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)
