Cardiovascular Physiology 3- Cardiac output and its control Flashcards
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
