CHAPTER 3

Breathing Lesson

How our branching bronchial pathway, lungs, and blood vessels keep us breathing in and out

PART 1 Take a Deep Breath

We do not give much conscious thought to the approximately 20,000 breaths we take each day. There's usually no need to. Our bodies are built to extract oxygen from the atmosphere around us in near silence, and deploy it to all of our vital systems. When we are in good health, and especially when we're breathing clean air, we barely notice this fundamental, life-sustaining act. We take in air through the nose and mouth. It is pulled through the pharynx then deeper to the trachea. From there, the oxygen, nitrogen, argon and carbon dioxide molecules in the air we breathe travel either left or right to one of the two bronchi, and follow a path of ever-smaller tubes that comprise the bronchial tree. READ MORE

Bronchioles, the smallest tubes in the bronchial tree, end in clusters of tiny air sacs called alveoli. Wispy blood vessels surround the alveoli. Oxygen in the inhaled air passes through the air sacs to red blood cells in the capillaries to be transported throughout the body. At the same time, the red blood cells are delivering the waste gas carbon dioxide back into the alveoli so it can be exhaled out of the body. This inhale-exchange-exhale pattern is repeated about twenty times a minute.

An infant takes a first breath of air seconds after birth. During pregnancy, a fetus receives oxygenated blood from the mother, through the umbilical cord. While babies seem to practice breathing movements, and even get the hiccups during pregnancy, their lungs are filled with fluid until they are born, so they are not really breathing air. The transition to filling the lungs with oxygen and exhaling breath is a crucial milestone at birth. LESS

PART 2 Oxygen Delivery Service

After oxygen passes through the thin, permeable wall of the lungs' alveoli, it is ready to catch a ride to wherever it will do its work. A tight but delicate web of capillaries surrounds the alveolar sacs, ready to absorb oxygen into the bloodstream to start this journey. Inside the capillary, oxygen molecules hook up with red blood cells. Inside each red blood cell are about 300 million hemoglobin molecules. In a hemoglobin molecule, an iron atom is surrounded by four heme molecules and four proteins called globins. Four oxygen molecules can jump aboard each hemoglobin molecule, and travel through the blood to their destination. READ MORE

Oxygenated blood travels through the capillaries to the pulmonary vein. It reaches the left side of the heart where it is pumped out to the main, or systemic, circulatory system. Every cell in the body needs oxygen, and red blood cells deliver it. LESS

PART 3 Let It All Out

It Adds Up

700 million breaths...

Red blood cells pick up passengers for the return trip as well. Carbon dioxide molecules bind to hemoglobin and are transported back through the blood vessels toward the pulmonary capillaries. Again, the gas molecules pass through the thin walls of the capillaries through the porous walls of the alveoli. The breath carries out the carbon dioxide and other wastes as we exhale. Before we can even breathe out, our red blood cells have captured fresh oxygen and started the journey again. READ MORE

The security system for this vital process packs a one-two punch. First, sticky mucus which coats the airways, secreted by the goblet cells, traps microbes and solid bits of airborne junk. Then hairlike cilia, which line the airways, make a wavelike motion to push unwanted pollution back toward the mouth and nose, where they can be expelled. The diaphragm muscle, which sits just below the lungs, expands and contracts to enable breathing, as do the intercostal muscles-the ones between the ribs. LESS