AQA Biology Unit 1.5. The Heart And Heart Disease

Covers the following topics:

  • 5.1 The structure of the heart
  • 5.2 The cardiac cycle
  • 5.3 Heart disease
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5.1 The Structure Of The Heart

Diagram of structure of the human heart:


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5.1 The Structure Of The Heart

The main structures include:

  • Left and right atrium, which are thin-walled and elastic and stretches to collect blood. Thin muscular wall as blood only has to be pumped a short distance to left and right ventricals so blood pressure not as high as in left and right ventricals.
  • Left ventrical (LV) and right ventricale (RV), have much thicker muscular walls as blood is pumped a longer distance. LV has a thicker muscular wall than RV as LV must pump blood to entire body and withstand the resistance of entire bodies capillary network. The RV withstands a smaller resistance (so lower blood pressure created) of lung capillary network so has a thinner muscular wall than LV.
  • Aorta connected to LV, and carries oxygenated blood to entire body except lungs.
  • Vena Cava is connected to right atrium and brings deoxygenated blood back from tissues of the body.
  • Pulmonary artey is connected to RV and carries deoxygenated blood to the lungs.
  • Pulmonary vein is connected to left atrium and brings oxygenated blood to the lungs where oxygen is replenished and carbon dioxide removed.
  • Left atrioventricular (bicuspid)  and Right atrioventricular (tricuspid) valves, two cupped shaped flaps on the left side of the heart and three cupped shaped flaps on right side between ventricals and atrium.
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5.2 The Cardiac Cycle

The cardiac cycle is the cycle of a single heartbeat involving diastole, atrial systole and ventricular systole.

Diastole (Relaxation of heart):

  • Blood returns to atria of heart via pulmonary vein and vena cava.
  • Left atria and right atria fill with blood causing pressure to rise, pushing open the atrioventricular valves open allowing the ventricals to fill with blood.
  • Muscular walls of atria and ventricals relaxed, pressure within ventricals therefore lower than aorta and pulmonary artery forcing shut the semi lunar valves in the aorta and pulmonary artery.

Atrial Systole (Contraction of the left and right atrium):

  • Muscle walls of left and right atrium contract, forcing the remaining blood into the ventricals due to an increase in pressure. Has a thin muscular wall due to short distance travelled by blood to LV and RV.
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5.2 The Cardiac Cycle

Ventricular Systole (Contraction of left and right ventrical):

  • After a short delay for ventricals to fill with blood, mucle walls contract simultaneously forcing shut the atrioventricular valves.
  • This causes pressure to rise within ventricals forcing open semi-lunar valves pushing blood into pulmonary artery and aorta.

Valves in the control of blood flow:

  • Atrioventricular valves shut to prevent backflow of blood from ventricals into atria when ventricals contract increasing ventricular pressure above atrial pressure so that blood only flows in one direction to aorta and pulmonary artery.
  • Semi-lunar valves in the aorta and pulmonary artery prevent backflow of blood intro ventricals when the recoil action of the elatic walls of these vessels create a greater pressure into the vessels than ventricals.
  • Pocket valves in veins throughout venous system to ensure that blood flows back to the heart rather than away from it.
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5.2 The Cardiac Cycle

Volume of blood pumped by one ventrical of heart in one minute = beats per minute x volume of blood pumped out per beat.

Cardiac Output = Heart Rate x Stroke volume

How is the cardiac cycle controlled?

  • SAN (Sinoatrial node) initiates each heart beat and acts as the hearts  pacemaker. Sends electricul impulses along nerves of left and right atrium to contract.
  • A layer of non conductive tissue (atrioventricular septum) delays electrical impulses crossing to the ventricals.
  • Allowed to pass via AVN (Atrioventricular node), so electricul impulses travel down bundle of hiss giving time for atral systole to allow ventricals to fill with blood before ventricular systole.
  • Electrical impulses reach ventricals when they are full of blood and cause ventricals to contract from the bottom up during ventricular systole.

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5.2 The Cardiac Cycle

Mammals have a Double Circulatory System: This means the blood goes through the heart twice per cycle, to the lungs via pulmonary loop, back to the heart and them back around the body via the systemic loop.

Advantages of a double circulatory system over a single circulatory system:

  • Can maintain a higher blood pressure so that oxygenated blood is able to travel rapidly around the body.
  • Keeps blood pressure higher as capillary networks reduce the blood pressure.
  • Higher blood pressure delivers oxygen and glucose to aerobically respiring cells.
  • This is useful for organisms with a high metabolism.
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5.3 Heart Disease

Coronary heart disease (CHD) occurs when bloodflow is impaired via the pair of coronory arteries supplying oxygen and glucose to heart mucle cells, which are essential for aerobic respiration and ATP production for mucle contraction.


  • Fatty deposits of LDL's form within endothelial wall of an artery.
  • These enlarge to for an atheromatous plaque.
  • These bulge reducing the diameter of lumen of artery.
  • This increases risk of thrombosis, aneurysm and mycoardial infarction.


  • Atheroma breaks through endothelial lining of blood vessel forming a rough surface that interrupts smooth flow of blood.
  • Can result in formation of a blood clot/thrombus.
  • Blocks blood vessel, so stops oxygen and glucose from reaching respiring tissue cells beyond blockage site. These cells die.
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5.3 Heart Disease


  • Atheromas that lead to formation of a thrombus also weaken the artery walls.
  • Weakened artery walls can swell to form a ballon-shaped, bloodfilled structure called an aneurysm.
  • These can burst leading to haemorrhage and internal bleeding.

Myocardial Infarction:

  • Formation of atheromas lead to blood clot and blockage of coronory artery.
  • This prevents glucose and oxygen from reaching aerobically respiring mucle cells of heart so these die, known as a heart attack.

Risk of CHD and smoking: smoking increases CHD risk, mainly due to carbon monoxide combining irreversibly to haemoglobin in red blood cells reducing delivery of oxygen to tissues. Heart must work harder to deliver same amount of oxygen so raises blood pressure. Nicotine stiumlates production of adrenline increasing heart rate and raises blood pressure so increases chance of CHD. Also makes red blood cells more sticky so a higher chance of blood clot/thrombosis so myocardial infarction.

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5.3 Heart Disease

High blood cholesterol increases chance of atheroma forming so increases chance of myocardial infarction. HDL's remove cholesterol from tissues and transport it to liver for excretion to help protect arteries from CHD. LDL's transport cholesterol from liver to tissues and artery walls so increase chance of development of atheroma and myocardial infarction.

Diet: high levels of salt raises blood pressure, high levels of saturated fats increases LDL levels and hence cholesterol blood concentration.

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Exactly what I need and was looking for! Perfect! Thank you very much :)

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