L15: Special circulation Flashcards
What are the two circulations to the lungs?
Pulmonary circulation–> blood goes to lungs to get oxygenated
–> blood to alveoli via pulmonary trunk (artery)
Bronchial circulation–> blood supply to the lung tissue
–> supplies the tissue to far away to get oxygen via diffusion
–> systemic circulation
How do the pulmonary and systemic circulation work?
Work in series
Pulmonary circulation receives the entire cardiac output
How does the diastolic pressure in the artia compare to the diastolic pressure in the ventricles?
Pressure in the atria greater than the ventricle
Need a pressure gradient from high (atria) to low (ventricles)
Can be the same
What is the diastolic pressure in each chamber of the heart and the great vessels?
RA–> 0-8 mmHg
RV–> 0-8 mmHg
LA–> 1-10 mmHg
LV–> 1-10 mmHg
Pulmonary trunk/artery–> 4-12 mmHg small amount of elastic recoil
Aorta –> 60-90 mmHg –> higher diastolic pressure because of elastic recoil of aorta maintains the pressure
What is the systolic pressure each of the chambers and the great vessels?
RA and LA --> no systolic pressure because they don't really contract RV--> 15-30 mmHg LV--> 110-140mmHg Pulmonary trunk--> 15-30mmHg Aorta--> 100-140 mmHg
What are the main features of the pulmonary system?
Low pressure–> MAP (12-15mmHg), Mean capillary pressure (9-12mmHg), mean venous pressure (5mmHg)
Low resistance–> need blood to flow through easily
- Short and wide vessels
- Lots of capillaries (many parallel elements)
- Arterioles relatively little smooth muscle–> keep lumen open allowing good blood flow
What is included in the pulmonary system?
RA, RV, pulmonary trunk, lungs, pulmonary veins
How has the pulmonary circulation adapted for efficient gaseous exchange?
Very high density of capillaries –> large capillary SA
Short diffusion distance –> thin layer of tissue separating gas phase from plasma (endo and epithelium 0.3 micrometres)
Good O2 and CO2 exchange/ transport capacity
What is the ventilation: perfusion ratio?
V/Q ratio- optimal 0.8
Efficient oxygenation
Ventilation needs to match perfusion
What is hypoxic pulmonary vasoconstriction?
Important regulator of vascular tone
Ensure optimal V/Q ratio
Vasoconstriction–> divert blood away from poorly ventilated alveoli (hypoxic alveoli)
Optimise gaseous exchange
What is one of the downsides to hypoxic pulmonary vasoconstriction?
Chronic hypoxia –> high altitude or consequence of lung disease (emphysema)
–> ↑ vasoconstriction–> increased resistance–> chronic pulmonary hypertension–> high afterload on right ventricle–> right sided heart failure–> pumping against higher pressure
How does gravity affect the pulmonary circulation?
Vessels–> thin compliant vessels, easily distend and easily collapse
Upright (orthostasis)
Vessels in lung above the heart–> collapse during diastole (decreased hydrostatic pressure), open in systole due to increased pressure
Vessels at level of heart–> patent all the time
Vessels below the level of the heart–> vessels distend (increased hydrostatic pressure)
How does exercise impact pulmonary blood flow?
Increase cardiac output
Pulmonary circulation takes all the blood
Small ↑ pulmonary arterial pressure
Opens apical capillaries (normally collapsed)
Increase O2 uptake in the lungs
Blood flow increases transit time is reduced
–> RBC loaded with oxygen quicker
(normally 1 second, reduced to 0.3 seconds)
What are the two forces acting on the blood in the capillaries?
Hydrostatic pressure–> Blood in capillary, pushes fluid out the capillary
Oncotic pressure–> pressure exerted by large molecules (plasma proteins), draws fluid into the capillary
Normally balanced
What influence the hydrostatic pressure more?
The venous pressure in the systemic circulation
Venous pressure increases the hydrostatic pressure increases
How is the formation of lymph in the lung kept to a minimum?
Hydrostatic pressure is much lower compared to peripheral capillaries
As well as oncotic plasma pressure also an oncotic interstitial pressure
Filtration = reabsorption
Minimum lung fluid
What happens if the hydrostatic pressure in the lung increases?
More fluid is filtered out leading to oedema
Filtration > reabsorption
Increase in pressure in left side of the heart
ALSO increase in arterial pressure (before the lungs in pulmonary artery) affects pulmonary hydrostatic pressure because resistance is lower
What could cause pulmonary oedema?
Capillary pressure increase due to:
Mitral stenosis–> difficult for blood to get out so ↑ pressure in LA which increase hydrostatic pressure in the lungs
Left ventricular failure–> more blood in LV in systole so difficult to get blood in during diastole –> back pressure
What pressure does pulmonary oedema occur at?
Normally kept low at 9-12mmHg
Pressure increase to 20-25mmHg get oedema
What does pulmonary oedema result in? How is it treated?
Impairs gas exchange –> affected more by posture, mainly at base when upright, lying down throughout lung
Diuretics–> reduced BP by removing water
Treat underlying cause
How much of the CO does the brain (cerebral circulation) receive?
15% of CO–> neurones are active
even though brain only accounts for 2% of body mass
Grey matter consumes 20% of total body comsumption at rest
How does the cerebral circulation meet the high demand?
High capillary density --> Large SA for diffusion --> Reduced diffusion distance (<10 micrometres) High basal flow rate --> x10 average of whole body --> anastomosis High O2 extraction --> 35% above average for body
Why is it crucial to have a good blood supply to the brain? How is a secure blood supply to the brain achieved?
Neurones sensitive to hypoxia
Irreverisble damage after 4 minutes
Stucturally
–> Anastomosis between the basilar (from vertebral artery) and carotid artery –> Circle of Willis
Functionally
–> Myogenic autoregulation maintains perfusion during hypotension
–> Metabolic factors control blood flow
–> Brainstem regulates other circulations
How does myogenic autoregulation work?
↑ in blood pressure in the whole body–> vasoconstriciton –> don’t want too much blood going to the brain–> ↑ intracranial pressure skull stops expansion
↓ in blood pressure in whole body–> vasodilation –> ensure enough blood reaches the brain
At what pressure does autoregulation fail?
MAP below 50mmHg
How is the cerebral blood flow metabolically regulated?
Sensitive to changes in pCO2
↑ pCO2 (hypercapnia)–> vasodilation–> neurones are active require more O2
↓ pCO2 (hypocapnia)–> vasoconstriction–> neurones less active require less O2
What does an fMRI scan show?
Areas with increase neuronal activity –> ↑ blood flow
↑ pCO2, [K+], and adenosine –> vasodilator
↓pO2
What is Cushing’s reflex?
Cranium–> protection but not expansion
↑ intracranial pressure–> impair cerebral blood flow
Impaired blood flow–> vasomotor control regions of brain stem–> ↑ sympathetic activity–> ↑ arterial BP to maintain blood flow
Can get high BP with tachycardia
What does the coronary circulation need to ensure?
Must deliver O2 at high basal rate
Meet increased demand (can increase 5 fold)
When does blood flow into the coronary arteries usually occur?
Coronary arteries penetrate the myocardium
Therefore blood flow has to occur in diastole
Systole occlude blood flow to the myocardium
↑ HR–> shorten diastole–> reduce time for blood flow
Not a problem with normal pathology but it is with reduced blood flow
How is the coronary circulation adapted to deal with it?
High capillary density–> efficient O2 delivery
Diffusion distance <9 micrometers
Continuous production of NO by coronary endothelium maintains high basal flow
How does the coronary blood flow increase with myocardial O2 demand?
Linear relationship until very high O2 demand
Vasodilation due to metabolic hyperaemia
Vasodilators–> adenosine, ↑[K+] and ↓pH
What type of arteries are the coronary arteries?
Functional end arteries
Few anastomoses
What is the problem with functional end arteries in the coronary system?
Prone to atheromas
Narrowed coronary arteries leads to angina on exercise
↑ O2 demand
–> blood flows most during diastole, diastole reduces as HR increase
Stress and cold can also cause sympathetic coronary vasoconstriciton= angina
Sudden obstruction–> MI
What does the skeletal coronary circulation need to ensure? How does it do this?
Increase O2 and nutrient delivery and removal of metabolites during exercise
Help regulate arterial blood pressure
SM–> 40% of body mass
Sympathetic vasoconstrictor regulation–> NA released–> α1 receptor as part of baroreceptor reflex–> maintains BP
How does the skeletal muscle circulation ensure that these demands are met?
Capillary density–> depends on muscle type
(postural muscles ↑ density)
High vascular tone–> vasodilation–> 20x increase in flow during exercise
At rest- 1/2 capillaries perfused (allows for increased recruitment)
Opening of precapillary sphincters–> more capillary perfusion –> increased blood flow, reduced diffusion distance
How does metabolic hyperaemia increase blood flow?
Vasodilator agents - ↓pH - ↑ K+ - ↑ osmolarity - inorganic phosphates - Adenosine - ↑ H+ Adrenaline acts as vasodilator in arterioles- β2 receptor Vasoconstriction--> NA on α1 receptors
What is the role of the cutaneous circulation?
Temperature regulation via skin
Core temperature maintained around 37 degrees
–> Balance heat production and loss
Maintain BP
What are arterovenous anastomoses? How do they help with cutaneous circulation?
Increase SA: volume ratio
Under neural control (sympathetic vasomotor fibres) (–> not metabolites)
Decrease temp–> ↑ sympathetic tone in AVAs–> decrease blood flow to apical skin
Increase temp–> open AVAs
Reduce vasomotor drive to AVAs allows them to dilate–> divert blood closer to the skin
