2 6.4.1 Ventilation, gas exchange and cell respiration. The flow of air in and out of the alveoli is called ventilation and has two stages: inspiration (or inhalation) and expiration (or exhalation).Lungs are not muscular and cannot ventilate themselves, but instead the whole thorax moves and changes size, due to the action of two sets of muscles: the intercostal muscles and the diaphragm.
3 2. Gas Exchange:This is the diffusion of gases (oxygen and carbon dioxide)There are two sites for gas exchange(a)Alveoli: Oxygen diffuses into the blood from the alveoli and carbon dioxide diffuses from the blood into the alveoli(b)Tissues: Oxygen diffuses from blood into the cells and carbon dioxide diffuses from cells to the blood
4 3. Cell RespirationAerobic respiration uses oxygen in the mitochondria and produces carbon dioxideAnaerobic respiration does not use oxygen but still produces carbon dioxide
5 6.4.2 The need for a ventilation system. A ventilation system is needed to maintain concentration gradients in the alveoliThe steep concentration gradient across the respiratory surface is maintained in two ways: by blood flow on one side and by air flow on the other side. The ventilation system replaces diffuses oxygen (keeping the concentration high) and removes carbon dioxide (keeping the concentration low).This means oxygen can always diffuse down its concentration gradient from the air to the blood, while at the same time carbon dioxide can diffuse down its concentration gradient from the blood to the air.
6 6.4.3 Features of the alveoli. Large surface area due to the combined spherical shape (600 million alveoli = 80 m2)Flattened epithelial cells of alveoli and close association with capillariesShort diffusion distance from alveoli to blood ( um)Dense capillary networkMoist surface for the solution of gases
7 6.4.4 Structure of the ventilation system. (a) Trachea(b) Cartilage ring support(c) Bronchi (plural) Bronchus (single)(d) Lung(e) Heart(f) Sternum(g) Rib cage(h) Bronchioles (j) Alveoli (k) Diaphragm
8 6.4.5 Mechanism of ventilation. The diaphragm contracts and flattens downwards.The external intercostal muscles contract, pulling the ribs up and outthis increases the volume of the thorax this increases the lung and alveoli volumethis decreases the pressure of air in the alveoli below atmospheric (Boyle's law)air flows in to equalise the pressure
9 Ventilation, cont’d The diaphragm relaxes and curves upwards the external intercostal muscles relax, allowing the ribs to fallthis decreases the volume of the thoraxthis decreases the lung and alveoli volumethis increases the pressure of air in the alveoli above atmospheric (Boyle's law)air flows out to equalise the pressure.The abdominal muscles contract, pushing the diaphragm upwardsThe internal intercostal muscles contract, pulling the ribs downwardThis gives a larger and faster expiration, used in exercise