Circulation

• Human heart


  • Has four chambers
  • Two thin-walled atria on top receive blood
  • Two thick-walled ventricles underneath pump blood
• Blood flow


  1. Veins carry blood into the atria

  2. Arteries carry blood away from the ventricles
• Atrioventricular valves


  • Prevent back-flow of blood from the ventricles to the atria
  • Left valve has two flaps (bicuspid or mitral valve)
  • Right valve has three flaps (tricuspid valve)
• Valve tendons and papillary muscles

  • Valve tendons ("heart strings") hold the valves in place
  • Papillary muscles contract at the same time as the ventricles, holding the valves closed
• Semilunar valves


  • Two semilunar valves in the arteries (pulmonary and aortic valves)
• Interventricular septum


  • Separates the left and right halves of the heart
• Right ventricle


  • Walls are 3 times thinner than the left ventricle
  • Produces less force and pressure in the blood
  • Internal volume is the same as the left ventricle
• Cardiac muscle


  Composed of cells called myocytes
• Heartbeat


  Myocytes receive an electrical impulse and contract together
• Myocytes


  • Constantly active, have a great requirement for oxygen
  • Fed by numerous capillaries from two coronary arteries
• Conditions:

  • Bradycardia slow resting rate below 60bpm 
  • can happen due to athletic training because the heart pumps a greater blood volume per beat, the number of beats per minute decreases
  • tachycardia heart rate greater than 100 bpm
  • caused by fear, panic, exercise or issues with SAN.
  • ectopic heart beat is an extra heart beat that is not part of the hearts usual rhythm
  • Atrial fibrillation, irregular waves of electrical excitation pass over atria causing it to contract rapidly
• Cardiac diastole:

  • Heart= relaxed
  • blood→ atria, increasing pressure, pushing open AV valves
  • so blood → ventricles
• Atrial systole:

  • atria contract, pushing any remaining blood into ventricles
  • atria volume decreases, pressure increases higher than ventricular, AV valves open
• Ventricular systole:

  • ventricles contract, volume decreases, pressure increases higher than atria, forcing AV valves to close (prevent back flow)
  • pressure higher than aorta, opening semilunar valves so blood forced into arteries
• IN the sinoatrial node, Cells depolarise, triggering a wave of electrical excitation across atria, causing atria to contract → atriaI systole
• The atrioventricular node Detects electrical excitation across atria
• The AVN then transmits waves of electrical excitation down the purkyne fibres, causing ventricles to contract from the bottom up
• Blood Clotting:

  • when calls are damaged, the blood is exposed to collagen
  • this stimulates the release of clotting factors such as thromboplastin.
  • thromboplastin causes a protein in the blood (prothrombin) to turn into thrombin (an enzyme) but calcium ions must be present 
  • fibrinogen is a soluble protein in the blood
  • thrombin causes fibrinogen to turn into fibrin which is an insoluble protein.
• Arteries carry blood from the heart to every tissue in the

  body. They continually branch into smaller and smaller vessels.

  Arteries have thick walls (over 100 cells thick) composed mainly

  of elastic tissue allowing the artery to expand without bursting

  and so withstand the high pressure of blood from the heart. The

  arteries close to the heart are particularly elastic and expand

  during systole and recoil again during diastole, helping to even

  out the pulsating blood flow.
• Veins carry blood from every tissue in the body to the heart.

  The blood has lost almost all its pressure in the capillaries, so it is at low pressure inside veins and is moving slowly. Veins don’t need thick walls and have a larger lumen than arteries, to reduce the resistance to flow. They also have semi-lunar valves to stop the blood flowing backwards. It is particularly difficult for blood to flow upwards through the legs to heart, and the flow is

  helped by contractions of the leg and abdominal muscles:
• Capillaries are where the transported substances actually

  enter and leave the blood. Capillaries are very narrow and their

  walls are composed of single squamous endothelial cells with

  gaps between them, making capillaries very permeable. There

  are a vast number of capillaries so they have a huge surface area : volume ratio, helping the rapid diffusion of substances between blood and cells.
• The lymphatic system has three different functions:

   It drains excess tissue fluid

   It absorbs fats from the small intestine, via the lacteals inside each villus.

   It is part of the immune system. There are networks of lymph vessels at various places in the body (such

  as tonsils and armpits) called lymph nodes where white blood cells develop. These become swollen if

  more white blood cells are required to fight an infection.
• Plasma is the liquid part of blood, comprising about 55% of blood by volume. Plasma is a pale yellow-

  coloured aqueous solution containing numerous dissolved solutes, all being transported from one part of

  the body to another.

   Nutrients (e.g. glucose, amino acids, vitamins, lipids, nucleotides)

   Waste e.g. urea, lactic acid

   Ions help control the solute potential of the blood and some also help buffer the blood pH.

   Hormones Transported from glands to target organs

   Proteins (eg albumins and blood clotting factors, antigens and antibodies)

   Water.

   Heat.
• Modifiable Risk factors of cardiovascular disease

   Smoking. nicotine in cigarette smoke increases heart rate

  and blood pressure, increasing the risk of atherosclerosis.

   Diet.—>high levels of saturated fat increase cholesterol carried in the blood so increase the risk of atherosclerosis. High levels of salt increase blood pressure and the risk of aneurism.

   Stress. —>causes the release of cytokines that trigger an inflammatory response in blood vessels leading to atheroma. It also raises blood pressure.
• Myogenic means hearts contraction is initiated within the muscle itself, rather than by nerve impulses