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

The bronchioles divide many times forming respiratory bronchioles, which in turn divide to to form alveolar ducts that terminate in groups of sacs – the alveoli Respiratory bronchioles Alveolar duct Alveoli A single alveolus 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 Gas Exchange Surface

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 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

The exchange of gases between the lungs and the circulating blood takes place across the thin-walled alveoli whose walls consist of squamous epithelium This photomicrograph shows the thin walls and large surface area displayed by the alveoli in human lung tissue ALVEOLI Gas Exchange occurs across the Alveolar Epithelium

The wall of the alveolus is composed of squamous epithelium together with its basement membrane squamous cells basement membrane This thin wall is surrounded by an extensive network of capillaries whose walls are also composed of flattened, squamous epithelium Gas Exchange occurs across the Alveolar Epithelium

capillary The thinness of the capillary and alveolar walls, together with their basement membranes, provides a barrier between the alveolar air and the blood in the capillaries of approximately 0.3  m The thinness of this barrier is one of the features that aids rapid diffusion of gases into and out of the blood Gas Exchange occurs across the Alveolar Epithelium

capillary and alveolar walls This electron micrograph shows a portion of human lung Alveolar wall Capillary wall Red blood cell Nucleus of capillary The short diffusion path for efficient exchange of gases between the air in the alveoli and the blood in the capillaries is provided by the thin squamous epithelium layers of these tissues O2O2 CO 2 Gas Exchange occurs across the Alveolar Epithelium