Double Circulatory System

• Closed system meaning blood remains in vessels
• Double system meaning passes through the heart twice
• Blood flows at low pressures in the lungs to prevent capillary damage and increased time for gas exchange
• Blood flows at high pressures when oxygenated to pump around the body to respiring cells
• Key Blood Vessels:

  • Coronary arteries
  • Heart - vena cava, aorta, pulmonary artery/vein
  • Lungs - pulmonary artery/vein
  • Kidneys - renal artery/vein
• Cardiac muscle is thick walled, is myogenic meaning it contracts and relaxes without nervous or hormonal stimulation and never fatigues as long as there is oxygen supply
• Coronary arteries supply cardiac muscles with oxygen, branching off from the aorta, a block in these causes myocardial infarction (heart attack)
• Atrias contain thin muscle, less contractions and are elastic
• Ventricles contain thick muscle for big contractions, with an increased blood pressure to flow longer distances; right ventricle has a lower pressure than left
• Veins go towards the heart
• Arteries go away from the heart
• Semi-lunar, Bi- and Tri-cuspid valves open with pressure behind them, and close to prevent backflow
• Septum splits blood flow
• Artery Structure:

  • Thicker muscle for constriction and dilation
  • Thicker elastic to maintain pressure and stretch
  • Thicker walls to prevent bursts due to high pressure
  • No valves
• Vein Structure

  • Thin muscle
  • Thin elastic as low pressure
  • Thin walls as less pressure and easily flattened
  • Valves to prevent backflow
• Capillary Structure:

  • Narrow diameter to slow blood flow
  • RBCs squashed to cell walls to maximise diffusion
  • No muscle, elastic or valves
  • 1 cell thick so short diffusion path
• Arterioles Structure:

  • Thicker muscle than arteries to restrict blood flows
  • Thinner elastic than artery as less pressure
  • Thinner wall as less pressure
• Diastole causes atria and ventricular muscles to relax, blood enters the atria increasing pressure
• Atrial Systole causes atria muscles to contract, increasing pressure causing atrioventricular valves to open and blood flows into ventricles
• Ventricular Systole causes ventricle muscles to contract, increasing pressure, atrioventricular valves close and semi-lunar valves open causing blood flow into arteries
• Cardiac Output = Heart Rate (BPM/min-1) x Stroke Volume (vol/dm3)