Gas Exchange in Humans Notes

Large surface area lets more gas molecules cross at once. A thin surface gives a short diffusion distance. A good blood supply and good ventilation keep a steep concentration gradient, by taking oxygen away and bringing carbon dioxide and fresh air.

The alveoli have all four.

The pathway is larynx, trachea, bronchi, bronchioles, alveoli. The trachea is kept open by rings of cartilage. The two bronchi are the main branches, one into each lung; bronchioles are the smaller branches. The diaphragm and intercostal muscles control ventilation.

Cells use oxygen and produce carbon dioxide and water in respiration. These diffuse into the alveoli and are breathed out. Limewater turns cloudy with $\ce{CO2}$, and expired air clouds it faster. Nitrogen stays about 79%.

In inspiration the diaphragm contracts and flattens and the external intercostal muscles raise the ribs up and out. The thorax volume increases, pressure falls below atmospheric and air flows in. In expiration the diaphragm rises and the internal intercostals pull the ribs down and in; volume falls, pressure rises and air flows out.

Muscles respire faster and release more $\ce{CO2}$. Chemoreceptors in the brain detect the rise and the medulla sends more frequent impulses to the diaphragm and intercostals. Resting rate is about 12 to 15 breaths per minute, rising to 20 to 30 in moderate exercise.

Goblet cells in the trachea and bronchi secrete mucus, which traps dust, bacteria and viruses. Ciliated cells have cilia that beat in waves to move the mucus up towards the throat, where it is coughed out or swallowed. Smoking damages cilia, so smokers get more respiratory infections.