HomeCells & MoleculesDiseasesEcosystemHuman BiologyAQA BIOL1AQA HBIO1AQA HBIO2AQA HBIO4AQA HBIO5
HBIO1 > Circulation
Welcome, Guest!
Login with Facebook | Login

Anatomy

  • Heart consists of 4 chambers
    • Right atrium (RA)
    • Right ventricle (RV)
    • Left atrium (LA)
    • Left ventricle (LV)
  • Blood flow
    • Right atrium receives blood from
      • Superior vena cava (SVC) - carries blood from upper body (head, arms)
      • Inferior vena cava (IVC) - carries blood from lower body (chest, abdomen, legs)
    • Blood flows from right atrium, across tricuspid valve, into right ventricle
    • Blood leaves right ventricle and enters pulmonary artery
      • Backflow into RV prevented by semilunar pulmonic valve
      • Deoxygenated blood arrives at lungs via pulmonary artery
      • Oxygenated blood leaves lungs via pulmonary vein
    • Blood from pulmonary vein enters left atrium
    • Blood flows from left atrium, across mitral valve, into left ventricle
    • Left ventricle has a thick muscular wall / generates high pressures during contraction
    • Blood from LV is ejected, across aortic valve, into aorta
  • Muscle of left ventricle is thicker than right ventricle
    • Pressure in aorta is higher than pulmonary artery
    • Left ventricle must generate more pressure to overcome pressure of aorta
    • Therefore, thicker muscle required in left ventricle
  • Tricuspid and mitral valves are atrioventricular (AV) valves
    • Have fibrous strands (cordae tendinae) that attach to papillary muscles
    • Papillary muscles contract during ventricular contraction
    • Generate tension on valve via cordae tendinae to prevent AV valves from flapping back into atria
  • Semilunar valves (pulmonic and aortic) do not have these attachments

Cardiac Cycle

  • Atria receive blood from veins and store it prior to each heart beat
  • Systole: period of contraction by heart muscle
  • Diastole: period of relaxation by heart muscle
  • Atrial systole
    • Both atria contract and move blood across AV valves into ventricles
    • This reduces volume of atria but increases pressure
      • Pressure of RA > RV - forces tricuspid valve to open
      • Pressure of LA > LV - forces mitral valve to open
  • Ventricular systole
    • Contraction of ventricles increases pressure
    • AV valves close as blood is forced against them → 1st heart sound
    • This prevents backflow into atria
    • Instead, blood is ejected into arteries through aortic and pulmonary valves
  • Ventricular diastole
    • End of cardiac cycle, all chambers relax
    • Aortic and pulmonary valves close2nd heart sound
    • This prevents backflow into ventricles
    • Atria fill up again to start next cycle
    • Volume increases while pressure decreases

Electrical Activity

  • Heart has unique ability to beat (contract) on its own
  • Assisted by nerves and hormones in blood but can function without them
  • Sinoatrial (SA) node
    • Located at the top right atrium
    • Also known as the "natural pacemaker" controlling heart rate
    • Increases with physical activity and decreases when relaxing
    • Sends impulses across the atria to the AV node
    • Cause contraction of atria
  • Atrioventricular (AV) node
    • Located between atrium and ventricle
    • Ventricles are isolated from atria
    • Impulse must pass through AV node to travel across ventricles
  • AV node is connected to the Bundle of His
    • Branches into a right bundle (to right ventricle) and left bundle (to left ventricle)
    • Fibres that branch out to distant ventricles are called Purkinje Fibers
    • Cause contraction of ventricles

Pressure Changes

  • Isovolumetric contraction
    • Ventricles start to contract
    • Intraventricular pressure rises and causes AV valves to close
    • Ventricles are no longer filled with blood and volume says the same
    • Pressure is not high enough to open semilunar valves
  • Pressure in LV > aorta
    • Semilunar valves open
    • Ventricular volume decreases
    • Blood is ejected into aorta
  • Pressure in LV < aorta
    • Back pressure causes blood to move back and semilunar valves to shut
  • Isovolumetric relaxation
    • AV and semilunar valves are closed
    • Lasts until pressure in atria > ventricles
  • Pressure atria > ventricles
    • Ventricles are filled
    • Atrial contraction/systole - final amount of blood is emptied into ventricles immediately prior to next phase of isovolumetric contraction of ventricles^

Blood vessels

Arteries

  • Pulmonary artery
    • Transports deoxygenated blood from right ventricle into lungs
  • Systemic arteries
    • Transport oxygenated blood from left ventricle to body tissues
    • Carry ≈10% of total blood volume
    • Blood is pumped from left ventricle into large elastic arteries
    • Elastic arteries become smaller muscular arteries
    • Muscular arteries branch into smaller arterioles (smallest arteries)
    • Arterioles regulate blood flow into tissue capillaries
  • Made up of 3 layers:
    • Tunica intima (innermost layer)
      • Simple squamous epithelium
      • Surrounded by a connective tissue basement membrane with elastic fibres
    • Tunica media (middle layer)
      • Smooth muscle and usually thickest layer
      • Changes vessel diameter to regulate blood flow and BP
    • Tunica adventitia (outermost layer)
      • Attaches vessel to surrounding tissue
      • Connective tissue with varying amounts of elastic and collagenous fibers
  • Compared to veins, arteries have a relatively small lumen

Veins

  • Pulmonary veins
    • Transport oxygenated blood from lungs to left atrium
  • Systemic veins
    • Transport deoxygenated blood towards the heart
    • Carry ≈70% of total blood volume
    • After blood has passed through the capillaries, it runs into venules (smallest veins)
    • Become progressively larger until they reach the right atrium
    • Medium and large veins have valvesthat prevent backflow of blood due to gravity
  • Made up of same 3 layers as arteries
    • BUT less smooth muscle and connective tissue
    • Makes walls of veins thinner with less pressurelarger lumen
    • Hold more blood than arteries