Week 8 - Special Circulation Flashcards
What are some features of the pulmonary circulation?
- Supply driven
- Minimal flow resistance
- Low resistance, low pressure system
What is the pulmonary circulation required for?
- Blood supply to the alveoli
- Gas exchange
Which circulation meets most of the metabolic needs to the lung?
A separate part of the systemic circulation
- The bronchial circulation
What is the mean arterial pressure in the pulmonary circulation vs systemic circulation?
- P: 12-15 mmHg
- S: 95 mmHg
What is the mean capillary pressure in the pulmonary circulation vs systemic circulation?
- P: 9-12 mmHg
- S: 30 mmHg
What is the mean venous pressure in the pulmonary circulation vs systemic circulation?
- P: 5 mmHg
- 2-5 mmHg
What is the pressure in the left atrium?
1-10 mmHg
What is the pressure in the right atrium?
0-8 mmHg
What is the pressure in the left ventricle?
(100-140)/(1-10) mmHg
What is the pressure in the right ventricle?
(15-30)/(0-8) mmHg
What is the pressure in the aorta?
(100-140)/(60-90) mmHg
What is the pressure in the pulmonary artery?
(15-30)/(4-12) mmHg
What is ventilation-perfusion matching?
For effective exchange to occur, blood flow (perfusion) and air flow (ventilation) to each part of the body must be ‘matched’
- Controlled by hypoxic pulmonary vasoconstriction
- Blood is generally directed away from areas where oxygen uptake is reduced by hypoxic pulmonary vasoconstriction
What is the role of gravity in circulation?
It creates hydrostatic pressure in a column of blood
- This allows posture to influence the distribution of blood flow through the lungs
- When standing upright, the transmural pressure within the blood vessels at the base of the lungs is elevated by the increased hydrostatic pressure
- Τhis may lead to some filtration of tissue fluid
- Will also distend the vessels and increase flow to those areas
How can tissue fluid form in the lungs?
- Hydrostatic pressure of blood within the capillary pushes fluid out of the capillary
- Oncotic pressure draws fluid into the capillary
- – This is the pressure exerted by large molecules such as plasma proteins
- Low capillary pressure minimises the formation of lung lymph
- – Increased capillary pressure causes more fluid to filter out
- – This causes oedema
- – Pulmonary oedema impairs gas exchange
What determines the oxygen demand of the myocardium?
How much metabolic work is done
Depends on
- The external work done
— Depends upon the stroke volume and the arterial pressure
- The efficiency with which metabolic energy is converted to external work
When does coronary blood flow occur?
During diastole
- During systole the tension in the walls of the ventricles compresses the coronary vessels
Does the coronary circulation have a high or low blood flow?
In diastole, it is high
- To compensate for reduced blood flow in systole
What is the effect of an increased heart rate on the coronary circulation?
As heart rate increases, diastole shortens much more than systole does
- So the peak flow in diastole must increase very rapidly with rising heart rate in order to maintain the necessary average flow
What controls the flow rate in the coronary vessels?
- Almost entirely controlled by the action of local vasodilator metabolites upon coronary arteries
- – E.g. adenosine, increased [K+], decreased pH
- The flow is increased to meet myocardial O2 demand
- – Vasodilation due to metabolic hyperaemia
What are the complications of the coronary arteries being end arteries?
- Prone to atheromas
- Narrowed coronary arteries leads to angina on exercise
- – Diastole is reduced as heart rate increases
- Stress and cold can also cause sympathetic coronary vasoconstriction and angina
- Sudden obstruction by thrombus causes myocardial infarction
How does capillary blood flow change in skeletal muscle?
- At rest, most capillaries within a muscle are shut off by contraction of pre-capillary sphincters so hence there is low blood flow
- Increases in blood flow are mainly brought about by opening up more capillaries
- – Under the influence of vasodilator nervous activity and local metabolites
- – These tend to reduce tonic sympathetic vasoconstrictor tone
How does blood flow through the skin?
Mainly through arterio-venous anastomoses rather than capillaries
How can blood flow through the skin be altered?
- Mediators released from active sweat glands increase flow
- Circulating vasodilator mediators from other sources sometimes increase skin blood flow
- Little effect by local metabolites
What happens if blood flow to the brain is reduced?
- For a few seconds: subject will faint
- For more than 3 or 4 minutes: can lead to permanent brain damage or death
How is cerebral circulation regulated?
- Cushing’s reflex
- Via effects of local metabolites upon resistance vessels
- CO2 is a potent modulator of brain blood flow
- – Rises in pCO2 increase blood flow
- Increased blood pressure leads to vasoconstriction (myogenic autoregulation)
How does the cerebral circulation meet the high demand for O2?
- High capillary density
- High basal flow rate
- High O2 extraction
What is Cushing’s reflex?
- Right cranium protects the brain
- – Doesn’t allow for volume expansion
- So increases in intracranial pressure impairs cerebral blood flow
- – May cause cerebral tumour or haemorrhage
- Leads to impaired blood flow to vasomotor control regions of the brainstem
- – This increases sympathetic vasomotor activity, so arterial blood pressure increasing which helps to maintain cerebral blood flow
How is a secure blood supply within the cerebral circulation ensured?
- Structurally: anastomoses between basilar and internal carotid arteries
- Functionally:
- – Brainstem regulates other circulations to preserve own blood flow
- – Myogenic autoregulation maintains perfusion during hypotension
- – Metabolic factors control blood flow
