Lecture 8.2a Part 1 - Special Circulations PDF

Lungs: ◦Have both bronchial and pulmonary circulations - therefore dual supply ◦Bronchial: supplies O2 and nutrients to the lungs ◦Pulmonary : carries deoxygenated blood away from the RV and oxygenated blood back to LA Bronchial circulation: ◦Perfuses with respiratory...

Lungs: ◦Have both bronchial and pulmonary circulations - therefore dual supply ◦Bronchial: supplies O2 and nutrients to the lungs ◦Pulmonary : carries deoxygenated blood away from the RV and oxygenated blood back to LA Bronchial circulation: ◦Perfuses with respiratory tract to the level of terminal bronchioles ◦Bronchial arteries arise from the aorta ◦Bronchial veins anastomoses and brings deoxygenated blood to pulmonary veins (slight mixing in the pulmonary veins) ◦Pressure in bronchial arteries is equal to systemic pressure Pulmonary circulation: ◦The right and left ventricles have the same CO ◦So the entire CO need to be oxygenated in the lungs ◦Special adaptations to facilitate the high volume of blood and to promote efficient gas exchange Pulmonary circulation: special adaptations: ◦To facilitate high volume of blood: ‣ Low pressure system: Mean arterial pressure: 12-15 mmHg Mean capillary pressure: 9-12 mmHg Mean venous pressure: 5mmHg Thin vascular walls, high compliance ‣ Low resistance: Pulmonary vessels are shorter and wider Capillaries run in parallel Relatively less SMC on arterioles Pulmonary circulation: special adaptations: ◦To promote efficient gas exchange: ‣ Branching structure provides large surface area 300 million alveoli in the lungs ‣ Short diffusion distance Alveoli and capillaries consist of one layer of epithelium ~0.3um thickness ‣ High density of capillaries Alveoli always close to capillaries ‣ Hypoxic vasoconstriction Optimal ventilation/perfusion (V/Q) ratio: ~0.8-1.0 If poorly ventilated, blood re-directed to well ventilated areas by constriction Problems due to hypoxic vasoconstriction: ◦Reasons for chronic hypoxia:  ‣ High altitude ‣ Lung disease (e.g. Emphysema) ◦Chronic hypoxia -> widespread vasoconstriction of pulmonary vessels -> increase in pulmonary resistance -> chronic pulmonary hypertension Lymphatic system prevents fluid build up: ◦The mean filtration pressure at the pulmonary capillaries is +1 mmHg ◦Lymphatic system drains the fluid in interstitial space preventing fluid build up ◦Any imbalance in hydrostatic pressure or oncotic pressure will lead to pulmonary oedema - due to fluid pooling around the alveolar sacs ◦Left ventricular heart failure or mitral valve stenosis can cause pulmonary oedema Low pressure pulmonary vessels strongly influenced by gravity: ◦In the upright position (orthostasis) there is greater hydrostatic pressure on vessels in the lower part of the lung, as fluid pools to the bottom of the lung ◦Apical capillaries open during increased CO (e.g. exercise) Coronary circulation: ◦Rest 70-80 ml min-1 gram-1 ◦Heavy exercise 300-400 ml min-1 gram-1 ◦Coronary arteries are intramural (buried within muscles) -> compressed during systole, perfusion occurs during diastole ◦Structural adaptations: ‣ High capillary density - low diffusion distance ‣ Continuous production of NO - potent vasodilator ‣ Reactive hyperaemia - increase in metabolite production -> local vasodilation ◦These are end arteries (few or no anastomosis) -> prone to atheromas ◦Obstruction -> ischaemia and infarction