Flashcards in Cardiovascular System Deck (119):
Give an example of a substance that moves through aqueous pores in the capillaries
Give an example of a substance that dissolves through the lipid bilayer of the endothelium
What does the rate of diffusion depend on?
Area - generally large, depends on capillary density. More metabolically active means more capillaries.
Diffusion 'resistance' - Not a rate limiting factor. Depends on the nature of the molecule (small is best), nature of the barrier (pore size or number), path length (shorter is most active).
Concentration gradient - high concentration gradient increases rate of diffusion. The gradient must be maintained for exchange to continue.
Describe the nature of blood flow in the brain, heart muscle and kidneys.
Brain - high, constant flow
Heart muscle - high, increase during exercise
Kidneys - high, constant
What vasculature controls flow?
Arterioles and pre-capillary sphincters
Ability to store blood. Veins chanage distention to increase blood in them, providing a temporary store.
Describe the pacemaker of the heart
A small group of highly specialised cells which spontaneously generate action potentials that spread over the whole heart for coordinated contraction. One action potential at regular intervals.
Describe the spread of excitation in the heart.
Pacemaker in the SAN, located in the right atrium.
Spreads over the atria causing atrial systole
Reaches the AVN, delayed for ~120ms.
From the AVN, spreads down the septum.
Bundles of His are specialised to spread action potentials quickly down the apex. Spreads from the endocardial to the epicardial surface.
As the ventricle contracts it rotates slightly, pushing the blood towards the valves.
Describe isovolumetric relaxation
A rapid drop in pressure as the ventricles relax without blood moving out.
Describe isovolumetric contraction
Ventricular systole, all valves closed. No change in ventricular volume.
Filling of the ventricles stops as the atrial and ventricular pressure .
How to calculate cardiac output
Stroke volume x heart rate
Describe the changes to membrane proteins and ion movements in a ventricle during an action potential.
Voltage gated sodium channels open causing depolarisation.
Sodium channels become inactivated, transient outward K+ channels open. Slight hyperpolarisation.
Voltage-gated L-type calcium channel open, causing the platau. Some potassium channels are open.
Repolarisation as calcium channels inactivate (slowly). More potassium channels open.
Describe the changes to membrane proteins and ion movements in cardiac pacemaker cells during an action potential.
At -60mV, sodium channels are largely inactivated. There is no stable resting potential (pacemaker potential). A funny current if formed due to channel permeability to sodium and potassium, mainly allowing an influx of sodium. This activates on hyperpolarisation and is cyclic nucleotide gated (HCN channel).
Opening of L-type calcium channels causing an increased rate of depolarisation to +30mV.
Voltage-gated potassium channel repolarises to -60mV before depolarisation begins again.
Describe the HCN channel
Hyperpolarisation activated cyclic nucleotide gated channel
Can be activated by cAMP
Allows sodium influx
Describe briefly the route a wave of excitation takes through the heart
Initiation by the SAN, spreads to the AVN where it is delayed. Then spreads down the bundles of His, then purkinje fibres down the interventricular septum, and up the walls of the ventricle.
What is the function of gap junctions in myocytes and what protein is found in them?
What is the function of desmosomes in myocytes and what protein is found in them?
Describe how high intracellular calcium is triggered by the initiation of an action potential.
Depolarisation opens L-type calcium channels in the t-tubule system.
This causes a localised increase in calcium which opens CICR channels in the sarcoplasmic reticulum.
Describe the difference in initiation of calcium release in skeletal and cardiac muscle.
Skeletal - conformational change from depolarisation causes calcium release
Cardiac - calcium is required in the cell to cause release from the SR.
Describe how low calcium is resumed in cardiac myocytes after contraction.
SERCA in the sarcoplasmic reticulum brings calcium back in
NCX transports calcium out of the cell, with the assistance of Na/K ATPase.
Describe how high intracellular calcium in cardiac myocytes causes contraction.
Calcium binds to and activates TnC, causing a conformational change. This moves tropomyosin to reveal the myosin binding site on actin filaments.
What controls the tone of arteries, arterioles and veins?
Smooth muscle in the tunica media.
In smooth muscle of the tunica media, what is actin anchored to?
Dense bodies in the cytoplasm and dense bands on the membrane
Describe the pathway of events after activation of a GPCR with G-alpha(q)
DAG and IP3 release.
IP3 activates MLCK which enables phosphorylation of the light chain of myosin head, allowing it to bind.
Explain the role of protein kinase A in contraction of smooth muscle
PKA inhibits MLCK.
PKA regulated by cAMP
What reverses the action of MLCK?
Describe the resistance and pressure in arteries.
Describe the term total peripheral resistance.
Total resistance opposing blood flow in the peripheral circulation
Describe the importance of arteries having distensible walls.
If arteries were solid the pressure in systole would force the stroke volume into the arterioles, so in diastole the pressure would fall to zero.
Distensible walls allow the arteries to stretch in systole so more blood flows in than out (capacitance). As they recoil in diastole, flow continues into the arterioles, preventing such a rapid change in pressure.
What causes the notch in the pressure wave of an artery?
Opening of valves
Give the factors that affect systolic pressure.
Strength of the heart pumping
Total peripheral resistance
Compliance of arteries
What factors affect diastolic pressure?
Total peripheral resistance
Why is diastolic pressure a better measure of total peripheral resistance than systolic?
Its not diluted by compliance
Define pulse pressure
The difference between systolic and diastolic pressure (usually 40mmHg).
What is the effect of adrenaline on pulse pressure?
Increases because the heart works harder
State how you would calculate the average pressure in the systemic system.
Diastolic pressure + 1/3 pulse pressure
Why do arterioles have a high resistance?
They have a narrow lumen
What is the function of pre-capillary sphincters and where are they mainly found?
In skeletal muscle, control resistance. Can totally close off some capillaries.
Required there because there must be such significant changes in blood flow.
What is vasomotor tone, and describe what happens when it increases and decreases.
Tonic contraction of smooth muscle.
Increase - vasoconstriction. Increased resistance and contraction.
Decrease - vasodilatation. Decreased resistance and contraction (not dilation)
Give the factors affecting vascular smooth muscle contraction.
Majorly by vasomotor tone.
Antagonised by vasodilator factors
Actual resistance by a balance of the two
Describe reactive hyperaemia.
When circulation is cut off from an area, restoration causes an enormous increase for a short while.
This is because vasodilator metabolites are produced which relax vascular smooth muscle, and are not being removed as there is no blood flow. Eventually restores to normal.
Give some vasodilator metabolites.
H+ from anaerobic respiration
Supply pressure changes so blood flow to the tissue will change. This changes metabolite concentration and alters the resistance of arterioles so blood flow returns to appropriate levels for metabolism.
What effect does low TPR have on venous pressure and why?
Increases venous pressure. Its easier to leave the arterioles.
What is central venous pressure?
Pressure in the great veins which fill the heart in diastole.
What does central venous pressure depend on?
Return of blood from the body
Pumping of the heart
Gravity and muscle pumping
Why does gravity affect superficial veins more significantly than deep veins?
Deep veins are surrounded by interstitial fluid, so when gravity pulls the blood down it also does so to the fluid surrounding it, which helps to keep the pressure constant.
What happens to total peripheral resistance as blood requirement increases?
Describe the difference between flow and velocity.
Flow - volume of fluid passing a given point per unit time
Velocity - rate of movement of fluid particles along a tube
Describe the relationship between velocity and cross sectional area with a constant flow.
Velocity proportional to inverse of cross sectional area.
Describe laminar flow.
Gradient of velocity from the middle to the edge of the vessel.
Highest in the centre, stationary at the edge.
Describe turbulent flow
Velocity gradient broken down, flow resistance greatly increases.
Occurs when mean velocity increases past a threshold.
What can you hear when there is laminar flow in an artery?
Describe what you would hear if there was turbulent flow in an artery.
Describe what is meant by viscosity.
The extent to which fluid layers resist sliding over each other.
At a constant gradient of velocity, what is the relationship between mean velocity and cross sectional area?
Mean velocity is proportional to cross sectional area
Describe the relationship between resistance and radius.
Resistance is proportional to 1/r^4
Why is pressure in the arteries high?
Because of high resistance in the arterioles.
If the heart increases cardiac output but resistance remains the same, what effect will this have on arterial pressure?
It will increase
Describe what happens to veins and venules with distensible walls at low pressures.
In very low pressure the tube will collapse.
With an initial increase in pressure, nothing will happen.
There is a bigger increase in flow per increment of pressure.
What is the difference between bronchial and pulmonary circulation?
Bronchial is part of systemic, meets metabolic requirements
Pulmonary is the blood supply to the alveoli for gas exchange.
Why is the pulmonary circulation low pressure and low force?
Thin right ventricle
Arterioles have a small amount of smooth muscle
Many small, branching capillaries
What is the pressure in the right ventricle?
What is the pressure in the left ventricle?
What is the pressure in the right atrium?
What is the pressure in the left atrium?
What is the pressure in the pulmonary artery?
What allows the pulmonary artery to maintain pressure during diastole?
What is the special task that the pulmonary circulation is adapted for?
Sufficient gas exchange
What is the optimum ventilation/perfusion ratio?
Describe hypoxic pulmonary vasoconstriction.
In poorly ventilated alveoli, pO2 decreases and perfusion is decreased by vasoconstriction locally.
How can emphysema cause right ventricular heart failure?
It causes chronic increased vascular resistance by hypoxic pulmonary vasoconstriction.
This increases afterload on the right ventricle, causing hypertrophy which can lead to heart failure.
Where does pulmonary oedema tend to form and why?
It is caused by high hydrostatic pressure which is greatest in the base of the lungs due to the effect of gravity on the low pressure vessels.
How is oxygen uptake by the lungs increased during exercise without increasing the rate of respiration?
There is an increase in cardiac output which is accepted by the lungs.
This causes an increase in arterial blood pressure so the apical capillaries open more, increasing oxygen uptake by the lungs.
Blood flow is increased so the capillary transit time is reduced.
Describe the difference between oncotic pressure and hydrostatic pressure.
Oncotic - pressure exerted by large molecules which draws fluid into the capillaries
Hydrostatic - blood within the capillaries
What features of the blood supply in the brain helps to keep a secure oxygen supply?
High capillary density
Small diffusion distance
High basal flow
High oxygen extraction
What gives secondary oxygen supply to the brain?
Circle of Willis - anastamoses between the basilar and inferior carotid arteries
Myogenic autoregulation mechanisms maintain perfusion during hypetension
Metabolic factors regulate flow
Brain stem regulated other circulation
Describe myogenic autoregulation of perfusion in the brain.
Originates in smooth muscle cells which respond to stretch by constricting.
Describe metabolic regulation of perfusion in the brain.
Hypercapnia causes vasodilatation. Very sensitive to arterial pCO2
How can panic hyperventilation cause dizziness and fainting?
Causes hypocapnia and cerebral vasoconstriction
Describe regional action of metabolites in the brain that regulates perfusion.
Cause a local increase in blood flow, so high neuronal activity causes an increase in blood flow.
High pCO2, K+ concentration and adenosine and low pO2 causes vasodilatation.
Describe Cushing's reflex
Space occupying lesions increase the pressure in the brain, impairing blood flow.
This increases sympathetic vasomotor action which increases arterial blood pressure, maintaining blood flow.
What allows the coronary arteries to fill with blood?
During diastole, backflow of blood into the aortic sinus
What features of the coronary circulation allows consistent high perfusion of the heart muscle?
High capillar density
Small fibre diameter
Continual production of NO by coronary endothelium which increases vasodilatation
Metabolic hyperaemia (high adenosine/potassium concentration, low pH)
Why are coronary arteries functional end arteries?
They have few anastamoses
What are vasodilators in the skeletal muscle circulation?
High inorganic phosphate
Adrenaline via beta-2 adrenoceptors
What causes vasoconstriction in skeletal muscle circulation?
NA acting on alpha-1 adrenoceptors
Describe how apical skin can cause increased heat loss.
Arterovenous anastamoses have rich sympathetic innervation which respond to changes in body temperature by neural control
Open to lose heat as it causes skin temperature to rise, increasing dissipation. Blood moves straight to the venous plexus
Describe how non-apical skin can cause increased heat loss.
Sweating is increased,
Local mediators from the sweat glands vasodilate capillaries.
What does oxygen demand of the heart depend on?
Wall tension (preload/afterload)
What does oxygen supply of the heart depend on?
Coronary blood flow (perfusion pressure/coronary artery resistance)
Carrying capacity of blood (haemoglobin/gas exchange)
Give some pathologies which cause decreased oxygen supply to the myocardium.
Give some pathologies which can increase oxygen demand
How can thyrotoxicosis cause an increase in oxygen demand of the myocardium?
Increased heart rate and BMR
How can aortic stenosis cause an increase in oxygen demand?
Increase afterload means the muscle has to work harder.
Low pressure at the bottom of the aorta can reduce filling of the coronary arteries so also decreases perfusion
Give some modifiable risk factors for CAD
Obesity and metabolic syndrome
Give some non-modifiable risk factors for CAD
What is the difference between a stable and vulnerable plaque?
Stable - small necrotic core, thick fibrous cap, less likely to fissure/rupture
Vulnerable - large necrotic core, thin fibrous cap, more likely to fissure/rupture
How can a vulnerable plaque precipitate thrombosis?
Fibrous cap undergoes erosion or fissuring
Expose blood to thrombogenic material in the necrotic core
Platelet clot followed by fibrin thrombus
What percentage of the lumen should be occluded before ischaemia?
How does a stable plaque clinically present?
No symptoms or stable angina
How does an unstable plaque clinically present?
Acute coronary syndrome (a medical emergency)
Describe the symptoms of angina.
Central and tightening pain
Can be relieved within five minutes by rest or nitrates
What clinical signs of angina risk factors may help to indicate it as a diagnosis?
High blood pressure
Left ventricular dysfunction
What characteristics can be seen on a stress ECG if a person has stable angina?
ST depression below or equal to 1mm
What are the aims when treating stable angina?
Reduce oxygen demand (decrease preload/afterload, heart rate and contractility)
Increase blood flow
Stop progression, stabilise the plaque and inhibit thrombosis
Give some drugs which can be used in the treatment of stable angina and their function
Beta blockers - decrease heart rate and contractility
Calcium channel blockers - decrease afterload as they cause peripheral vasodilatation
Nitrates - venodilator which decreases preload
Aspirin - decrease platelet aggregation which reduces thrombus formation if a plaque were to rupture or fissure
Statins - reduce LDL cholesterol by inhibiting HMG-CoA reductase. Decreases plaque progression.
Describe percutaneous coronary intervention
Introduce a catheter through the femoral artery and progress to beyond the plaque. Inflate the balloon to open the artery and place the stent.
Describe a CABG
Use an internal artery or vein to bypass a blockage
What conditions cause a rise in cardiac biomarkers?
What causes a STEMI?
Complete and persistent occlusion.
Large area of myocardium without circulation is affected.
Severe ischaemia with myocardial necrosis
What causes an NSTEMI or unstable angina?
Non-occlusive thrombus which is brief.
Small area of myocardium is affected.
Why is there ST elevation in STEMI but not NSTEMI?
Injury is transmural in a STEMI
What are the symptoms of a myocardial infarction?
Extreme central crushing chest pain which persists at rest.
Anxiety, sweating, pallor, nausea, vomiting
Low blood pressure and breathlessness can cause fainting.
May have tachycardia and arrhythmia with signs of heart failure.
Describe the different cardiac biomarkers which can be released during a myocardial infarction and when they can be detected.
cTnT and cTnI. Peak between 18 and 36 hours, decline over the next 2 weeks.
CK-MB only used if cardiac troponins are unavailable. Rise 3-4 hours after an attack, peak at 24 hours. Fall to zero after 48-72 hours.
Describe the ECG changes during a STEMI and what causes them.
Wide and deep pathological Q waves due to necrosis
ST elevation due to injury
T wave inversion due to ischaemia
All seen in the leads facing the damage.
Describe the long term treatments after an MI
Aspirin - decrease mortality and reinfarction
Beta blockers - decrease mortality and reinfarction
ACE inhibitors - improve survival
Statins - reduce mortality and reinfarction
Manage risk factors with lifestyle modification e.g. weight loss, exercise, diet, stop smoking and reduce alcohol intake.