Cardiovascular Physiology 3- Cardiac output and its control Flashcards Preview

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Flashcards in Cardiovascular Physiology 3- Cardiac output and its control Deck (17):

What is cardiac output?

The volume of blood pumped by each ventricle per minute

All blood that leaves the heart flows through the tissues, cardiac output is an indicator of the total blood flow through the body BUT does not indicate how much blood is distributed to various tissues


What are the determinants of cardiac output?

Heart rate= Beats per minute

Stroke volume= Volume of blood pumped per beat (or stroke)

Cardiac output= Heart rate X Stroke volume


How can CO be maintained if there is sudden drop in blood pressure?

Drop in blood pressure= Low venous return= Decreased stroke volume
To maintain CO= Increase heart rate


What is heart rate mainly determined by?

Heart rate is initiated by autorhythmic cells in SA node but is modulated by neural and hormonal input

Innervated by autonomic nervous system= Sympathetic and parasympathetic systems= Send axons to the heart= Changes firing rate of SAN + Innervates the ventricles

Parasympathetic= Slows heart rate= Cell body origin is in the brain

Sympathetic: Increases heart rate= Cell body origin is in spinal cord


What is the parasympathetic neurotransmitter and what effect does it have?

Neurotransmitter= Acetylcholine (ACh)

Effect: Influences K+ and Ca2+ channels in the pacemaker cell, K+ permeability increases (more K+ outside)= Pacemaker potential begins at a more negative value

While Ca2+ permeability decreases= More Ca2+ outside, less comes in

Result: More hyperpolarisation, rate of depolarisation decreases as it takes longer for the cell to reach threshold= Heart rate decreases


What is the sympathetic neurotransmitter and what effect does it have?

Adrenaline also known as epinephrine and noradrenaline which is also known as norepinephrine increase HR

Increase ion flow through both If and Ca2+ channels=Reaches threshold faster

This increases the rate of depolarisation= Increases heart rate


Which dominates at rest?

Rest: Parasympathetic dominates the heart rate= Inhibits SAN= low heart rate, by delaying the rise time to threshold firing

Both systems work together at the same time but the activation of one inhibits the other, change the balance of which one is dominating


What is stroke volume determined by?

The extend of venous return by sympathetic activity:

Venous return: Amount of blood that enters the heart from the venous circulation


Intrinsic control: What does the Frank-Starling Law of the heart state?

The heart normally pumps out during systole, the volume of blood returned to it during diastole : increased venous return= Increase stroke volume

Stroke volume is proportional to EDV

The amount of blood pumped into heart, the heart will always pump that out= The more you pump in, the more forcefully the heart will contact= More will be pumped out


What is the intrinsic control related to extend venous return?

Venous return determines end-diastolic volume= Amount of blood left in the ventricle after contraction determines the length of cardiac muscle

The longer a muscle, the greater the tension created during contraction

Cardiac muscle: The more you stretch it, the more it will contract= Increasing stroke volume

As more blood returns to the heart, the heart pumps out more blood, but heart does not eject all the blood it contains

Normally: Cardiac muscle fibre length is normally less than the optimal length (Io) for developing maximal tension

Increase in venous return= Increase in EDV= Cardiac muscle fibre length increases closer to Io= Increase contractile tension of fibres on the next systole= More blood is squeezed out= MORE STROKE VOLUME


How is stroke volume extrinsically controlled?

Related to extent of sympathetic stimulation of the heart

Contractility= The intrinsic ability of a cardiac muscle fibre to contradict at any given fibre length and is a function of Ca2+ interaction with the contractile filaments

Sympathetic stimulation of the heart= Increases the contractility of the heart= Increases SV

Contractility= Increases the amount of Ca2+ available for contraction

Sympathetic stimulation= Voltage gated Ca2+ channels open and stay open longer= More Ca2+ enter the cell= More active cross bridges= Stronger contraction= More blood is forced out= Increases SV

Shifts Frank-Starling curve to the left


What happens to the contractility of the heart during heart failure?

1) Decrease in cardiac contractility= Heart loses its ability to contract as ventricles become thinner
2) Kidneys conserve salt/water expand blood volume= EDV
3) Sympathetic nervous system should switch itself off= But stays on= Abnormal activation


What are baroreceptors?

Stretch-sensitive mechanoreceptors that respond to changes in pressure (Stretch artery wall)

2 Types:
-Carotid and Aortic


What is the baroreceptor reflex?

The primary reflex pathway for homeostatic control of blood pressure

Blood pressure is highest in arteries and decreases continuously as blood flow through the circulatory system

Bararoceptors detect pressure, if it falls or increases, sends information to the brain so that it can accommodate the change needed

Negative feedback control arterial blood pressure so that it remains at normal levels


What happens there is increased blood pressure?

Stretches the baroreceptor membrane= The firing rate of receptor increases, activating the medulla cardiovascular control center= Increases parasympathetic activity and decreases sympathetic= Slows down heart and dilates arterioles= Cardiac output falls


What is the difference between static and dynamic responses?

Normal blood pressure= Static= Firing at moderate rate

Blood pressure increases= Dynamic= Dynamic rate increase so that it is proportional to the rate of increase in blood pressure

Static= Maintaining the blood pressure
Dynamic= Changes the blood pressure


What happens to the baroreceptor reflex during haemorrhage (significant loss of blood)?

SV, CO, Mean Arterial Pressure decrease due to less venous return= Less EDV= Less forceful contraction= Baroreceptors detect this

Decrease ABP unloading baroreceptors (Taking away the signal from baroreceptors) = keeps reflect compensated for blood loss