2. Gas exchange and gas carriage

Question 1

How are the lungs kept at a negative pressure compared to the atmosphere?

Answer 1

Lung is an elastic tissue - tries to collapse

- bounded by the ribs, sternum and diaphragm which are under tension - try to spring out

Question 2

How to calculate compliance?

Answer 2

Compliance = change in volume/change in pressure

Question 3

What does a high compliance mean?

Answer 3

Small change in pressure (thoracic) leads to a large change in volume (expansion in the lung), tissue has high compliance (veins, lung)

Question 4

What is surfactant?

Answer 4

Increases lung compliance (easier to expand with inspiration)
- reduces energy needed to inflate the lung

Question 5

Where is surfactant produced?

Answer 5

Released by type II alveolar cells/type II pneumocytes

Question 6

What is infant respiratory distress syndrome?

Answer 6

Effect of lack of surfactant

• stiff lungs with low compliance
• alveolar collapse
• alveoli filled with transudate

Treated with synthetic surfactant

Question 7

How does inspiration occur?

Answer 7

1. Diaphragm contracts and flattens (phrenic nerve) and enlarges thoracic cavity
   - dome shaped → flattened shape
2. Ribs move outwards and parietal pleura pulls visceral pleura and lung outwards, expanding the lung (lowered pressure within the thorax)
3. Air enters the lung and travels to the alveoli where gas exchange occurs
   - - active process

Question 8

How does expiration occur?

Answer 8

Reverse of inspiration

• passive process, horse has an active phase even at rest
  1. Diaphragm relaxes, reducing volume of thorax (increases pressure), elastic lung recoils, expelling air
• flattened shape → dome shape
  2. Internal intercostals help in reduction of thoracic volume - accessory respiratory muscles

Question 9

What is biphasic ventilation?

Answer 9

Occurs in horse

- first passive then active due to abdominal muscle contraction

Question 10

What is Exercise Induced Pulmonary Haemorrhage (EIPH)?

Answer 10

Small blood vessels in the lung burst under high pressure and bleed into the airway
- visible epistaxis (nosebleed) in small proportion of EIPH cases

Question 11

Describe bronchial circulation

Answer 11

Low volume
High pressure input from aorta
Arterial blood to lungs to supply metabolic needs of tissues of bronchial tree
Blood is then returned to the left as well as the right side of the heart
Right-to-left shunt

Question 12

Describe circulation

Answer 12

Large volume system
Low pressure input from pulmonary trunk, carries venous blood to the lungs
- deoxygenated blood from right side of the heart through the pulmonary artery, then pulmonary capillary network → pulmonary vein → left atrium → oxygenated blood around the body

Question 13

What is an anastomoses?

Answer 13

Between bronchial and pulmonary circulation

- mixing of blood from left + right sides

Question 14

What is hypoxic vasoconstriction?

Answer 14

Occurs in the lung (low oxygen levels lead to vasoconstriction)

• areas that receive little oxygen received reduced flow from pulmonary artery
• maximises oxygen exchange

Question 15

What is a pulmonary embolism?

Answer 15

Obstruction to a branch of PA

- enlargement of bronchial arteries by dilation - increases overall flow rate

Question 16

What is angiogenesis?

Answer 16

New bronchial arterial anastomoses
Increased blood flow is able to by-pass the obstructed PA
- important in pathology as there is formation of new common capillary network and small right to left shunt

Question 17

What makes up the blood-gas barrier?

Answer 17

Very thin barrier
Large surface area
Large pressure difference

Question 18

How quick does diffusion of O2 between alveolar air and pulmonary capillaries occur?

Answer 18

Rapid diffusion of oxygen from alveolus into the pulmonary capillary
- 1/3 of the transit time

Question 19

Describe the pressures of diffusion of oxygen from peripheral capillaries into tissue fluid

Answer 19

Systemic arterial blood PO2 95mmHg
Interstitial fluid PO2 40 mmHg
Very large pressure difference → rapid diffusion of O2 into tissue
PO2 of blood leaving the capillaries drops to almost 40mmHg

Question 20

How does Fick’s law affect gas exchange?

Answer 20

• Area of the respiratory surface - ↓ then G.E. will ↓
• Thickness of the respiratory surface - ↑ then G.E. will ↑
• PO2 difference between alveolus and capillary - ↓ then G.E. will ↓
• Pulmonary oedema - no efficient diffusion occurs

Question 21

How does adequate ventilation affect gas exchange?

Answer 21

Atelectasis (alveolar collapse)
Tracheal/bronchial collapse
Bronchoconstriction

Question 22

How does perfusion of the lung affect gas exchange?

Answer 22

Pulmonary embolism

Hypoxia - not enough oxygen

Question 23

How do the partial pressure of O2 affect binding to Hb?

Answer 23

High PO2 (eg pulmonary capillaries) O2 binds to Hb
Low PO2 (eg peripheral capillaries) O2 is released from Hb

Question 24

How does CO2 affect the shift of the oxygen-haemoglobin dissociation curve?

Answer 24

• More CO2 makes blood more acidic - shifts curve to the right - lower affinity for oxygen
• Less CO2 makes blood less acidic (↑pH) haemoglobin will have a higher affinity for oxygen - shifts curve to right - higher affinity for O2

Question 25

What is carbaminohaemoglobin?

Answer 25

CO2 reacts directly with amine radicals of Hb to form CO2Hb

• reversible reaction with loose bond, so CO2 is easily released into the alveoli
• reaction is slow so only 23% of CO2 is transported in this way