HUBS 192 - LECTURE 10

Controlling the Heart and Blood Pressure

which chamber generate pressure for systemic circulation

• left ventricle

what is the difference between the varying pressure in the heart compared to the arteries

• pressure in atria and ventricles goes from no pressure to very high pressure

• the arteries stay at high pressure

why do arteries need to stay at high pressure

• criticaly important determinant of blood flow

• important to how we function

what allows the arteries to withstand the oscillatory changes of pressure

• various wall layers

at what point does blood pressure fall?

• falls steeply across the arterioles, capillaries and venules

• the oscillatory nature is reduced

how is the massive difference in pressure betweeen arterial and venus pressure functional

• it is the reason we are able to have controlled unidirectional flow.

• high pressure to low pressure means we can generate the force to move blood against gravity

• gradient drives direction

what would happen if main arterial pressure falls

• the difference in pressure to the vein reduces and not enough force will be produced to reach the extremeties of the body such as the brain which puts the body at risk

what is haemodynamics

• how blood flows in a single vessel

• ultimately blood is just fluid flowing in a pipe

what is the formula for flow in a pipe

• flow = pressure difference/resistance

what is the formula for main arterial pressure

• main arterial pressure is equal to the pressure difference

• so main arterial pressure is equal to flow times resistance

• (rearranging the previous equation)

what actually is blood pressure

• how much blood is in your arteries

what determines blood pressure

• how much blood is flowing in and out

what maintains arterial blood volume and blood pressure

• ejection of blood into the arterial system

how does blood flow in affect arterial pressure

• fills arteries

• increases arterial blood volume

• increases arterial pressure

how does blood flow out affect arterial pressure

• drains arteries

• decreases arterial blood volume

• descreases arterial pressure

• arterial pressure is determined by the balance between in and out

what is arterial resistance

• how easy or hard it is for blood to flow through arteries

what determines how much blood flows in

• ventricular contraction

• ejection of blood

• CARDIAC OUTPUT

what determines blood flow out

• resistance of the arteries

• capillary flow (how easily blood is entering capillaries)

how does arterial resistance affect main arterial pressure

• increase in resistance increases arterial pressure as blood is less able to leave, increasing the volume of blood in the arteries and therefore increases the pressure

• vice versa also applies

how does cardiac output and arterial resistance collectively affect arterial pressure

MAP = CO x TPR

main arterial pressure = cardiac output x total peripheral resistance

what determines cardiac output

• storke volume and heart rate

• the more your heart contracts - the higher the cardiac output

• the longer your heart contracts for - higher cardiac output

what is stroke volume

• litres per beat

• the length of the contraction

• how much blood is pushed out per contraction

what is heart rate

• beats per minute

• contraction speed

• how long a contraction lasts

how do stroke volume and heart rate collectively influence cardiac output

• cardiac output = heart rate x stroke volume

• CO = HR x SV

how does the body ensure cardiac output needs are met

• balance between stroke volume and heart rate

what happens when someone goes into heart failure

• myocytes die and gets replaced with scar tissue

• but scar tissue doesnt contract or stretch

• contractions arent as efficient and this is seen through a decrease in stroke volume

• this means the heartrate must increase to replace low stroke volume which places stress on the heart

is cardiac output constant or variable

• variable - at rest cardiac output is lower than during exercise

• changes depending on metabolic demand

how is mean arterial pressure regulated

• tightly regulated within a narrow range

• coordinated communication between cardiac and nervous system

• afferent input from CNS and periphery

• efferent output to heart and vessels

what are baroreceptors

• blood pressure sensors - efferent input

where are baroreceptors found

• they are ocncetrationed in key locations (present everywehre tho)

• aortic arch and carotid artery

why is baroreceptors concentrated in the aortic arch

• able to track the pressure of blood coming straight out of the ventricles

• monitors contractile pressure

why are baroreceptors concentrated in the carotid artery

• the carotid artery leads to the brain

describe the mechanism behind baroreceptors

• embedded in arterial walls

• pressure from blood stretches or collapses arterial walls - baroreceptors sense this stretch and interpret pressure from this

• they are tonic receptors - constantly sending signals to the brain under normal conditions - singalling increases when pressure is high and decreases when pressure is low

• brain responds to the changes in signalling accordingly

how does the brain influence cardiac output

• parasympathetic and sympathetic pathways

• can only change the speed of the beating - doesnt constantly tell the heart to beat

describe the parasympathetic pathway of the brain

• the vagus nerve is the parasympathetic pathway of the brain

• it decreases heart rate

• it originates at the dorsal motor nucleus of vagus in the cardiac inhibitory centre in the medulla oblongata

describe the sympathetic pathway of the brain to the heart

• the sympathetic cardiac nerves are the sympathetic pathway

• they increase heart rate and force of the contraction

• originates at the sympathetic trunk ganglion in the thoracic spinal chord which links to the cardioacceleratory centre