4.2 The Respiratory System

The respiratory system delivers oxygen to the blood and removes carbon dioxide, working hand-in-hand with the cardiovascular system you just studied. The TEAS often pairs these two systems, because gas exchange is meaningless without a pump to carry the gases. Your job on the exam is to trace the path of air, explain where and how gas exchange happens, describe the mechanics of a single breath, and reason about what controls breathing rate. The recurring theme is diffusion: gases always move from areas of higher concentration to areas of lower concentration, and no energy is required to make that happen.

Anatomy: Upper and Lower Airways

Anatomists split the respiratory tract into an upper portion (in the head and neck) and a lower portion (in the chest).

The epiglottis is a favorite test point: this small flap closes over the larynx when you swallow so that food enters the esophagus, not the airway. When it fails, you choke.

The Pathway of Air

Trace a single breath of air from outside to the exchange surface:

1. Air enters through the nose or mouth.
2. It passes through the pharynx (throat).
3. It moves through the larynx (voice box).
4. It travels down the trachea (windpipe).
5. The trachea splits into the left and right bronchi.
6. Each bronchus branches into smaller and smaller bronchioles.
7. Air finally reaches the alveoli, where gas exchange occurs.

Gas Exchange in the Alveoli

The alveoli are roughly 480 million tiny sacs wrapped in capillaries, giving the lungs a gas-exchange surface area of about 70 square meters — close to the size of a tennis court. Their walls are a single cell thick, so the diffusion distance is about one micrometer. This is the whole design goal: huge area, tiny distance, maximum diffusion.

• External respiration (in the lungs): oxygen diffuses from the alveoli into the blood; carbon dioxide diffuses from the blood into the alveoli to be exhaled.
• Internal respiration (at the tissues): oxygen diffuses from the blood into body cells; carbon dioxide diffuses from cells into the blood.

The Mechanics of Breathing

Breathing is driven by pressure differences created when the diaphragm — the dome-shaped muscle beneath the lungs — changes shape. This trips up many students: when the diaphragm contracts, it flattens and moves down.

The key principle is Boyle's law: when the chest cavity gets bigger, the pressure inside drops below atmospheric pressure, so air flows in to equalize. The intercostal muscles between the ribs assist by lifting the rib cage. Quiet inhalation is active (it requires muscle work), while quiet exhalation is mostly passive (the tissues simply recoil).

Oxygen and Carbon Dioxide Transport

Gases do not just float in the blood — they are carried in specific ways the TEAS expects you to know.

Oxygen transport:

• About 98.5% binds to hemoglobin, the iron-containing protein inside red blood cells; each hemoglobin molecule carries up to four oxygen molecules
• About 1.5% dissolves directly in the plasma

Carbon dioxide transport:

Respiratory Volumes

Clinicians measure how much air the lungs move using standard volumes:

Control of Breathing

The medulla oblongata in the brainstem is the primary respiratory control center, with the pons fine-tuning rhythm. Chemoreceptors monitor the blood, and the dominant signal may surprise you: it is not low oxygen but rising carbon dioxide (and the falling pH it causes) that most strongly drives you to breathe faster.

Worked Example: A patient holds their breath. Within seconds they feel an overwhelming urge to breathe. Is this because oxygen has run low, or because carbon dioxide has built up? The urge is driven mainly by rising CO₂. As CO₂ accumulates, it forms carbonic acid, lowering blood pH; chemoreceptors in the medulla detect this and force the breathing reflex — well before oxygen reaches a dangerous low. That is why CO₂, not O₂, is the primary respiratory stimulus.