1. Gas Exchange in Mammals Aim – to understand the structure and function of the lungs. Objectives- by the end of this lesson you should be able to Name and identify parts of the gas exchange system. Explain how the alveoli are adapted for efficient gas exchange. Describe the structure and function of ciliated epithelium, goblet cells, cartilage, smooth muscle and elastic fibres Explain how the lungs are ventilated.

2. The human gas exchange system consists of the nasal passages, the pharynx or throat, the larynx or voice box, the trachea, the right and left bronchus and the lungs Larynx Trachea (with rings of cartilage) Left lung Ribs Diaphragm (a powerful sheet of muscle separating the thorax from the abdomen) Intercostal muscles Section through ribs Right bronchus Bronchioles The Human Gas Exchange System

3. The trachea or windpipe is about 10 cm long and is supported by C-shaped rings of cartilage to prevent the tube from collapsing during breathing Trachea The trachea subdivides to give rise to the right and left bronchus – these tubes are also strengthened by cartilage The two bronchi subdivide to form an extensive network of bronchioles that deliver air to the gas exchange surfaces – the alveoli Right and Left bronchus Bronchioles Air enters the body through the nasal passages and mouth, and passes via the pharynx and larynx to the trachea Air is delivered to the alveoli as the trachea branches into bronchi and bronchioles The Trachea

4. This photomicrograph of a transverse section through the trachea shows the C-shaped ring of cartilage Magnify C-shaped cartilage ring

5. This magnified view of the wall of the trachea shows the cartilage cells together with the cells that line the lumen of the trachea – ciliated epithelium Cartilage cells Ciliated epithelium

6. Lumen of trachea This highly magnified view of the lining of the trachea shows the cilia and mucus-secreting goblet cells that make up the epithelium Goblet cell that secretes mucus to trap dust and other foreign material that may enter the respiratory system The wafting of these cilia removes the mucus and trapped foreign material from the respiratory system

7. Move the cursor over the area of lung (yellow circle) to show the alveoli... The Gas Exchange Surface

8. Section of lung A Single alveolusThorax The Gas Exchange Surface Each alveolus is a hollow, thin-walled sac that is surrounded by a dense network of capillaries and is the site of gas exchange in the lungs The bronchioles divide many times forming respiratory bronchioles, which in turn divide to form alveolar ducts that terminate in groups of sacs – the alveoli Alveolar duct Respiratory bronchiole

9. Gases are exchanged across the alveoli by diffusion According to Fick’s Law... Rate of diffusion = surface area x difference in concentration thickness of exchange surface Maximum rate of diffusion of respiratory gases is achieved by: the large surface area presented by the alveoli (there are about 350 million alveoli in the two lungs presenting an enormous surface area of approximately 90 square metres – about the area of a tennis court) the large differences in concentration of metabolites between the alveoli and the blood capillaries the thinness of the diffusion barrier (alveolar and capillary walls provide a total thickness of only 0.005 mm) As deoxygenated blood from the body tissues flows through the network of capillaries surrounding each alveolus, oxygen diffuses into the blood and carbon dioxide diffuses from the blood into the alveolus; oxygenated blood travels from the lungs to the left of the heart for delivery to the body tissues The Gas Exchange Surface

10. Alveolar Tissue A photomicrograph of a section of alveolar tissue showing the delicate nature of the lungs and the 'one cell thick' walls of the alveoli which facilitate diffusion of respiratory gases. A photomicrograph of a section of alveolar tissue showing the delicate nature of the lungs and the 'one cell thick' walls of the alveoli which facilitate diffusion of respiratory gases.

11. Breathing in (inspiration) and breathing out (expiration) are mechanical processes involving the ribs, intercostal muscles and the diaphragm Two sets of antagonistic muscles are located between the ribs; these are the external and internal intercostal muscles External intercostal muscles Internal intercostal muscles The intercostal muscles are antagonistic in the sense that contraction of the external muscles raises the rib cage, whereas contraction of the internal muscles lowers the rib cage The diaphragm is a powerful sheet of muscle that separates the thorax from the abdomen; it is dome-shaped when relaxed and flattens on contraction Diaphragm The Mechanics of Breathing

12. During inspiration, the external intercostal muscles contract and raise the rib cage upwards and outwards; the diaphragm also contracts and flattens The volume of the thorax increases, lowering the air pressure in the chest cavity to less than that of the atmosphere outside A pressure gradient is created between the atmosphere and the lungs, and air rushes in via the trachea to equalise the pressure difference Air moves from a higher to a lower pressure region and inflates the lungs as inspiration takes place Inspiration - Breathing In

13. During an expiration, the external intercostal muscles relax and lower the rib cage; the diaphragm relaxes and becomes dome-shaped The volume of the thorax decreases, raising the air pressure in the chest cavity to above that of the atmosphere outside A pressure gradient is created between the lungs and the atmosphere, and air rushes out via the trachea to equalise the pressure difference Air moves from a higher to a lower pressure region and deflates the lungs as expiration takes place The mechanism described is that for breathing at rest At rest, inspiration is an active process involving contraction of the muscles of breathing Expiration is a purely passive process involving relaxation of the muscles of breathing together with elastic recoil of the lungs During forced breathing, as in exercise, expiration becomes an active process Expiration - Breathing Out Expiration is assisted by the elastic recoil of the lungs following the stretching of elastic fibres during the process of inspiration

14. ExpirationInspiration External intercostal muscles contract and raise the ribs upwards and outwards External intercostal muscles relax and the ribs move downwards and inwards The diaphragm muscle contracts and flattens The diaphragm muscle relaxes and becomes dome- shaped The volume of the thorax increases The air pressure in the thoracic cavity falls below that of the atmospheric air Air rushes into the lungs along a pressure gradient The volume of the thorax decreases The air pressure in the thoracic cavity rises above that of the atmospheric air Air rushes out of the lungs along a pressure gradient Summary