Physiology Flashcards
Where are cardiac electrical impulses generated?
Within the heart
What is the name given to the term used to describe the way by which the heart can beat without external stimuli?
Authorhythmicity
Where does excitation originate?
Pacemaker cells found in the sino-atrial node
What is the SA node and where can it be found?
A cluster of pacemaker cells that initiates a heart beat. It is located in the upper right atrium close to where the superior vena cava enters the right atrium
What is it called when a heart is controlled by the SA node?
Sinus rhythm
What can be said about the resting potential of pacemaker cells?
They have no stable resting membrane potential, it drifts until an action potential is reached.
How is an action potential generated in pacemaker cells?
The spontaneous pacemaker potential takes the membrane potential to a threshold where an action potential is generated
Why does permeability not remain constant between action potentials?
- decrease in potassium efflux - sodium and potassium influx - transient calcium influx
What is the other name for the sodium/potassium influx?
The funny current
Once the threshold is reached what results in the rising phase of the action potential?
- activation of L type calcium channels - calcium influx
What is the falling phase due to?
- inactivation of L type calcium channels - activation of potassium channels leading to potassium efflux
Describe the pathway of impulse
SA - AV - Bundle of His - Right/left branches - purkinje fibres
What is the AV node and where can it be found?
It is the only point of electrical contact between the atria and ventricles. It is a small bundle of specialised cells found just above the junction of the atria and ventricles.
Why is conduction slow?
The cells are small in diameter
How does current flow from cell to cell?
Gap junctions
What does the conduction delay allow?
Atrial systole to precede ventricular systole
How many stages are there of cardiac myocyte action potential?
5 phase 0-4
Describe phase 0
- resting potential remains - 90mV - fast sodium influx causes the potential to become 20mV
What happens in phase 1?
- closure of sodium channels - transient potassium efflux
Give another name for phase 2 and describe what happens
plateau phase - calcium influx through L type calcium channels
What happens in phase 3?
- closure of calcium channels - potassium efflux
Describe phase 4
resting membrane potential
What is the normal heart rate?
60-100bpm
Name HR<60 and HR>60
<60 bradycardia >60 tachycardia
What nerve continuously influences the heart and what does it do?
Vagus nerve Slows the heart rate from 100bpm to 70 bpm by supplying the SA and AV node
What is the effect of vagal stimulation?
decreased heart rate and increased AV nodal delay
What does vagal stimulation do to the action potential graph?
Decreases the slope of pacemaker potential so it takes longer to reach the threshold so the frequency of action potentials decreases
State the term used to describe an effect that slows the heart rate
Negative chronotopic
Describe the sympathetic innervation of the heart
Cardiac sympathetic nerves supply the SA node, AV node and myocardium
What does sympathetic stimulation result in?
increased heart rate, decreased AV nodal delay and increased force of contraction
What does sympathetic stimulation do to the action potential graph?
increased the slope of pacemaker potential so it reaches threshold faster so the frequency of action potentials increases
State the term used to describe an effect that speeds up the heart rate
positive chronotopic
Are cardiac myocytes striated?
Yes, due to the regular arrangement of contractile protein
Are there neuromuscular junctions in cardiac cells?
No - gap junctions
What is the function of gap junctions?
Protein channels that form low resistance electrical communication pathways between neighbouring myocytes. They ensure electrical impulses reach all cardiac myocytes
Other than gap junctions what other junction is there between cells?
Desmosomes - provide mechanical adhesion between adjacent cardiac cells. Ensure tension is transmitted from one cell to the next
What is another name for cardiac muscle cells?
muscle fibres
Describe myofibrils
contractile units of muscle, they have alternating segments of thick and thin protein filaments
What causes the thick/thin appearance? What do these proteins form?
actin - thin/light myosin - thick/darker Forming sarcomeres
What does muscle contraction depend on?
ATP and calcium
Describe muscle in the relaxed state in terms of actin and myosin
no cross bring binding because binding site is covered by troponin
Describe muscle contraction
binding of calcium to troponin means the binding site is exposed and myosin can bind. This triggers a power stroke that pulls actin forward.
What is the refractory period?
the period following an action potential in which it is not possible to generate another action potential
How does the refractory period occur?
Sodium channels are closed Potassium channels are open Therefore the membrane cannot be depolarised
What is the benefit of the refractory period?
It protects the heart and prevents tetanic contractions
Define stroke volume
the volume of blood ejected by each ventricle per heart beat
How can stroke volume be calculated?
SV = EDV - ESV
Describe the intrinsic control of stroke volume
Diastolic stretch is determined by the volume of blood in each ventricle at the end of diastole. EDV is determined by venous return.
What does the Frank Starling curve show?
the more the ventricle is filled with blood (higher EDV) the higher the stroke volume
As well as stroke volume what else does stretch increase?
Affinity of calcium for troponin, the optimal length is achieved by stretching unlike skeletal muscle which can be overstretched
Define preload and afterload
preload - initial stretching prior to contraction afterload - pressure required to eject blood
What happens if afterload increases?
The heart is unable to eject full stroke volume so EDV increases. Force of contraction increases and eventually ventricular mass too.
Name two types of extrinsic control of stroke volume
- Nerves - Hormones
What nerve supply influences stroke volume?
sympathetic fibres with noradrenaline
Describe the effect of noradrenaline
increases the force of contraction known as a positive inotropic effect
How is force of contraction increased?
- Increased calcium influx - Increased ventricular pressure - Increased rate of pressure change - Decreased duration of systole - Decreased duration of diastole
What hormones control stroke volume?
adrenaline & noradrenaline released from adrenal medulla have inotropic and chronotropic effects
Define cardiac output
volume of blood pumped by each ventricle per minute
How can cardiac output be calculated what is the usual value?
CO = SV x HR Usually about 5L (70x70)
Define blood pressure
The outwards pressure exerted by the blood on the vessel walls
Define systemic systolic arterial blood pressure
the pressure exerted by the blood on the walls of the aorta and systemic arteries when the heart contracts
What should systolic BP be less than?
140mmHg
Define systemic diastolic arterial blood pressure
the pressure exerted by the blood on the walls of the aorta and systemic arteries when the heart relaxes
Give the values required for a diagnosis of hypertension
clinic BP >140/90 day time BP >135/85
What is pulse pressure?
The difference between systolic and diastolic normally between 30mmHg and 50mmHg
For a BP of 120/80 give examples of korotkoff sounds
- At 120 the first sound will be heart as systolic exceed the cuff 2/3 intermittent sound will be heart as the BP is turbulent but exceeds the cuff 4. The sound becomes muffled 5. No sound - diastolic
What drives the blood around the systemic circulation?
A pressure gradient between the aorta and the right atrium MAP - CVP
Define mean arterial pressure
the average arterial blood pressure during a single cardiac cycle
How can MAP be calculated?
MAP = 2diastolic+systolic/3 MAP = DBP+1/3 pulse pressure MAP= SVxHRxSVR
What is the normal range for MAP?
70-105mmHg At least 60mmHg is required to perfuse the coronary arteries, brain and kidneys.
Define systemic vascular resistance
The sum of resistance of all vasculature in the systemic circulation
What are the main resistance vessels?
arterioles
What are the two types of baroreceptor? Where are they found?
carotid and aortic found in their respective arteries
What is the control centre and effectors for baroreceptor signals?
control centre - medulla effectors - heart and blood vessels
How are the baroreceptors transmitted?
aortic - vagus nerve - 10th cranial nerve carotid - glossopharyngeal nerve - 9th cranial nerve
What disease do baroreceptors prevent?
postural hypotension
Describe postural hypotension is overcome
when someone stands up the venous return to the hearts decreases so MAP decreases and baroreceptor firing decreases. This decreases vagal tone but increases sympathetic tone. HR and SV increase, sympathetic constrictor tone increases so SVR increases and venous return increases
What is postural hypotension?
Failure of baroreceptors responses to gravitational shifts in blood
What are the risk factors for postural hypotension?
- Age - Medications - Certain diseases - Reduced intravascular volume - prolonged bed rest
What is a positive result for postural hypotension?
Standing up from lying down within 3 mins a drop of; systolic BP of at least 20mmHg (with or without symptoms) diastolic BP of at least 10mmHg with symptoms
What are the symptoms of postural hypotension?
- light head - dizzy - falls - faints - blurred vision
What happens to baroreceptor control if abnormal blood pressure is sustained?
It will reset to the new level and only fire when there is an acute change in MAP
Can baroreceptors supply information about steady state BP?
No
What regulates long term MAP?
Hormones that regulate plasma volume
What is total body fluid made up of?
intracellular fluid (2/3) + extracellular fluid (1/3)
Name the two components of extracellular fluid volume
plasma volume - in the vessels interstitial volume - bathes the cells acts as the go between blood and vessels
What can be said about the plasma and interstitial?
They are at equilibrium so if plasma fluid falls, fluid from the interstitial compartment will move into the plasma
Name two factors that affect extracellular fluid volume
- water excess or deficit - sodium excess or deficit
How are the factors affecting extracellular fluid controlled?
By hormones
What three classes of hormones control blood pressure?
Renin-Angiotensin- Aldosterone Natriuretic Peptides Antidiuretic Hormone (ADH)
Describe the function of renin
It is released by granular cells in the kidneys and stimulates the formation of angiotensin I in the blood from angiotensin produced in the liver. Angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE) which is mainly produced in the pulmonary vascular endothelium
Describe the function of angiotensin II
- stimulates the release of aldosterone from the adrenal cortex - causes sympathetic vasoconstriction (increases SVR and BP) - stimulates thirst and ADH release to increase plasma volume and BP
Where is aldosterone produced?
Adrenal Cortex
What is the function of aldosterone?
Steroid hormone that acts on the kidneys to increase sodium and water reabsorption to increase plasma volume and blood pressure
What is the rate limiting step in RAAS?
renin secretion
Name three stimuli that regulate renin secretion
- renal artery hypotension cause by sympathetic hypotension
- stimulation of renal sympathetic nerves
- decreased sodium in the renal tubular fluid - sensed by macula densa
Where are natriuretic peptides synthesised? When are they released?
The heart, brain and other organs, they are released in response to cardiac distension and neurohormonal stimuli.
What do natriuretic peptides cause?
- excretion of salt and water in the kidneys to reduce blood volume and pressure
- decrease in renin release to decrease blood pressure
- vasodilation to decrease SVR They are a counter
- regulatory system to RAA
What are the two types of natriuretic peptide?
atrial & brain type
Describe atrial natriuretic peptide
28 amino acid peptide synthesised and stored by atrial muscle cells. ANP is released in response to atrial distension (hypovolaemic states)
Describe brain type natriuretic peptide
first synthesised as prepro BNP then cleaved to pro BNP (108 amino acids) and finally BNP (32 amino acids)
What can be measured in patients with suspected heart failure?
serum BNP and N terminal piece of pro BNP
Where does ADH come from?
A pre hormone precursor synthesised by the hypothalamus and stored in the posterior pituitary.
How is secretion of ADH stimulated?
- reduced extracellular volume - increased extracellular fluid osmolality - increased plasma osmolality
What does plasma osmolality indicate and how is it monitored?
solute - water balance monitored by osmoreceptors mainly in the brain close to the hypothalamus
What does ADH do?
- Acts in the kidney tubules to increase reabsorption of water - this increases extracellular &; plasma volume
- which increases cardiac output and blood pressure
- It also acts on blood vessels to cause vasoconstriction and increase SVR and BP
When is the vasoconstriction by ADH important?
patients in hypovolaemic shock
What is the main site of SVR?
Arterioles
What does contraction cause?
Vasoconstriction that increases SVR and increases MAP
What does relaxation cause?
Vasodilatation that decreases SVR and decreases MAP
Describe the relationship of resistance
It is proportional to blood viscosity and length of vessel It is inversely proportional to the radius of the blood vessel to the power of four
What does extrinsic control involve?
Nerves and hormones
Describe the nerve supply to vascular smooth muscle
sympathetic nerves cause partial constriction at rest vasomotor tone
What causes vasomotor tone?
Tonic discharge of sympathetic nerves results in continuous release of noradrenaline
What is the effect of increased discharge?
increased vasomotor tone and vasoconstriction
What is the effect of decreased discharge?
decrease vasomotor tone and vasodilatation
Name the main hormone involved in extrinsic control
Adrenaline from the adrenal medulla causes organ specific responses
Describe the effect on alpha receptors
vasoconstriction, receptors in the skin gut & kidney arterioles
Describe the effect on beta 2 receptors
vasodilatation, receptors in cardiac and skeletal muscle
What other hormones are involved in extrinsic control?
- angiotensin II causes vasoconstriction - antidiuretic hormone causes vasoconstriction
How does intrinsic control work?
Matches the blood flow of different tissues to their metabolic needs
State the two components of intrinsic control
- chemical - physical
Describe chemical control
local metabolic & humoral changes within an organ influence the contraction of arteriolar smooth muscle
Name five metabolic changes that can lead to vasodilatation and relaxation
- decreased pO2
- increased local CO2
- increased local [H+]
- increased extracellular potassium
- increased osmolality of ECF Adenosine release
What are humoral responses to?
- histamine - bradykinin - nitric oxide All cause vasodilatation
Where is nitric oxide produced?
Vascular endothelium from amino acid L arginine through the enzyme nitric oxide synthase (NOS)
What is the impact of stress on NO release?
Increased flow causes calcium release & activation of NOS
What can also induce NO formation?
chemical stimuli
Where does NO diffuses from and to?
vascular endothelium into the adjacent smooth muscle cells where it activates the formation of cGMP that signals smooth muscle relaxation
Name four chemical signals that cause vasoconstriction
- serotonin - thromboxane A2 - leukotrienes - endothelin
What do endothelial products control?
- thrombus - inflammation - oxidation
Name three physical components of intrinsic control
- temperature -myogenic response to stress -sheer stress
Explain how temperature influences resistance
cold causes vasoconstriction warmth causes vasodilatation
How does myogenic response to stress influence resistance?
MAP rises, resistance vessels constrict MAP falls resistance vessels dilate
What is the effect of dilated arterioles?
Causes stress in the arteries upstream making them dilate, increasing flow to metabolically active tissues
State four factors that increase venous return
- venomotor tone - skeletal muscle pump - respiratory pump - blood volume
What does increase in venous return lead to?
increase in atrial pressure, increase EDV and increase stroke volume
Describe venomotor tone
Veins contain most of the blood volume at rest. Smooth muscle is supplied with sympathetic fibres and stimulation gives venous constriction
What does an increased venomotor tone result in?
increased venous return, SV and MAP
Describe the respiratory pump
when intra-thoracic pressure decreases, intra-abdominal pressure increases creating a suction effect that moves towards the heart
Describe the skeletal pump
contraction aids venous return, one way venous valves allow blood to move towards the heart. Muscle activity increases venous return.
How does sympathetic innervation effect the body in the acute response to exercise?
- Increases heart rate and stroke volume
- Reduces flow to the kidneys and gut by vasoconstriction
- Metabolic hyperaemia overcomes vasomotor drive causing vasodilatation, blood flow increases in proportion to metabolic activity
- Cardiac output increases SBP
- hyperaemia decreases SVR and SBP
What is the main chronic response to exercise?
Reduced blood pressure
Define shock
an abnormality of circulatory system resulting in inadequate tissue perfusion & oxygenation
How does shock result in cellular failure?
inadequate tissue perfusion→ inadequate tissue oxygenation → anaerobic metabolism → accumulation of metabolic waste products → cellular failure
What does adequate tissue perfusion depend on?
- adequate blood pressure
- adequate cardiac output
Ultimately what does cardiac output depend on?
MAP = CO x SR
CO = SV x HR
Stroke volume
- preload (venous return)
- myocardial contractility
- afterload
Name five types of shock
- hypovolaemic
- cardiogenic
- neurogenic
- obstructive
- vasoactive
Describe hypovolaemic shock
loss of blood volume → decreased blood volume → decreased venous return → decreased EDV → decreased stroke volume → dereased cardiac output → inadequate tissue perfusion
Define cardiogenic shock and give an example
sustained hypotension caused by decreased cardiac contractility
e.g. myocardial infarction due to damage to the pump
Describe cardiogenic shock
decreased cardial contractility → decreased stroke volume → decreased cardiac output & BP → inadequate tissue perfusion
Give an example and describe obstructive shock
e.g. pneumothorax
increased intra-thoracic pressure → decreased venous return → decreased EDV → decreased stroke volume → decreased cardiac output & BP → inadequate tissue perfusion
Describe neurogenic shock
loss of sympathetic tone to blood vessels & heart →venous and arterial vasodilatation → decreased heart rate → decreased venous return & SVR → decreased cardiac output & BP → inadequate tissue perfusion
Describe vasoactive shock
release of vasoactive mediators → venous & arterial vasodilatation → increased capillary permeability & leakage of fluid → decreased venous return & SVR → decreased cardiac output & BP → inadequate tissue perfusion
What are the four key steps in treatment of shock?
- ABCDE
- high flow oxygen
- volume replacement
- call for help early
What treatment can be used when the specific type of shock is known?
- inotropes in cardiogenic shock
- immediate chest drain in pneumothorax
- adrenaline for anaphylactic shock
- vasopressors in septic shock
What is the most common type of shock?
Hypovolaemic
Give two causes of hypovolaemic shock
- Haemorrhage (trauma, surgery, GI)
- Vomitting, diarrohoea, excessive sweating (decreases ECF)
How long can compensatory mechanisms control blood pressure for?
Until blood lost is >30%
What will haemorragic shock do to the body?
Increased heart rate via baroreceptors - tachycardia
Decreased stroke volume - small volume pulse
Increased SVR via baroreceptors - cool peripheries
All of which lead to decreased MAP if >30% blood lost
What special adaptations are present in the coronary circulation?
- High capillary density
- High basal blood flow
- High oxygen extraction (75%)
What is the result of a high oxygen extraction?
Extra oxygen cannot be supplied by increasing extraction
How is the coronary circulation intrinsically controlled?
- Decrease pO2 causes vasodilatation
- Metabolic hyperaemia matches flow to demand
- Adenosine causes vasodilatation
How is the coronary circulation extrinsically controlled?
Coronary arterioles are supplied by sympathetic vasoconstrictor nerves but over-ridden by metabolic hyperaemia as a result of increased heart rate & stroke volume.
Sympathetic stimulation results in vasodilation by adrenaline activating beta 2 adrenoceptors
When does most coronary blood flow and perfusion occur?
During diastole because the subendocardial vessels from the LCA are not compressed
What arteries supply the brain?
Internal carotid & vertebral arteries
Why does it need a secure supply of oxygen?
Grey matter is very sensitive to hypoxia, consciousness is lost after a few seconds of ischaemia
How soon does irreversible damage to the brain occur during hypoxia?
within 3 mins
What is the special adaptation of the cerebral circulation regarding the arteries?
Basilar & carotid arteries anastomos to form the circle of willis this mean that if one caroitd artery gets obstructed the brain can still receive a blood supply
What is the special adaptation of the cerebral circulation regarding blood pressure?
If MAP increases vessels constrict to limit blood flow
If MAP decreases vessels dilate to maintain blood flow
What happens if MAP falls below 50mmHg?
- confusion
- fainting
- brain damage
What does the skull consist of?
80% Brain
12% Blood
8% Cerebrospinal fluid
What are the normal values of intracranial pressure?
8-13mmHg
What is the effect of increasing ICP?
Decreases cerebral perfusion pressure & blood flow
Describe the blood brain barrier
Capillaries have very tight intercellular junctions which are highly permeable to oxygen & carbon dioxide. Glucose crosses by facilitated diffusion but in order to protect brain neurons it is impermeable to hydrophilic substances
What is the normal range for pulmonary blood pressure?
20-25/6-12mmHg
What are the special adaptation of the pulmonary circulation?
- Low capillary pressure
- Absorptive forces exceed filtration to protect against oedema
- Vasoconstriction in response to hypoxia means blood will be diverted to poorly ventilated lung
What percent of body mass is skeletal muscle?
40%
What is the cause for the low blood flow to skeletal muscle?
sympathetic vasomotor tone
What happens in response to exercise?
Metabolic hyperaemia overcomes sympathetic vasomotor tone and circulating adrenaline causes vasodilation
How do muscles aid venous return?
Large veins lie between skeletal muscles and one-way venous valves allow blood to move towards the heart
What happens if the valves become incompetent?
Blood will poll in the lower limb veins as varicose veins
