Respiratory

Question 1

Describe the mechanism of expiration

Answer 1

1. Respiratory centre in the brain stops initiating breath
2. The respiratory muscles relax
3. Diaphragm moves down and in
4. Thoracic volume decreases
5. Lungs recoil due to elastic recoil
6. Intrapulmonary pressure rises
7. Intrapleural pressure becomes less negative
8. Air leaves the lungs
9. Air movement stops

Question 2

Describe the mechanism of inspiration

Answer 2

1. The respiratory centre in the brain signals to initiate a breath
2. The muscles of respiration ate initiated and start to contract (intercostals)
3. Diaphragm contracts and descends
4. Rib cage moves up and out, increasing the volume in thoracic cavity
5. Intrapulmonary pressure starts to drop
6. Pleural pressure in pleural space becomes more negative as ribs move out
7. Intrapulmonary pressure becomes less than atmospheric pressure
8. Air moves into lungs until equilibrium is reached
9. Air movement stops

Question 3

External respiration

Answer 3

Transfer of gases in the lungs
Partial pressures of O2 and CO2 drive the diffusion of these gases across the respiratory membrane
Deoxygenated blood (PO2 40mmHg) in pulmonary artery creates a steep partial pressure gradient apprised to PO2 in alveoli (104mmHg)
As a result O2 diffuse into pulmonary blood until 104mmHg is reached both sides of respiratory membrane

CO2 diffuses in opposite direction in a gentler partial pressure gradient 45mmHg in pulmonary blood to 40mmHg in alveoli pulmonary veins

Deoxygenated blood is continuously pumped to lungs and oxygenated blood pumped away to maintain partial pressure gradient

Question 4

Internal respiration

Answer 4

Gas exchange between capillaries and body tissues
Tissue cells continuously need O2 for metabolic activities
Because PO2 in tissues is always lower (40mmHg) than arterial blood (104mmHg)
O2 moves rapidly from blood into tissues until equilibrium is reached

At the same time CO2 moves quickly along its pressure gradient from a PCO2 of 45mmHg in tissues to 40mmHg in arterial blood

As a result venous blood has PCO2 of 45mmHg and PO2 of 40mmHg

Question 5

Explain the oxygen dissociation curve

Answer 5

Oxygen dissociation curve shows how PO2 controls oxygen and its unloading from haemoglobin.

• at resting conditions PO2 = 100mmHg
• Hb is 98% saturated
• as blood flows through capillaries its releases a yield of 75% Hb saturation
• as the 1st molecule binds the Hb molecule changes shape as a result it more readily binds with 2 more O2 molecules
• uptake of the 4th is more facilitated
• this explains the s-shaped curve
• An ⬆️ in temp, PCO2, H+, BPG, lowers Hb affinity for O2, shifting curve to right (Bohr)
• A ⬇️ in temp, PCO2, H+, BPG, increases its affinity, shift to left (feutal Hb)