IB Biology SL

Revision Notes

6.2.6 Skills: The Blood System

Mammalian Heart Structure

Heart structure

  • The human heart has a mass of around 300g and is roughly the size of a closed fist
  • The heart is a hollow, muscular organ located in the chest cavity
  • It is protected in the chest cavity by the pericardium, a tough and fibrous sac

The Human Heart, downloadable AS & A Level Biology revision notes

The human heart has four chambers and is separated into two halves by the septum

  • The heart is divided into four chambers. The two top chambers are atria and the bottom two chambers are ventricles
  • The left and right sides of the heart are separated by a wall of muscular tissue, called the septum. The portion of the septum which separates the left and right atria is called the interatrial septum, while the portion of the septum which separates the left and right ventricles is called the interventricular septum
  • The septum is very important for ensuring blood doesn’t mix between the left and right sides of the heart

Valves in the heart

  • Valves in the heart:
    • Open when the pressure of blood behind them is greater than the pressure in front of them
    • Close when the pressure of blood in front of them is greater than the pressure behind them
  • Valves are important for keeping blood flowing forward in the right direction and stopping it flowing backwards. They are also important for maintaining the correct pressure in the chambers of the heart
  • The right atrium and right ventricle are separated by the atrioventricular valve, which is otherwise known as the tricuspid valve
  • The right ventricle and the pulmonary artery are separated by the pulmonary valve
  • The left atrium and left ventricle are separated by the mitral valve, which is otherwise known as the bicuspid valve
  • The left ventricle and aorta are separated by the aortic valve
  • There are two blood vessels bringing blood to the heart; the vena cava and pulmonary vein
  • There are two blood vessels taking blood away from the heart; the pulmonary artery and aorta

Coronary arteries

  • The heart is a muscle and so requires its own blood supply for aerobic respiration
  • The heart receives blood through arteries on its surface, called coronary arteries
  • It’s important that these arteries remain clear of plaques, as this could lead to angina or a heart attack (myocardial infarction)

The Outside of the Heart, downloadable AS & A Level Biology revision notes

The coronary arteries cover the outside of the heart, supplying it with oxygenated blood

Exam Tip

When looking at the heart, remember the right side of the heart will appear on the page as being on the left. This is because the heart is labelled as if it were in your body and flipped around.

The Walls of the Heart

  • The muscular walls of the atria are thinner than those of the ventricles
  • When the atria contract, the thin muscular walls do not generate much pressure, but enough to force blood down into the ventricles, through the atrioventricular valves
  • In contrast, the walls of the ventricles are thicker and more muscular
  • Following contraction of the atria, the ventricles contract and squeeze blood inwards, increasing its pressure and pushing it out of the heart through right and left semilunar valves

Left & right ventricle

  • The muscle of the left ventricle is significantly thicker than the right ventricle
  • This is because the blood leaving the right ventricle travels less distance than blood leaving the left ventricle
  • The blood pumped out from the right ventricle travels to the lungs, whereas blood leaving the left ventricle has to travel to the rest of the body to deliver oxygen for respiration.
  • To reach the rest of the body, the blood leaving the left ventricle must be under high pressure
  • This is generated by the contraction of the muscular walls of the left ventricle
  • The right ventricle generates less pressure from the contraction of its thinner walls, as blood only has to reach the lungs

Worked Example

A graph showing pressures changes in the heart is shown below

Pressure in Left Atrium & Ventricle, downloadable AS & A Level Biology revision notes

Explain the differences in pressure between left atrium and ventricle in the graph

The walls of the left atrium are thin, so the pressure generated by their contraction is low. Low pressure is sufficient because blood is forced only a short distance; from the left atrium down to the left ventricle. The muscular walls of the left ventricle are much thicker and generate much higher pressure, as shown on the graph. This is because much more pressure is generated when this thick muscle contracts and squeezes blood with enough force to reach the rest of the body.

Structure & Function of the Heart

  • The heart organ is a double pump
    • Oxygenated blood from the lungs enters the left side of the heart and is pumped to the rest of the body (the systemic circuit)
      • The left ventricle has a thicker muscle wall than the right ventricle as it has to pump blood at high pressure around the entire body,
    • Deoxygenated blood from the body enters the right side of the heart and is pumped to the lungs (the pulmonary circuit)
      • The right ventricle is pumping blood at lower pressure to the lungs
    • A muscle wall called the septum separates the two sides of the heart
  • Blood is pumped towards the heart in veins and away from the heart in arteries
  • The coronary arteries supply the cardiac muscle tissue of the heart with oxygenated blood
    • As the heart is a muscle it needs a constant supply of oxygen (and glucose) for aerobic respiration to release energy to allow continued muscle contraction
  • Valves are present to prevent blood flowing backwards

Structure of the heart, IGCSE & GCSE Biology revision notes

Structure of the Heart

The pathway of blood through the heart

  • Deoxygenated blood coming from the body flows through the vena cava and into the right atrium
  • The atrium contracts and the blood is forced through the tricuspid (atrioventricular) valve into the right ventricle
  • The ventricle contracts and the blood is pushed through the semilunar valve into the pulmonary artery
  • The blood travels to the lungs and moves through the capillaries past the alveoli where gas exchange takes place
    • Low pressure blood flow on this side of the heart prevents damage to the capillaries in the lungs
  • Oxygenated blood returns via the pulmonary vein to the left atrium
  • The atrium contracts and forces the blood through the bicuspid (atrioventricular) valve into the left ventricle
  • The ventricle contracts and the blood is forced through the semilunar valve and out through the aorta
    • Thicker muscle walls of the left ventricle produce a high enough pressure for the blood to travel around the whole body

Exam Tip

Remember : Arteries carry blood Away from the heart

When explaining the route through the heart we usually describe it as one continuous pathway with only one atrium or ventricle being discussed at a time, but remember that in reality, both atria contract at the same time and both ventricles contract at the same time

Also, the heart is labelled as if it was in the chest so the left side of a diagram is actually the right hand side and vice versa

Heart Rate & Exercise

Control of the heart rate

  • A heart rate is measured by counting the number of times a heart beats in a minute (bpm)
  • The natural resting heart rate is controlled by a group of cells located in the right atrium called the pacemaker
    • The role of the pacemaker is to coordinate the contraction of the heart muscle and regulate the heart rate
    • Pacemaker cells send out electrical impulses which initiate a contraction in the cardiac muscle
  • Other factors can also influence the heart rate, such as the  hormone adrenaline

Exercise and heart rate

  • The heart pumps blood around the body in order to supply oxygen and glucose to respiring cells
  • The blood also removes waste products from the respiring cells
  • During exercise, the cells of the muscles respire more rapidly in order to provide energy for muscle contraction
    • Respiration may be aerobic if exercise is moderate, or anaerobic is exercise is more intense
  • An increase in respiration means an increase in requirement for oxygen and glucose as well as an increase in production of waste products that need to be removed
  • The nervous system responds to this requirement by stimulating the following changes
    • Heart rate increases to deliver oxygen and glucose and remove waste more frequently
    • The volume of blood pumped out of the heart also increases to deliver bigger quantities of oxygen and glucose
  • Production of the hormone adrenaline increases heart rate as part of a ‘fight or flight’ response
  • At the end of a period of exercise, the heart rate may remain high for a period of time as oxygen is required in the muscles to break down the lactic acid from anaerobic respiration
    • This is how the oxygen debt is paid off
  • The time taken for the heart rate to return to the resting rate is called the recovery time
    • A physically fit person will have a lower resting heart rate and a shorter recovery time compared to an unfit person

Heart Disease

Causes of coronary heart disease

  • Like all cells in the body, cardiac muscle cells need a supply of blood to deliver oxygen and glucose and to remove waste products such as carbon dioxide
  • The blood is supplied to the heart by the coronary arteries which branch off directly from the aorta
    • The heart needs to constantly respire, so it is vital that it receives oxygen

The coronary arteries, IGCSE & GCSE Biology revision notes

The coronary arteries supply the heart with oxygenated blood

  • In coronary heart disease (CHD), layers of fatty material (plaque) build up inside the coronary arteries
  • These fatty deposits are mainly formed from cholesterol
  • There are two sources of cholesterol in the body:
    • Dietary cholesterol (from animal products eaten)
    • Cholesterol synthesised by the liver

Buildup of plaque in the coronary arteries, IGCSE & GCSE Biology revision notes

Buildup of plaque in the coronary arteries narrows the lumen

  • If a coronary artery becomes partially or completely blocked by these fatty deposits, it loses its elasticity and cannot stretch to accommodate the blood which is being forced through every time the heart contracts
  • The flow of blood through the arteries is reduced, resulting in a lack of oxygen for the heart muscle
    • Partial blockage of the coronary arteries creates a restricted blood flow to the cardiac muscle cells and results in severe chest pains called angina
    • Complete blockage means cells in that area of the heart will not be able to respire aerobically, leading to a heart attack
  • Treatment of CHD involves either increasing the width of the lumen of the coronary arteries using a stent, or prescribing statins to lower blood cholesterol

Effect of narrowing of arteries, IGCSE & GCSE Biology revision notes

The effect of a narrowed lumen in a coronary artery is reduced blood flow to the heart

Risk factors of coronary heart disease

  • There are several risk factors which will increase the chances of coronary heart disease:
  • Obesity
    • Carrying extra weight puts a strain on the heart
    • Increased weight can lead to diabetes which further damages your blood vessels
  • High blood pressure
    • This increases the force of the blood against the artery walls and consequently leads to damage of the vessels
  • High cholesterol
    • Speeds up the build up of fatty plaques in the arteries leading to blockages
  • Smoking
    • Chemicals in smoke cause an increase in plaque build up and an increase in blood pressure
    • Carbon monoxide also reduces the oxygen carrying capacity of the red blood cells

Arteries, Arterioles & Veins

  • The body contains several different types of blood vessel:
    • Arteries: transport blood away from the heart (usually at high pressure)
    • Veins: transport blood to the heart (usually at low pressure)
    • Arterioles: arteries branch into narrower blood vessels called arterioles which transport blood into capillaries
  • The walls of each type of blood vessel have a structure that relates to the function of the vessel
  • Blood flows through the lumen of a blood vessel; the size of the lumen varies depending on the type of blood vessel (with arteries having a narrow lumen, and the veins a wider one)

Comparing arteries and veins, IGCSE & GCSE Biology revision notes

The blood vessels form a continuous network; the structure of each allows it to carry out its function.

How structure relates to function

  • Arteries must be able to withstand high pressures generated by the contracting heart, and maintain these pressures when the heart is relaxed
    • The wall of the artery is relatively thick with layers of collagen, smooth muscle and elastic fibres
    • The elastic fibres allow the artery wall to expand around blood surging through at high pressure when the heart contracts, these fibres then recoil when the heart relaxes – this alongside a narrow lumen maintains high blood pressure
  • In contrast, veins receive blood that has passed through capillary networks; blood is at very low pressure and must be returned to the heart
    • The wall of the vein is relatively thin with thinner layers of collagen, smooth muscle and elastic fibres
    • The lumen of the vein is much larger than that of an artery
    • Veins contain valves that prevent the backflow of blood, helping return blood to the heart
  • Arterioles can contract and partially cut off blood flow to specific organs
    • Eg. During exercise blood flow to the stomach and intestine is reduced which allows for more blood to reach the muscles
    • Unlike arteries, arterioles have a lower proportion of elastic fibres and a large number of muscle cells
    • The presence of muscle cells allows them to contract and close their lumen to stop blood flow

Exam Tip

For “Explain” questions, remember to pair a description of a structural feature to an explanation of how it helps the blood vessel to function. For example, “Capillaries are one-cell thick, which enables quick and efficient diffusion of substances.”


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