Structure and function of the heart Flashcards
what is the ratio of muscle in left to right of heart
4-6 times more pressure in left side
3:1 ratio in muscle mass between Left and right
what happens to blood pressure throughout the body?
drops once blood leaves the heart
what is capillary mean pressure?
systemic = 17mmHg Pulmonary = 7mmHg
Conduction of cardiac AP?
intercalated disces interconnected cardiac muscle cells secured by desmosomes linked by gap junctions propagate action potentials
how are action potentials conducted in cardiac muscle?
local changes in currents cause passive depolarisation of adjacent muscle cells through gap junctions
excitation-contraction coupling
T tubule transfers action potential into cell
high concentration of calcium in T tubule
calcium moves intracellularly through L type calcium channels and increases intracellular concentration or once calcium has moved from extracellular to intracellular it
binds to ryanodine receptors on sarcoplasmic reticulum which causes release of calcium –> calcium induced calcium release
altering calcium release
anything that alters calcium release or storage alters contractility and relaxation
what impacts calcium release or storage?
calcium ion channel blockers - non-dihydropyridines
beta blockers - blocks effect of adrenaline and noradrenaline
caffeine
What are the basic mechanics of cardiac contraction?
preload afterload contractility heart rate PACE
How to calculate stroke volume?
SV = end diastolic volume - end systolic volume
what is end diastolic volume?
volume of blood in heart just before contraction
what is end systolic volume?
volume of blood after contraction - left over
what is isovolumetric contraction?
pressure is changing - contracting but volume is same
valves are closed
preload
increases in end diastolic volume leads to increases in myocardial performance/ contractility
why does an increase in EDV cause increased myocardial performance?
physical and activating factors
physical factors that increase myocardial performance
more optimum myofilament overlapping
decrease lattice spacing - decreased distance between myofilaments so increased probability of interaction between contractile components
activating factors that increase myocardial performance
increase in calcium ion sensitivity by multiple mechanisms
increased calcium release
increase calcium sensitivity
what happens if you increase end diastolic volume?
increases contractility and increases stroke volume as the volume in the heart increases due to increase in venous return
what is afterload?
what the heart has to pump against
higher the pressure in systemic/ pulmonary circulation = more force/ work required by the heart
what happens when there is an increase in end systolic volume?
needs to increase pressure in ventricle to meet that in aorta and so increases volume so thre is less opportunity for the muscle to shorten
shifts the pressure-volume loop to the right
stroke volume decreases
when is there an increase in end systolic volume?
chronic hypertension
when is there a fall in contractility?
MI
heart failure
weak, floppy ventricle
what impact does decreased contractility have?
reduced stroke volume
reduction in cardiac output
what happens when there is a fall in compliance?
stiff, fibrotic ventricle
more difficult to contract and recoil
decrease in stroke volume
when is there a fall in compliance?
ageing
contractility
noradrenaline/ adrenaline binds to beta 1 adrenoreceptor on GPCR
causes adenyl cyclase to be activated, converting ATP to cAMP which ultimately causes an increase in calcium and increase contractility
what does dual innervation mean?
both sympathetic and parasympathetic innervations
SAN and AVN
are dual innervated
Innervation of atria
sympathetic and small amount of parasympathetic innervation
innervation of ventricles
only really have sympathetic innervation
what does caffeine do?
increases calcium
increases contractility
control of contractility
sympathetic drive to ventricular muscle fibres - noradrenaline at beta 1 receptors in cardiac muscle cells
hormonal control by circulating adrenaline and noradrenaline
how to calculate the ejection fraction?
stroke volume/ end diastolic volume
expressed as a %
what is ejection fraction?
quantification of contractility
measure of the ability of the ventricle to contract
Ranges of ejection fraction
> 75% could indicate hypertrophic cardiomyopathy
55-70% normal or heart failure with preserved ejection fraction
40-55% abnormal - maybe clinically insignificant
<40% = heart failure, can be very low
importance of contractility
most important factor in mortality
heart rate regulation
neuronal and endocrine
positive ionotropic
increase in heart contractility
chronotropic
affect on heart rate
positive = increase HR
negative = decrease HR
which hormones affect HR?
adrenaline and noradrenaline = increase
acetylcholine = decrease
what happens to the heart at rest?
increase parasympathetic activity
bowditch effect
aka staircase phenomenon
increased heart rate causes increase contractility strength
atrial reflex
aka bainbridge reflex
stretch receptors in right atrium tirgger increase in heart rate through sympathetic activity due to venous return increase
sympathetic activity
increases permeability of membrane to sodium ions
increase so sodium moves into cell spontaneous depolarisation
reduces time to initiate depolarisation
Parasympathetic activity
decreases permeability of the membrane to sodium and increases potassium leaving the cell
decreases spontaneous depolarisation
increases time to initiate depolarisation - causes hyperpolarisation
what does a selective beta 1 antagonist affect?
reduced contractility
reduced heart rate
reduced renin release
how does a beta 1 antagonist affect renin release?
reduced renin secretion via selective beta 1 inhibition at juxtaglomerular cells
what determines how blood travels through blood vessels?
flow
pressure
resistance
flow
volumber per unit of time
blood flow is determined by pressure change and resistance to flow
pressure
driving force behind blood flow, generated by heart. Blood flows from high to low pressure regions
resistance
an impediment to flow, high resistance means a higher pressure gradient is needed to achieve the same flow
what affects resistance to blood flow
blood viscosity
vessel length
vessel radius
blood viscosity
thicker the blood the higher the resistance to flow. Increase in RBC count, LDL, smoking etc.
vessel length
the longer the vessel, the higher the resistance to flow. Unlikely to change by much in an adult
vessel radius
the narrower a vessel is the higher the resistance to flow.
what is the most important variable affecting resistance to blood flow?
vessel radius
resistance to blood flow formula
8nL/Pi x r^4
n = blood viscosity
L = vessel length
r = vessel radius
flow equation
Q = change in pressure/R Q = flow
what are the functions of the CVS?
delivery of oxygen and nutrients to tissues and removal of waste products
distributes hormones, fluids and electrolytes
immune function
thermoregulation
what are the 2 circulations?
pulmonary and systemic circuits
pulmonary circuit
is a specialised circulation that is relatively short, simple and operates at a lower pressure than the systemic circulation
specialised circulations
circulation is specialised within specific organs
most tissues receive enough blood to meet metabolic needs, whilst other tissues receive more blood than they need - reconditioning organs
reconditioning organs
tissues that receive more blood than they need metabolically
cardiac output in pulmonary and systemic circulation
equal in both
resistance to flow in pulmonary and systemic circulation
low in pulmonary as short and simple and high in systemic as long and complex
pressure in pulmonary and systemic circulation
low in pulmonary - 25/10mmHg
high in systemic - 120/80mmHg
mean arterial pressure/ pulse pressure
approximates to diastolic pressure plus 1/3 of the difference between systolic and diastolic pressure because more time is spent in diastole
BP
refers to mean arterial blood pressure
what is the dicrotic notch?
when the aortic valve shuts causing a small dip in pressure
pressure changes in circulation
decreases from LV, aorta/ arteries, arterioles, capillaries and veins
pressure changes in LV
ranges from 120-0 mmHg
pressure changes in aorta and arteries
smaller pressure changes- pressure is maintained
when is the biggest pressure drop?
arterioles because of their small radius and so high resistance
blood rubs against walls and loses energy
exchange vessels
capillaries - main interface with tissues
veins
hold the largest share of blood in the whole circulation = capacitance vessels
what determines mean arterial blood pressure?
cardiac output
total peripheral resistance
blood volume
what is total peripheral resistance?
can be controlled by constricting or dilating of muscular arteries and arterioles
another phrase for total peripheral resistance
systemic vascular resistance
what is the most important factor in BP?
total peripheral resistance
what regulates BP?
autonomic NS
humoral
autonomic NS regulation of BP
short-term
influences cardiac output and vascular resistance
humoral control of BP
aldosterone adrenaline ADH atrial natriuretic peptide angiotensin II short and long-term regulation influences vascular resistance and blood volume
what are arterial baroreceptors?
stretch receptors/ mechanoreceptors
in aortic arch and carotid sinus
continuously monitor BP by monitoring stretch of vessel walls
where are chemoreceptors found?
carotid body
baroreceptor neural pathway
input to the cardiovascular centre in medulla oblongata
via glossopharyngeal nerve from carotid sinus or via vagus nerve from aortic arch
causes an autonomic nervous system response
what do baroreceptors do?
responsible for rapid, short-term control of BP
firing rate for BP
increases firing rate when BP increases
decreases firing rate when BP decreases
Controlling high BP
increased firing to brain via glossopharyngeal and vagus nerves to medulla
increased firing down vagus to reduce HR, acts on SAN and decreased down sympathetic innervation to SAN and muscles of heart via Beta 1
decreased sympathetic activity to arterioles and veins via alpha 1 causing vasodilation and decreased systemic vascular resistance so BP falls
controlling low BP
parasympathetic activity is decreased to the heart and sympathetic activity increases to heart and vascular smooth muscle
