Lecture 15 Flashcards
What is the 2 circulations to the lungs?
- bronchial circulation (part of systemic circulation, meets metabolic requirements of the lungs)
- pulmonary circulation (blood supply to alveoli, for gas exchange)
What are the pressures in each compartment of the heart?
RA: 0-8 mmHg
LA: 1-10 mmHg
No systolic pressure for atria as you don’t have systolic pressure which the ventricles have
RV: Systole 15-30 mmHg, Diastole 0-8 mmHg (same or lower than RA, as blood as has to flow into RV)
LV: Systole 100-140 mmHg, Diastole 1-10 mmHg
What are the aortic and pulmonary artery pressures?
Aorta: Systole 100-140 mmHg, Diastole 60-90 mmHg (doesn’t go as low due to elastic recoil)
Pulmonary artery: Systole 15-30 mmHg, Diastole 4-12 mmHg
(Pulmonary is at much lower pressure)
What are the average pressures in the vessels in the pulmonary circulation?
Mean arterial: 12-15 mmHg
Mean capillary: 9-12 mmHg
Mean venous: 5 mmHg
All low in pulmoary circulation
Why is there low resistance in pulmonary cirulation?
- short wide vessels
- lots of capillaries
- arterioles have little smooth muscle
What are some adaptations fo the lungs for efficient gas exchange?
- high density of capillaries in alveolar wall (large SA)
- short diffusion distance due to thin tissue layers (endothelium/epithelium)
What is the optimal ventilation-perfusion (V/Q) ratio and how do you maintain it?
0.8
V= air reaching the alveoli
Q= amountof blood reaching alveoli via capillaries
Diverting blood away from alveoli which aren’t ventilated well via hypoxic pulmonary vasoconstriction
What is the hypoxic pulmonary vasoconstriction?
Vasoconstriction of pulmonary vessels in presence of alveolar hypoxia
-poorly ventilated alveoli are less perfused
How can chronic hypoxic pulmonary vasoconstriction cause RV failure?
At high altitude / lung disease (emphysema)= chronic hypoxia
=chronic increase in vascular resistance
=chronic pulmonary hypertension
=high afterload on RV
What are low pressure vessels strongly influenced by?
Gravity
-capillaries at bottom of lung are more distended due to increased hydrostatic pressure
(ones at top will collapse (during diastole) until hydrostatic pressure is increased)
- vessels at base of lung= distended
- vessels at apex of lungs= collapsed
- vessels on level of heart= continuously patent
How does exercise effect pulmonary blood flow?
- increased CO
- small increase in pulmonary aterial pressure
- opens apical capillaries
- increased oxygen uptake by lungs
- blood flow increases, capillary transit time is reduced but this doesn’t compromise gas exchange
How is tissue fluid formed?
Starlings forces (hydrostatic pressure in capillary-pushes fluid out of capillary/oncotic pressure-draws fluid into capillary)
What is capillary hydrostatic pressure affected most by?
Capillary hydrostatic pressure is much more affected by venous pressure than any change in arteriole pressure
Why is there not much lung lymph formed?
- capillary hydrostatic pressure is lower
- plasma oncotic pressure increases
When do you get pulmonary oedema?
When filtration > reabsorption
Due to increased capillary pressure
What can cause pulmonary oedema?
- mitral valve stenosis
- left ventricular failure
Why is pulmonary oedema dangerous?
-impairs gas exchange
(Affected by posture- changes in hydrostatic pressure due to gravity)
-forms at base when upright
-forms throughout lung when lying down
How do you treat pulmonary oedema?
Use diuretics (reduce blood volumeby increasing amount of urine)
OR
Treat underlying cause if possible
How much of the cardiac output does the brain receive via the cerebral circulation?
15% (despite the brain only accounting for 2% of the total body mass)= high oxygen demand
Oxygen consumption of grey matter- 20% of Boyd consumption
MUST HAVE SECURE OXYGEN SUPPLY
How does cerebral circulation meet high demand of oxygen?
- high capillary density (large SA for gas exchange)
- high basal flow rate (x10 avergae of body)
- high oxygen extraction (35% above average)
Why is secure oxygen supply to the brain vital?
Neurones are very sensitive to hypoxia
-loss of consciousness after a few seconds of cerebral ischaemia
-irreversible damage to neurones in 4 mins
=interruption to blood supply causes stroke (neuronal death)
How is a secure blood supply to brain ensured?
- anastomoses between basilar and internal carotid arteries
- myotonic autoregulation which maintains perfusion during hypotension
- metabolic factors control blood flow
- brainstem regulates other circulations
What is myotonic autoregulation?
Increase in BP= vasoconstriction
Decrease in BP= vasodilation
-maintains cerebral blood flow when BP changes
What is metabolic regulation in the brain?
Cerebral vessels are very sensitive to changes in arterial pCO2
Hypercapnia: increase pCO2= vasodilation (faster flow, due to reduced resistance)
Hypocapnia: decrease pCO2= vasoconstriction
How can you image an increase in blood flow to the brain?
Areas with increased neuronal activity have increased blood flow:
(Increased pCO2, increased K+, increased adenosine, decreased pO2 = vasodilation)
FMRI scan
What is Cushing’s reflex?
-rigid cranium protects brain but doesn’t allow expansion
-increased intracranial pressure (cerebral tumour/haemorrhage) impairs cerebral blood flow
-impaired blood flow to vasomotor control regions of brainstem increase sympathetic vasomotor activity
(Increases arterial BP, helps maintain cerebral blood flow)
What must the coronary circulation do?
- deliver oxygen at high basal rate
- must meet increased demand of myocardium (work rate of heart can increase 5 fold)
What are some problems with coronary arteries?
Left coronary artery gets blood flowing through it during diastole
(ventricular systole- high pressures, cuts off the flow to left coronary artery)
(Diastole shortens the most as heart rate increases)
How is coronary circulation adapted?
- high capillary density
- short diffusion distance
- continuous production of NO (vasodilator) by coronary endothelium maintains high basal flow
What are the issues with the coronary arteries?
Functional end arteries, few anastomoses
- prone to atheroma
- narrowed coronary arteries leads to angina upon exercise/stress/cold due to increased oxygen demand
- sudden obstruction by thrombus causes MI
What are the subdivisions of the systemic circulation?
- cerebral
- coronary
- skeletal
- cutaneous
How much blood does the pulmonary circulation pump?
It accepts the entire cardiac output
What is the maximum cardiac output for a non-athleteand cardiac output at rest?
20-25 l/min
CO at rest is 5 l/min
Some features of pulmonary circulation:
- low pressure
- low resistance
What is the normal pulmonary capillary pressure and at what pressure does pulmonary oedema occur?
Normal: 9-12 mmHg
Oedema: left atrial pressure rises to 20-25 mmHg
When does myogenic autoregulation fail?
When BP falls below 50 mmHg
What can panic hyperventilation lead to?
Hypocapnia, therefore cerebral vasoconstriction= dizziness/fainting
Function of adenosine?
Powerful vasodilator of cerebral arterioles
When does flow usually occur in the left coronary artery?
During diastole
Differences between cardiac muscle and skeletal muscle?
Cardiac -fibre diameter (18 micrometres) -capillary density (3000/mm'2) -capillaries continuously perfused Skeletal -fibre diameter (50 micrometres) -capillary density (400/mm'2) -not all capillaries continuously perfused at rest
What is the relationship between coronary blood flow and myocardial oxygen demand?
Linear relationship
- until very high oxygen demand
- vasodilation due to metabolic hyperaemia: increased blood flow (adenosine/high K+/low pH)
What must skeletal muscle circulation do during exercise?
Increase oxygen and nutrient delivery and removal of metabolites
What is a resistance artery?
Artery which has rich innervation by sympathetic vasoconstrictor fibres
What does capillary density in skeletal muscle depend on?
Muscle type
-postural muscles have higher capillary density as they are active all the time
What is the vascular tone of skeletal muscle vessels?
Very high
-permits lots of dilation so flow can increase >20 times in active muscle
What is vascular tone?
Degree of vasoconstriction experienced by a blood vessel compared to its maximal dilated state
How many capillaries are perfused in skeletal muscle?
At rest only 1/2- allows for increased recruitment
-opening of precapillary sphincters allows more capillaries to be perfused increasing blood flow and reducing diffusion distance
How does adrenaline affect arterioles in skeletal muscle?
Acts as a vasodilator
-via B2 receptors
(vasoconstrictor response via NA on A1 receptors)
What is the main rolein cutaneous circulation?
Temp regulation
-skin is main heat dissipating surface
Also has a role in maintaining BP, vasoconstriction to maintain BP
What are AVA’s?
Artereovenous anastomoses
- found in apical skin only
- direct blood flow to superficial veins
Function of AVA’s?
Regulate heat loss from apical skin
-under neural control: sympathetic vasoconstrictor fibres
Decrease in core temp= increases sympathetic tone, decreasing blood flow to skin
Increased core temp= opens AVA’s via dilation
