Mech of Heart (1) 2 Flashcards
What is needed for contraction?
Calcium
What is the difference between cardiac and skeletal muscle cells?
Skeletal muscle cells can contract without extracellular calcium
Describe cardiac cell structure:
- length = 100 micrometres
- width = 15 micrometres
- ttubule diameter is 200 nm
- ttubules spread about 2 micrometres apart
Most important ion channel in the T-tubule?
L type Calcium Channel
Describe excitation-contraction coupling in the cardiomyocytes
1) depolarisation is sensed by the L type Calcium channels and calcium entres the cell
2) Some calcium binds directly to cause contraction
3) some binds to ryanodine receptors on the sarcoplasmic reticulum which causes a further release of calcium
4) This causes further contraction
5) the same amount of calcium that entered the cell then leaves the cell by the Sodium - Calcium Exchanger
How would you describe the relationship between force of contraction and intracellular calcium?
Sigmoidal
logarithmic scale on x axis
What type of contraction occurs in the heart?
Isometric (muscle doesnt shorten). Therefore, an increase in muscle length = increased force. This occurs when the heart fills up and initially contracts
Isotonic = muscles shortens for force production. This occurs when the heart is full and ventricular pressure > arterial pressure
Why can you not overstretch the heart?
The pericardium restricts the stretching
What is preload?
The weight that stretched the muscle before it was stimulated to contract. More preload causes more stretching of the muscle so increased force (to a point)
What is afterload?
Weight that is not apparent to the muscle in the resting state. More afterload = less shortening. More afterload = lower velocity of shortening
What are the in vivo correlates of preload?
- End-diastolic Volume (EDV)
- End-diastolic Pressure (EPV)
- Right atrial pressure
What is preload dependent on?
Venous return to the heart
Afterload definition
The load that the left ventricle ejects against after opening of the aortic valve
Measure of afertload?
Diastolic arterial blood pressure
What is Starlings Law?
Increased diastolic fibre length increases ventricular contraction
What is the frank-starling relationship dependent on?
- Changes in the number of cross-bridges in myofilaments that contract
- changes in the calcium sensitivity of the myofilaments
What happens at shorter than ideal lengths in myofilaments?
Actin filaments overlap and therefore there is a reduced number of myosin cross bridges that can be made
What are the two possible causes for myofilament changes in calcium sensitivity?
1) At longer sarcomere lengths, the affinity for calcium of Troponin C is increased due to a conformational change
2) At longer the lengths, the space between actin and myosin filaments decreases so there is a stronger possibility of forming good cross-bridges. This is called lattice spacing.
What is stroke work?
Work done by the heart to eject blood against the pressure of the pulmonary artery and aorta
How is stroke work calculated?
Stroke Work = Volume of blood ejected (SV) multiplied by the pressure at which the blood is ejected (P)
What is the Law of Laplace?
When the pressure in a cylinder is constant, the pressure on the walls increases with increasing radius.
What is the physiological relevance of the law of laplace?
The radius of curvature of the left ventricle is less than that of the right ventricle. This allows the LV to generate more force with the same wall stress (tension) to the RV.
What happens in failing hearts related to the law of laplace?
Hearts become dilated and this increases the wall stress ( tension). Therefore, heart needs to work harder to maintain the same pressure
