5 Blood Supply

Question 1

How do we denote alveolar air, arterial blood and mixed venous blood?

Answer 1

Alveolar air - A
Arterial blood - a
Mixed venous blood - ṽ (e.g. in pulmonary artery)

Question 2

How do we denote the partial pressure of alveolar air?

Answer 2

PAO2 (A in subscript)

Question 3

From what section of the heart do the L&R pulmonary arteries arise?

Answer 3

Right ventricle, carry entire cardiac output

Question 4

What section of the heart does pulmonary circulation return to?

Answer 4

Left Atrium

Question 5

Describe the pressure & flow of the pulmonary circulation?

Answer 5

High flow
Low pressure

Question 6

What do systolic/diastolic mean?

Answer 6

Systolic is max blood pressure when heart is contracted
Diastolic is min blood pressure when heart is relaxed

Question 7

What are the normal systolic pressures?

Answer 7

Pulmonary systolic pressure = 25mmHg
Systemic systolic pressure = 120mmHg

Question 8

What is the difference between pressure of the pulmonary vein and artery that causes blood to flow?
Why is it clinically important?

Answer 8

About 10mmHg

Because its so small it only takes a little pathology to disrupt blood flow
(Systemic normally 100mmHg)

Question 9

What causes gas to move between the blood and lungs?

Answer 9

Gases move down the partial pressure gradient

Question 10

What effects the rate of diffusion of O2 across the membrane?

Answer 10

Directly proportional to:
- partial pressure gradient
- gas solubility
- surface area

Inversely proportional to membrane thickness
Most rapid over short distance

Question 11

What happens in emphysema?

Answer 11

Destruction of Alveoli
Surface area reduced
Gas exchange decreases
PaO2 gets low

Question 12

What happens in fibrotic lung disease?

Answer 12

Alveolar membrane thickens
Gas exchange slows
Loss of compliance may decrease alveolar ventilation
PAO2 may be reduced and reduce PaO2

Question 13

How does pulmonary oedema affect gas exchange?

Answer 13

Interstitial fluid increases
Diffusion distance increases
PAO2 is normal but PaO2 is low

Question 14

How does asthma affect gas exchange?

Answer 14

Bronchioles constrict
Airway resistance increases
Alveolar ventilation decreases
PAO2 & PaO2 are reduced

Question 15

What is the ideal ventilation-perfusion relationship?

Answer 15

Alveolar ventilation matches perfusion in L/min

Question 16

Where is pulmonary blood flow highest and why?

Answer 16

Base of lung
Arterial pressure exceeds alveolar pressure so vascular resistance is low

Question 17

Where is pulmonary blood flow lowest and why?

Answer 17

Apex of lung
Alveolar pressure exceeds arterial pressure
Arterioles are compressed and vascular resistance increased

Question 18

How much of the lung has an equal blood flow & ventilation?

Answer 18

Around 75% of lung height has 1:1 ratio of blood flow to ventilation

Question 19

Where does most of the bloodflow to ventilation mismatch occur?

Answer 19

Apex

Question 20

What is it called when blood flow to a section of lung tissue exceeds ventilation?

Answer 20

Shunt

(Blood gets shunted away)

Question 21

Define shunt:

Answer 21

Passage of blood through areas of the lung that are poorly ventilated

Question 22

In a shunt how do the Partial pressures alter in the alveoli?

Answer 22

• PA CO2 increases
• PA O2 decreases

Question 23

What happens to blood flowing past a shunt?

Answer 23

It is deoxygenated

Question 24

How does the body auto-regulate a shunt?

Answer 24

Hypoxia in alveoli detected
-> Relevant arterioles constrict
-> Less blood flows to under-ventilated parts of lung

-> Increased PA CO2 also causes bronchodilation
-> Ventilation increases

Question 25

What normally happens to blood vessels when hypoxia is detected?

Answer 25

Systemic circulation - dilate to allow greater oxygen delivery
Pulmonary circulation - constrict to divert blood to better ventilated parts of lung

Question 26

What do we call it when an area of lung tissue has a greater ventilation than blood flow?

Answer 26

Alveolar dead space

Question 27

Define alveolar dead space?

Answer 27

Alveoli that are well ventilated but insufficiently perfused

Question 28

What happens to PA during alveolar dead space?

Answer 28

PA O2 increases

PA CO2 decreases

Question 29

How does the body regulate alveolar dead space?

Answer 29

• Increased PA O2 causes pulmonary vasodilation diverting blood to the highly ventilated areas
• Decreased PA CO2 causes bronchial constriction which reduces ventilation

Question 30

What is physiological dead space?

Answer 30

Sum of Alveolar Dead Space + Anatomical Dead Space