Control of cardiac output Flashcards
How can cardiac output be rearranged?
CO = HR x SV
CO = HR x (EDV-ESV)
Amount ejected from the heart is the volume of blood at the beginning in the heart minus what is left.
How can cardiac output be controlled?
HR
EDV
ESV
What is myogenic rhythmicity?
Cardiac muscle cells depolarise and contract rhythmically without nerves, by the SA node.
Conduction then occurs in a highly coordinated way.
Contract can be impacted by autonomic control - parasympathetic and sympathetic.
How is the parasympathetic nervous system organised?
The cell bodies of the long preganglionic neurone is in the medulla of the brain stem.
The axons of the neurone are the Vagus nerve, which stimulates the heart and blood vessels.
The ganglion (synapse) is very close to the organ or part of the organ.
How is the sympathetic nervous system organised?
The cell bodies of the short preganglionic neurones are in the spinal cord, controlled by the brain.
The ganglion chain has a synapse, then has long postganglionic neurones, which go to the blood vessels and heart.
What is the control of the autonomic nerves?
They both innervate the heart.
Each nerve can split into branches and go to the SA node, AV node, and to the ventricles.
They can change activity of the separate parts of the heart.
How does the sympathetic nerve affect the SA node?
Increased sympathetic activity increases heart rate.
The sympathetic system releases noradrenaline, which acts on B1 receptors, and brings the cell closer to threshold.
This means there are more action potential per minute, and higher HR.
How does the sympathetic nerve bring the SA node closer to threshold?
Positive chronotropic effect:
Increases permeability to Na+ and Ca2+
Reaches threshold quicker
So there are more action potentials per minute.
How does the parasympathetic nerve affect the SA node?
Increased parasympathetic activity decreases heart rate.
The parasympathetic nerve releases acetylcholine, which acts on the muscarinic receptors on the SA node, brings it further from threshold.
This reduces the number of action potentials and decreases heart rate.
How does the parasympathetic nerve bring the SA node away from threshold?
Negative chronotropic effect:
Hyperpolarises cell (opens K+ channels).
Reduces permeability to Na+ and Ca2+.
Takes longer to reach threshold.
So there are less action potentials per minute.
Why is heart rate less than the number of action potentials per minute?
There is ongoing vagal activity - the Vagus nerve sends signals at rest to reduce heart rate.
There is also ongoing sympathetic activity, but the vagal tone predominates.
How can HR be changed?
Changing the number of action potentials by the SA node. Only by changing electrical activity.
But electrical activity could change by:
Exercise
Stress
Temperature - increased increases electrical activity and heart rate.
Drugs
By autonomic nervous system
How is EDV changed?
The central venous pressure (CVP) - the pressure in the thoracic vena cava near the right atrium, determines venous return.
This then determines EDV - the volume of blood in the ventricle at the end of diastole.
What determines venous return?
Valves
Skeletal muscle pump
Respiratory pump
Blood volume
Venous tone
Gravity
What is the skeletal muscle pump?
Valves prevent backflow of blood due to gravity
The skeletal muscle pump squeezes the segments of veins that are divided by valves, causing blood to return to the heart.
In exercise the muscles contract more, so more blood returns to the heart.
What are varicose veins?
When the veins in the legs are faulty, and blood pools (remains in the veins) rather than returning to the heart.
Venous return is therefore reduced.
What is the respiratory pump?
When inspiring, the pressure in the thorax decreases, so the pressure in the abdomen is greater than in the thorax, so blood is sucked up through the veins into the right atrium.
During exercise, breathing rate increases, so more blood is returned.
How does increased blood volume determine venous return?
Increase total blood volume increases venous return.
E.g. due to renal failure
Fluid retention through activation of the renin angiotensin system
How does decreased blood volume determine venous return?
Decreased total blood volume decreases venous return
e.g. haemorrhage
Dehydration
Reflexes can help with dehydration, by increasing ADH, so more water is reabsorbed to retain volume.
What is the storage of blood in veins?
Veins are capacitance vessels, and 60% of blood is here.
When the pressure is low, there is less blood, and veins are flat.
As the veins fill, they become rounder, and there is lots of blood in them.
This increases venous return, cardiac filling and cardiac output.
What is venous tone?
Blood flow can be increased by stimulating sympathetic fibres in the vein, this causes them to venoconstrict so they carry less blood, and the blood returns to the heart.
How does gravity determine venous return?
Lying down, no gravity, very good venous return.
When stand up, veins constrict to return blood to the heart.
There can be venous pooling, where more blood flows through the veins, so is harder to return to the heart.
How EDV affect SV?
Starting with more blood, eject more blood, increases stroke volume.
What is starlings law?
The force of ventricular contraction is dependent on the length of ventricular muscle fibres in diastole.
Length is determined by venous return - when more blood is returned (increased EDV) it stretches the cardiac muscle, so EDV is a pre-load on cardiac muscle.
Increased EDV causes more stretching, so more cross-bridges form, and there are more heart contractions.
How does starlings law change in exercise?
During exercise the walls of the heart stretch more, so they form more cross-bridges, and have a greater force of contraction. This increases stroke volume.
How are more cross-bridges formed?
There is a length-dependent increase in Ca2+ sensitivity in contractile apparatus, so more cross bridges form.
What is the length tension relationship of skeletal muscle?
see picture
How can cardiac output be affected by ESV?
Start with same amount of blood but eject more - increased stroke volume.
This is why it is important that not all the ventricle is emptied, so more can be ejected when needed.
What is contractility?
Or inotropy, an increase in stroke volume at any EDV.
Increased force of contraction is increased contractility
Or when ventricle fails, contractility decreases, so less blood is ejected.
How can contractility be increased?
Stimulate sympathetic nerves, releases noradrenaline which acts at B1 receptors on myocytes in the ventricle.
This increases Ca2+ in the cell, more crossbridge form, so greater force of contraction and stroke volume, and ESV decreases, which increases cardiac output.
How does contractility affect stroke volume?
see picture
What is afterload?
The load against which the heart must contract to eject the stroke volume.
What is the afterload of the left ventricle?
The afterload is the aortic pressure - the pressure the heart must generate to be higher than aortic pressure and open the valve.
This is very high so the left ventricle is very muscular.
What is the afterload of the right ventricle?
The afterload is the pulmonary pressure - the pressure the heart must generate to be higher than the pulmonary pressure and open the valve.
This is very low so the right ventricle is not very muscular.
What is pulmonary hypertension?
The pulmonary pressure increases, so the right ventricle becomes more muscular to generate higher pressure to open the pulmonary valve.
The ventricle will hypertrophy.
