Respiratory System — Reading Comprehension Lesson

Reading Passage

Respiratory System

The respiratory system is the organ system responsible for taking oxygen from the air and delivering it to the bloodstream, while removing carbon dioxide — a waste product of cellular metabolism — from the blood and expelling it. Every cell in the body requires a continuous supply of oxygen to produce energy through cellular respiration. Without oxygen, cells begin to die within minutes. The respiratory system works in close coordination with the circulatory system: the lungs add oxygen to the blood and remove carbon dioxide, and the heart pumps that oxygenated blood throughout the body.

Air enters the body through the nose or mouth. The nose is preferred for normal breathing because it filters, warms, and humidifies the air before it reaches the lungs. Air travels down the pharynx (throat), through the larynx (voice box), and into the trachea (windpipe) — a tube reinforced by C-shaped rings of cartilage that keep it open. The trachea divides into two bronchi, one entering each lung. Inside the lungs, the bronchi branch repeatedly into smaller tubes called bronchioles, eventually ending in tiny air sacs called alveoli. An adult’s lungs contain about 300 million alveoli, giving them a total surface area roughly the size of a tennis court.

Gas exchange occurs in the alveoli. Each alveolus is surrounded by a network of tiny capillaries — the smallest blood vessels in the body. Oxygen diffuses from the air in the alveoli across the thin alveolar walls and into the blood, where it binds to hemoglobin molecules in red blood cells. At the same time, carbon dioxide diffuses from the blood into the alveoli and is exhaled. The alveolar walls are so thin — only one cell thick — that gases can pass through them almost instantly. The entire gas exchange process takes less than a second.

Breathing is powered by the diaphragm, a dome-shaped muscle beneath the lungs. When you inhale, the diaphragm contracts and flattens, increasing the volume of the chest cavity. This lowers the air pressure inside the lungs below atmospheric pressure, causing air to rush in. When you exhale, the diaphragm relaxes and rises, decreasing lung volume and pushing air out. At rest, an adult breathes about 12 to 20 times per minute, moving about half a liter of air with each breath. During vigorous exercise, breathing rate and depth both increase dramatically to meet the body’s increased oxygen demand. The respiratory system is vulnerable to infection, pollution, and smoking, which can cause asthma, bronchitis, pneumonia, and lung cancer.

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