Cardiac Oxygen Supply and Demand Flashcards
Concept about the heart as a pump
The heart comprises 2 muscular pumps in series: Right ventricle and left ventricle
How much blood does the heart pump on average per beat
Pumps 70 ml/beat
Myocyte
-special cell that makes up the myocardium (heart muscle)
Composition of the myocardium
- nucleus
- cell membrane (sarcolemma)
- sarcoplasmic reticulum
- mitochondria
- various storage granules
- myofibrils made up of a linear series of sarcomeres *functional contractile unit
Z line
Delineates each sarcomere
Steps depolarization –> contraction
- Depolarization of the sarcolemma of the myocytes = opening Ca2+ channels
- Ca2+ entry into the sarcoplasm
- rise in Ca2+ stimulates release of Ca2+ from sarcoplasmic reticulum - leads to bigger rise in Ca2+ in the sarcoplasm
- Regulatory protein troponin C which is normally attached to active sites on actin - preventing myosin head attachment will bind to Ca2+ causing TpC to move and expose active site
- Myosin binds to actin
- Heads undergo flexion (in the presence of ATP) driving the actin filaments past the myosin filaments - actin filaments are attached to Z lines - so Z lines get closer together (mininum length of sarcomere)
- Heads detach and return to unflexed config and then cycle begins again
Cross bridge cycling in different crossbridges
Cross bridge cycling is in different phases in different cross bridges
Allows a process of smooth and progressive shortening during systole with many cycles of attachment
Cause of diastole
- Ca2+ leaves the attachment sites to reenter the sarcoplasmic reticulum
- all cross bridges release and the sarcomere relaxes
Origin of energy for contraction
- energy comes from high energy phosphate from ATP
- ATP synthesis requires continuous supply of fuels and oxygen
Cardiac fuel -preferred cardiac fuel in the fasting state
-free fatty acids 60-70% of cardiac needs
<30% glucose (and glycogen)
-10% lactate
Cardiac fuel - after a high carbohydrate meal
-glucose supplies about 70% of the needs
Cardiac fuel -after a fatty meal
-FFA supply up to 80%
Importance of O2
- O2 must be continuously available for cross bridge cycling
- when O2 supply is interrupted the generation of ATP by anaerobic glycolysis is minimal and within 3-4 s contractile perfomance falls dramatically
Determinants of myocardial oxygen supply
1) Coronary blood flow
2) Coronary O2 content and availability
Determinants coronary blood flow
1) Coronary perfusion gradient
2) Coronary resistance
Causes of coronary resistance
1) Myocardial compression
2) Tone of resistance vessles
- metabolic (principal mechanism)
- endothelial
- neurogenic
- myogenic
3) Stenoses and tone of conductance vessles
Origin of coronary arteries
-sinus of valsalva (coronary sinus) located just distal to aortic valve
Branches of coronary artery
- right and left coronary artery
- left branches very quicklly into left anterior descending and cricumflex
Left anterior descending coronary artery supply
- supplies the anterior wall of the left ventricle
- most of the interventricular septum (septal branches)
Circumflex coronary artery supply
- lateral wall of the left ventricle
- posterior surface of left ventricle
Right coronary artery supply
- the right ventricle
- posterior part of interventricular septum
- SA and AV nodes
Grouping of coronary arteries
1) Conductance vessels
2) Resistance vessels (arterioles and capillaries)
Conductance vessles
- vessels that run over the surface of the heart (epicardial vessels)
- and those that penetrate through muscle mass to bring blood to myocardium (myocardial penetrating vessels)