CVS Special Circulations Flashcards
State the mean normal pressures in:
The pulmonary artery
The pulmonary capillaries
The pulmonary veins
The pulmonary arteries: 12-15mmHg
The pulmonary capillaries: 9-12mmHg
The pulmonary veins: 5mmHg
What are the major differences between the systemic and pulmonary circulations?
The lungs have 2 circulations- pulmonary and bronchial
Bronchial is part of the systemic circulation and meets metabolic requirements of the lungs.
The pulmonary supplies the alveoli for gas exchange.
The pulmonary circulation accepts the entire cardiac output and has low resistance due to short wide vessels and lots of capillaries in parallel. This low resistance ensures the circulation is low pressure.
Explain the concept of ventilation perfusion and state the optimal ratio
For efficient oxygenation, ventilation of the alveoli needs to be matched with perfusion of alveoli
Optimal V/Q ratio is 0.8
How is the optimal ventilation perfusion ratio maintained?
A ratio of 0.8 is maintained by diverting blood away from alveoli that are not well ventilated, by hypoxic pulmonary vasoconstriction.
Alveolar hypoxia results in vasoconstriction of pulmonary vessels which diverts flow to better ventilated areas.
However chronic hypoxic vasoconstriction can lead to right ventricular failure because the chronic increase in vascular resistance puts a high after-load on the right ventricle.
How is tissue fluid formed in the lungs and systemic circulation?
Starling forces determine the formation of tissue fluid”
- Hydrostatic pressure of blood pushes fluid out
- Oncotic pressure draws fluid back in
Venous pressure has greatest effect of capillary hydrostatic pressure so in low pressure pulmonary system only a small amount of fluid leaves the capillaries.
In an upright position there is greater hydrostatic pressure on vessels in the lower lung due to gravity.
Pulmonary oedema is also affected by posture and impairs gas exchange.
What happens if the coronary arteries are partially or totally occluded?
Coronary arteries are functional end arteries prone to atheromas.
Partial occlusion - can lead to angina, sharp pain upon exertion. Also triggered by stress and cold as this can cause sympathetic coronary vasoconstriction
Total occlusion by a thrombus - MI
What features of cerebral circulation ensure the brains high demand for oxygen is met?
The brain receives ~15% of cardiac output. The cerebral circulation: - high capillary density -high basal flow rate -high O2 extraction
Needs a SECURE blood supply - a few seconds of cerebral hypoxia will lead to syncope and irreversible damage occurs in ~4minsWhat
How is a secure blood supply maintained to the brain?
Myogenic auto-regulation: (fails below BP 50mmHg)
Cerebral resistance vessels have a developed myogenic response to changes in transmural pressure, ensuring efficient vasodilation/constriction.
Metabolic regulation:
Cerebral vessels sensitive to arterial pO2. Hyperventilation can lead to hypocapnia (decreased CO2 in blood) leading to vasoconstriction.
Regional activity:
Areas with increased neuronal activity have increased blood flow. Adenosine is a powerful vasodilater.
Cushings reflex:
Increased intracranial pressure impairs blood flow, impaired blood flow to the brainstem increases vasomotor activity (peripheral) helping to maintain cerebral blood flow.
What factors affect blood flow to the skin?
The cutaneous circulation has a role in temperature regulation (skin is main heat dissipating surface). The blood flow to the skin is heavily influenced by SNS.
Decreased temp - increased tone in aterio-venous anastomoses, decreasing blood flow.
Increased temp - reduced vasomotor drive to arterio-venous anastomoses so shunt of blood to venous plexus.
NB Arterio-venous anastomoses are vessels that shunt blood from arteries to veins to bypass capillaries
What factors affect blood flow to skeletal muscle?
At rest most capillaries are shut off by contraction of pre-capillary sphincters, however flow can be increased ~20x in active muscle.
Blood flow is increased by opening more capillaries due to vasodilator nervous activity and local metabolites that reduce sympathetic vasoconstrictor tone.
Adrenaline also acts as a dilator - acts through B2
