Cvs Special Circulations Flashcards
state the major differences between the properties of the systemic and pulmonary circulations
Unlike the systemic circulation, which is demand led, the pulmonary circulation is supply driven, it must accommodate the entire cardiac output.
offers minimal flow resistance ( wide vessels, lots of capillaries, arterioles have less smooth muscle) and operates as a low resistance, low pressure system.
When standing upright, transmural pressure within the blood vessels at the base of the lungs is elevated by the increased hydrostatic pressure - inc flow.
explain how pulmonary vessels respond to changes in oxygen
levels
pulmonary arterioles can control the distribution of the cardiac output over the lung. Blood is generally directed away from areas where oxygen uptake is reduced by hypoxic pulmonary vasoconstriction - Maintaining optimal VQ - Helps to optimise gas exchange - Effect is opposite to that in systemic circulations
compare pressure in the chambers on the left and right side of the heart in a healthy adult
Left ventricle
100-140mmHg
1-10mmHg
LA
1-10mmHg
Aorta 100-140mmHg 60-90mmHg
Right ventricle
15-30mmHg
0 - 8mmHg
Pulmonary artery
15-30mmHg
4 -12mmHg
compare normal adult pressures in the pulmonary and systemic circulations
Pulmonary
mean arterial pressure ∼ 12-15mmHg
– mean capillary pressure ∼ 9-12mmHg
– mean venous pressure ∼ 5mmHg
Systemic
70 -100
15-35
20
explain the concept of ventilation perfusion matching in the pulmonary circulation notion
In order for effective exchange to occur blood flow (‘perfusion’) and air flow (‘ventilation’) to each part of the lungs must be ‘matched’. Because of the way the gasses are carried in the blood, if there is a ‘ventilation/perfusion mismatch’ the blood leaving the lungs will contain less oxygen and hypoxia will result.
• Optimal V/Q ratio = 0.8
in normal lungs there is a small mismatch because gravity increases blood flow to the base of the lungs when more air is delivered to the apices. This leads to some blood passing through the lungs without being properly oxygenated.
Chronic hypoxic vasoconstriction
Chronic hypoxia can occur at altitude or as a
consequence of lung disease such as emphysema.
chronic increase in vascular resistance - chronic pulmonary
hypertension
high afterload on right ventricle - can lead to right ventricular
heart failure
Effect of exercise on pulmonary blood flow notion
• Increased cardiac output Apex
• Small increase in pulmonary arterial pressure
• Opens apical capillaries Level of heart
• Increased O2 uptake by lungs
• As blood flow increases, capillary transit time is reduced
describe the forces which are involved in the formation of tissue fluid in the lungs and in the systemic circulation notion
Low capillary hydrostatic pressure minimises the formation of lung lymph
close to the colloid osmotic pressure, so little tissue fluid in lungs. This must be no more than the lymphatic system in the lungs can drain.
Systemic:
capillary hydrostatic pressure is influenced more by venous pressure in the systemic circulation so Hypertension does not usually result in peripheral oedema.
explain how some conditions can increase pressure in the pulmonary circulation and the consequence of this
Can get pulmonary oedema if capillary pressure increases eg
if the left atrial pressure rises to 20 - 25 mmHg in LV failure/ mitral valve stenosis.
Impairs gas exchange.
Use diuretics for symptoms and treat underlying cause.
describe the factors which influence blood flow through the brain
Blood flow reduced for few secs = syncope
three or four minutes = death
high capillary density
High O2 extraction
high basal flow rate
anastomoses between basilar and internal carotid arteries
myogenic autoregulation maintains perfusion during
hypotension/ changes in transmural pressure - inc in BP causes vasoconstriction and vice versa.
metabolic factors control blood flow - Adenosine, CO2, K+ cause vasodilation so inc blood flow.
Rises in the partial pressure of carbon dioxide (hypercapnia) increase blood flow - vasodilation and vice versa.
Cushing’s Reflex
Increases in intracranial pressure impair cerebral blood flow
In response:
Brainstem has inc sympathetic vasomotor activity.
increases arterial BP so helps maintain cerebral blood flow
Therefore when there is space occupying lesion there is reflex bradycardia and acute hypertension - hallmarks.
Cushing’s triad which is characterised by high BP, irregular breathing and bradycardia.
describe the relationship between the mechanical work and oxygen demand of the myocardium
oxygen demand of the myocardium is determined by how much metabolic work is done
metabolic energy is converted to external work -needs to be efficient
external work done by the heart per beat depends upon the stroke volume and the arterial pressure - AT HIGHER PRESSURE - LOWER EFFICIENCY- NEED BIGGER BLOOD FLOW.
describe the particular features of the coronary circulation
During systole tension in the walls of the ventricles compresses coronary vessels - dec blood flow.
SO MUST HAVE HIGH BLOOD FLOW IN DIASTOLE TO COMPENSATE especially in LV.
As heart rate increases diastole shortens much more than systole esp LCA.
peak flow in diastole must inc rapidly with inc HR to maintain the necessary flow (this is why angina worse with exercise).
Control by the action of local vasodilator metabolites - Continuous production of NO by coronary endothelium maintains a high basal flow. Vasodilation due to metabolic hyperaemia - adenosine, ↑[K+], ↓pH
Prone to atheromas
stress and cold can cause sympathetic coronary
vasoconstriction and angina
Sudden obstruction by thrombus can cause MI
Skeletal muscle circulation
Must increase O2 and nutrient delivery and removal of metabolites during exercise.
Resistance vessels have many sympathetic
vasoconstrictor fibres - maintains ABP.
At rest most capillaries within a muscle are shut off by contraction of pre-capillary sphincters.
Opening up more capillaries under the influence of vasodilator nervous activity and local metabolites (↑[K+],↑ osmolarity, Inorganic phosphates, Adenosine,↑[H+], Adrenaline )
= dec tonic sympathetic vasoconstrictor tone
= inc blood flow.
Compare coronary and skeletal
Fibre diameter 18μm • Capillary density 3000/mm2 • Capillaries continuously open and
perfused
Fibre diameter 50μm • Capillary density 400/mm2 • Not all capillaries perfused at rest
