Edexcel IGCSE Biology 复习笔记 2.8.10 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

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

   

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

   

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 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 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

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

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