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IGCSE Biology 0610 · Chapter 8 notes · ~12 min read

Transport in animals

Single versus double circulation, a fully labelled heart, how the valves work, coronary heart disease, arteries vs veins vs capillaries, the liver and kidney vessels, blood cells and clotting.

Double circulationHeartCoronary heart diseaseBlood vesselsBlood components
ASby Aiza Ma'am
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • The circulatory system is a pump (the heart), a network of vessels and valves that keep blood moving one way. Arteries carry blood away from the heart; veins carry it back; capillaries link them and exchange materials with cells.
  • Blood = plasma (liquid) + red cells (carry oxygen) + white cells (fight disease) + platelets (help clotting).
  • Coronary heart disease happens when coronary arteries are blocked, so heart muscle gets too little oxygen. Risk factors: diet, stress, smoking, genes, age, gender.

By the end you can

  • Describe the circulatory system as a pump, vessels and valves giving one-way flow.
  • Identify the parts of the heart and the main vessels to and from the heart, lungs, liver and kidney.
  • Describe coronary heart disease, its risk factors and how diet and exercise reduce the risk.
  • Describe how the heart rate is monitored and how exercise changes it.
  • Describe the structure and function of arteries, veins and capillaries.
  • Name the components of blood and state the function of each.
  • Compare single and double circulation and explain the advantages of double. EXT
  • Explain how the valves, septum and thick ventricle walls work. EXT
  • Explain how blood clots, in terms of fibrinogen and fibrin. EXT

1The circulatory system

Why animals need a circulatory system

  • Large animals have too small a surface area to volume ratio for diffusion alone to supply every cell. Blood transports oxygen, glucose, CO₂, urea, hormones and heat around the body (diffusion and surface area).
  • It has three parts: a pump (the heart), vessels (arteries, veins, capillaries) and valves that make sure blood flows in one direction.
Memory trickArteries go Away from the heart. Veins Visit (go to) the heart.

Single and double circulationEXTENDED

SINGLE CIRCULATION (fish)DOUBLE CIRCULATION (mammal)HEARTGILLSBODYblood goes through the heartONCE per circuitpressure falls after the gills,so flow to the body is slowerLUNGSHEARTBODYto lungsbackbackto bodyblood goes through the heartTWICE per circuit: the heartre-boosts the pressureoxygenateddeoxygenated
EXT A fish heart pumps blood once per circuit. A mammal heart pumps it twice.
  • EXT Fish have a single circulation: heart → gills → body → heart. Blood loses pressure in the narrow gill capillaries, so it reaches the body slowly.
  • EXT Mammals have a double circulation: a low-pressure circuit to the lungs and a high-pressure circuit to the body. The blood returns to the heart to be pumped again, which boosts the pressure.
  • EXT Advantages of double circulation: blood reaches the body at high pressure (so it delivers oxygen and glucose faster, which a warm-blooded animal needs); the lung capillaries are not damaged because pressure there is lower; oxygenated and deoxygenated blood are kept separate.
LUNGS: CO₂ out, O₂ inBODY: O₂ out, CO₂ in(head, organs, legs)RARVLALVHEARTpulmonaryarterypulmonaryveinaortavenacavaoxygenateddeoxygenated
Follow one drop of blood round the whole circuit. It passes through the heart twice.
  • Pulmonary circuit (right side of the heart): right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium.
  • Body circuit (left side of the heart): left ventricle → aorta → body → vena cava → right atrium.
Common mistake"Arteries always carry oxygenated blood and veins always deoxygenated." The pulmonary artery carries deoxygenated blood (to the lungs) and the pulmonary vein carries oxygenated blood (to the heart). The rule is direction, not colour.

2The heart

Structure

vena cavapulmonary arteryright atriumright ventricletricuspidvalveseptumpulmonary veinaortaleft atriumcoronaryarterythick musclewallbicuspidvalveleft ventricle
The heart seen from the front. The left side of the heart is on the right of the picture.
PartJob
right atriumreceives deoxygenated blood from the body through the vena cava
right ventriclepumps deoxygenated blood to the lungs through the pulmonary artery
left atriumreceives oxygenated blood from the lungs through the pulmonary vein
left ventriclepumps oxygenated blood to the body through the aorta
septumthe muscular wall between the two sides; keeps the two kinds of blood apart
valvesmake sure blood flows one way only
coronary arteriessupply the heart muscle with oxygen and glucose
  • Atria (top chambers) receive blood. Ventricles (bottom chambers) pump it out.
  • EXT Why is the left ventricle wall thicker? It must pump blood at high pressure all round the body. The right ventricle only sends blood to the nearby lungs at lower pressure.
RVLVLEFT VENTRICLEthick muscle wall: pumps bloodat high pressure to theWHOLE BODYRIGHT VENTRICLEthinner wall: lower pressure,to the LUNGS only (a shorttrip; lung capillaries aredelicate)cross-section through the two ventricles
EXT Same heart, cut across: the left ventricle wall is much thicker.

How the valves workEXTENDED

1 ATRIA CONTRACTveinatriumventriclearteryAV valveSL valveatria squeezeblood into theventricles2 VENTRICLES CONTRACTAV valve slams shut:the “LUB” sound.blood forced outinto the artery3 HEART RELAXESSL valve snaps shut:the “DUB” sound.atria refill fromthe vein
EXT The three stages of one heartbeat.
  • EXT Atrioventricular (AV) valves sit between atria and ventricles. They shut when the ventricles contract, so blood cannot go back into the atria. This is the “lub” sound.
  • EXT Semilunar (SL) valves sit at the start of the aorta and pulmonary artery. They shut when the ventricles relax, so blood cannot fall back into the ventricles. This is the “dub” sound.
  • EXT Valves open because blood pushes them open and close because blood pushes them shut. They have no muscle.

Monitoring the heart

  • Pulse: the pressure wave in an artery (felt at the wrist or neck). Pulse rate = heart rate in beats per minute.
  • An ECG records the electrical activity of the heart. A stethoscope lets you hear the valves closing.
EXERCISE6010014018002468101214time (minutes)heart rate (beats / min)unfit person: slow recoveryfit person: quick recoveryresting rate
Heart rate before, during and after exercise for an unfit and a fit person.
  • During exercise the heart rate rises and breathing gets faster. After exercise it falls slowly back to the resting rate. The fitter you are, the lower your resting rate and the faster you recover.
  • EXT Why? Exercising muscles respire faster. They need more oxygen and glucose and make more CO₂. The brain detects the higher CO₂ and the hormone adrenaline is released, so the heart beats faster and harder to deliver and remove materials more quickly.
Worked example

A student counts 21 pulses in 15 seconds at rest and 38 pulses in 15 seconds after exercise. Find each heart rate and the percentage increase.

  1. rest: 21 × 4 = 84 beats/min
  2. after: 38 × 4 = 152 beats/min
  3. increase = (152 − 84) ÷ 84 × 100 = 81% (to the nearest whole number)
  4. because the muscles need more oxygen and glucose

Coronary heart disease

aortacoronaryarteriesblockagestarvedmuscleblocked coronary arteryWHAT GOES WRONG1 fat + cholesterol build up in the artery wall2 the lumen becomes narrower3 a clot can block it completely4 muscle starved of oxygen: cells die = HEART ATTACKHEALTHYblood flows freelyNARROWEDless blood gets throughBLOCKEDno blood gets throughfatty deposit
A blocked coronary artery starves part of the heart muscle of oxygen.

Coronary heart disease — a disease in which the coronary arteries are blocked or narrowed by fatty deposits, so the heart muscle gets too little oxygen and glucose.

  • Fatty deposits (cholesterol) build up in the artery wall, narrowing it. A blood clot can then block it completely. The muscle cells beyond the blockage cannot respire and may die: a heart attack.
Risk factorHow it raises the risk
diettoo much saturated fat raises cholesterol; too much salt raises blood pressure
stressraises blood pressure
smokingnicotine raises heart rate and blood pressure; carbon monoxide carries less oxygen; chemicals damage artery walls
genetic predispositionsome people inherit a higher risk
agerisk increases as arteries age
gendermales are generally at greater risk
  • Lower your risk: eat a balanced diet low in saturated fat and salt, take regular exercise (it strengthens the heart and helps control weight and blood pressure), do not smoke and manage stress.
Exam tipYou cannot change age, gender or genes. You can change diet, exercise, smoking and stress.

3Blood vessels

Arteries, veins and capillaries

ARTERYthick muscular,elastic wallnarrow lumenHIGH pressure (pulse)blood AWAY from heartVEINthin wall, wide lumenLOW pressurevalves stop backflowblood TO the heartCAPILLARY(drawn larger than life)wall ONE cell thicklumen so narrow thatred cells go single fileshort diffusion pathblood pulses witheach heartbeatvalves open one wayonly (towards the heart)red cells passin single filepressure: artery > capillary > vein
Three vessels, three jobs: cross-sections above, long sections below.
ArteryVeinCapillary
carries bloodaway from the heartto the heartbetween arteries and veins
wallthick, muscular and elasticthinone cell thick
lumennarrowwidevery narrow
pressurehigh (a pulse)lowfalls as blood passes
valvesnone (except at the heart)yesnone
  • EXT Artery: the thick muscular and elastic wall stretches as blood is forced in and recoils to keep the pressure up, so blood reaches the whole body.
  • EXT Vein: the blood is at low pressure, so a wide lumen gives little resistance and valves stop backflow. Muscles squeezing the veins help to push blood along.
  • Capillary: the wall is one cell thick and the lumen is just wide enough for one red cell, so oxygen, glucose, CO₂ and urea diffuse across a very short distance.
ARTERYarteriolesCAPILLARIESvenulesVEINO₂, glucoseout to cellsCO₂, ureain from cellsbody cellsHIGH pressureLOW pressureblood pressure falls steadily from artery to vein
Blood slows and loses pressure as it passes through the capillaries.

Vessels to and from the organs

HEARTLIVERGUT (small intestine)KIDNEYaortavena cavahepatic arteryhepatic veinto the guthepatic portal veinrenal arteryrenal veinliver: TWO bloodsupplies: artery(oxygen) and portalvein (digested food)
The main vessels linking the heart, liver, gut and kidney.
OrganVessel inVessel out
heartvena cava, pulmonary veinaorta, pulmonary artery
lungspulmonary arterypulmonary vein
kidneyrenal arteryrenal vein
EXT liverhepatic artery, hepatic portal veinhepatic vein
  • EXT The hepatic portal vein carries blood from the gut to the liver, rich in digested food such as glucose and amino acids. The liver then processes it (liver and excretion).

4Blood

What blood is made of

blood samplePLASMA (about 55%)WHITE CELLS and PLATELETS (<1%)RED CELLS (about 45%)a pale yellow liquid: water withdissolved glucose, amino acids,proteins, CO₂, urea, hormones,ions; carries heat around the bodywhite cells fight pathogens;platelets help clottingcarry oxygen (haemoglobin)
A centrifuge spins blood into layers: plasma on top, then white cells and platelets, then red cells.
ComponentWhat it does
plasmaliquid that transports blood cells, ions, glucose, amino acids, CO₂, urea, hormones and heat
red blood cellscarry oxygen using haemoglobin
white blood cellsfight pathogens
plateletshelp the blood to clot
RED BLOOD CELLbiconcave discno nucleusfull of haemoglobincarries oxygenPHAGOCYTElobed nucleusengulfs and digestspathogensLYMPHOCYTElarge round nucleusmakes antibodiesPLATELETScell fragmentshelp the bloodto clotRemember the sizes: red cells are the most NUMEROUS (roughly 600 for every white cell),white cells are the LARGEST, platelets are the smallest and are not whole cells.Red cells and platelets have NO nucleus. White cells do.
The four cell types in blood.
  • Red cells have no nucleus, which leaves more room for haemoglobin. The biconcave shape gives a large surface area for taking up oxygen. Haemoglobin + oxygen → oxyhaemoglobin in the lungs; it releases the oxygen again in the tissues.
  • White cells include phagocytes, which engulf and digest pathogens, and lymphocytes, which make antibodies (immunity).
  • EXT Platelets are fragments of cells; they are not whole cells.

ClottingEXTENDED

1 CUTblood escapes2 PLATELETSstick to the cut andrelease chemicals3 FIBRINmesh of insolublethreads traps redcells4 SCABclot dries; skinrepairs underneathsoluble FIBRINOGEN (a plasma protein) → insoluble FIBRIN threadsa clot seals the wound: it stops blood loss and keeps pathogens out(Extended) fibrinogen → fibrin is the key chemical step in clotting
EXT A clot forms when platelets and fibrin seal a wound.
  • EXT When a vessel is cut, platelets stick to the damaged area and a soluble plasma protein, fibrinogen, is changed into insoluble fibrin threads. The threads form a mesh that traps red cells and forms a clot.
  • EXT The clot stops blood loss and keeps pathogens out while the skin repairs. It dries into a scab.

5Test yourself

Test yourself — cover the answers first

Quick check

  1. Which type of blood vessel carries blood away from the heart?
  2. Name the vessel that carries oxygenated blood from the lungs to the heart.
  3. Which chamber of the heart pumps blood to the whole body?
  4. Name the vessels that supply the heart muscle with blood.
  5. State two risk factors for coronary heart disease.
  6. Which blood vessels have valves?
  7. What do red blood cells carry?
  8. What do platelets do?
  9. Name the vessel that carries blood from the gut to the liver.
  10. EXT Why is the wall of the left ventricle thicker than the right?
  11. EXT Name the protein that forms the clot.

Exam-style

  1. Describe the path of blood from the body to the lungs and back. [4]
  2. Describe how a capillary is adapted for exchange. [3]
  3. Explain why heart rate increases during exercise. [3]
  4. EXT Explain two advantages of a double circulation. [2]
  5. EXT Describe how a blood clot forms. [3]
Show answers
  1. artery
  2. pulmonary vein
  3. left ventricle
  4. coronary arteries
  5. e.g. smoking, diet high in saturated fat, stress, genes, age, gender
  6. veins
  7. oxygen (in haemoglobin)
  8. help the blood to clot
  9. hepatic portal vein
  10. it pumps blood at high pressure to the whole body
  11. fibrin (from fibrinogen)
  12. vena cava into right atrium
    right ventricle
    pulmonary artery to lungs
    pulmonary vein to left atrium
  13. wall one cell thick
    short diffusion distance
    very narrow so red cells pass slowly in single file
  14. muscles respire faster
    need more oxygen and glucose and produce more CO₂
    faster heart rate delivers these and removes CO₂ more quickly
  15. blood is pumped at high pressure to the body
    lung capillaries are not damaged because pressure there is lower
  16. platelets stick to the damaged vessel
    fibrinogen is changed to insoluble fibrin
    fibrin threads form a mesh that traps red cells
Practise this chapter
Heart & circulationBlood vesselsBlood
Next chapterDiseases, immunity & drugs →
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