Special Circulations Flashcards
State the major differences between the systemic and pulmonary circulations
The lungs have two circulations; pulmonary and bronchial. The bronchial is part of the systemic circulation and meets the metabolic requirements of the lungs. The pulmonary is the supply to the alveoli for gas exchange. The pulmonary system accepts the entire cardiac output and works with low resistance due to wide vessels and capillaries in parallel.
What are the normal pressures in the pulmonary artery. capillaries and veins?
Artery: 12-15 mmHg
Capillary: 9-12 mmHg
Veins: 5 mmHg
Explain the concept of ventilation perfusion matching in the pulmonary circulation
To maximise oxygenation, ventilation of alveoli must match their perfusion. The optimal ventilation perfusion rate is 0.8. which is maintained by diverting blood away from poorly ventilated alveoli. This occurs through ‘hypoxic pulmonary vasoconstriction’, whereby vaso constriction increases pressures thus blood flow to well ventilated areas. This increases the pressure on the right ventricle and, if prolonged, cause right ventricular failure.
Describe the forces which are involved in the formation of tissue fluid in the lungs and in the systemic circulation
Starlings Law determines tissue fluid formation. Hydrostatic pressure of blood in capillary pushed fluid out, and colloid osmotic pressure in the post capillary venules drags fluid back in (albumin etc).
Hydrostatic pressure governed by venous pressure (back pressure). As Pulmonary pressure is low, only small amounts of fluid leaves. If venous/ capillary pressure increases, oedema occurs.
Hydrostatic pressure is greater in lung bases when standing up. Oedema will collect here when standing, but diffuse across lung fields when lying flat.
Describe the relationship between mechanical work and oxygen demand of the myocardium.
Coronary circulation must have a high basal O2 saturation, which must increase as cardiac effort increases in response to exercise etc. There is almost a totally linear relationship until high demand, where bF plateaus off slightly.
Describe the particular features of the coronary circulation
R and L coronary arteries arise from the R and L aortic sinuses and fill DURING DIASTOLE. In systole, the increased myocardial pressures increase coronary pressure (?pushing against pericardium) making it too high for filling.
Capillary density is high in the myocardium and there is a constant release of NO from the endothelium.
Describe the consequences of partial or total occlusion to coronary blood flow
Coronary arteries are functional end arteries which are prone to atheromas. Narrowed arteries lead to angina on exercise due to increased O2 demand. Stress and cold can also cause sympahetic coronary constriction. Sudden thrombus obstruction leads to MI.
Describe the factors which influence blood flow through the brain
The brain has a high O2 demand and uses around 15% of TCO. The O2 consumption of grey matter accounts for around 20% of TCO at rest. This is supported by having a high capillary density in the cerebrum, with large surface areas for gas exchange and reduced diffusion resistance; a high basal flow rate and a high O2 extraction rate.
Syncope occurs after a few seconds of cerebral hypoxia and permanent brain damage occurs after around 4 minutes.
The brain can regulate its blood supply further through:
- myogenic auto-regulation (vaso constriction and dilation)
- Metabolic regulation (hyperventilation and hypercapnia)
- Vasodilatory metabolites i.e. adenosine
Describe the factors controlling flow of blood through skeletal muscle
At rest, most blood is diverted away from the muscles through the action of pre capillary sphincters. Increases in blood flow is predominantly brought about through relaxation of these sphincters. This occurs through the influence of local vasodilatory metabolites and reduction of sympathetic vasoconstrictor tone.
Describe the factors controlling flow of blood to the skin
Most of the flow through the skin is not nutritive and flows through ateriovenous anastamoses (AVA) rather than through capillaries. The main purpose of this flow is in temperature regulation, which is controlled through the sympathetic nervous system.
