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The Circulatory System — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

30 min read 29 parts Lesson 2 of 23 Unit quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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The Circulatory System Edexcel A Level Biology SNAB notes: Explain why many animals need a heart and circulation, and why water is a suitable transport medium.Cardiac output = stroke volume × heart rateCO (cm3 min-1) = SV (cm3) × HR (beats min-1).Heart rate (beats min-1) = 60 ÷ d…

  • Body mass index
  • Water is a good transport medium
  • Cardiac cycle
  • Capillaries
  • Consequences of CVD
  • Required practical: Daphnia heart rate

Section 1 of 29The Circulatory System

Small organisms have a large surface area to volume ratio and short diffusion distances; larger organisms need a circulatory system to deliver oxygen and nutrients and remove wastes by mass flow.
Water is dipolar, so it forms hydrogen bonds and dissolves ionic and polar substances (glucose, amino acids, ions), making blood plasma an effective transport medium.
In a double circulation, blood returning from the lungs is re-pumped so it reaches the body at high pressure while the lungs receive lower-pressure blood that will not damage capillaries.
Cardiac cycle
Cardiac cycle during atrial systole, atria contract and top up the ventricles; during ventricular systole, AV (tricuspid/bicuspid) valves shut (first heart sound) and semilunar valves open; during diastole, semilunar valves shut (second heart sound) and chambers refill.
Arteries
Arteries thick elastic walls stretch during systole and recoil in diastole, maintaining pressure and smoothing flow; smooth muscle allows vasoconstriction.
Veins
Veins low pressure, wide lumen, valves; flow aided by contraction of surrounding skeletal muscles.
Capillaries
Capillaries one-cell-thick endothelium gives short diffusion distance; huge number gives large surface area; slow flow allows exchange.
Atherosclerosis develops as endothelial damage leads to inflammation, LDL cholesterol accumulation, plaque formation, narrowing and hardening of arteries.
Clotting
Clotting platelets and damaged tissue release thromboplastin; prothrombin → thrombin (needs Ca2+); fibrinogen → fibrin; fibrin mesh traps cells.
Consequences of CVD
Consequences of CVD angina and myocardial infarction (coronary arteries), stroke (arteries to brain), aneurysm (weakened, ballooning artery wall behind a blockage).
Risk is the probability of an unwanted event; people often misjudge risk, overestimating unfamiliar, dramatic risks and underestimating familiar, voluntary ones (e.g. smoking).
Treatments include antihypertensives (diuretics, ACE inhibitors, beta blockers, calcium channel blockers), statins (lower LDL), anticoagulants (warfarin) and platelet inhibitors (aspirin); each has side effects such as bleeding, muscle pain or dizziness.
Required practical
Daphnia heart rate
Place a Daphnia in a cavity slide with a set caffeine concentration, allow time to acclimatise,
  • and count heartbeats under low power for 15 s,
    • (e.g. making dots)
  • then convert to beats per minute.
  • Repeat with several individuals per concentration plus a water control; keep temperature and Daphnia size constant.
Daphnia ethics
Daphnia are transparent, so their heart is visible, and invertebrates are thought to feel less pain than vertebrates. They still cannot consent, so handle gently, keep observation short, avoid extreme concentrations or temperatures, and return them promptly to the culture.
Open vs closed circulation
In a closed system blood stays inside vessels; in an open system it bathes the organs in a body cavity. In a single circulation blood passes through the heart once per circuit (e.g. fish); mammals have a closed, double circulation.
Cohesion and adhesion
Hydrogen bonds attract water molecules to each other (cohesion) and to vessel walls (adhesion), so water flows as a continuous column. As a solvent, the δ+ and δ− regions of water surround ions and polar molecules, keeping them dissolved and free to react.
Heart structures
The septum keeps oxygenated and deoxygenated blood separate. Valve tendons (cords) anchor AV valves so they do not invert under high ventricular pressure. Coronary arteries on the heart surface supply cardiac muscle; the pericardium encloses and protects the heart.
Artery wall layers
Arteries have an inner smooth endothelium (folded to allow stretching), a thick middle layer of smooth muscle and elastic tissue, and an outer collagen layer. Arterioles constrict to redirect blood, e.g. away from the gut during exercise. Arterioles feed capillaries; venules drain into veins.
Capillary pores
Gaps between endothelial cells let plasma leak out to form tissue fluid and allow white blood cells to squeeze into infected tissue. Red cells pass in single file, slowing flow and giving more time for exchange.
Cardiac pressure graphs
On a pressure trace, valves change state where lines cross: ventricle above atrium closes the AV valve; ventricle above aorta opens the semilunar valve; ventricle falling below aorta closes it; atrium above ventricle reopens the AV valve. Most ventricular filling is passive during diastole.
Deep vein thrombosis
A clot forming in a deep vein, usually in the leg, is a DVT. Risk rises with long periods of immobility, old age and some medicines. Part of the clot may break off and travel to the lungs.

Section 2 of 29Process & sequence

1
Endothelium lining an artery is damaged, e.g. by high blood pressure or toxins from cigarette smoke.
2
An inflammatory response follows: white blood cells (macrophages) move into the artery wall and accumulate cholesterol from LDLs.
3
A fatty deposit (atheroma) builds up beneath the endothelium; calcium salts and fibrous tissue harden it into a plaque.
4
The lumen narrows and the wall loses elasticity, so blood pressure rises further (positive feedback) and flow reduces.
5
Plaque may rupture the endothelium, exposing collagen; platelets stick and the clotting cascade forms a thrombus.
6
The thrombus blocks the artery (or breaks off as an embolus); tissue beyond is starved of oxygen: myocardial infarction, stroke or aneurysm.

Route of blood through the heartwrite these steps in this order; each step is a marking point.

1
Deoxygenated blood enters the right atrium from the vena cavae.
2
It passes through the tricuspid (right AV) valve into the right ventricle.
3
Right ventricle pumps it through the pulmonary valve into the pulmonary artery to the lungs.
4
Oxygenated blood returns via pulmonary veins to the left atrium.
5
It passes through the bicuspid (mitral) valve into the left ventricle.
6
Left ventricle pumps it through the aortic valve into the aorta to the body.

Section 3 of 29Compare

Arteries, veins and capillaries
FeatureArteryVeinCapillary
Direction of flowAway from heartTowards heartBetween arterioles and venules
Blood pressureHigh, pulsatileLow, steadyFalling
WallThick; much elastic tissue and smooth muscle; collagenThin; little elastic tissue and muscle; collagenEndothelium only (one cell thick)
LumenNarrow relative to wallWideAbout one red blood cell wide (~7 µm)
ValvesNone (except at heart exits)Present, prevent backflowNone
AdaptationElastic fibres stretch and recoil to smooth flow and maintain pressureBlood squeezed by skeletal muscle; valves keep flow one wayShort diffusion distance, large total surface area

Collagen in arteries and veins resists overstretching and bursting.

Risk factors for cardiovascular disease (CVD)
Risk factorHow it raises risk
GeneticsInherited alleles can cause high blood cholesterol or high blood pressure
Age and sexArteries lose elasticity with age; males at greater risk (oestrogen partly protective before menopause)
High blood pressureExtra force damages endothelium, starting atherosclerosis
SmokingCarbon monoxide lowers oxygen carriage; nicotine raises heart rate and blood pressure and makes platelets stickier; chemicals damage endothelium
InactivityRaises blood pressure and risk of obesity
DietHigh salt raises blood pressure; high saturated fat raises LDL cholesterol

Section 4 of 29Worked examples

A person's stroke volume is 70 cm3 and one cardiac cycle takes 0.80 s. Calculate their cardiac output in dm3 min-1.
  • Heart rate = 60 ÷ 0.80 = 75 beats min-1.
    • Cardiac output = 70 × 75 = 5250 cm3 min-1.
    • Divide by 1000: 5.3 dm3 min-1 (2 s.f.).
186%
During exercise cardiac output rises to 15.0 dm3 min-1 and heart rate to 150 beats min-1. Calculate stroke volume and percentage increase in cardiac output from 5.25 dm3 min-1.
  • SV = CO ÷ HR = 15 000 cm3 ÷ 150 = 100 cm3. Percentage increase = (15.0 − 5.25) ÷ 5.25 × 100 = 186% (3 s.f.).
3.0
A study found 42 deaths from CVD per 1000 smokers and 14 per 1000 non-smokers. Calculate the relative risk.
  • Relative risk = risk in exposed ÷ risk in unexposed = (42/1000) ÷ (14/1000) = 3.0.
    • Smokers were three times as likely to die from CVD;
      • this is a correlation and does not by itself prove causation.

Section 5 of 29Keep going

The Circulatory System Edexcel A Level Biology SNAB: what to do next

These The Circulatory System Edexcel A Level Biology SNAB notes covered the key ideas, worked examples and exam technique for this topic. Check your The Circulatory System Edexcel A Level Biology SNAB understanding with the practice questions below, then reread any The Circulatory System Edexcel A Level Biology SNAB point you were unsure about before moving on.

The Circulatory System Edexcel A Level Biology SNAB: revision checklist

1
Revise the main The Circulatory System Edexcel A Level Biology SNAB definitions until you can write them from memory.
2
Practise The Circulatory System Edexcel A Level Biology SNAB exam questions under timed conditions.
3
Link each The Circulatory System Edexcel A Level Biology SNAB idea to a diagram or a worked example.
4
List the common The Circulatory System Edexcel A Level Biology SNAB mistakes and how to avoid them.
5
Explain one The Circulatory System Edexcel A Level Biology SNAB idea out loud as if teaching a friend.
6
Summarise The Circulatory System Edexcel A Level Biology SNAB in five key points on a revision card.
7
Check which command words appear most often in The Circulatory System Edexcel A Level Biology SNAB questions.
8
Come back to The Circulatory System Edexcel A Level Biology SNAB after a week for spaced practice.
9
Mark your answers to The Circulatory System Edexcel A Level Biology SNAB questions against the key ideas above.
1
Practice: The Circulatory System Edexcel A Level Biology SNAB practice questions
2
Previous lesson: 2.1 Gas Exchange, Cell Membranes & Transport — 9BI0 Notes
3
Next lesson: 2.2 Inheritance — 9BI0 Notes
4
Course: Edexcel A Level Biology A (snab) 9BI0 — all units
5
Official syllabus: Pearson Edexcel A Levels

Section 6 of 29Why a circulatory system is needed

The cells of all living organisms need a constant supply of reactants for metabolism, for example oxygen and glucose.

Why a circulatory system is needed
Single-celled organisms obtain them directly from their surroundings,
  • and the molecules diffuse quickly to all parts of the cell because the diffusion distances are short.
Larger organisms are made up of many layers of cells, so the time taken for oxygen and glucose to diffuse to every cell would be far too long.
They therefore need a transport system that moves substances quickly around the body.

Circulatory systems transport fluids containing materials the organism needs, as well as waste materials that must be removed. They can be open (blood flows freely around the body cavity) or closed (blood stays inside vessels).

Section 7 of 29Double circulation in mammals

Mammals have a double circulatory system

  • blood travels around the body in two separate circuits, the pulmonary circulation (heart to lungs and back) and the systemic circulation (heart to the rest of the body and back).
  • Blood passes through the heart twice in each complete circuit, which allows the blood to be pumped at high pressure to the body after it has lost pressure passing through the lungs.

Section 8 of 29The importance of water in transport

Water is the medium in which all metabolic reactions take place in cells and in which all substances are transported around the body. A water molecule is made of hydrogen and oxygen atoms. Because of its polarity (a dipole), hydrogen bonds form between adjacent water molecules, and these bonds give water many properties that make it important to living organisms.

Solvent
Solvent many ions, for example sodium and chloride ions, and polar covalently bonded substances such as glucose dissolve in water.
  • Water molecules surround charged particlesthe positive parts of water are attracted to negatively charged particles and the negative parts to positively charged ones.
Cohesion and adhesion
Cohesion and adhesion water molecules stick to one another, and water adheres to the sides of a vessel,
  • which helps it flow through narrow tubes as a continuous column.
Transport
Transport the dipole nature of water makes it good at transporting substances.

Section 9 of 29Mammalian heart: structure and function

The human heart is adapted for pumping blood around the body.

Mammalian heart: structure and function
It is a muscular organ, and the heart muscle itself is supplied with oxygen and glucose by the coronary arteries.
It has four chambers: two thin-walled atria that receive blood and two thick-walled ventricles that pump it out.
The wall of the left ventricle is thicker than that of the right because it pumps blood at higher pressure around the whole body,
  • whereas the right ventricle pumps only to the nearby lungs.

Section 10 of 29Worked examples

  1. Explain why a single-celled organism does not need a circulatory system but a mammal does. [3]3 marks

  2. State what is meant by a double circulatory system. [2]2 marks

  3. Explain why water is a good solvent for transport. [3]3 marks

  4. Why is the left ventricle wall thicker than the right? [2]2 marks

Section 11 of 29Valves and vessels of the heart

The heart is a muscular organ pumping blood through a series of major vessels.

Valves and vessels of the heart
Valves keep blood flowing forward and maintain the correct pressure in the chambers.
The right atrium and right ventricle are separated by an atrioventricular (AV) valve, the tricuspid valve (remember that the right side has the tricuspid valve),
  • and the left side has the bicuspid valve.
Semilunar valves are found at the exits to the aorta and pulmonary artery.
VesselDirectionCarries blood
Vena cavaInto the heartFrom the body (deoxygenated)
Pulmonary veinInto the heartFrom the lungs (oxygenated)
Pulmonary arteryAway from the heartTo the lungs (deoxygenated)
AortaAway from the heartTo the body (oxygenated)

Section 12 of 29Blood vessels: types

The body contains different types of blood vessel, which form a continuous network. Arteries transport blood away from the heart, usually at high pressure, to the tissues. Veins return blood to the heart at lower pressure, and capillaries are the exchange vessels between them. The structure of each vessel allows it to carry out its function.

Section 13 of 29Structure of an artery wall

An artery wall has three layers.

1
The inner layer is the endothelium, sometimes called the tunica intima, a thin single layer of cells that gives a smooth surface for blood flow.
2
The middle layer (tunica media) has smooth muscle and elastic tissue.
3
Smooth muscle can contract to narrow the vessel,
  • which is useful for diverting blood flow away from certain locations, for example away from the digestive system during exercise.
4
The elastic tissue helps to maintain blood pressure: it stretches as the ventricle pumps and recoils afterwards, evening out fluctuations in pressure.
5
The outer wall (tunica adventitia or tunica externa) contains the structural protein collagen, which provides strength to resist pressure.

Section 14 of 29Veins and capillaries

FeatureArteryVeinCapillary
WallThick layer of elastic and muscle tissueThin layer of elastic and muscle tissueOne cell thick, permeable
LumenNarrowWideVery narrow
ValvesNo (except semilunar at exit of heart)Yes, to prevent backflowNo

Capillary walls are thin and permeable, allowing substances to leave the blood and reach the tissues.

Veins and capillaries
They can form networks called capillary beds, which are important exchange surfaces.
The cells of the wall have gaps (pores) that allow blood plasma to leak out and form tissue fluid.
White blood cells can fight infection in affected tissues by squeezing through the pores in the capillary walls.

For explain questions, pair each structural feature with its effect. For example: capillary walls are one cell thick, which enables quick diffusion of substances because of the short diffusion distance. Remember that muscle tissue contracts and elastic tissue recoils.

Section 15 of 29Worked examples

  1. Name the four vessels that enter or leave the heart. [4]4 marks

  2. Explain the function of elastic tissue in the artery wall. [2]2 marks

  3. Explain two ways in which a capillary is adapted for exchange. [4]4 marks

  4. Why does muscle in an arteriole wall help during exercise? [2]2 marks

Section 16 of 29Stages of the cardiac cycle

The cardiac cycle is the series of events in one heartbeat, including muscle contraction and relaxation. Contraction of the heart is called systole and relaxation is called diastole. One cycle is followed immediately by another in a continuous process.

1
Atrial systole
Atrial systole during the preceding diastole the atria fill with blood.
  • The atria contract, atrial pressure rises above ventricular pressure, the AV valves are open, and blood is forced into the ventricles.
2
Ventricular systole
Ventricular systole the walls of the ventricles contract, ventricular volume decreases and the pressure in the ventricles rises above that in the atria.
  • This forces the AV valves to close, preventing backflow.
  • When ventricular pressure exceeds that in the aorta and pulmonary artery, the semilunar valves open and blood is ejected.
3
Diastole
Diastole the ventricles relax, pressure falls below that in the arteries, the semilunar valves close, and the atria refill with blood from the veins.

Section 17 of 29Interpreting the cardiac cycle graph

The pressure changes of the cardiac cycle can be analysed from a graph.

Interpreting the cardiac cycle graph
At the start, pressure in the atrium is higher than in the ventricle, so the AV valve is open (point A to B is atrial systole).
The left atrium contracts, increasing atrial pressure and forcing blood into the left ventricle,
  • so ventricular pressure increases slightly as it fills with blood.
Pressure in both chambers then rises slowly as they continue to fill.

Exam questions commonly ask about pressure and volume changes, when valves open and close, and when blood flows into or out of particular chambers. Valves open when the pressure behind them is greater than the pressure in front, and close when it is lower.

Section 18 of 29Calculating heart rate from a graph

Step 1: work out the length of one heartbeat (the time between two identical points, e.g. two consecutive peaks). Step 2: heart rate in beats per minute = 60 divided by the cycle time in seconds. For example, if one cycle lasts 0.8 s, heart rate = 60 / 0.8 = 75 beats per minute.

Section 19 of 29Investigating heart rate

Many factors influence heart rate.

Investigating heart rate
Experiments can be designed to test a named variable using invertebrates such as Daphnia, whose internal organs can be viewed through a light microscope,
  • as well as vertebrates such as humans.
In the Daphnia investigation, prepare five concentrations of caffeine solution and a distilled-water control; place a Daphnia in each and count heart beats under the microscope for a fixed time.
Avoid extreme ranges of the variable being tested, for example extremes of temperature or strong caffeine solutions.

Section 20 of 29Worked examples

  1. A cardiac cycle on a graph lasts 0.6 s. Calculate the heart rate. [2]2 marks

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

  3. Name the term for contraction and relaxation of the heart. [2]2 marks

  4. Why should extreme caffeine concentrations be avoided? [1]1 mark

Section 21 of 29Measuring Daphnia heart rate

The heart of Daphnia is located on the dorsal side just above the gut and in front of the brood pouch.

Measuring Daphnia heart rate
Use a stopwatch to time 20 seconds and count the number of beats; because the heart beat is very rapid, count by making dots on a piece of paper.
Count the dots, then multiply by three to convert beats per 20 seconds into beats per minute.
Avoid extremes of the variable being tested, such as extremes of temperature or strong caffeine solutions.

Section 22 of 29Atherosclerosis

There are a number of diseases of the heart and blood vessels (cardiovascular diseases).

1
Atherosclerosis,
  • or hardening of the arteries, is caused primarily by damage to the delicate endothelium of an artery followed by an inflammatory response.
2
It is a progressive disease, which means it can worsen over time.
3
White blood cells migrate to the damaged site and take up lipids, particularly cholesterol, forming atheroma plaques in the artery wall.
4
The plaques narrow the lumen and make the wall less elastic, which raises blood pressure.

Section 23 of 29Blood clotting (thrombosis)

Blood clotting prevents excess blood loss and the entry of pathogens, and provides a barrier or scab under which healing can occur.

Blood clotting (thrombosis)
A break in the skin or mucous membrane releases molecules that trigger a cascade of chemical reactions, ending with soluble fibrinogen converted into insoluble fibrin that forms a mesh.
The process is called thrombosis.
Atheromas increase the risk of clotting: the plaque deposit can rupture through the endothelium, damaging it and leaving a rough surface on which clots can form.

Section 24 of 29Consequences: heart attack, stroke and DVT

1Heart attack
Heart attack an atheroma and clot in the coronary arteries restrict blood flow to the heart muscle,
  • which is starved of oxygen and glucose and may die.
2Stroke
Stroke a sudden loss of brain function in a localised area due to disruption of blood flow to the brain.
  • A clot blocks the arteries supplying the brain.
3Deep vein thrombosis (DVT)
Deep vein thrombosis (DVT) a clot forming in a vein deep inside the body, most commonly in the veins of the legs.

Section 25 of 29Preventing atherosclerosis and clots

The risk of atherosclerosis is reduced by not smoking, eating less saturated fat, exercising regularly and controlling blood pressure. These reduce damage to the endothelium and the build-up of cholesterol. People at risk may be given statins, anticoagulants or platelet inhibitors to reduce the risk of clots forming on atheromas.

Section 26 of 29Worked examples

  1. Describe how an atheroma forms. [4]4 marks

  2. Explain how atherosclerosis can cause a heart attack. [3]3 marks

  3. A Daphnia heart beat 52 times in 20 s. Calculate the beats per minute. [1]1 mark

  4. Define stroke. [2]2 marks

Section 27 of 29Key terms

TermMeaning
Circulatory systemSystem that transports materials around the body
Double circulationBlood passes through the heart twice per circuit
Hydrogen bondWeak attraction between slightly positive hydrogen and slightly negative oxygen in neighbouring water molecules
PolarHaving uneven charge distribution
SolventLiquid that dissolves a solute
Coronary arteryArtery supplying the heart muscle
TermMeaning
Atrioventricular valveValve between atrium and ventricle
EndotheliumThin inner lining of a blood vessel
Tunica mediaMiddle layer with smooth muscle and elastic tissue
Tunica adventitiaOuter layer containing collagen
Capillary bedNetwork of capillaries forming an exchange surface
Tissue fluidPlasma that has leaked out of capillaries
TermMeaning
Cardiac cycleSequence of events in one heartbeat
SystoleContraction of the heart
DiastoleRelaxation of the heart
Atrial systoleContraction of the atria
DaphniaSmall crustacean with a visible heart, used in heart rate experiments
Heart rateNumber of heart beats per minute
TermMeaning
AtherosclerosisBuild-up of plaque in artery walls
AtheromaPlaque deposit in the endothelium
ThrombosisFormation of a blood clot
StrokeLoss of brain function caused by blocked blood supply
Heart attackDeath of heart muscle caused by blocked coronary supply
Deep vein thrombosisClot in a deep vein, usually in the legs

Section 28 of 29Command terms

Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.

Section 29 of 29Exam technique

1
Saying all diffusion is slow - over short distances it is fast.
2
Confusing surface area with surface area to volume ratio.
3
Saying hydrogen bonds are between hydrogen and oxygen atoms in the same molecule - they form between neighbouring molecules.
4
Saying the heart muscle is supplied by blood in its chambers - it is supplied by the coronary arteries.
1
Saying veins carry deoxygenated blood only - the pulmonary vein carries oxygenated blood.
2
Saying the elastic tissue contracts.
3
Saying capillaries have thick walls.
4
Giving structure without the effect on function in explain questions.
1
Saying valves open and close because of muscle in the valves - they are passive and move due to pressure differences.
2
Mixing up systole and diastole.
3
Reading heart rate directly in seconds, without converting to beats per minute.
4
Saying blood flows from atria to ventricles only because of atrial contraction - most flows passively during diastole.
1
Saying atherosclerosis is caused by clots - clots may be a consequence.
2
Saying a stroke is in the heart.
3
Forgetting to multiply by three to convert 20 s to a minute.
4
Saying DVT occurs in arteries.

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