Zoology · Ch 6 — Respiration
Human Respiratory System
Human Respiratory System
Air enters the body through the external nostrils and passes into the nasal chamber, where fine hairs and a mucus lining filter out dust and microorganisms before the air is warmed and humidified. From the nasal chamber, air moves into the nasopharynx and then through the glottis - the opening of the larynx (voice box) - into the trachea. During swallowing, a thin flap of elastic cartilage called the epiglottis folds down over the glottis so that food travels down the gullet instead of the airway, preventing choking.
Everything from the external nostrils down to the terminal bronchioles is called the conducting zone: its job is purely to carry, filter, warm, and moisten incoming air, not to exchange gases. Ciliated cells and mucus-secreting goblet cells line this entire pathway, trapping particles and sweeping them back up to be swallowed or expelled.
The trachea is a semi-flexible tube held open by C-shaped cartilage rings that stop it collapsing as air pressure changes; at around the level of the fifth thoracic vertebra it splits into a right and a left primary bronchus, one entering each lung. Inside the lungs, each bronchus divides repeatedly into secondary and tertiary bronchi and finally into terminal bronchioles. The larger bronchi still carry curved cartilage plates for support, but the bronchioles lose the cartilage altogether and instead have smooth muscle in their walls, which can contract or relax to change the airway's diameter.
The terminal bronchioles lead into fine respiratory bronchioles, which end in clusters of thin-walled, richly vascularised air sacs called alveoli - the respiratory zone, and the actual site of gas exchange. The wall separating alveolar air from capillary blood is only three layers thick: the squamous epithelium of the alveolus, the endothelium of the capillary, and a thin basement layer between them. Most of this epithelium is made of very thin Type I cells suited to rapid diffusion, while scattered Type II cells are thicker and secrete surfactant, a protein-and-phospholipid film that lowers surface tension in the alveoli and stops them collapsing. Because surfactant synthesis only begins around the 25th week of gestation, very premature babies can develop newborn respiratory distress syndrome (NRDS) from a shortage of it. …
What this figure shows. A labelled diagram of the whole human respiratory system, showing the pathway of air from the larynx and trachea, through the bronchus, into the lung, down to the bronchiole and alveoli; it also shows the heart, the diaphragm, and the double-layered pleural membranes with the pleural fluid enclosed between them aroun …
What this figure shows. A branching diagram (the bronchial tree) tracing the airway from the trachea (with its cartilage rings) through the left/right primary bronchus and the root of the lung, into secondary and tertiary bronchi, then smaller bronchi, bronchioles, terminal bronchioles and respiratory bronchioles, ending in clusters of alveoli within a pulmonary lobule; the trachea and larger bronchi are shown with support …
What this figure shows. A close-up diagram of a single alveolus surrounded by capillaries, showing the pulmonary artery branch carrying deoxygenated blood into the capillary bed around the alveolus and the pulmonary vein branch carrying away the oxygenated blood, illustrating how the thin capillary network wraps around the alveolar sac …