Flashcards in Physiology Deck (179):
What is meant by autorhythmicity of the heart?
It is able to generate its own electrical impulses without external stimuli
Where does excitation of the heart normally originate?
What are the specialised cells within the SA node that initiate the heart beat called?
What is meant by sinus rhythm?
Describes the heart's pace being controlled by the SA node
SA node cells have a stable resting membrane potential. True/False?
They exhibit spontaneous pacemaker potential
What is the function of the spontaneous pacemaker potential?
Takes the membrane potential to threshold (depolarisation) to generate an action potential
What gives rise to pacemaker potential?
Decrease in K+ efflux
Slow Na+ influx
What causes the rising phase of the action potential (depolarisation) in SA node cells?
Opening of Ca++ channels, resulting in Ca++ influx
What causes the falling phase of the action potential (repolarisation) in SA node cells?
Opening of K+ channels, resulting in K+ efflux
Summarise the phases of the SA node action potential
Pacemaker potential: decreased K+ efflux, slow Na+ influx
Rising phase: Ca++ influx
Falling phase: K+ efflux
Which junctions allow cell-to-cell spread of excitation?
The AV node is the only point of electrical contact between atria and ventricles. True/False?
AV node cells are large and slow to conduct. True/False?
They are small and slow to conduct
Why is AV nodal delay present?
To allow time for atrial systole to precede ventricular systole
Which fibres enable the excitation to spread to the ventricles?
Bundle of His and Purkinje fibres
What gives rise to the rising phase of the action potential in ventricular contractile cells?
Fast Na+ influx
Describe Phase 0 of the cardiac action potential
Fast Na+ influx causes reversal of the resting membrane potential from -90mV to +30mV
Describe Phase 1 of the cardiac action potential
Closure of Na+ channels + transient K+ efflux causes some repolarisation
What gives rise to the plateau phase (phase 2) of the cardiac action potential?
What gives rise to the falling phase (phase 3) of the cardiac action potential?
Describe Phase 3 of the cardiac action potential
Closure of Ca++ channels and opening of K+ channels allows K+ efflux which causes repolarisation of the membrane potential back to -90mV
Sympathetic stimulation causes increased heart rate. True/False?
What is meant by vagal tone?
Parasympathetic stimulation to the heart dominating in resting conditions
The vagus nerve supplies only the SA node. True/False?
Supplies both SA and AV nodes
What does parasympathetic stimulation do to the AV node?
Increases AV nodal delay
Which neurotransmitter acts on which receptor in parasympathetic control of the heart?
ACh on M2 receptors
Name a competitive inhibitor of ACh that is used in bradycardia
Vagal stimulation causes the slope of the pacemaker potential to increase. True/False?
Slope decreases (increased AV node delay)
What is meant by negative chronotropic effect?
Decreased contraction of the heart due to less frequent action potentials
Which areas of the heart does the sympathetic system supply?
Which neurotransmitter acts on which receptor in sympathetic control of the heart?
Noradrenaline on B1 receptors
Sympathetic stimulation does what to the slope of the action potential?
What is meant by positive chronotropic effect?
Increased contraction of the heart due to more frequent action potentials
Where does Lead I of an ECG connect?
Right arm - Left arm
Where does Lead II of an ECG connect?
Right arm - Left leg
Where does Lead III of an ECG connect?
Left arm - Left leg
Cardiac muscle is striated. True/False?
What creates the striated appearance of cardiac muscle?
Contractile protein elements (actin and myosin)
Give the name of protein channels that which form electrical communication between neighbouring myocytes
What do desmosomes do in the heart?
Provide mechanical adhesion between adjacent cardiac cells
Ensure tension is developed
What is contained within muscle fibres?
Myofibrils (contractile protein elements of muscle)
Actin filaments are thick and appear light. True/False?
They appear light but are thin
Myosin filaments are thick and appear dark. True/False?
What is the arrangement of of actin and myosin within each myofibril called?
Myosin filaments slide over actin filaments to produce muscle tension. True/False?
Actin slides over myosin!
What is required to generate the force by which sliding of filaments can occur?
What is the role of calcium in sliding of filaments?
Required to 'switch on' cross-bridge formation
1. binds to troponin complex on myosin
2. causes conform change which exposes actin binding site
3. cross-bridge forms via site and myosin binding site
What is the role of ATP in sliding of filaments?
Binds to myosin head to either energise it or break down the cross-bridge between myofibrils (that is created by calcium)
Where does the calcium that activates contractile machinery come from (where is it stored)?
What is meant by calcium-induced calcium release?
Ca++ influx during the plateau phase of the AP causes Ca++ to be released from the sarcoplasmic reticulum to cause contraction
What is meant by the refractory period?
Period following action potential where it is not possible to generate another action potential
What are the two moments where a new action potential cannot be generated?
Plateau phase (Na channels in closed state)
Falling phase (K channels open, thus membrane cannot depolarise)
What is the clinical benefit of the refractory period?
Prevents tetanic contractions of the heart
Define stroke volume
Volume of blood ejected by each ventricle per heart beat
EDV - ESV
What is meant by end diastolic volume (EDV)?
Volume of blood remaining in each ventricle following diastole
What determines EDV?
Describe the Frank-Starling Law of the Heart
The greater the EDV (as a result of more venous return), the greater the stroke volume will be during systole
Optimal skeletal muscle fibre length (for contraction) is achieved by stretching the muscle. True/False?
Optimal length is at rest
What is meant by preload?
Volume of blood in each ventricle before contraction
What is meant by afterload?
The resistance against which the heart has to pump after contraction
How does the Frank-Starling law compensate partially for decreased stroke volume?
EDV increases (due to failure to pump full SV) so force of contraction increases
What is meant by positive inotropic effect?
Force of contraction increases (due to sympathetic stimulation)
Parasympathetic system has a negative chronotropic and inotropic effect. True/False?
No inotropic effect
What does sympathetic stimulation do to the Frank-Starling curve?
Shifts it to the left (increased SV)
What is meant by cardiac output?
Volume of blood pumped out by each ventricle per minute
SV x HR
Cardiac valves produce a sound when they open and close. True/False?
Only produce a sound when they close (normally)
What is the cardiac cycle?
Encompasses all the events from one heartbeat to the next
What happens in Passive Filling?
AV valves open and blood flows into ventricles
80% of ventricular filling is done by atrial contraction. True/False?
80% is contributed to by passive filling
In Passive Filling, what are the pressures in the atria and ventricles?
Close to zero
What happens in Atrial Contraction?
Remaining atrial volume fills ventricles by atrial systole, completing the EDV
Which part of the ECG signals atrial depolarisation?
During which part of the ECG do the atria contract?
Between the P wave and QRS complex
What happens in Isovolumetric Ventricular Contraction?
AV valves shut since Patria less than Pventricles (produces S1)
Ventricle is essentially a closed box of high pressure
Which part of the ECG signals ventricular depolarisation?
When does ventricular systole take place on the ECG?
Between end of QRS and beginning of T wave (ST segment)
What happens in Ventricular Ejection?
Aortic/pulmonary valve opens since Pventricles is greater
Stroke volume is ejected; ESV remains
Ventricular pressure falls and aortic/pulmonary valve shuts since P here less than Pventricles (produces S2)
What produces the dicrotic notch in the aortic pressure curve?
Aortic valve closing
What happens in Isovolumetric Ventricular Relaxation?
Ventricle is a closed box again (since aortic/pulmonary valves have closed)
Pressure falls until Pventricles less than Patria, where AV valves open and whole cycle restarts
S1 heralds the end of systole. True/False?
Heralds the start of systole (AV valves shut)
S2 heralds the start of diastole. True/False?
Define 'blood pressure'
Outward force exerted by blood on blood vessel walls
Arterial laminar flow is audible. True/False?
If external pressure exceeds systolic blood pressure, sound will be heard through a stethoscope. True/False?
Which sort of blood flow can be heard through a stethoscope?
What is the 1st Korotkoff sound?
Peak systolic pressure
What are the 2nd-3rd Korotkoff sounds?
Intermittent sounds of turbulent flow
What is the 4th Korotkoff sound?
Last muffled sound heard before sound stops
What is the 5th Korotkoff sound?
Represents diastolic pressure
What are the formulae for calculating MAP?
[2 x diastolic + systolic]/3
diastolic + [systolic - diastolic]/3
CO x TPR
SV x HR x TPR
What is the normal range for MAP?
70-105 mm Hg
MAP of at least 50 mm Hg is needed to perfuse the vital organs. True/False?
At least 60 mm Hg is needed
Arteries are the main resistance vessels. True/False?
What are the main resistance vessels?
Which receptors regulate blood pressure short-term?
The higher the blood pressure, the greater the firing of baroreceptors. True/False?
Which CN do the carotid baroreceptors fire through?
Which CN do the aortic baroreceptors fire through?
When arterial blood pressure decreases, what happens with regards to baroreceptors?
Decreased firing, causing decreased vagal activity, causing increased sympathetic activity, causing increased vasoconstriction, leading to increase in blood pressure
When you suddenly stand up, what happens to the venous return to the heart and thus MAP?
How much of total body fluid does extracellular fluid contribute to?
What is the function of renin in the RAAS?
Released from kidneys to stimulate conversion of angiotensinogen to angiotensin I
What is the function of ACE?
Converts angiotensin I to angiotensin II
What is the function of angiotensin II?
Stimulates release of aldosterone
Causes systemic vasoconstriction
What is the function of aldosterone in the RAAS?
Acts on kidneys to increase Na+ and water retention
Where is renin released from?
Juxtapulmonary apparatus in the kidney
Renal artery hypertension causes renin to be released. True/False?
Hypotension would cause its release
Where is ANP stored?
What does ANP do?
Causes excretion of Na+ and water in the kidneys
Decreases renin release
When is ADH release stimulation?
Reduced extracellular fluid
Increased extracellular fluid osmolarity (solute)
What does ADH do?
Causes reabsorption of water, i.e. concentrates urine, to increase plasma volume
Vasoconstriction (small degree)
Which blood vessel holds the most blood volume at rest?
Resistance to blood flow is directly proportional to what?
Thickness and length of blood vessel
Resistance to blood flow is inversely proportional to what?
[Radius of blood vessel]^4
How is resistance to blood flow mainly controlled?
Through changes in the radius of the vessel
What is meant by vasomotor tone?
Vascular smooth muscle being partially constricted at rest due to tonic discharge of the sympathetic system (releases noradrenaline)
There is no parasympathetic innervation of vascular smooth muscle. True/False?
There is in the penis and clitoris
Adrenaline acting on alpha receptors causes what?
Adrenaline acting on beta receptors causes what?
Alpha receptors are predominant in skeletal and cardiac muscle arterioles. True/False?
Beta receptors are predominant here
Where are alpha receptors predominately found?
Skin, gut and kidney arterioles
What is the effect of angiotensin II on vascular smooth muscle?
Local metabolic conditions can override extrinsic control of vascular smooth muscle. Explain?
You can have local vasodilation at an organ, despite widespread vasoconstriction, and this will not influence overall blood pressure
Decreased local PO2 causes vasoconstriction in systemic circulation. True/False?
What is the effect of decrease in local PO2 in pulmonary circulation arterial smooth muscle?
Increased local [H+] and [K+] in systemic circulation causes vasodilation. True/False?
Name some humoral agents that are potent vasodilators
Name some humoral agents that are potent vasoconstrictors
Describe myogenic response to stretch
If MAP falls, resistance vessels in brain and kidneys dilate to increase flow (i.e. not in line with normal baroreceptor reflex)
Sympathetic stimulation increases during exercise. What does this do to the HR, SV and CO?
Increases all of them
How does blood flow change to the kidney and gut during exercise?
Decreases - vasomotor tone causes vasoconstriction in these areas
How does blood flow change to skeletal and cardiac muscle during exercise?
Increases due to vasodilation in these areas
What causes vasodilation in skeletal and cardiac muscle during exercise? i.e. what overrides sympathetic effects?
Systolic and diastolic murmurs coincide with the carotid pulse. True/False?
Only systolic murmurs coincide with carotid pulse
Physiological splitting of the 2nd heart sound occurs on inspiration. True/False?
What happens in physiological splitting of the 2nd heart sound?
Inspiration causes decrease in intrathoracic pressure, causing increase in venous return which prolongs RV ejection time (so pulmonary sound delayed fractionally behind aortic sound)
A 4th heart sound is always pathological. True/False?
An abnormality in the circulatory system, resulting in inadequate tissue perfusion and oxygenation
Which 3 factors influence the stroke volume?
Preload (venous return)
How does hypovolaemic shock arise?
Loss of blood volume leads to decreased venous return leads to decreased stroke volume leads to decreased CO + BP = low perfusion and oxygenation
How does cardiogenic shock arise?
Decreased myocardial contractility leads to decreased stroke volume leads to decreased CO + BP = low perfusion and oxygenation
How does tension pneumothorax lead to obstructive shock?
Increased intrathoracic pressure leads to decreased venous return leads to decreased stroke volume leads to decreased CO + BP = low perfusion and oxygenation
How does neurogenic shock arise?
Loss of sympathetic (vasomotor) tone leads to increased vasodilation leads to decreased venous return leads to decreased stroke volume leads to decreased CO + BP = low perfusion and oxygenation
How does vasoactive shock arise?
Release of vasoactive mediators leads to increased vasodilation leads to decreased venous return leads to decreased stroke volume leads to decreased CO + BP = low perfusion and oxygenation
Compensatory mechanisms exist to deal with blood volume loss until greater than 40% is lost. True/False?
Mechanisms only compensate until greater than 30% volume is lost
What are the first branches/arteries that come off the aorta?
Right + left coronary arteries
What drains coronary venous blood into the right atrium?
Which organ has the greatest oxygen demand?
The heart can increase the amount of oxygen extracted in order to improve its oxygenation. True/False?
It already extracts 75% of total, could not extract more
How is the oxygen supply to the heart increased, if not by increasing oxygen extraction?
Increase coronary blood flow
Decreased PO2 causes coronary vasoconstriction. True/False?
Need to improve oxygenation, therefore vasodilation occurs to increase blood flow
Decreased PO2 causes pulmonary vasoconstriction. True/False?
Describe the mechanism by which sympathetic stimulation (indirectly) causes coronary vasodilation
Sympathetic tone causes increased HR + SV and hence CO (which itself dilates coronary arteries);
This increases cardiac work and metabolism, consuming O2 (leading to decreased PO2, increased ADP) and releasing metabolites, all of which cause vasodilation;
Adrenaline also acts on B2 to cause vasodilation
What effect do K+, H+ and CO2 have on coronary arteries?
When does peak coronary flow occur?
Which arteries supply the brain?
Internal carotid arteries
Which brain matter - grey or white - is very sensitive to hypoxia?
Which arteries make up the Circle of Willis?
Internal carotids + basilar artery (formed by both vertebral arteries joining)
The baroreceptor reflex affects the brain. True/False?
If MAP rises, cerebral vessels constrict. True/False?
If MAP falls, cerebral vessels dilate. True/False?
When does autoregulation of cerebral blood flow fail?
When MAP is less than 60 or greater than 160 mm Hg
Decreased PCO2 results in cerebral vasodilation. True/False?
Results in vasoconstriction - this is why hyperventilation leads to fainting
Head injury and tumours increase intracranial pressure. How does this affect cerebral blood flow?
Decreased O2 does what to pulmonary arterioles? Why?
Vasoconstriction (opposite to systemic circulation)
Redirects blood to alveoli to get more oxygen
2/3 of body water is extracellular. True/False?
2/3 is intracellular
What is the function of capillaries?
Rapid exchange of gases, water and solutes with the interstitial fluid
Describe net filtration pressure
Forces favouring filtration - forces favouring absorption
Forces favouring filtration are stronger at the venule end. True/False?
Filtration is stronger at the arterial end
Forces favouring absorption are stronger at the venule end. True/False?
What are the forces favouring filtration?
Capillary hydrostatic pressure (Pc)
Interstitial fluid osmotic pressure (πi)
What are the forces favouring absorption?
Capillary osmotic pressure (πc)
Interstitial fluid hydrostatic pressure (Pi)
What is the main contributor to capillary hydrostatic pressure (Pc)?
Tends to force blood out of capillary
What is the main contibutor to capillary osmotic pressure (πc)?
Presence of plasma proteins
Tends to force blood into capillary
How is net filtration pressure calculated using forces described previously?
(Pc + πi) - (πc - Pi)
Pulmonary capillary hydrostatic pressure is high compared to systemic hydrostatic pressure. True/False?
What is oedema? How does it affect diffusion?
Accumulation of fluid in the interstitial space
Increases distance over which diffusion must take place
LV failure causes pulmonary oedema. True/False?