2.2. Respiratory System

Question 1

Explain the gas exchange in the alveoli

Answer 1

The alveolar wall is a single layer of flattened squamous epithelial cells. There is a dense capillary network surrounding each alveolus and these are only 1 cell thick.
Gas exchange occurs via simple diffusion across the respiratory membrane and depends on the existence of pressure gradients between the lungs and atmosphere.

Question 2

Explain the inspiration process

Answer 2

diaphragm flattens, extending the superior/inferior dimension of the thoracic cavity. The external intercostal muscles contract and elevate the ribs and sternum. This increases the volume of the thoracic cavity and decreases the pressure in the lungs. As a result, air moves down the pressure gradient into the lungs.

Question 3

Explain the expiration process

Answer 3

the diaphragm and external intercostal muscles relax which causes a decrease in volume of the thoracic cavity and an increase in pressure in the lungs. Air moves out of the lungs down the pressure gradient.

Question 4

List 3 parameters that INCREASE in pregnancy

for each give the name, definition, normal range and change in pregnancy

Answer 4

1. TIDAL VOLUME: the volume of air entering the lungs at rest in a single breath. Normal range = 500ml. Increases by 150-200ml (especially in 1st tri)
2. RESPIRATORY RATE: number of breaths per minute. Normal range = 12 breaths per min. Slight increase to 15/min.
3. VITAL CAPACITY: the total volume of gas that can be moved in and out of the lungs. Normal range = 3.1L. Increased to 4.9-5L.

Question 5

Describe the anatomical changes that happen in pregnancy

Answer 5

The diaphragm raises by about 4cm and the transverse diameter of the chest is increased by 2cm.
The subcostal angle widens from 68* to 103* in late pregnancy.
Change from abdominal to thoracic breathing.

Question 6

What could lead to maternal respiratory acidosis in labour and why

Answer 6

Due to the increase in muscle activity, there is an increase in oxygen requirement. However, if contractions are occurring too frequently with not enough time for relaxation, there will be a decrease in oxygen supply to the muscles (ischaemia). This leads to tissue hypoxia with an increase in carbon monoxide and fall in pH.