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The Circulatory System in Animals — AQA A Level Biology 7402 Revision Notes & Topical Questions

10 min read 20 parts Lesson 33 of 48 Unit quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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The Mammalian Heart and Cardiac Cycle — AQA A LEVEL BIOLOGY 7402 notes: Describe the structure of the heart, the cardiac cycle and valve action.The mammalian circulatory system is a mass transport system ensuring that cells receive a constant supply of reactants for metabolism (s…

  • Atrial systole
  • Ventricular systole
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Atrioventricular valve

Section 1 of 20Mammalian circulation

The mammalian circulatory system is a mass transport system ensuring that cells receive a constant supply of reactants for metabolism (such as oxygen and glucose) and have waste removed.

Closed
It is a closed circulatory system, where blood flows inside enclosed vessels.
Open
In an open circulatory system (for example insects), blood is not contained within vessels but is pumped directly into body cavities.
Mammals have a closed, double circulatory system: blood passes through the heart twice per circuit (pulmonary circuit to lungs, systemic circuit to the body).
The pulmonary vein is the only vein that contains oxygenated blood.

Section 2 of 20Structure of the heart

The heart has four chambersthe two upper chambers are the left and right atria, and the two lower chambers are the left and right ventricles.

Structure of the heart
The septum separates the two halves.
When looking at a diagram, the right side of the heart appears on the left of the page because it is labelled as if in your body facing forward.
The thickness of the muscle wall depends on the pressure that must be generated: the left ventricle is thicker than the right ventricle, which is thicker than the atria.

Why is pressure different in the left atrium and left ventricle? The left atrium wall is thinner, so contraction is weaker; and the left atrium only needs to pump blood a short distance into the ventricle while the left ventricle pumps blood all around the body.

Section 3 of 20The cardiac cycle

The cardiac cycle is the series of events in one heart beat, including contraction and relaxation. Contraction is systole; relaxation is diastole.

1
Atrial systole
Atrial systole atria contract, pressure rises and blood is forced through the open atrioventricular (AV) valves into the ventricles.
2
Ventricular systole
Ventricular systole ventricles contract, pressure rises above that in the atria so the AV valves close (the first heart sound); when ventricular pressure exceeds arterial pressure the semilunar (SL) valves are forced open and blood is forced into the arteries and out of the heart.
  • The atria relax (atrial diastole) and begin to fill again.
3
Diastole
Diastole both ventricles and atria are relaxed; SL valves close as pressure in the arteries is higher (second heart sound); blood returns to the heart and flows passively into the atria and ventricles.

Section 4 of 20Valve action

Pressure changes cause the valves to open and close. Valves open when the pressure of blood behind them is greater than the pressure in front of them.

StageAV valvesSL valves
Atrial systoleOpenClosed
Ventricular systoleClosedOpen
DiastoleOpenClosed

Section 5 of 20Worked examples

  1. Explain why the wall of the left ventricle is thicker than that of the right ventricle. [2]2 marks

  2. Explain why the AV valves close at the start of ventricular systole. [2]2 marks

  3. State the only vein that carries oxygenated blood. [1]1 mark

Section 6 of 20Reading the cardiac cycle graph

Make sure you can point out the times on a cardiac cycle graph at which different valves open and close.

1
Point A
Point A end of diastole
.
  • The atrium has filled with blood during the preceding diastole;
    • pressure is higher in the atrium than in the ventricle, so the AV valve is open.
2
Atrial systole raises atrial pressure further, pushing the remaining blood into the ventricle.
3
Ventricular systole: ventricular pressure exceeds atrial pressure, the AV valve closes,
  • and when ventricular pressure exceeds the aortic pressure, the SL valve opens.
4
Diastole: ventricle relaxes and its pressure falls below the aortic pressure, closing the SL valve.
  • The internal volume of the left ventricle increases and ventricular pressure decreases; blood flows slowly through the newly opened AV valve into the ventricle,
    • causing a brief decrease in pressure in the left atrium.
  • Pressure in both atrium and ventricle then rises slowly as they continue to fill.

Section 7 of 20Calculating heart rate from a graph

Heart rate = 60 ÷ the time for one cardiac cycle (seconds). Example: three cardiac cycles occur in 3 seconds, so each lasts 1 s and the heart rate is 60 beats per minute.

Section 8 of 20Investigating heart rate

The effect of a named variable on heart rate can be investigated, for example the effect of exercise on human pulse rate or the effect of caffeine on the heart rate of the water flea, Daphnia. An activity such as a sit-up or bleep test can quantify different levels of exercise intensity.

Daphnia are small, transparent aquatic invertebrates whose internal organs can be viewed through a light microscope. Method:

1
Prepare five different concentrations of caffeine solution and a control solution of distilled water.
2
Place a Daphnia in a cavity slide with a small amount of solution and allow it to acclimatise.
3
Use a stopwatch to time 20 seconds and count the heart beats. The heart beat is very rapid, so make dots on a piece of paper.
4
Count the dots and express heart rate as beats per minute: multiply by three to convert beats per 20 seconds into beats per 60 seconds.
5
Repeat and calculate the mean; allow recovery time between concentrations and use a fresh animal where possible.

Ethical issuestreat animals with care, return them to water promptly, and use the minimum number.

Section 9 of 20Worked examples

  1. A graph shows a cardiac cycle lasting 0.8 s. Calculate heart rate. [2]2 marks

  2. Explain why Daphnia are suitable for this investigation. [2]2 marks

  3. A Daphnia heart beats 63 times in 20 seconds. Calculate heart rate. [1]1 mark

Section 10 of 20Cardiac output

Cardiac output is the volume of blood pumped by the heart per minute: cardiac output = heart rate × stroke volume.

1
It can be rearranged: heart rate = cardiac output ÷ stroke volume; stroke volume = cardiac output ÷ heart rate.
2
Example: cardiac output 4.3 dm3 min-1, heart rate 72 bpm.
3
Convert 4.3 dm3 to 4300 cm3.
4
Stroke volume = 4300 ÷ 72 = 59.7 cm3.

Section 11 of 20Blood vessels: structure and function

VesselStructureFunction
ArteryThick muscular, elastic wall; narrow lumenTransport blood away from the heart at high pressure; elastic recoil smooths flow
ArterioleLower proportion of elastic fibres, many muscle cellsMuscle contracts to adjust blood flow to specific organs, for example in exercise flow to the intestine is reduced while flow to muscles increases; pressure is lower than in arteries so elasticity is less essential
VeinThin wall with fewer smooth muscle and elastic fibres; much larger lumen; valvesBlood is not at high pressure so thick walls are not needed; the large lumen allows a high volume to flow; valves prevent backflow
CapillaryOne cell thick; tiny lumen about the diameter of a red blood cellExchange with cells; blood travels slowly, giving more time for diffusion

Arteries branch into narrower arterioles, which transport blood into capillaries.

Section 12 of 20Capillaries and tissue fluid

Capillaries form networks called capillary beds that function as exchange surfaces with cells.

Their features give effective exchange: very thin walls (short diffusion distance), large total surface area, and a small lumen that slows blood.
Tissue fluid
As blood passes through capillaries, plasma leaks out through gaps in the walls to surround the cells; this fluid is tissue fluid.
Its composition is very similar to plasma, but it has fewer large proteins, which are too large to leave the capillaries.
Exchange of substances between cells and blood occurs via the tissue fluid.

Formation and return of tissue fluid depend on a changing balance between the outward force of hydrostatic pressure and the inward pull exerted by dissolved solutes (low water potential in the blood due to plasma proteins).

Capillaries and tissue fluid
At the arteriolar end, hydrostatic pressure is higher,
  • so fluid leaves; at the venous end, the water potential of the blood is lower and water returns by osmosis.
Excess tissue fluid drains into lymph vessels.
High blood pressure can force extra fluid out of capillaries, leaving an increased volume of tissue fluid after osmosis.

Section 13 of 20Worked examples

  1. Calculate the cardiac output when stroke volume is 70 cm3 and heart rate is 75 bpm. [2]2 marks

  2. Explain why veins have valves but arteries do not. [2]2 marks

  3. Explain how tissue fluid forms. [3]3 marks

Section 14 of 20Cardiovascular disease

Cardiovascular disease (CVD) is any disease of the heart and blood vessels, for example coronary heart disease (CHD), in which blockage of the coronary arteries can lead to a heart attack. Risk factors include smoking, high blood pressure, high blood cholesterol, poor diet, lack of exercise, and genetics.

Section 15 of 20Interpreting data

Typical questions

  • (a) describe what the data show about the link between smoke exposure and CVD risk;
    • (b) state what can be concluded;
    • (c) comment on the validity of the data.

For (a), quote values showing how risk changes with exposure.
Correlation
For (b), it would not be correct to say that increased smoke exposure causes increased CVD risk; we can only state that there is a correlation (association) between the two variables.
For (c), no information is given on how other risk factors (age, diet, biological sex, exercise) may interact with smoking,
  • and no statistical tests were used so we cannot state the significance of differences between levels of exposure.

Section 16 of 20Conflicting evidence and quality of studies

Research may produce conflicting evidence on the role of risk factors in CVD; conflicting evidence shows a different pattern to evidence gained elsewhere.

Conflicting evidence and quality of studies
When evaluating a study ask: does it control additional variables, so that variables not being tested are taken into account by the design?
Is the study biased (are the lead scientists benefitting, for example financially, from the results)?
Is the sample large and representative?

Section 17 of 20Required practical: mammalian heart dissection

Dissecting a heart helps understand form and function by allowing observation of chambers, valves and associated vessels. Equipment: scalpel, scissors, forceps, dissecting board and a specimen such as a pig or sheep heart.

Required practical: mammalian heart dissection
Place the specimen on a dissection board with the front facing upwards.
  • The front can be identified by the coronary arteries, which cross the front of the heart between the left and right ventricles.
Identify the chambers and vessels (aorta, pulmonary artery, vena cava, pulmonary vein).
Make vertical cuts through the front of the left and right sides so that the muscle walls can be compared and connections between chambers and vessels located; the left ventricle wall is thicker.

Dead tissue may not accurately reflect the appearance or flexibility of living organs; for example, sometimes the atria and blood vessels are removed in butchering.

Section 18 of 20Worked examples

  1. A study finds that CVD is more common in people with high smoke exposure. State two limitations of concluding that smoke causes CVD. [2]2 marks

  2. Explain why the left ventricle is thicker than the right in a heart dissection. [2]2 marks

  3. Identify the vessels that supply the heart muscle. [1]1 mark

Section 19 of 20Key terms

TermMeaning
SystoleContraction phase of the heart
DiastoleRelaxation phase of the heart
Atrioventricular valveValve between atrium and ventricle
Semilunar valveValve at the base of the aorta or pulmonary artery
Closed circulationBlood contained in vessels
TermMeaning
Heart rateNumber of beats per minute
Cardiac cycleSequence of events in one heartbeat
DaphniaWater flea used to study heart rate
AcclimatiseAllow to adjust to experimental conditions
TermMeaning
Cardiac outputHeart rate multiplied by stroke volume
Stroke volumeVolume of blood pumped per beat
ArteryVessel carrying blood away from the heart
VeinVessel carrying blood towards the heart
CapillaryTiny vessel for exchange
Tissue fluidFluid surrounding cells formed from plasma
TermMeaning
Cardiovascular diseaseDisease of the heart and blood vessels
CHDCoronary heart disease
Coronary arteryArtery supplying the heart muscle
ValidityWhether data answer the question and are well controlled
BiasInfluence that distorts study results

Section 20 of 20Exam technique

Avoid these mistakes
Mixing up left and right when looking at a diagram.
Saying valves open and close because of muscle - they are passive, controlled by pressure.
Describing diastole as contraction.
Avoid these mistakes
Forgetting to multiply by 3 when converting 20 s counts.
Mislabelling the point at which the SL valves open.
Not including a control solution of distilled water.
Avoid these mistakes
Forgetting to convert dm3 to cm3 in stroke volume calculations.
Saying arteries carry oxygenated blood only - the pulmonary artery carries deoxygenated blood.
Saying tissue fluid contains red blood cells and large proteins.
Avoid these mistakes
Stating that correlation proves cause.
Ignoring sample size or statistical testing in validity.
Placing the heart upside down in dissection.

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