# Alveoli --- **Alveoli** (singular: **alveolus**) are the tiny, hollow little sacs at the depths of the [[lungs]] where [[alveolar gas exchange]] takes place. Each lung contains about 300 to 400 million alveoli, and usually reach that amount by 8 years old. Combined together they give the lungs an airy, spongy appearance. This all *drastically* increases the available surface area for gas exchange, which is 75-100 square meters! ## Alveolar Sacs Alveoli are clustered together in **alveolar sacs**, which are attached at the end of a [[bronchi|terminal bronchiole]], with about 20-30 alveoli per sac. The alveoli are all squished together, separated/connected by **interalveolar septa**. Some adjacent ones have small pores (called **alveolar pores**) in the septum to facilitate air circulation. Each alveolar sac is supplied by a pulmonary arteriole, and a pulmonary venule, as well as the pulmonary [[capillaries|capillary beds]]. ![[alveoli.png]] The alveolar are made up of two main types of cells, **alveolar type I cells** and **alveolar type II cells**. - **Alveolar type I cells** are very common, and they make up part of the [[alveolar gas exchange|respiratory membrane]]. - **Alveolar type II cell** are less numerous, but they serve a very important function of producing a substance called **pulmonary surfactant**, keeps the alveoli inflated. This is because the moist environment of the alveoli actually pulls on itself because of surface tension. The pulmonary surfactant is made up of a complex of [[phospholipid|phospholipids]] and [[protein|proteins]] that disrupt the surface tension of water lining the alveoli which would otherwise prevent them from re-expanding after every exhale. Failure of the pulmonary surfactant can collapse the alveoli, a condition called [[atelectasis]]. ## Respiratory Membrane The **respiratory** (or **respiration**) **membrane**, sometimes called the **air-blood barrier** is an extremely thin membrane made up of type I cells, the [[epithelial tissue|endothelium]] of the respiratory capillaries, and their single, fused [[basement membrane]]. Both the type I and the endothelial cells are [[epithelial tissue|simple squamous]] cells, so they are about as thin as it gets—all together the respiratory membrane is only about 0.5mm thick—and so it easily [[passive transport#Simple Diffusion|permeable]] to the respiratory gasses. Moving with their concentration gradients, oxygen from the air and carbon dioxide in the blood change places in [[alveolar gas exchange]]. ![[respiratory membrane.png]] ## Problems with the Alveoli - [[pneumonia]] causes the alveoli to fill with puss, fluid or exudate, which impedes gas exchange. - [[emphysema]] breaks down the walls between the individual alveoli, effectively reducing the surface area for gas exchange to occur. - [[atelectasis]] is a collapse of these alveoli ___