Week 8 - Special Circulations Flashcards
How are the various circulations within the systemic circulation arranged?
In parallel
What features of the pulmonary circulation allow it to work with low pressure and low resistance?
- Short, wide vessels
- Lots of capillaries in parallel
- Arterioles have little smooth muscle
What adaptations of the pulmonary circulation promote gas exchange?
Large capillary surface area
Short diffusion distance
List the pressures within the pulmonary circulation
Pulmonary artery - 12 - 15mmHg
Pulmonary vein - 5 mmHg
Pulmonary capillaries - 9 - 12mmHg
Why is the ventilation perfusion ratio important?
What is its optimal value?
Ventilation and perfusion must be matched to allow effective O2 and CO2 exchange.
Optimal value = 0.8 (Ventilation/perfusion)
What is hypoxic pulmonary vasoconstriction?
The phenomenon whereby blood is diverted away from alveoli which are not well ventilated, via vasoconstriction of the pulmonary vessels.
This optimises gas exchange as hypoxic alveoli are less well perfused.
What results from chronic hypoxic vasoconstriction?
Why might it occur?
Pulmonary hypertension, which itself leads to increased afterload on the RV, and hence RV hypertrophy and right heart failure.
May be due to high altitudes or conditions such as emphysema.
Why is there a slight ventilation-perfusion mismatch in normal lungs?
How is this evident from a patient’s observations?
Due to gravity - more blood at base of lungs, but more air at apices. Therefore some blood passes through lungs without being fully oxygenated.
Normal O2 sats are normally around 98%
What happens to pulmonary blood flow during exercise?
Must accept a huge increase in cardiac output, therefore pulmonary arterial pressure increases.
How do the pulmonary capillaries respond to exercise?
Apical capillaries open
In an upright position, describe the distribution of pressure across the lung.
Greater hydrostatic pressure in lower part of lung as below level of heart
In orthostasis, in which regions of the lung are vessels open or closed?
Apex - vessels collapse during diastole due to pressure falling as a result of being above the heart
At level of heart, always patent
At base, vessels are distended - high pressure as below heart
What is hydrostatic pressure mainly reliant on?
Venous pressure
Why is little lung lymph produced in normal conditions?
Low pressure in pulmonary system, so low hydrostatic pressure forcing fluid out of capillaries - little fluid leaves.
What happens in the lungs if LA pressure was to rise due to mitral stenosis or LV failure?
Venous pressure in the pulmonary veins would rise, and hence pressure in the capillaries would also rise. This would result in more fluid being forced out of the capillaries –> pulmonary oedema
How does pulmonary oedema affect gas exchange?
Impairs gas exchange, as reduces the alveoli surface area available
How does the cerebral circulation meet the brain’s high O2 demand?
- High capillary density
- High basal flow rate
- High O2 extraction
How is blood supply to the brain ensured, in general terms?
Functionally and structurally
What structural feature of the cerebral circulation ensures blood supply reaches the brain even if one vessel blocks?
What is it?
The Circle of Willis - a ring of arteries formed by anastomoses between the basilar and internal carotid arteries.
Cerebral vessels are very sensitive to changes in ________ ?
Explain how vessels respond to these changes.
Arterial CO2
If CO2 increases (hypercapnia), vessels dilate, and vice versa
What is myogenic autoregulation?
What does it allow?
Cerebral vessels respond to changes in transmural pressure.
Increased pressure results in vasoconstriction to limit flow, and vice versa.
Allows maintenance of constant cerebral flow when arterial bp changes
Why might someone faint during a panic attack?
Panic attack causes hyperventilation, which decreases the amount of CO2 in the blood (hypocapnia). The arterioles in the brain respond by constricting, reducing blood flow to the brain, which results in syncope.
How does the brain respond to decreased flow due to a bleed within the brain?
What is this phenomenon called?
Bleed within the brain increases intracranial pressure as the cranium is rigid and hence the brain cannot expand. The rise in pressure impairs blood flow.
In response to this, the vasomotor control regions increase sympathetic vasomotor activity - constrict peripheral vessels, which raises BP, sending more blood to the brain.
This is Cushing’s reflex
What is the relationship between oxygen demand in the heart and cardiac blood flow?
Almost linear
But at very high oxygen demands, amount of oxygen extracted increases slightly, so flow does not need to match it - slightly lower.
