Day 15: The Respiratory System

How Do Humans Breathe?


Lung Anatomy

The lungs are made up of millions of tiny balloons called alveoli, which fill with air each time you inhale and deflate each time you exhale. The many microscopic alveoli make the lungs look like sponges. Alveoli are made up of squamous epitheliel tissue, which is very thin and elastic, like the walls of a balloon.

When you breath, air goes down your windpipe or trachea. The trachea branches off to either lung; these branches are called branchi. Each branchus branches again and again into smaller and smaller branches, like a tree. The tiniest of these branches are called bronchioles. Each bronchiole supplies air to a cluster of alveoli, called an alveolar sac. The individual alveoli are connected to the bronchioles by tiny ducts.

Each alveolar sac is surrouned by a capillary bed, which is a network of tiny blood vessels. The walls of the blood vessels and the balloon-like walls of the alveoli are so thin that oxygen molecules (O2) can pass from the air-filled alveoli to red blood cells inside the vessels. Likewise, carbon dioxide molecules (CO2) can pass from the red blood cells into the alveoli. When the body exhales, the CO2 is removed.

Respiration: The Mechanics of Breathing

Tickle my ribs with the mouse to see me breathe!

Inhalation/Exhalation Animation

What makes the air go in and out?

The lungs are housed in the pleural cavity. The pleural cavity is created by the rib cage, which surrounds and protects the lungs, and by the diaphragm, a thin, flat muscle which separates the heart and lungs from the organs of the abdomen like a sheet. When the diaphragm is relaxed, it balloons upward. When the diaphragm contracts, it flattens out and pushes downward. When the muscles of the rib cage contract, they spread and raises the ribs. When both the diaphram and the muscles of the rib cage contract at the same time, the pleural cavity becomes larger and a vacuum is created which sucks air into the lungs. This process is called inhalation. When the rib muscles relax, the ribcage lowers and comes back together. At the same time, the diaphragm relaxes. The space inside the pleural cavity becomes smaller and the air is pushed back out of the lungs. This process is called exhalation.

Cellular Respiration


The lungs are responsible for bringing oxygen into the body and taking carbon dioxide out. Every cell in the body needs oxygen to live and function. Oxygen is used in many of the cell's chemical reactions. The most important of these chemical processes is the breakdown of glucose (simple sugar) to produce the energy needed to keep the cell alive and allow it to perform its special tasks. Carbon dioxide, on the other hand, is a waste product of energy. If carbon dioxide was allowed to build up, it would poison the cell and eventually the entire body.

Oxygen is brought from the lungs to individual cells by the bloodstream. Oxygen passes from the alveoli in the lungs into the surrounding capillaries and attaches to the hemaglobin found in red blood cells. Hemaglobin is a chemical, found in red blood cells, which can bind with oxygen and carbon dioxide, but it loves oxygen and will readily trade the CO2 it has carried back from the cells for an oxygen molecule. When hemaglobin is carrying oxygen, it is red; when it is carrying CO2, it is blue.

The lungs are placed near the heart so that when blood returns from the rest of the body full of CO2, it can be pumped directly to the lungs to trade its CO2 for O2. It then goes back to the heart, full of O2, to be pump back into the rest of the body and its individual cells.

Text by Janet Sinn-Hanlon
Illustrations by Dawn Gorski

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