Pulmonary Circulation and Ventilation - Perfusion Ratio

Question 1

pulmonary circulation

Answer 1

flow of blood between the right ventricle and the left atrium

• low pressure, low resistance and high capacitance system
• pulmonary vessels (artery and its branches and vein and its tributaries) have thin walls, —> high compliance —> easily distended —> can accommodate large amounts of blood
• pulmonary capillaries are larger than systematic capillaries
• they also have rich anastomoses
• thickness of PA and right ventricle is 1/3rd of that of aorta and left ventricle
• the capillaries form lattice in the alveolar wall so that blood flows as a thin sheet

functions:

• gas exchange
• metabolic function: secretion of Angiotensin- converting enzyme by capillary endothelium
• filtration and dissolution of clots - small clots and emboli formed on the right side of the heart are filtered in pulmonary vessels
• blood reservoir: holds 500 ml of circulating blood thereby acting like a reservoir.

pressure in pulmonary system:
right Ventricular pressure -
- 25 mm hg during systole
- 0-1 mm hg during diastole

pressure in pulmonary artery

• 25 (systole)
• 8 (diastole)

mean pulmonary arterial pressure - 15 mm hg

pressure in pulmonary vein - 5 mm hg

so pressure gradient - 15- 5 = 10 mm hg

if this pressure rises above 25 - cause edema
left arterial pressure is about 5 mm hg

blood volume of lungs -
450 ml - 70 pulmonary capillaries and remaining in PA and PVs

Applied
pulmonary hypertension: sustained elevation of pulmonary arterial pressure. there is an increase in pulmonary vascular resistance leading to right side heart failure
- o2 therapy, vasodilators, and ca2+ channel blockers - treatment

pulmonary edema - occurs due to an increase in interstitial fluid pressure. it causes accumulation of fluid in pulmonary interstitial space and alveoli

causes:

a) left side heart failure
b) pneumonia
c) o2 toxicity
d) high altitude

Question 2

ventilation perfusion ratio

Answer 2

ratio of alveolar ventilation and volume of blood perfusing the alveoli

• at rest - alveolar ventilation (Va) - 4 L/min and pulmonary blood flow (Q) - same as right Ventricular output - 5 L/min
  so normal VP ratio = 0.8

more at apex - 3.6
and base - 0.6

graph

importance

• physiological - for gas exchange
• at the apex the va/q ratio is high because there is more ventilation relative to blood flow therefore alveolar oxygen tension is high and co2 tension is low at the apex of the lungs
• at the base- the ratio is low as there is more perfusion relative to ventilation. thus some amount of the blood circulates through the base of the lungs without becoming fully oxygenated.

clinical:
- due to the differences in regional va/q some diseases are localised to a part of the lungs.
for eg tuberculosis occurs more commonly at the apex because high oxygen facilitates growth

decreased va/q - when the va/q ratio becomes less - due to delivery of less oxygen, alveolar po2 falls and due to removal of less co2. pco2 rises

increased va/q

wasted perfusion refers to the quantity of the venous blood which is not fully oxygenated. the mixing of unoxygenated blood with oxygenated blood is known as venous admixture. it occurs either due to a shunt or a low ventilation perfusion ratio

Question 3

physiological shunt

Answer 3

whenever va/q is below normal, there js inadequate ventilation to provide o2 needed to fully oxygenate the blood flowing through the alveolar capillaries.

• therefore a certain fraction of venous blood does not become oxygenated. - shunted blood
  also, some additional blood flows through bronchial vessels - 2% of cardiac output - unoxygenated, shunted blood

the total quantitative amount of shunted blood per minute is called the physiological shunt
measured by - analysing the conc of o2 in both mixed venous and arterial blood
+ measurement of cardiac output

the greater the physiological shunt the greater is the amount of blood that fails to be oxygenated as it passes through the lungs

disorders causing a shunt

1. emphysema
2. pneumonia
3. pulmonary embolism
4. atelectasis

Question 4

va/q inc and dec

Answer 4

inc - Ventricular septal defect and fallot’s tetralogy

dec - pulmonary embolism and pneumonia