Page 42 - Q & A STPM 2022 Biology

Biology STPM Chapter 7 Gaseous Exchange
Answer:
(a) – The alveoli provide a large surface area for gaseous exchange as there are
numerous alveoli (about 700 million alveoli).
– The inner surface of the alveolus is always moist, allowing the gases to
dissolve easily before diffusion can occur.
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– The alveolar walls are thin and permeable allowing the gases to diffuse
across it easily.
– The alveoli are surrounded by a network of blood capillaries for gaseous
exchange.
(b) – The involuntary control of breathing is carried out by respiratory centres
in the medulla oblongata.
– The respiratory centres are divided into the inspiratory centre that
controls inspiration and expiratory centre that stimulates expiration.
– The peripheral chemoreceptors, carotid and aortic bodies, in the carotid
arteries and aorta are stimulated by the high partial pressure of carbon Semester
dioxide and the concentration of H .
+
– The central chemoreceptors in the medulla are stimulated by high
concentration of H . As the partial pressure of CO in the artery 2
+
2
increases, some of the molecules diffuse into the cerebrospinal fluid
surrounding the medulla. As a result, the concentration of H increases
+
and pH decreases.
CO + H O ⎯⎯→ H CO ⎯⎯→ H + HCO 3 -
+
2
3
2
2
– Nerve impulses from the peripheral and central chemoreceptors are sent
to the inspiratory centre.
– The inspiratory centre sends out nerve impulses via the phrenic nerve
to the diaphragm and via the thoracic nerve to the external intercostal
muscle causing increase in rate of contraction. This increases rate of
breathing.
– As the lungs expand, stretch receptors in the walls of alveoli and
bronchioles are stimulated.
– Nerve impulses are sent via vagus nerve to the expiratory centre in the
medulla.
– This inhibits inspiration. Expiration occurs.
– The lungs are no longer stretched. Stretch receptors are no longer
stimulated.
– Expiratory centre becomes inactive. Inspiration begins again.

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Chapter 7.indd 167 11/2/21 9:27 AM