CVS Session 8 Flashcards
What does the bronchial circulation do?
Part of systemic circulation that meets metabolic requirements of the lungs
What are the two circulations of the lungs?
Bronchial
Pulmonary
What does the pulmonary circulation do?
Blood supply to alveoli required for gas exchange
Why just the pulmonary circulation accept the entire cardiac output?
It is in series w/ the systemic circulation
What is the maximum cardiac output for a non-athlete?
20-25 l per minute
Why does the pulmonary circulation work with low pressure and resistance?
High number of branching capillaries giving lots of parallel branches
What are the typical mean arterial, capillary and venous pressures?
Arterial = 12-15 mmHg Capillary = 9-12 mmHg Venous = 5 mmHg
How does the amount of smooth muscle in the arterioles in the pulmonary circulation compare to that in the systemic?
Relatively little
How is low resistance achieved in the pulmonary circulation?
Short, wide vessels
Lots of breaching capillaries
Arterioles with little smooth muscle
What is the combined endothelium and epithelium thickness separating the gas phase from plasma in the pulmonary circulation?
~0.3 micrometers
Why does pulmonary oedema tend to accumulate in the lower part of the lung?
In orthostatsis gravity causes increased hydrostatic pressure on vessels in this part
What must oxygen and carbon dioxide pass through to reach an erythrocyte?
Type 1 pneumocyte –> basement membrane –> endothelial cell –> erythrocyte
Compare the lumen of vessels in the lungs closest to the apex level, closest to the heart level and closest to the base level.
Nearest apex = collapse during diastole
Nearest heart = continuously patent
Nearest base = vessels distended
Why is the diastolic pressure in the pulmonary artery higher than in the right atrium?
Elastic recoil of the artery
What is needed for efficient oxygenation?
Matching of ventilation of alveoli w/their perfusion
Air in/out matched w/bloodflow in same site in lungs
How is the ventilation-perfusion ratio maintained?
Divert blood from alveoli that are not well ventilated to those that are
What is the optimal ventilation-perfusion ratio?
0.8
What can cause decreased alveoli ventilation?
Mucus plug
Fluid build up
What can cause chronic hypoxia?
Altitude
Lung disease e.g. emphysema
How does hypoxic pulmonary vasoconstriction lead to right sided heart failure?
Alveolar hypoxia –> vasoconstriction –> becomes chronic and widespread –> chronic pulmonary hypertension –> high workload on R ventricle –> R ventricular hypertrophy –> R sided heart failure
What is the effect of exercise in pulmonary blood flow?
Increased cardiac output –> small increase in pulmonary arterial pressure –> apical capillaries open –> increased oxygen uptake –> capillary transit time decreases
What does opening of the apical capillaries help to achieve during exercise?
Matching of ventilation-perfusion ratio
What range can the capillary transit time lie within?
~1 s at rest to ~0.3 s w/out compromising gas exchange
What determines formation of tissue fluid?
Starling forces
How do the Starling forces determine tissue fluid formation?
Hydrostatic pressure of blood in capillary pushes fluid out
Oncotic pressure/ colloid osmotic pressure draws fluid in to capillaries
What influences capillary hydrostatic pressure more in the systemic circulation?
Venous pressure
Will hypertension cause peripheral oedema?
No
Why does pulmonary oedema accumulate throughout the lungs when lying down?
Capillaries throughout lung become distended
What is the treatment plan for pulmonary oedema?
Diuretics to treat symptoms and treat the underlying cause
What does pulmonary oedema impair?
Gas exchange
What minimises formation of lung lymph so that it remains at a level that can be dealt with and does not become pulmonary oedema
Low capillary pressure
How does the oncotic pressure of tissue fluid in the lungs compare to that in the periphery?
Greater than in periphery
How does capillary hydrostatic pressure in lung compare to that in the systemic capillaries?
It is lower
What changes in the heart lead to pulmonary oedema?
Mitral valve stenosis or left ventricular failure causing left atrial pressure to rise to 20-25 mmHg
How does the plasma oncotic pressure compare in the lungs and systemic circulation?
It is equal
How does the percentage of cardiac output received by the brain compare to its mass?
Receives ~15% of cardiac output but only account for ~2% of body mass
Why is a secure oxygen supply needed by the brain?
Grey matter accounts for ~20% of total body consumption at rest
How sensitive are neurones to hypoxia?
Very:
Loss of consciousness after a few seconds of cerebral ischaemia
Neuronal death after ~4 mins
What is the diffusion distance in the cerebral circulation?
< 10 micrometers
How does the basal flow rate of the cerebral circulation compare to the average for the rest of the body?
10x greater
How does oxygen extraction in the cerebral circulation compare to the average for the whole body?
35x greater
What is the Circle of Willis?
Anastomoses b/w basilar and internal carotid arteries
What does the Circle of Willis ensure?
Interruption of the vertebral/carotid blood supply does not cause lack of blood flow to an area
What does the brain stem coordinate?
Sympathetic stimulation to cause vasoconstriction to divert blood to brain
What do cerebral resistance vessels have in order to respond to changes in transmural pressure?
Well developed smooth muscle
What is the name given to the use of week developed smooth muscle to maintain cerebral perfusion?
Myogenic autoregulation
At what pressure does myogenic autoregulation fail causing loss of consciousness?
Below 50 mmHg
Why is there very little variation on a graph of mean arterial pressure vs cerebral blood flow?
Myogenic response
What is hypercapnia?
Increased p(carbon dioxide)
What does hypocapnia stimulate?
Vasoconstriction
How does panic cause dizziness/fainting?
Hyperventilation –> hypocapnia –> vasoconstriction –> lack of cerebral perfusion
Why do areas with increased neuronal activity have increased blood flow?
They have increased p(carbon dioxide), potassium and adenosine
They have decreased p(oxygen)
Causing vasodilation
What is a powerful vasodilator of cerebral arterioles?
Adenosine
What is Cushing’s reflex?
Increase in intracranial pressure impairs blood flow to vasomotor control regions of brain stem –> increased sympathetic vasomotor activity by increased arterial BP and decreased HR from decreased vagal stimulation –> maintains cerebral blood flow
What forms the tight blood-brain barrier?
Cerebral capillaries w/tight junctions b/w them
How can lipid soluble and lipid insoluble molecules move across the blood-brain barrier?
Lipid soluble diffuse freely
Lipid insoluble cannot cross barrier
Give two examples of lipid insoluble dilutes that cannot cross the blood-brain barrier.
Potassium
Catecholamines
Give two examples of lipid soluble molecules that freely diffuse through the blood-brain barrier.
Oxygen
Carbon dioxide
Where do the right and left coronary arteries arise from?
Aortic sinuses
How much can the work rate of the heart increase when cardiac output needs to be increased?
5-fold
Why is blood flow into the left coronary arteries usually during diastole?
Greater muscle mass causing increased pressure in left side
When do the right coronary arteries mainly fill?
During systole
What facilitates efficient oxygen delivery in the coronary circulation?
High capillary density
How does capillary density in the coronary circulation compare to that in skeletal muscle?
3000 per mm squared in coronary
400 per mm squared in skeletal muscle
What maintains a high basal flow in the coronary circulation?
Continuous production of NO by coronary endothelium to keep BV relatively dilated
What is the relationship between myocardial oxygen demand and coronary blood flow?
Linear until very high oxygen demand
What molecules cause metabolic hyperaemia to allow vasodilation?
Adenosine
High potassium
Low pH
How many arterio-arterial anastomoses are there in the coronary circulation?
Few
What percentage of coronary artery occlusion leads to angina on exercise?
80-90%
Why does atheroma cause angina on exercise?
Diastole time decreases with increasing heart rate so there is less time to fill coronary arteries
What two things excluding exercise can cause sympathetic coronary vasoconstriction and angina?
Stress
Cold
What must the skeletal muscle circulation do during exercise?
Increase oxygen and nutrient delivery
Remove metabolites
What important role does the skeletal muscle circulation have?
Helping regulate TPR
How are the resistance vessels in the skeletal muscle circulation innervated?
High number of symoathetic vasoconstrictor fibres
What maintains blood pressure in the skeletal muscle circulation?
Baroreceptor reflex
Why is capillary density high in postural muscles?
They are continually active
Why do skeletal muscle BV have a very high vascular tone to keep the normally quite constricted?
Permits up to 20x dilatation
Why are only 1/2 the capillaries available perfuse day rest at any one time in the skeletal muscle circulation?
Allows for increased recruitment to increase bloodflow and decrease diffusion distance
What vasodilator molecules can cause metabolic hyperaemia in the skeletal muscle circulation?
High potassium Increased osmolarity High inorganic phosphates Adenosine High hydrogen ion levels
How does adrenaline at physiological levels act in the skeletal muscle circulation?
Through beta 2 receptors at arterioles causing vasodilation
Why does the cutaneous circulation not require much bloodflow?
It is not highly metabolically active
What is the special role of the cutaneous circulation?
Temperature regulation to keep core temp ~37 degrees Celsius
What is the main heat dissipating surface regulated by cutaneous blood flow?
Skin
How does the cutaneous circulation act in shock?
Vasoconstricts to maintain BP
What are acral areas?
Apical areas e.g. fingers, toes, nose, ears
What do acral areas have due to their large SA:volume ratio?
Artereovenous anastomoses (AVAs)
What controls AVAs?
Sympathetic vasoconstrictor fibres
Do local metabolites act on AVAs?
Nope
What happens in the cutaneous circulation when the core temperature drops?
Increased AVA tone –> decreased blood flow to apical skin
What happens in the cutaneous circulation when the core temperature increases?
Decreased sympathetic AVA tone –> decreased resistance shunt to venous plexus to bypass capillaries –> vasodilatation in non-apical skin, helped by bradykinin
