Cardiovascular

• Closed circulation


  Blood contained within vessels, co-evolved with hemoglobin (most vertebrates)

  
• Single-circuit, closed circulation


  • Heart → gas exchanger → tissues (fishes)

  
• Double-circuit, closed circulation


  • Right heart → gas exchanger → left heart → tissues (amphibians, reptiles, birds, mammals)
• 'Compact' myocardium


  • High flow & high pressure pump, vascularized (birds and mammals)

  
• 'Spongy' (trabeculated) myocardium


  • Low flow & low pressure pump, non-vascularized (fishes, amphibians, reptiles)
• Components of the cardiovascular system

  • Heart
  • Vessels
  • Blood

  
• Blood


  Composed of erythrocytes (red blood cells), leukocytes (white blood cells) and the thrombocytes (platelets) – contained within plasma

  
• Diffusion


  Passive movement of substances along a gradient
• Perfusion


  Convective or bulk flow of substances under the pumping action of the heart
• Cardiac output (CO)


  Volume of blood pumped per unit time
• Stroke volume (SV)


  Volume of blood pumped per contraction
• Heart rate (HR)


  Number of contractions per unit time
• Systole


  Contraction of cardiac chambers
• Diastole


  Relaxation of cardiac chambers
• Contractility (ΔP/Δt or SV/EDV)

  Strength of the contracting ventricles (force and velocity of contraction)
• Compliance (ΔV/ΔP)


  Degree of stiffness of the relaxing ventricles or vessels (ease of stretch)
• Preload


  Force on ventricle at end-diastole (end-diastolic blood volume)
• Afterload


  Force on ventricle during systole (arterial pressure)
• Chronotropy


  Refers to heart rate
• Inotropy


  Refers to contractility
• Lusitropy


  Refers to relaxation
• Dromotropy


  Refers to conduction velocity
• Bathmotropy


  Refers to excitability
• Location of the heart

  Located in the middle of the thoracic cavity, between the two lungs, housed within the mediastinum space

  
• Generally, in a standing animal, the heart is located between (and just above) the two elbows
• Size, shape and position of the heart

  • Heart has the shape of a prolate spheroid (elongate sphere), the cranial end (at the top) is the base of the heart, the caudal end (at the bottom) is the apex of the heart, the left ventricle forms the apex of the heart, the heart does not sit straight along the median plane, the base is shifted to the right and faces more dorsally, the apex is shifted to the left and sits more ventrally

  
• Pericardium (and pericardial sac) of the heart

  Heart is enclosed with the pericardium, which has three parts: fibrous pericardium, 'parietal layer' of the serous pericardium, 'visceral layer' of the serous pericardium (i.e. epicardium), the pericardial sac is comprised of the fibrous pericardium and the 'parietal layer' of the serous pericardium, and fits loosely around the heart so that it can beat inside this sac, the serous pericardium (both the parietal & visceral layers) are smooth membranes that slide past each other when the heart beats, pericardial fluid between parietal & visceral layers acts as a lubricant, excessive fluid accumulation in the pericardial space (e.g. hemorrhage, trauma, pericarditis) leads to cardiac tamponade
• Wall of the heart

  Responsible for pumping action, wall of the heart has three layers: Myocardium (middle and thickest layer, comprises cardiac muscle, cardiomyocytes are anatomically joined by intercalated disks allowing all cells to functionally 'beat as one' (syncytium), cardiomyocytes are resistant to fatigue), Epicardium (outer layer, i.e. visceral layer of the serous pericardium), Endocardium (inner layer, thin and flat (squamous) epithelium lining chambers of the heart, continuous with lining of valves and vessels)
• Chambers of the heart

  Four chambers – two atria receive blood into heart and two ventricles pump blood out of heart, the two atria sit on top of the two ventricles, the left atrium and right atrium (LA and RA) are thin-walled chambers separated by the interatrial septum, receive blood from large veins that carry blood to heart, atria direct blood through one-way atrioventricular valves (AV valves) into the ventricles, atria are seen from the outside by their auricles, the left ventricle and right ventricle (LV and RV) are thick-walled chambers separated by the interventricular septum

  
• Chambers of the heart

  • Four chambers – two atria receive blood into heart and two ventricles pump blood out of heart
  • The two atria sit on top of the two ventricles
  • The left atrium and right atrium (LA and RA): thin-walled chambers, separated by the interatrial septum, receives blood from large veins that carry blood to heart, atria direct blood through one-way atrioventricular valves (AV valves) into the ventricles (passively at first, then forcefully 'top-up' ventricles), atria are seen from the outside by their auricles (= ear flap), which are pouch-like structures that form part of the atria
  • The left ventricle and right ventricle (LV and RV): thick-walled chambers, separated by the interventricular septum, interventricular septum is evident from the outside by the interventricular groove (large 'coronary' blood vessels and fat), when ventricles have filled with blood (from atria), they contract and push blood through one-way semilunar valves into the major arteries, LV pumps into aorta → systemic circulation (around body), RV pumps into pulmonary artery → pulmonary circulation (to lungs)
  • LV has thicker walls than the RV – due to the different pressures required to pump against the different resistances

  
• Valves of the heart

  • Four valves control one-way direction of blood flow through heart – two of the valves are located between atria and ventricles ('left and right atrioventricular (AV) valves') and two of the valves are located between the ventricles and the major arteries they pump into ('aortic and pulmonary semilunar valves')
  • Valves open & close in response to pressure difference only
  • Left AV valve (bicuspid/mitral valve) has two cusps (flaps) and opens when pressure in LA exceeds that in LV (and closes vice versa)
  • Right AV valve (tricuspid valve) has three cusps (flaps) and opens when pressure in RA exceeds that in RV (and closes vice versa)
  • Papillary muscles control tension along chordae tendineae (i.e. the 'heart strings') that attach to the AV cusps, preventing the AV valves from opening backward when they snap shut
  • Aortic semilunar valve has three cusps (crescent moon shape), and opens when pressure in the LV exceeds that in aorta (and closes vice versa)
  • Pulmonary semilunar valve also has three cusps, and opens when pressure in the RV exceeds that in pulmonary artery (and closes vice versa)

  
• Right atrioventricular (tricuspid) valve
• Skeleton of the heart

  • Ringed structures located between the atria and ventricles
  • Composed "mostly" of dense fibrous connective tissue
  • Provides four (4) functions – separate the atria and ventricles, provide point of attachment for the heart valves, provide point of attachment for myocardium (cardiac muscle), provides electrical insulation (delays the electrical impulse) between the atria and the ventricles

  
• Ring for left atrioventricular valve (bicuspid/mitral valve)
• The path of blood flow

  • The function of the heart is to: receive de-oxygenated blood from the systemic circulation (flows into RA) and pump it through the pulmonary circulation (flows from RV), receive oxygenated blood back from the pulmonary circulation (flows into LA) and pump it though the systemic circulation (flows from LV)
  • De-oxygenated blood from body, flows via the venae cavae, and empties into the RA
  • Blood flows from RA, through the right AV valve (tricuspid valve), and into the RV
  • Blood flows from RV, through the pulmonary semilunar valve, and into the pulmonary artery, then travels to the lungs via 'pulmonary circulation'
  • Oxygenated blood from lungs, flows via the pulmonary veins, and empties into the LA
  • Blood flows from LA, through the left AV valve (bicuspid/mitral valve), and into the LV
  • Blood flows from LV, through the aortic semilunar valve, and into the aorta, then travels to the body via 'systemic circulation'

  
• The heart pumps blood in a rhythmic fashion despite the left and right sides pumping to different parts (viz. left side → body, right side → lungs)
• De-oxygenated blood enters the RA and oxygenated blood enters the LA at the same time

  
• The RA and LA contract and relax at the same time
• The right AV valve (tricuspid valve) and left AV valve (bicuspid/mitral valve) open and close at the same time
• The RV and LV contract and relax at the same time