special circulation Flashcards
what are the 2 circulations to the lungs?
1) bronchial circulation:
- parts of systemic circulation
- meets metabolic requirements of lungs
2) pulmonary circulation:
- blood supply to alveoli
- required for gas exchange
pulmonary circulation has to accept the full cardiac output. what are the features of the pulmonary circulation which allow for this?
low pressure:
- mean arterial pressure = 12-15mmHg
- mean capillary pressure = 9-12 mmHg
- mean venous pressure = 5 mmHg
low resistance:
- short wide vessels
- lots of capillaries
- arterioles have relatively little smooth muscle
what are adaptations of the pulmonary circulation that aid efficient gas exchange?
- Very high density of capillaries in alveolar wall:
– large capillary surface area - Short diffusion distance:
– very thin layer of tissue separating gas phase from plasma
– combined endothelium & epithelium thickness is ~ 0.3 μm - Large surface area and short diffusion distance produce high O2 and CO2 transport capacity
what is ventilation - perfusion ratio (V/Q ratio)?
for efficient oxygenation the ventilation of alveoli needs to match perfusion of alveoli
optimal V/Q ratio = 0.8
this is maintained by diverting blood from alveoli which are not well ventilated
how is an optimal V/Q ratio maintained?
- Hypoxic pulmonary vasoconstriction is the most important mechanism regulating pulmonary vascular tone
- Alveolar hypoxia results in vasoconstriction of pulmonary vessels
- Ensures that perfusion matches ventilation
- Poorly ventilated alveoli are less well perfused
- Helps to optimise gas exchange
- Effect is opposite to that in systemic circulations
what issues can chronic hypoxic vasoconstriction cause?
chronic hypoxia can occur at altitude or as a consequence of lung disease such as emphysema
- chronic increase in vascular resistance causes chronic pulmonary hypertension
- high afterload on RV causes right ventricular heart failure
where is pressure greatest in lungs in standing position?
greater hydrostatic pressure on vessels in lower part of lung
what is the effect of exercise on pulmonary blood flow?
- Increased cardiac output
- Small increase in pulmonary arterial pressure
- Opens apical capillaries
- Increased O2 uptake by lungs
- As blood flow increases capillary transit time is reduced
– at rest transit time ~ 1s
– can fall to ~ 0.3s without compromising gas exchange
what is the correlation between capillary pressure and tissue fluid formation?
low capillary pressure = decreased formation of lung lymph
increased capillary pressure = increased formation of fluid = oedema
what are the consequences of pulmonary oedema?
pulmonary oedema impairs gas exchange
- affected by posture (changes to hydrostatic pressure)
- forms mainly at bases when upright
- forms throughout lung when lying down
how is pulmonary oedema treated?
use diuretics to relive symptoms
treat underlying cause if possible
the brain needs a high blood supply. How does cerebral circulation meet the high demand for oxygen?
- high capillary density
– large surface area for gas exchange
– reduced diffusion distance (<10μm) - high basal flow rate
– X10 average for whole body - high O2 extraction
– 35% above average
why is securing the oxygen supply to the brain important?
- Neurones are very sensitive to hypoxia
- Loss of consciousness after a few seconds of cerebral
ischaemia - Begin to get irreversible damage to neurones in ~ 4 minutes
- Interruption to blood supply e.g. stroke causes neuronal death
How is a secure blood supply to the brain ensured?
- Structurally
– anastomoses between basilar and internal carotid arteries - Functionally
– myogenic autoregulation maintains perfusion during hypotension
– metabolic factors control blood flow
– brainstem regulates other circulations
what are the metabolic regulations of cerebral vessels?
very sensitive to changes in PCO2:
- hypercapnia causes vasodilation
- hypocapnia causes vasoconstriction (results in dizziness or fainting)
what are the myogenic auto regulations of cerebral vessels?
respond to changes in pressure:
- increased blood pressure = vasoconstriction
- decreased blood pressure = vasodilation
how does blood flow change to active brain areas?
areas with increased activity have increased blood flow due to vasodilation caused by:
- increase Pco2
- increased [K+]
- increased adenosine
- decreased Po2
what is Cushing’s reflex?
- Rigid cranium protects the brain:
– but does not allow for volume expansion - Increases in intracranial pressure impair cerebral blood flow:
– Eg cerebral tumour or haemorrhage - Impaired blood flow to vasomotor control regions of the brainstem increase sympathetic vasomotor activity:
– increases arterial BP
– helps maintain cerebral blood flow
its important oxygen supply to heart is efficient. how is coronary circulation adapted to supplying heart?
- High capillary density facilitates efficient O2 delivery
- Diffusion distance < 9μm
- Continuous production of NO by coronary endothelium maintains a high basal flow
what is the relationship between coronary blood flow and myocardial oxygen demand?
- Extra O2 required at high work load is supplied mainly by increased blood flow
- Almost linear relationship until very high O2 demand
- Small increase in amount of O2 extracted
*Vasodilation due to metabolic hyperaemia *Vasodilators - adenosine, ↑[K+], ↓pH
what are functional end arteries?
those arteries whose terminal branches do anastomose, but the anastomosis is not sufficient to maintain the blood supply to the part they supply in case of any blockage in the artery
- coronary arteries are functional end arteries
what are the complications with issues of coronary arteries?
- Few aterio-arterial anastomoses
- Prone to atheromas
- Narrowed coronary arteries leads to angina on exercise – blood flow mostly during diastole
- diastole is reduced as heart rate increases
– Reduced blood flow in diastole combined with increased demand
– Note: stress and cold can also cause sympathetic coronary vasoconstriction and angina - Sudden obstruction by thrombus can cause myocardial infarction
its important oxygen supply to skeletal muscles is efficient. how is circulation adapted to supplying skeletal muscle?
- Capillary density depends on muscle type
– Postural muscles have higher capillary density - Very high vascular tone
– Permits lots of dilatation
– Flow can increase > 20 times in active muscle - At rest only ~ 1⁄2 of capillaries are perfused at any one time
– allows for increased recruitment - Opening of precapillary sphincters allows more capillaries to be perfused.
– increases blood flow and reduces diffusion distance
what is the role of cutaneous circulation?
- Special role in temperature regulation
- Core temperature is normally maintained around 37’C
– Balance of heat production and heat loss - Skin is the main heat dissipating surface
– This is regulated by cutaneous blood flow
– also has role in maintaining blood pressure - vasoconstriction in cutaneous circulation to maintain BP
– see lecture on shock
what are atereovenous anastomoses (AVA’s)?
specialised structures in the acral (apical) skin
how are AVA’s adapted to regulate heat loss from the apical skin?
- Apical (acral) skin has a high surface area to volume ratio
- AVAs are under neural control
– sympathetic vasoconstrictor fibres - Not regulated by local metabolites
- Decrease core temperature increases sympathetic tone in AVAs
– decreases blood flow to apical skin - Increased core temperature opens AVAs
- Reduced vasomotor drive to AVA’s allows them to dilate – diverts blood to veins near surface
– Helps in heat loss
