Regulation of circulation Flashcards
Depolarisation
- Activation of cells by depolarisation
* Na+ channels open and Na+ ions move inside making the inside of the cell more positive than the outside
Hyperpolarisation
- Inhibition of cells by hyperpolarisation
- After repolarisation, some K+ channels remain open and K+ continues to go out. Then the inside of the cell is more negative than when it began.
- It is harder to depolarise
How is muscle contraction triggered by depolarisation?
- occurs when muscle cell membrane is depolarised (more positive inside)
- depolarisation triggers calcium ion release from sarcoplasmic reticulum - causes contraction
- muscle contraction is then inhibited by hyperpolarisation of the membrane
What controls rhythmic heart contractions?
• autorhythmicity • gap junctions • 2 groups of pacemaker cells - Sinoatrial node (SAN) - Atrioventricular node (AVN)
SAN (Sinoatrial node)
- spontaneous rhythmic depolarisation (approximately 100 times per minute if independent of nervous control)
- SAN kicks of action potential which spreads through the atrial syncytium - immediately the atria contract
How does the Atrioventricular node (AVN) control heart contractions?
- Action potentials sent to the ventricles via the bundle of His
- run to the ventricular apex via right and left bundle branches
- APs spread through ventricles via Purkinje fibres
- ventricular syncytium
What is cardiac arrest?
Sudden loss of cardiac function when the heart abruptly stops beating
What is the most common loss of cardiac function?
- Loss of ventricular fibrillation
- caused by irregular activity in the heart (Arrhythmia)
- when this happens, the heart pumps little or no blood to the body
What is the treatment of cardiac arrest?
• defibrillation using electricity to ‘shock’ the heart to try and restore its regular rhythm
How do you calculate mean arterial pressure (MAP)?
MAP = (Systolic + 2diastolic)/3
Why is MAP calculated that way?
- diastolic part of cycle is twice as long as systolic
* therefore we calculate the weighted average
What is involved in the response to a change in blood pressure?
- baroreceptors which then initiate the baroreceptor reflex
- uses autonomic nervous system to effect/restore the mean arteriole pressure
- this is a rapid response
What is involved in the response for an increased demand for oxygen?
- chemoreceptors
- autonomic nervous system
- hormones
What are the different factors that act on blood vessels causing vasoconstriction/dilation?
- Noradrenaline from the sympathetic nervous system - vasoconstriction
- release of paracrine factors from endothelial cells
• edothelin 1 - vasoconstriction
• nitric oxide - vasodilation - Hormones also affect vascular smooth muscles
• Angiotensin II from kidney - vasoconstriction
Regulation of blood vessels via vasoconstriction
- Norepinephrine, Angiotensin II, Endothelin-1 act via different (G-protein linked) receptors
- Phospolipase C (PL-C) activation causes formation of Inositol tri-Phosphate (IP3)
- IP3 stimulates the release of calcium from the sarcoplasmic reticulum (SR)
- [Ca2+] increases causing smooth muscle contraction
What response occurs in the kidney when the arterial pressure declines?
- Kidney blood pressure declines
- kidney releases renin which activates angiotensin
- angiotensin causes vessels to constrict and stimulates thirst
- arterial pressure rises
What response occurs in the pituitary gland when arterial pressure declines?
- Firing in arterial stretch sensors decrease since there is a decrease in arterial pressure
- ADH (anti-dieuretic hormone - antiweeing) released from posterior pituitary
- ADH stimulates water reabsorption by kidneys
- arterial pressure rises
What is meant by vasomotor tone?
- Blood vessels are always a bit constricted
* this means they can either relax more or constrict more
How is the frequency of neural electrical signals related to the diameter of blood vessels?
- increased frequency of signals mean there is more norepinephrine released which bind to alpha receptors
- vessels constrict
- decreased frequency of signals mean less norepinephrine is released and bind to blood vessels
- vessels dilate
What is the positive feedback loop that occurs when arterial blood pressure falls?
- decreased blood flow to tissue
- local accumulation of metabolic wastes
- autoregulatory widening of vessels
Autonomic Innervation (supply of nerves to the heart) for sympathetic nerves
Sympathetic Nerves
• Noradrenaline binds to Beta 1 adrenoreceptors
How are pacemaker cells involved in the reducing the heart rate?
- Parasympathetic - reduce heart rate
- Ach bind to muscarinic receptors in the SAN - become hyperpolarised
- it is harder for depolarisation to occur
- there is a decrease in heart rate and there is a greater interval between each beat
How are pacemaker cells involved in increasing the heart rate?
- adrenaline binds to beta adrenoreceptors
- beta adrenoreceptors act on SAN and causes depolarisation which increases heart rate
- myocytes also have increased [Ca2+] - causes heart to beat with greater force
What is the baroreceptor reflex?
Initiates response to bring MAP back to normal
Postural/Orthostatic hypotension
- ‘dizzy spell’ when someone stands up too fast - increases with age
- occurs when person’s BP suddenly falls as they stand up
- overall effect is a transitory insufficient blood perfusion in the upper part of the body, particularly the brain
What are baroreceptors and how do their signalling change when MAP is too high/low?
• Baroreceptors detect the amount of stretch in the blood vessels and signals it to the brain
When BP is too high
• More stretch in the vessels, so more action potentials sent to the brain
When BP is too low
• less stretch in the vessels and hence less signals are sent to the brain
What happens after the detection of a sudden fall in BP?
- MAP drops
- baroreceptor firing is lower
- CNS wants to then increase the MAP
- Sympathetic increases - total peripheral resistance and cardiac output increases
- Parasympathetic decreases - cardiac output increases
Arterial pressure then increases
What chemoreceptors are involved in regulating the ANS?
- arterial chemoreceptors - detect changes in blood chemical composition (CO2 and O2)
- medullary chemoreceptors - detect changes in increases in CO2 levels
What happens when there is an increased deman of oxygen due to exercise?
- rapid response
- involves central command, chemoreceptors, autonomic nervous system and hormones
- Redistribution of blood flow - decreased blood flow to visceral beds and increased flow to skeletal muscle
- this causes increased cardiac output - elevated heart rate
- increased breathing rate
Does norepinephrine influence vasoconstriction or vasodilation?
- Sympathic neurons
- Neural type
- Causes vasocontriction
Does endothelin influence vasoconstriction or vasodilation?
- Vascular enothelium
- Paracrine
- Vasoconstriction
Does angiotensin influence vasoconstriction or vasodilation?
- Sourced from plasma
- Endocrine
- Vasoconstriction
How does CO2 affect blood flow?
- Sourced from multiple tissues
- Metabolite
- Vasodilation
How does nitric oxide influence blood flow?
- Sourced from vascular endothelium
- Paracrine
- Vasodilation
How does bradykinin influence blood flow?
Sourced from multiple tissues
- Paracrine
- Causes vasodilation
How does vasodilation occur?
- Bradykinin binds to the receptor inside the endothelial cell which increases calcium production in that call
- Increase in Ca2+ opens calcium gates K channels
- Potassium flows out creating a difference in charges
Autonomic Innervation (supply of nerves to the heart) for parasympathetic nerves
Parasympathetic Nerves
• Acetylcholine (ACh) binds to Muscarinic (M2) receptors
What is pre-eclampsia?
A pregnancy-specific cardiovascular disease that is characterised by hypertension and excess levels of serum protein in urine
What are the effects of pre-eclampsia?
Reduced sensitivity to bradykinin and abnormal vasorelaxation occurs
Repolarisation
- following depolarisation
- sodium channels open and Na+ ions move into the cell causing the inside of the cell to become more positive than the outside.
What is the atrial syncytium?
A network of cardiac muscle cells connected by gap junctions that allow coordinated contractions of the atria.
List the steps in an action potential
- resting membrane potential
- depolarisation
- repolarisation
- hyperpolarisation
