pulmonary circulation

Question 1

Compare the** ciruclation differences** between systemic vs pulmonary circulation

Answer 1

Pulmonary:
* blood comes from the** right ventricle, moves through the pulmonary artery, through the capillaries in lungs** and back to the left atrium
Systemic:
* blood comes from the left ventricle, exits via the aorta, goes through the** systemic capillaries **in the periphery of the body and goes back to the right atrium

Question 2

why does the mean pressure in the systemic circulation **drop **so significantly?

Answer 2

• there are muscular arterioles (with thick media) that constrict and can also increase their diameter
• they allow the distribution of blood to the different parts of the body eg if you are running, it is important that the amount of cardiac output goes to the legs

Question 3

Compare vascular resistance and pressure in both circulations

Answer 3

• in the systemic circulation, there is a high pressure and high vascular resistance
• in the pulmonary circulation, there is low pressure and low resistance

Question 4

Describe the** structure** of the pulmonary vasculature

Answer 4

• pulmonary arteries and arterioles are thin walled - little or no media
• they are very compliant vessels
• thin walled due to the lower pressures

Question 5

what pressure are the pulmonary capillaries exposed to?

Answer 5

• the capillaries are very close to the alveolar gas
• therefore they are exposed to atmophseric (alveolar) pressure

Question 6

what makes the lung vulnerable to oedema?

Answer 6

• the thin membrane
• large surface area of the lung

Question 7

what occurs to the pulmonary pressure when there is a large increase in cardiac output?

Answer 7

• large increases in blood flow (cardiac output) are accomodated with only small increases in pressure (non-ohmic behaviour)
• DIFF to sytemic circulation

Question 8

what is the uneven distribution of blood flow in the lung explained by?

Answer 8

• the **hydrostatic pressure differences **within the blood vessels
• the difference between the top and bottom of the lung is around 23mmHg - large pressure difference for such a low pressure system

Question 9

how many conceptual (not actual) ‘zones’ are there in the lung?

Answer 9

3 zones

Question 10

where is zone 1?

Answer 10

at the top of the lung (apex)

Question 11

Describe zone 1

Answer 11

• zone 1 is the region at the top of the lung
• pulmonary pressures fall below alveolar pressure (close to atm pressure)
• if this occurs, the capillaries are squashed flat and no flow is possible
• this DOES NOT occur under normal conditions

Question 12

why does ‘zone 1’ not occur under normal conditions?

Answer 12

because the pulmonary arterial pressure is just sufficient to raise blood to the top of the lung

Question 13

Under what 2 conditions may zone 1 occur?

Answer 13

1. if the arterial pressure is reduced - eg following a severe** haemorrhage **
2. or if the alveolar pressure is raised - eg during** posiitve pressure ventilation**

Question 14

Describe zone 2

Answer 14

• zone 2 is farther down the lung from zone 1
• pulmonary arterial pressure increases because of the hydrostatic effect and it now exceeds alveolar pressure
• in this case, blood flow determined by difference between arterial and alveolar pressure

Question 15

Describe zone 3

Answer 15

• arterial pressure and venous pressure now exceed the alveolar pressure
• increase in blood flow down this region - caused by distension of capillaries , the pressure within them increases down the zone and the pressure surrounding them remains constant

Question 16

what can the distribution of pulmonary blood flow be affected by?

Answer 16

• changes in posture/ GRAVITY- eg if the subject is lying in a supine position - the apical flow increases , but basal flow remains unchanged
• ** exercise**- both upper and lower zone blood flows increase

Question 17

what law does fluid exchange across the capillary endothelium obey?

Answer 17

• Starling’s law

Question 18

how do we prevent oedema in the lungs?

Answer 18

• low pressure circulation
• pulmonary lymphatic system

Question 19

Describe the pulmonary lymphatics and how they prevent oedema

Answer 19

• the pulmonary lymphatics are an extensive network of lymphatic capillaries within the alveolar walls
• constant flow of lymph fluid draining lung
• they remove interstitial fluid

Question 20

Why is the vascular resistance so low in the pulmonary circulation?

Answer 20

• the pulmonary circulation lacks muscular arterioles
• important to note that the vascular resistance becomes even smaller as the pressure within the vessels rise

Question 21

why does pulmonary vascular resistance fall when the arterail pressure within the vessels increases?

Answer 21

as the pressure rises, the pulmonary capillaries open up and begin to conduct blood - recruitment (capillaries open up)
* IE the capillaries distend

Question 22

Why is there no basal tone of the pulmonary vessels?

Answer 22

As the vast majority of them have no media layer and therefore cannot be innervated by autonomic nerve fibres

ie there is an absence of sympathetic vasoregulatory tone

Question 23

what is hypoxic pulmonary vasoconstriction?

Answer 23

• when PO2 of alveolar gas decreases
• contraction of smooth muscle in the walls of the small arterioles in the hypoxic region

Question 24

what are the non respiratory functions of the pulmonary circulation?

Answer 24

• filtration
• fibrinolysis - break down fibrin clots
• activation & inactivation of hormones & transmitters eg angiotensin, bradykinin , serotonin etc