Work of the Heart (Fri 20th)

Question 1

Which mechanisms regulate the force of contraction?

Answer 1

1. Hetero-metric/ Intrinsic

2. Homeo-metric/ Extrinsic

Question 2

What are Intrinsic Mechanisms?

Answer 2

Starling’s Law: the force of contraction is proportional to initial fibre length (how stretched the fibre is) in diastole (just before contraction, when it is the most full. Like a rubber band until you pull too hard. The heart will pump whatever volume of blood it receives. Starling curves show stroke volume, and volume or pressure in the ventricle. Stretching the ventricular muscle generates greater force because it causes overlap of myosin and actin fibres, therefore a greater no. cross bridges are formed. Also increased stretch causes contractile proteins to be more sensitive to the same amount of calcium ions.

Question 3

Why is the Starling Effect important?

Answer 3

1. Helps match the output of right and left side of the heart within a few beats. For example if the right ventricle output was greater than LV output, more blood would be going to lungs than body each heart beat, resulting in severe oedema in lungs and drowning. Results to oedema because the hydro-static pressure is so much higher due to increased blood volume, fluid pushed out into lungs. Starling effect in this case makes sure the ventricles adapt to the high venous return from lungs and so match the LV output.
2. Allows adaption. When go from standing to lying down, gravity causes more more venous blood to enter the RA and RV output increases. This means more blood to lungs, left Venous Return/ Ventricular Preload is greater, so Starlings Law allows increased LV stroke volume.

Question 4

Oncotic pressure

Answer 4

The osmosis pressure due to proteins in blood attracting fluid in.

Question 5

Hydro-static. P

Answer 5

Fluid pushed out, oedema in lungs.

Question 6

Extrinsic Mechanisms that regulate the force of contraction.

Answer 6

1. Sympathetic Nerve Stimulation: for example nor-epinephrine release, causes increase in SV without a change in initial fibre length. Increase in contractility or positive inotropic effect. Affects the cAMP system in the heart. See diagram- cAMP system in heart
   2.

Question 7

Lusitrophy

Answer 7

The process by which muscle relaxation is speeded up, so the cardiac cycle of contracting and relaxing can happen faster overall.

Question 8

CO

Answer 8

HR times SV