Cardiovascular system

• Cardiac conduction system -

  • begins at the sinoatrial node (SAN) spreads a wave of excitation
  • atrial systole
  • atrioventricular node (AVN) held for 0.1s to allow atria to contract fully before ventricular systole
  • travels through the bundle of his
  • down the bundle branches
  • through the purkinje fibres
  • ventricular systole
• Neural control mechanism -

  The sympathetic and para-sympathetic nervous systems are co-ordinated by the cardiac control centre in the medulla oblongata.

  Cardiac control centre stimulated by chemo, baro, proprioceptors and will then send an impulse through either the sympathetic to the SAN to increase heart rate or para-sympathetic system to the SAN to decrease heart rate.
• Chemoreceptors - Detect increase in CO2 --> Cardiac control centre --> sympathetic system --> SAN increases heart rate.

  Baroreceptors - Detect increase in blood pressure --> Cardiac control centre --> para-sympathetic system --> SAN decreases heart rate.

  Proprioceptors - Detect increase in muscle movement --> Cardiac control centre --> sympathetic system --> SAN increases heart rate.
• Stroke volume - the volume of blood pumped out by the ventricles in each contraction. Depends upon: venous return (return of blood to the right side of the heart), elasticity of cardiac fibres (the more they can stretch, the greater force of contraction) therefore increasing the ejection fraction (percentage of blood pumped out by the left ventricle per beat - this is starling's law), contractility of cardiac tissue (greater contractility = greater force of contraction).
• Heart disease - occurs when coronary arteries become blocked or start to narrow by gradual build up of fatty deposits. This is called atherosclerosis and the fatty deposits are called atheroma.

  High blood pressure - The force exerted by the blood against the blood vessel wall. High blood pressure puts extra strain on the arteries and heart and increases chance of a heart attack.

  Cholesterol levels - LDL - low density lipoproteins - BAD as they transport cholesterol from the blood to the tissues. HDL - high density lipoproteins - GOOD - transport cholesterol back to liver to be broken down.
• strokes -

  • Ischaemic strokes - happen when blood clots stop the blood supply .
  • Haemorrhagic strokes - happen when a weakened blood vessel supplying the brain bursts.
• Cardiovascular drift - characterised by a progressive decrease in stroke volume and arterial blood pressure, together with a progressive rise in heart rate. Occurs during prolonged exercise in warm environments despite the intensity remaining the same. A reduction in plasma volume occurs from the increased sweating response and this reduces venous return and stroke volume. Heart rate increases to compensate and maintain cardiac output.
• Veins - transport de-oxygenated blood back to the heart, thinner muscle/elastic tissue, blood is at a lower pressure with valves and a wider lumen .

  Arteries - transport oxygenated blood around the body, have the highest pressure, thick muscular/elastic walls with smaller lumen and a smooth inner layer.

  Capillaries - wide enough to allow one red blood cell to pass through at any given time. Slowing down blood flow allowing diffusion of nutrients to the tissues to take place.
• Systolic - when the ventricles are contracting.

  Diastolic - when the ventricles are relaxing.
• Venous return mechanisms -

  • skeletal muscle pump - when muscles contract and relax, they press on nearby veins, squeezing the blood back to the heart.
  • respiratory pump - when muscles contract and relax, pressure changes occur in the thoracic and abdominal cavities. Compresses nearby veins and assists blood back to the heart.
  • pocket valves - ensuring blood flows in one direction.
  • thin layer of smooth muscle in veins squeezing blood back to the heart.
  • Gravity helps blood to return from the upper parts of the body.
• Oxyhaemoglobin dissociation curve - There is an S shaped curve because at rest, the high partial pressure of oxygen in the lungs means that haemoglobin is almost completely saturated with oxygen. and in the tissues, the partial pressure of oxygen is lower and so haemoglobin gives up some oxygen to the tissues. During exercise, the S moves to the right. This is known as the Bohr shift and occurs because muscles require more oxygen so dissociation of oxygen from haemoglobin occurs more readily.
• Three factors responsible for the increase in dissociation -

  • increase in blood temperature - blood and muscle temp increasing makes dissociation happen more readily.
  • Partial pressure of blood carbon dioxide increases - as the level of blood CO2 rises during exercise, oxygen will dissociate quicker.
  • Blood PH - more CO2 will lower the PH in the body. A drop in PH will cause oxygen to dissociate from haemoglobin more quickly.
• Redistribution of blood - the redirecting of blood flow to the areas where it is most needed is known as shunting or the vascular shunt mechanism and ensures -

  • more blood goes to the heart, needs oxygen to pump fater with more force.
  • more blood goes to the muscles, need oxygen for energy and more blood is needed to remove waste products such as CO2 and lactic acid.
  • more blood goes to the skin, energy is needed to cool the body down.
  • blood flow to the brain remains constant, needs oxygen for energy to maintain function.
• control of blood flow - Through vasodilation and vasoconstriction.

  • vasodilation is when the blood vessel widens to increase blood flow into the capillaries.
  • vasoconstriction - is when the blood vessel narrows to decrease blood flow. During exercise, the muscles require more oxygen so vasodilation takes place and vasoconstriction supplies the non-essential organs such as the liver and intestines as it is reducing the blood supply and redirecting it to other areas.
• Arterio-venous difference (A-VO2 diff) - the difference between the oxygen content of the arterial blood arriving at the muscles and the venous blood leaving the muscles. At rest, the arterio-venous difference is low, as not much oxygen is required by the muscles and the opposite during exercise. This will affect gaseous exchange at the alveoli, so more oxygen is taken in and more carbon dioxide is removed.