Session 8 Flashcards
State the normal pressures in the pulmonary artery, pulmonary capillaries and pulmonary veins
Pulmonary artery - 12-15 mmHg
Pulmonary capillaries - 9-12 mmHg
Pulmonary veins - 5 mmHg
State the major differences between the properties of the systemic and pulmonary circulations
The lungs have two circulations (pulmonary and bronchial)
Pulmonary circulation is blood supply to the alveoli required for gas exchange.
Bronchial circulation is part of the systemic circulation.
Pulmonary circulation must accept the entire cardiac output and works with low resistance and low pressure.
Explain the concept of ventilation perfusion matching in the pulmonary circulation
For efficient oxygenation, ventilation of the alveoli needs to be matched with perfusion. The optimal Ventilation/Perfusion (V:Q) ratio is 0.8. Maintaining this means diverting blood from alveoli that are not well ventilated.
This is achieved by hypoxic pulmonary vasoconstriction. The increased resistance means less flow to the poorly ventilated areas and greater flow to the well ventilated areas.
Chronic hypoxic vasoconstriction can lead to right ventricular failure.
Describe the forces which are involved in the formation of tissue fluid in the lungs and in the systemic circulation
Starling forces determine the formation of tissue fluid. Hydrostatic pressure of blood within capillaries pushes fluid out and oncotic pressure draws fluid back in.
In the low pressure pulmonary system, only a small amount of fluid leaves the capillaries. If capillary pressure increases this leads to pulmonary oedema.
Pulmonary oedema impairs gas exchange and is affected by posture (at base of lungs when upright, throughout lung when lying down)
Describe the relationship between the mechanical work and oxygen demand of the myocardium
The coronary circulation must deliver oxygen at a high basal rate, which must rise to meet increased demand.
There is an almost linear relationship until very high oxygen demand, when there is a small increase in the amount of oxygen extracted.
Describe the particular features of the coronary circulation
Right and left coronary arteries arise from right and left aortic sinuses, and fill during diastole.
Cardiac muscle has a high capillary density to efficiently deliver oxygen and the continuous production of nitrous oxide by coronary endothelium maintains a high basal flow.
Describe the consequences of partial or total occlusion of coronary arteries
Coronary arteries are functional end arteries, prone to atheroma.
Partial - angina on exercise
Total - myocardial infarction
Describe the factors which influence blood flow through the brain
The cerebral circulation meets the high oxygen demand by having a high capillary density, large surface area, reduced diffusion difference, high basal flow rate and high oxygen extraction.
Neurones are very sensitive to hypoxia.
Secure blood flow is ensured:
Structurally - anastomoses between basilar and internal carotid arteries
Functionally - brain stem regulates other circulations, myogenic auto-regulation (maintains perfusion during hypotension due to changes in transmural pressure), metabolic factors (sensitive to changes in arterial CO2)
Regional activity - areas with increased neuronal activity have increased blood flow, adenosine is a powerful vasodilators
Cushing’s reflex - rigid cranium does not allow for volume expansion, impaired blood flow to vasomotor control regions of the brainstem increases sympathetic vasomotor activity
Describe in broad outline the factors which influence blood flow through skin and skeletal muscle
Cutaneous circulation:
Most blood flow through skin in not nutritious, flows through arterio-venous anastomoses (AVA) rather than capillaries
Role in temperature regulation, main heat dissipating surface, influences by sympathetic nervous system
Skeletal muscle circulation:
At rest most capillaries within muscle are shut off by contraction of pre-capillary sphincters
Increases in blood flow are brought about mainly by opening up more capillaries (vasodilator nervous activity, local metabolites)
Flow can be increased >20x in active muscle
