Physiology 9 Flashcards
Where does coronary venous blood drain?
Drains via coronary sinus into the right atrium
When is oxygen demand of heart muscle increased?
During exercise etc
What is coronary heart disease?
Area of cardiac muscle deprived of blood supply due to blockage of coronary vessels
Special adaptations of coronary circulation? (3)
- High Capillary Density (to meet O2 demand)
- High Basal Blood Flow
- High Oxygen Extraction (~75% compared to 25% whole body average) under resting conditions - means extra O2 cannot be supplied by increasing O2 extraction
Since extra O2 cannot be supplied by increasing O2 extraction in coronary circulation, how is O2 increased? What is this controlled by?
- Increasing coronary blood flow
* Coronary blood flow controlled by intrinsic & extrinsic mechanisms
Intrinsic mechanisms controlling coronary blood flow? What is this process known as? Example of metabolite?
- Decrease in PO2 causes vasodilation of coronary arterioles
- Metabolic hyperaemia - matches flow to demand
- Adenosine - potent vasodilator
What would cause a decrease in PO2 of cardiac muscle?
Muscle consuming more O2
Extrinsic mechanism of controlling coronary blood flow? What is the effect of this? Why? What is this called?
- Coronary arterioles supplied by sympathetic vasoconstrictor nerves
- Sympathetic stimulation of heart results in coronary vasodilation despite direct vasoconstrictor effect
- Because over-ridden by metabolic hyperaemia as a result of increased HR and SV
- FUNCTIONAL SYMPATHOLYSIS
What effect does adrenaline acting on B2 adrenoceptors have?
Vasodilation of coronary arteries
What are other examples of metabolites other than adenosine? (3) What is there effect?
- K+, PCO2, H+
* Vasodilation
Describe coronary blood flow in LEFT coronary artery during systole? Diastole? What happens if diastole shortens i.e. in tachycardia? What are the symptoms of this?
- Becomes almost 0 during systole due to constriction of coronary arteries
- Most left coronary blood flow occurs during diastole
- If diastole shortens, shortens period of coronary flow to left ventricle
- Ischaemic-like chest pain
Describe coronary blood flow in RIGHT coronary artery during systole?
Right ventricular pressure not as high as LV pressure so coronary arteries will not be squeezed as tightly and coronary blood flow will be present furing both systole and diastole
When does peak left coronary flow occur? What does shortening diastole due to a very fast HR result in?
- Peak left coronary flow occurs during diastole
* Shortening diastole results in decrease in coronary flow
When does most myocardial perfusion occur?
Diastole - when the subendocardial vessels from the left coronary artery are not compressed
What arteries supply the brain? (2) Why is the Circle of Willis an important feature of the brain?
- Internal carotids and vertebral arteries
- Grey matter VERY sensitive to hypoxia - consciousness lost after few seconds of ischaemia, irreversible cell damage within ~ 3 minutes
What are special adaptations of cerebral circulation? (2)
- Circle of Willis
* Autoregulation of cerebral blood flow
What arteries compose the Circle of Willis? What arise from circle of willis? What is advantageous about circle of Willis? Disadvantageous?
- Basilar (formed from 2 vertebral arteries) and carotid arteries anastomose to form circle of willis
- Major cerebral arteries
- Cerebral perfusion maintained even if one carotid artery gets obstructed
- Obstruction of a smaller branch of a main artery would deprive a region of the brain of its blood supply
Stroke caused by? Main types of stroke? (2)
- Stroke caused by interruption of blood supply to region of brain
- Haemorrhagic stroke
- Ischaemic stroke e.g. blood clot
What is the purpose of autoregulaton of cerebral blood flow? Range? Explain effect? (2)
- Prevents changes in cerebral blood flow if mean arterial blood pressure changes within a range
- 60 - 160 mmHg
- If MAP drops, resistance vessels dilate to maintain blood flow
- If MAP rises, resistance vessels constrict to limit blood flow
Does brain participate in baroreceptor reflex? Why?
- No
* Do not want fluctuation in cerebral blood flow with every change in MAP
Does sympathetic stimulation have an effect on cerebral blood flow? Rank methods of cerebral blood control in order of most effect to least effect (3)
- Very little effect
- Hypercapnia (most effect)
- Autoregulation
- Sympathetic nerve stimulation (least effect)
When does autoregulation fail? (2) What happens if MAP drops below 50 mmHg? (3)
- If MABP falls below 60 mmHg (cerebral blood flow falls)
- If MAPBP rises above 160 mmHg (cerebral blood flow rises)
- MABP below 50 mmHg results in confusion, fainting and brain damage
What is other important regulation of cerebral blood flow other than autoregulation and sympathetic innervation? (2)
- PCO2
* Regional hyperaemia
What is the effect of PCO2 in regulation of cerebral blood flow? (2)
What common reaction does this explain?
- Increased PCO2 causes cerebral vasodilation
- Decreased PCO2 causes cerebral vasoconstriction
- This explains why hyperventilation can lead to fainting
What is regional hyperaemia? Mechanism?
- Increase in blood flow to active parts of the brain
* Unknown - could be rise in [K+] as a result of K+ efflux from active neurones??
What does the skull contain? (3) What is normal intracranial pressure (ICP)?
- Brain (80%), blood (12%) and cerebrospinal fluid (CSF - 8%)
- Normal ICP = 8 - 13 mmHg
Equation for cerebral perfusion pressure (CPP)? What does increased ICP result in? What causes raised ICP? What can this lead to?
- CPP = MAP - ICP
- Increasing ICP decreases CCP and so cerebral blood flow
- Head injury, brain tumour
- Can lead to failure of autoregulation of cerebral blood flow
What is blood brain barrier? What can cross blood brain barrier? What about glucose?
- Very tight intracellular junctions in cerebral capillaries
- O2 and CO2
- Glucose crosses BBB by facilitated diffusion using specific carrier molecules
What is BBB impermeable to? Why?
- Hydrophilic substances like ions, catecholamines, proteins etc
- Protects brain neurones from fluctuating levels of ions etc in blood
What percentage of CO does pulmonary circulation recieve? How are metabolic needs of airways met?
- Entire CO from right ventricle
* Metabolic needs of airways met by systemic bronchial circulation
What is the value of pulmonary resistance? What is pulmonary blood pressure?
- Pulmonary resistance only 10% of that of systemic circulation
- Pulmonary artery BP = 20-25/6-12 mmHg
Special adaptations of pulmonary circulation? What is the purpose of this?? (2)
- Pulmonary capillary pressure low (8-11 mmHg) compared to systemic pressure (17-25 mmHg)
- Absorptive forces exceed filtration forces
- Hypoxia causes vasoconstriction of pulmonary arterioles - COMPLETE OPPOSITE effect of hypoxia on systemic arterioles
WHY???
- Absorptive forces - protects against pulmonary oedema
- Vasoconstriction to hypoxia - to divert blood from poorly ventilated areas of the lung
What mass of body is skeletal muscle? How does skeletal muscle affect blood pressure? Why is resting blood flow low?
- 40%
- Resistance of skeletal muscle vascular bed has large impact on BP
- Low because of sympathetic vasoconstrictor tone
Why is resting blood flow in skeletal muscle increased during exercise?
Metabolic hyperaemia overcomes sympathetic vasoconstrictor activity
What is the skeletal muscle pump? What allows blood to move towards the heart? What are the benefits of the skeletal muscle pump?
- Large veins in limbs lie between skeletal muscles and contraction aids venous return
- One-way venous valves allow blood to move towards the heart
- Skeletal muscle pump reduces postural hypotension and fainting
What happens if venous valves become incompetent?
Blood pools in lower limbs causing varicose veins
Do varicose veins reduce CO? Why?
- No
* It is a chronic process so there is a compensatory increase in blood volume e.g. RAAS
