Cardiovascular system

• Cardiovascular system

  The key components are: blood, the blood vessels (arteries, veins and capillaries), and the heart
• Main blood components


  • Blood cells
  • Platelets
  • Plasma
• Blood components

  • Have two main functions: transport and defence against disease
• Main types of blood vessels

  • Arteries
  • Veins
  • Capillaries
• Arteries

  Carry blood away from the heart, have thick walls to withstand high pressure
• Capillaries

  Microscopic vessels with one cell thick walls, allow exchange between blood and tissues
• Veins

  Carry blood back to the heart, have thin walls due to low pressure
• Heart

  The organ that pumps blood through the body, has a double circulation (pulmonary and systemic)
• Heart chambers

  • Atria have thin walls to receive blood, ventricles have thick walls to pump blood out
• Heart valves

  Prevent backflow of blood, include atrioventricular valves and semilunar valves
• Atrioventricular valves

  • Anchored by papillary muscles and chordae tendinae to prevent turning inside out
• Semilunar valves

  Open when pressure in arteries exceeds pressure in ventricles, close to prevent backflow
• Major blood vessels entering/leaving the heart

  • Aorta
  • Pulmonary artery
  • Venae cavae
  • Pulmonary vein
• The pulmonary artery and pulmonary vein are unusual in that the artery carries deoxygenated blood and the vein carries oxygenated blood
• Cardiac cycle

  1. Diastole (heart relaxed)

  2. Atrial systole (atria contract)

  3. Ventricular systole (ventricles contract)
• The cardiac cycle normally occurs 60-80 times per minute in a human heart
• Atrial systole

  Increases atrial pressure, opens atrioventricular valves, keeps semilunar valves closed
• Ventricular systole

  Increases ventricular pressure, closes atrioventricular valves, opens semilunar valves
• Pressure changes in the cardiac cycle

  1. Atrial walls contract (atrial systole), increasing atrial pressure

  2. AV values are open (as atrial pressure > ventricle pressure) and semilunar valves remain closed (as aortic pressure > ventricular pressure)

  3. Atrial contraction is complete (atria are empty of blood) and ventricles begin to contract (start of ventricular systole)

  4. Ventricular pressure > atrial pressure, which leads to AV valves closing (first heart sounds)

  5. Continued contraction of the ventricles to the extent that ventricle pressure > arterial pressure resulting in the semilunar valves opening

  6. Arterial pressure increases so that arterial pressure > ventricular pressure, leading to semilunar (arterial) valves closing due to the back pressure (second heart sounds)

  7. Ventricular pressure falls (little blood present and walls begin to relax), atrial pressure > ventricle pressure, resulting in the AV valves opening

  8. Atrial pressure > ventricular pressure as blood is flowing into the atria, AV valves remain open and blood passively flows into the ventricles from the atria
• The aortic valve is the semilunar valve at the base of the aorta
• The heart valves open and close in response to pressure changes either side of the valves (ie. the AV valves open only if the pressure in the atria exceeds the pressure in the ventricles and close only if the ventricular pressure exceeds the atrial pressure)
• Valve action is a response to pressure; they do not cause the pressure changes
• Semilunar (arterial) valves

  1. Open during ventricular systole when the pressure in the ventricles exceeds the pressure in the arteries

  2. Close during diastole when the pressure in the ventricles falls below the pressure in the arteries
• The sequences within the cardiac cycle are stimulated by a coordinated wave of electrical excitation through the heart
• SA node (sinoatrial node)

  A small section in the wall of the right atrium that acts as a pacemaker, producing electrical signals that pass across the atria walls and then down into and across the ventricles
• AV node

  An area of special tissue in the central wall of the heart (septum) at the junction of the atria and ventricles that passes the electrical signal between the atria and the ventricles
• Contraction of a particular part of the heart is a consequence of the wave of electrical excitation reaching that part
• Electrocardiogram (ECG)

  A graphical representation of the electrical activity in the heart
• ECG trace of a typical heartbeat

  • P wave represents the wave of electrical stimulation that triggers the contraction of the atria
  • QRS complex represents the electrical activity that stimulates contraction of the ventricles
  • T wave represents the relaxation of the ventricles
• The R peak has a much greater amplitude than P, as there is much greater electrical activity in the ventricles (reflecting their larger size and thicker muscle)
• The short straight section between the P wave and the QRS complex represents the wave of excitation passing from the atria down into the ventricles
• Arrhythmia

  A general term for a heartbeat problem meaning the heart is beating irregularly, too fast or too slow
• Heart conditions

  • Normal heartbeat
  • Bradycardia (heart beating too slowly - typically 60 bpm at rest)
  • Tachycardia (heart beating too fast - typically >100 bpm at rest)
  • Ventricular fibrillation (abnormal rhythm and can indicate a heart attack is taking place)
• Heart sounds

  Produced by the atrioventricular and arterial valves closing
• Pulse rate

  The rate at which the heart beats, measured by feeling the pulse at a position in the body where an artery is close to the skin
• Pulse rate is normally between 60-100 beats per minute when resting
• Exercise causes a large increase in pulse rate

  The heart needs to beat more often so that the blood reaches the muscles more quickly (ie. more blood per unit time) and can deliver the additional glucose and oxygen required for the higher rate of respiration needed
• Blood pressure

  The force produced by the beating of the heart, measured using a sphygmomanometer
• Measuring blood pressure

  1. A cuff is placed around the upper arm and inflated to a pressure high enough to stop the blood flow in the artery down the arm

  2. The pressure in the cuff is reduced slowly until the blood flow (surge) is detected again, which is the systolic blood pressure

  3. The pressure is reduced further until a constant blood flow is detected, which is the diastolic blood pressure
• Human blood pressure is traditionally measured in millimetres of mercury (mm Hg)