Week 5: Ventilation Perfusion Relationships

Question 1

What are some characteristics of pulmonary arteries?

Answer 1

• The pulmonary artery is quite short, only running about 5cm beyond the right ventricle before it divides into the right and left branches
• The wall of this artery is extremely thin at 1/3 third of the aorta

Question 2

What is the pulmonary vascular pressure like in a normal person? And what value does it sit at?

Why is it like this?

Answer 2

should be around 25/8. THere is significantly less blood pressure so that these thin fine capillaries are not ruptured.

Question 3

What percent of the total blood volume is within the lungs?

Answer 3

9%

Question 4

Explain the reason for hypoxic pulmonary vasoconstriction and the purpose of this mechanism.

Answer 4

We want the best blood flow, flowing through the best oxygenated alveoli so that we can have the best gas exchange.

• When a bronchial or alveolar becomes hypoxic in the lung, the adjacent blood vessels actually constrict for 3 – 5 minutes
• This promotes blood flow to areas of the lung that are better ventilated, matching airflow with blood flow

Question 5

Why is ventilation and perfusion better at the bottom of the lungs?

Answer 5

• As a result of gravity, blood flow at the top of the lung is worse than the blood flow at the bottom of the lung, so the perfusion, or amount of blood that reaches the alveoli is better in the lower region of the lung compared to the top
• As a result of the upper alveoli already being stretched from holding the weight of the tissue (wet hanging jumper analogy)(it’s basically hanging over the bronchi, hence the top is stretched from carrying the weight), air is going to travel down to the lower alveoli as they are not stretched and therefore will stretch more easily (air travels to areas that stretch easily). Thus, ventilation is better at the bottom of the lung than at the top (as the alveoli at the top are already stretched to their limit)

Question 6

What does V’ and Q’ Represent

Answer 6

V’ is air that reaches the alveoli and Q’ is blood that reaches the alveoli

Question 7

If you have decrease ventilation but still perfusion, what would the V’Q’ ratio be like?

Answer 7

Would be low (would be 0 if no ventilation)

(think: if you divide any number by 0 it equals zero, or if you are lowering V’ and divide it by Q’ you will get a smaller number)

Question 8

If you had decrease perfusion but normal ventilation what would the V’Q’ ratio be like?

Answer 8

V’Q’ ration would be higher

(think: if you make Q’ smaller and then put it in the V’Q division, it will give a much larger number.)

Question 9

What does Pa, PA and PV represent?

Answer 9

• Pulmonary arterial pressure Pa
• Alveolar Pressure PA
• Pulmonary Venous pressure PV

Question 10

explain the difference between the 3 zones of the lungs

Answer 10

1. Zone 1
   - The alveolar capillary pressure never exceeds the alveolar air pressure
   - Thus, no blood flow is evident during any part of the cardiac cycle
   - Pressure in the lung is higher than the pressure in the blood vessel all the time
   - Alveolar pressure is greater than the pulmonary arterial pressure which is greater than the pulmonary venous pressure
2. Zone 2
   - This zone is characterised by intermittent blood flow that ‘bursts’ when pulmonary arterial pressure peaks (during systole)
   - The diastolic pressure (pressure of the capillary leaving the alveolar) is lower than the alveolar air pressure
   - Most common in the upper region of the lungs
   - Pulmonary arterial pressure is greater than alveolar pressure which is greater than pulmonary venous pressure
3. Zone 3
   - Alveolar capillary pressure always exceeds alveolar air pressure
   - As a result of this, there is continuous blood flow
   - Most common in the lower region of the lungs
   - Pulmonary arterial pressure is greater than pulmonary venous pressure which is greater than alveolar pressure

Question 11

in the upper regions of the lung, are we going to have higher or lower V’Q’ and why?

Answer 11

Higher as we have decreased blood flow in upper regions of heart

Question 12

What are the 3 mechanisms that the body does to compensate for the increased cardiac output during exercise?

(they also slow down the time that the blood passes through the capillaries)

Answer 12

1. Increase the number of open capillaries
   - At rest, a significant number of capillaries are not used as they are not required
   - When we start to exercise, the number of open capillaries can increase 3-fold
2. Distending all capillaries
   - The walls of these capillaries are really stretchy, so in order to accommodate the blood flow they are stretched
   - Distending a capillary can double the blood flow through the structure
3. Increasing pulmonary arterial pressure
   - This is not something that is desired in the body
   - If the above strategies don’t work, increasing the arterial pressure is something our body can do, but isn’t something it likes doing
   - If this is raised too high, the pulmonary capillaries will rupture

Question 13

Why do we not want blood to pass through pulmonary capillaries too fast?

(we don’t want it to drop below 0.25 seconds)

Answer 13

• If the transit time of an RBC through these capillaries drops below 0.25 seconds, it will be moving too fast for the best exchange of gas to occur. That is, it won’t be able to pick up the full amount of oxygen it needs and won’t be able to dump the amount of carbon dioxide it needs to. Lowering the oxygen levels in the blood, which is why you faint when your heart rate is too high

Question 14

what is the equation to work out the net filtration pressure?

Answer 14

Question 15

What creates the negative interstitial pressure in the capillaries?

Answer 15

lymphatic system drainage creates this negative interstitial pressure

Question 16

What will a disruption of the negative pulmonary interstitial pressure cause?

Answer 16

• cause a shift to positive pressure, pulmonary oedema will occur (as fluid will be pushed into the alveolar surface leading to pulmonary oedema)

Question 17

what are 3 things that can cause pulmonary odema?

Answer 17

• Left sided heart failure
• Rapid leakage of plasma proteins and fluid out of the pulmonary capillaries will cause a build up in the interstitial space and alveoli
• Reduced lymphatic drainage

Question 18

What is the value of the odema safety factor and why does it exist?

Answer 18

Capillary hydrostatic pressure can never exceed 21mmHg

• If pulmonary capillary pressure does rise (which would cause an increase in hydrostatic pressure) even slightly above the afore mentioned safety factor, it can result in lethal pulmonary oedema within hours. Never get more than 21 being pushed out
• If the factors that are pushing water out is greater than those pulling water back into the capillaries, oedema is likely to occur
• If the pressure is more than 25mmHg above the safety factor the patient can die within 20-30 minutes

Question 19

What is a pulmonary embolism and what effect would it have on V’Q’ ration?

Answer 19

• a pulmonary embolism is a condition in which one or more arteries in the lungs become blocked by a blood clot or foreign body

would increase the V’Q’ ratio as you are getting a decrease of perfusion of blood (decrease Q’)

Question 20

Many factors can affect the results of arterial blood gas analysis.

How would these affect it and why:

Delayed processing of the sample

air bubbles in the sample

Answer 20

decrease O2 as it would diffuse out as it has a higher pressure than atmosphere.

increase O2 - air bubbles contain O2, CO2 decreases as less CO2 from atmosphere

Question 21

Compare and contrast the structural and functional differences between the pulmonary arteries and systemic arteries

Answer 21

pulmonary arteries are short, have large diametes, high compliance and thin walls, low resistant nature

systemic arteries are not compliant and are a high resistant nature.

this is so that pulmar arteries can accomodate for increased blood pressure whereas systemic cannot

Question 22

Describe the bronchial circulation

Answer 22

bronchial artery and vein drain into pulmonary vein. This causes sligh reduciton in oxygenation of blood in the pulmonary vein. It also increases blood volume returning to left side of heart.

The bronchial artery supplies the lungs with oxygenated blood

bronchial veins drain the lungs

Question 23

How might the volume of blood in the lungs be altered both physiologically and pathophysiologically

Answer 23

physio - height, weight, gender

patho - loss of blood, congestive heart failure, decrease lymph drainage, leaky proteins

Question 24

at total lung capacity and very low lung capacity, polmonary vascular resistance is high, why?

Answer 24

At TLC pulmonary vascular resistance increase as the high alveolar pressure begins to collapse the pulmonary blood vessels

At low lung volumes –> decrease alveolar stretch –> decrease radial traction stretch –> decrease pulling of blood vessels

Question 25

in emphysema the hypoxia present in the lung is patchy in naute (some alveolar units are hypoxi while others are normal). In a climber of high altitude the hypoxia is consistent in nature. Describe the result patterns of hypoxic pulmonary vasoconstriction in these two types of lungs and discuss whether the hypoxic pulmonary vasoconstriction is advantageous?

Answer 25

In COPD, is advantageous to a certain point. It promotes blood flow to areas of lung that are better ventilated, however eventually blood will back up into heart due to vasoconstriction.

In climbers the entire lung is hypoxic and therefore, the blood can’t go anywhere except diffuse in the lungs due to vasoconstriction of all a lot of the alveoli

Question 26

Chronic pulmonary disease often leads to right sided heart failure and peripheral oedema. Explain

Answer 26

CPD results in reduced alveolar ventilation and hypoxia.

Induces hypoxic pulmonary vasoconstriction –> increases pulmonary vascular resistance –> backflow of blood due to buildup–> increases buildup of blood in heart –> increases afterload on right ventricle –> leads to heart failure

Due to this build up of blood in heart –> starts building up in periphery which increases capillary hydrostatic pressure –> oedema

Question 27

how would an increase in herat rate alter pulmonary capillary transit time?

Answer 27

decrease transit time

Question 28

which forces air fluid clearance from the alveoli?

Answer 28

negative hydrostatic pressure by lymphatics pulls fuild out of alveoli

Question 29

what pressure changes are required before significant pulmonary odema occurs?

Answer 29

interstitial hydrostatic pressure from negative to positive

Question 30

A patient exhibits signs of pulmonary oedema and you are concerned he is suffering from left sided heart failure. Why?

Answer 30

blood enters lungs the left side from lungs. Therefore buiild up of blood in left atrium causes backflow of blood to lungs. THis increases the hydrostatic pressure in the pulmnary capillaries. Exceeds capacity of lymphatics and causes oedema.

Question 31

what chronic adaptation occurs following a sustained pulmonary capillary pressure? how will this help prevent odema?

Answer 31

increase pulmary capillary pressure –> lympatics expand –> increases capacity to drain interstitial fluids.

Question 32

explain the mechanisms behind producing a Va/Q of infinity and a Va/Q of zero.

Answer 32

INfinity: when you have decreased perfusion of blood (no perfusion)

0: when you have decreased ventilation

Question 33

In a patient with a pulmonary embolism. Why would you have decrease arterial PO2 levels?

Answer 33

Have a higher V’Q’ ratio.

therefore, you have decreased blood perfusion to alveoli. Therefore the O2 is not taken up by blood and will have less O2 in blood