L7 Cardiac Haemodynamics Flashcards
How does the heart contract?
- In the stage of increased permeability to calcium, calcium comes into the cell and is used to start a new cardiac contraction. Actin is wrapped up in tropomyosin and troponin. This hides away the binding sites of actin for myosin to prevent it firing by itself. Calcium binds to troponin, inducing a conformational change in the troponin-tropomyosin complex in such a way that the myosin bindings sites are exposed. Rocking and rolling. The binding of the calcium is related to the action potential.
- The myosin heads, once calcium is bound to troponin, is able to interact with the actin and bind to it.
- The action of myosin binding to the actin requires ATP. Myosin exerts a pulling action on the actin, initiating a muscle contraction.
- The heart relaxes as the calcium is taken back up the sarcoplasmic reticulum and extracellular calcium leaves the cell.
Once the cell is repolarised there is room for the myocardium to contract again.
How is the heart able to pump out the maximum volume of blood?
The muscle performs a horizontal and longitudinal contraction as it has horizontal and longitudinal fibres. There is also a twisting action. This means there is maximum volume squeezed out of the ventricles.
What is the importance of diastole?
It means the ventricles can be filled. The aortic valve shuts, and so the coronary sinuses fill up enabling the coronary arteries to be filled, perfusing the myocardium.
What is the resting valve for cardiac output?
3-5 L/min
What is the equation for cardiac reserve?
Cardiac reserve = the maximum cardiac output - cardiac output at rest
How is cardiac reserve achieved though heart rate?
The sympathetic nerve fibres can reach the SAN and speed up the frequency of action potentials. Adrenaline also has a similar effect.
How is cardiac reserve achieved though stroke volume?
Sympathetic input leads to prolonged opening fo calcium ion channels. This means more calcium is able to bind to troponin and cause contraction. The stroke volume also depends on preload. A larger contraction, leads to a greater stroke volume.
What is the value for cardiac output at exercise?
20 L/min
What is preload?
The end diastolic volume. It is the volume in the heart before systole.
How does preload affect cardiac performance?
The greater the preload, the greater the sarcomere length. At physiological stretch, ventricular sarcomere length is in the ascending limb and so the greater the stretch, the greater the tension and so greater cardiac output. The is as the muscle stretches, the diameter of the myofibrils is reduced and thick and thin filaments are closer together. More myosin heads interact with actin and so more contraction occurs. However past a certain point, increasing stretch, rapidly decreases tension.
What is Starling’s law?
Starlings Law: ‘the law of the heart is thus the same as the law of muscular tissue in general, that the energy of contraction, however measured, is a function of the length of the muscle fibre”
How can we change preload?
During activity, the use fo the skeletal muscle pump increases and so there is a greater venous return. This leads to a greater preload and so the ventricles pump harder to prevent back up.
How can the Frank Starling curve change?
Left shift - with exercise or pharmaceutical stimulation
Right shift - pharmacological depression, myocardial loss (e.g. with a heart attack and some of the muscle has infarcted).
What is ejection fraction?
Ejection fraction = stroke volume / End diastolic volume
What is a the ejection fraction at rest, compared to exercise?
At rest EF = 55-75%
In Exercise = 90%
In heart failure, even at rest, there is a reduced EF.
