MAMMALIAN LUNGS: ALVEOLI
Air flows two ways through mammalian respiratory systems. Mammals ventilate with negative pressure breathing—a pump sucking air into the lungs. The rib cage muscles and diaphragm contract simultaneously, increasing lung volume and dropping lung air pressure below what's outside. Air flows from regions of high to low pressure, so air rushes into the lungs. When the rib cage muscles and diaphragm relax, the resulting increase in air pressure forces air out of the lungs through the nostrils and mouth.

Fresh air enters through the mouth and nostrils and moves down the trachea, which branches into two smaller pipes (bronchi), one for each lung. Within each lung, the bronchi branch into smaller and smaller tubes (bronchioles).

Eventually, the tiniest bronchiole branches end in millions of air sacs (alveoli), where actual gas exchange occurs. The walls of the alveoli consist of extremely thin squamous epithelial cells, the respiratory surface.

Between and around the alveoli walls lies a complex network of capillaries. On inhalation, the concentration of oxygen in the air is greater than in the capillary blood, so oxygen diffuses across the moist epithelium into the blood. Simultaneously, the concentration of CO₂ in the blood is higher than in the air, so CO₂ diffuses from the blood into the alveoli. Oxygenated blood leaves the lungs through the pulmonary veins, and CO₂ exits through the mouth and nostrils.