other way; a large surface area, thin walls and steep gradients (from greater demand) speed diffusion.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20194 marksExplain how cardiac output changes from rest to maximal exercise, referring to heart rate and stroke volume.Show worked answer →
Cardiac output (Q) is the volume of blood pumped by the heart per minute and equals heart rate multiplied by stroke volume.
At rest Q is about 5 litres per minute. During maximal exercise it can rise to 20 to 40 litres per minute in a trained athlete.
Heart rate rises in proportion to intensity up to maximum. Stroke volume also rises but plateaus at around 40 to 60 per cent of maximal effort, so the later increase in cardiac output comes mainly from rising heart rate.
Markers reward the equation, the rise in both variables, and the point that stroke volume plateaus while heart rate keeps climbing.
WJEC 20215 marksDescribe the vascular shunt mechanism and explain its importance during exercise.Show worked answer →
The vascular shunt is the redistribution of blood flow towards the working muscles and away from less active organs such as the gut during exercise.
It is controlled by the vasomotor centre in the medulla, which alters the diameter of arterioles and the action of pre-capillary sphincters.
Vasodilation widens the vessels supplying active muscle so more blood flows there, while vasoconstriction narrows vessels to inactive areas.
This is important because it delivers more oxygen and glucose to muscles and removes carbon dioxide and heat faster, raising the rate of aerobic respiration.
Markers reward redistribution, the vasomotor centre, vasodilation and vasoconstriction, and the link to oxygen delivery.
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