CVS S8 - Special Circulations Flashcards
What two circulations are present in the lungs?
Pulmonary
Bronchial
Describe the bronchial circulation
Part of systemic circulation
Meets the metabolic requirements of the lungs
Describe pulmonary circulation
Blood supply to alveoli required for gas exchange
Must accept entire cardiac output (5 - 25L/min)
Low resistance due to short, wide vessels with lots of parallel capillaries and arterioles with relatively little smooth muscle
Low resistance means low pressure in the pulmonary circulation
CVS link
What are the average systolic and diastolic pressures in the 4 chambers of the heart, pulmonary artery and aorta?
RA:
0 - 8mmHg
LA:
1-10mmHg
RV:
S = 15 - 30mmHg
D = 4 -12mmHg
LV:
S = 100 - 140mmHg
D = 1 - 10mmHg
Pulm Artery:
S = 15 - 30mmHg
D = 4 -12mmHg
Aorta:
S = 100 - 140mmHg
D = 60 - 90mmHg
State the normal pressures in the:
Pulmonary artery
Pulmonary capillaries
Pulmonary veins
PA:
12 - 15mmHg
PC:
9 - 12mmHg
PV:
5mmHg
What adaptations do alveoli possess to enable efficient gas exchange?
Very high density capillaries in the alveolar wall:
- Large capillary surface area
Short diffusion distance:
- Thin layer of tissue between gas and plasma
- Endothelium & epithelium distance = 0.3um
These adaptations produce high O2 and CO2 transport capacity
What is the perfusion ratio?
The optimal ratio of ventilation/perfusion
Ventilation of alveoli and perfusion of alveoli are matched
0.8 is optimal value
How is perfusion ratio maintained in normal and abnormal conditions?
Blood must be diverted from poorly ventilated alveoli
Regulated by pulmonary vascular tone
Hypoxia results in vasoconstriction of pulmonary vessels to ensure perfusion matches ventilation
This optimises gas exchange
What problems arise when hypoxic vasoconstriction of pulmonary circulation is chronic?
When might chronic hypoxia occur?
Chronic hypoxia causes chronic increase in vascular resistance
This in turn causes pulmonary hypertension
High afterload on the RV can cause right ventricular heart failure
At altitude or as a consequence of lung disease such as emphysema
How does gravity affect the pulmonary circulation?
In the upright position (orthostasis) there is greater hydrostatic pressure on vessels in the lower lung due to gravity
Vessels near the apex collapse during diastole
Vessels in transverse plane with the heart are continuously patent
Vessels in the lower lung distended
What is the effect of exercise on the pulmonary circulation?
Increased cardiac output leads to a small increase in pulmonary arterial pressure
This opens apical capillaries
Increased O2 uptake by the lungs
As blood flow increases capillary transit time decreases
At rest about 1s, can fall to 0.3s without compromising gas exchange
What forces determine tissue fluid formation in the pulmonary circulation?
Starling forces determine fluid formation
Hydrostatic pressure of blood within the capillary an interstitial oncotic pressure forces/draws plasma out of the capillary
Oncotic (colloid osmotic) pressure exerted by large molecules such as plasma proteins draws fluid into the capillaries
Describe how forces determining tissue fluid formation in the pulmonary circulation vary over the course of a capillary?
Capillary hydrostatic pressure is more influenced by the venous pressure in the systemic circulation
Fluid therefore tends to move out of a capillary toward the arterial end and move in at the venous end
How are forces that determine tissue fluid formation balanced so as to not cause pulmonary oedema?
This applies to the rest of the circulation as well
Low capillary pressure minimises hydrostatic pressure
(9 - 12mmHg)
This helps to balance the starling forces so only a small amount of fluid flows out (lung lymph)
What is the effect of increased pulmonary capillary pressure on interstitial fluid in the lungs?
How is this increase in pressure caused?
Increases hydrostatic pressure causing fluid to flow out of the capillary
This causes oedema
Increased pressure caused by LA pressure raising to 20 - 25mmHg:
- Mitral valve stenosis
- Left ventricular failure
What is the effect of pulmonary oedema on the lungs?
How can symptoms of this be relieved?
Impairs gas exchange:
- Affected by posture
- Mostly at the bases when upright
- Throughout the lung when lying down
Use diuretics to relieve symptoms
Treat underlying cause
What is the relative oxygen demand of the myocardium in basal conditions?
How does this vary when demand is increased?
Coronary circulation must supply a relatively high basal rate of O2 to satisfy myocardial demand in basal conditions
Myocardial workload can increase 5 fold
Extra O2 supplied by increased blood flow
There is an almost linear relationship between coronary blood flow and myocardial O2 demand (about a 5 fold increase in blood flow)
Until very high O2 demand where there is a small increase in the amount of O2 extracted (gradient is lowered slightly)
From where does coronary blood flow arise?
Right and left coronary arteries arise from the left and right aortic sinuses
How is high blood flow in the coronary circulation maintained?
Continuous NO production by the coronary endothelium
Metabolic hyperaemia caused by vasodilators:
- Adenosine
- K+
- Lowered pH
State fibre diameter and capillary density in skeletal and cardiac muscle
What do the relative capillary densities indicate?
What is the max diffusion distance of O2 through cardiac tissue?
Skeletal muscle:
- Fibre diameter = 50um
- Capillary density = 400/mm2
Cardiac muscle:
- Fibre diameter = 18um
- Capillary density = 3000/mm2
Cardiac muscle more capillary dense, therefore can deliver O2 more efficiently
Max diffusion distance = 9um
Explain how coronary arteries are involved in angina and myocardial infarction
Coronary arteries are functional end arteries (few arterio-arterial anastomoses) which are prone to atheroma
Narrowed coronary arteries lead to angina on exercise (increase O2 demand not met)
Sudden obstruction of the end arteries by a thrombus causes myocardial infarction
When does a majority of blood flow occur in the heart?
Occurs mainly during diastole:
- Contraction of myocardium compresses coronary vessels during systole
- Aortic sinuses are closed during systole
How does increasing heart rate affect rate of coronary blood flow?
Why is this a problem?
As blood flow is mostly during diastole (duration of which is reduced as heart rate increases) the rate of blood flow must rise very high to maintain adequate flow
Therefore minor problems with cardiac circulation might become apparent at higher heart rates
Coronary circulation is therefore much more sensitive to arterial occlusion than the rest of the body
Brielfy describe the O2 demand of the cerebral circulation
High O2 demand
Receives 15% of cardiac output (despite only being 2% of body mass)
Grey matter accounts for 20% of O2 consumption at rest
How does cerebral circulation meet the high O2 demand?
High capillary density:
- Large exchange area
- reduced diffusion distance (
Why is a secure O2 supply to the brain important?
Neurones very sensitive to hypoxia
Loss of consciousness after a few seconds of cerebral ischaemia (syncope)
Irreversible neurone damage after around 4 minutes
Interruptions to the blood supply such as in stroke lead to neuronal death
How does the structure of the cerebral circulation help maintain a secure blood supply to the brain?
Anastomoses between basilar and internal carotid arteries
How do functional factors contribute to secure blood supply to the brain?
Brainstem regulates other circulations
Myogenic auto-regulation maintains perfusion during hypotension
Metabolic factors control blood flow
Describe myogenic auto-regulation of cerebral circulation
Cerebral vessels respond to change in blood pressure
Increased BP = vasoconstriction
Decreased BP = vasodilation
this serves to maintain cerebral blood flow when BP changes
It fails below 50mmHg
Describe metabolic regulation of cerebral circulation
Cerebral vessels are sensitive to pCO2
Increased pCO2 (hypercapnia) = vasodilatation Decreased pCO2 (hypocapnia) = vasoconstriction
In addition, areas of increased neuronal activity have increased blood flow as active neurones release adenosine, a powerful cerebral arteriole vasodilator
What is the effect of panic hyperventilation on the brain?
Can cause a rise in pCO2 (hypercapnia) leading to vasoconstriction
This can cause dizziness or fainting (syncope)
Describe Cushing’s reflex
Rigid cranium doesn’t allow volume expansion of the brain
Increased intercranial pressure can therefore result from haemorrhage or tumour formation which can impair cerebral blood flow
Impaired blood flow to the vasomotor control regions of the brainstem increases sympathetic vasomotor activity causing peripheral vasoconstriction
This leads to more blood flow being directed to the brain
What are the special roles of cutaneous circulation?
Temperature regulation
Maintaining blood pressure
How is the cutaneous circulation involved in maintaining blood pressure?
Vasoconstriction can cutaneous circulation helps maintain BP if necessary (E.g. When in shock)
What structures in the cutaneous circulation are involved in temperature regulation?
How is cutaneous circulation controlled?
Most blood flow in the skin isn’t nutritive, it flows through arterio-venous anastomoses (AVAs) instead of capillaries
AVAs are under sympathetic control (sympathetic vasoconstrictor fibres in the AVAs)
Not regulated by local metabolites
What is the response of cutaneous circulation to increase or decrease in core temperature?
Lowered core temperature:
- Increases sympathetic tone causing AVA constriction
- This decreases blood flow to the apical skin
- Causes heat retention and pallor
Raised core temperature:
- Decreases sympathetic tone causing AVA dilation
- AVA forms a low resistance shunt to venous plexuses
- Increases blood flow to apical skin
- Causes increased heat dissipation
What is the function of skeletal muscle circulation during exercise?
What is one other function of skeletal muscle circulation in general?
Increase O2 and nutrient delivery and removal of metabolites
Important role in helping to regulate blood pressure (40% of body mass)
Describe the changes that occur in skeletal circulation as you begin to exercise starting from a basal state
Don’t worry about how this is controlled, just state the changes
At basal state, 1/2 capillaries closed off by pre-capillary sphincters
Increased blood flow in exercise is then brought about by opening of the pre-capillary sphincters
Vasodilation of vessels (up to 20x)
How is skeletal circulation controlled?
Resistance vessels have rich sympathetic innervation controlling vasomotor tone
Metabolic hyperaemia is also involved in control Vasodilators: - K+ - Increased Osmolarity - Inorganic phosphates - Adenosine - H+
Adrenaline (B2 adrenoceptor agonist) also acts as a vasodilator of arterioles
State the relative basal vasomotor tone of skeletal muscle
Give an explanation as to why
Relatively high
High as it allows up to a 20x increase in blood flow to help meet metabolic demand during exercise
