Science · Body foundations
Respiratory System and Gas Exchange
Estimated time includes reading and one quiz. Take the time you need.
Start here: key ideas
- Air travels through conducting passages to the alveoli.
- Pressure changes caused by chest volume move air.
- Oxygen and carbon dioxide diffuse down their concentration gradients.
- Rising carbon dioxide is a major signal to increase breathing.
What you’ll be able to do
- Trace air to the alveoli.
- Explain breathing pressure and gas diffusion.
- Connect carbon dioxide, pH, and breathing control.
Follow air inward, then follow oxygen into blood and carbon dioxide out.
Optional review
If you’d like to review the basics, visit Anatomical Orientation, Organization, and Homeostasis. You can start this lesson without completing those first.
From atmosphere to alveoli
Air enters through the nose or mouth, then passes the pharynx (throat), larynx (voice box), trachea (windpipe), bronchi (large branches), and smaller bronchioles. It ends in tiny air sacs called alveoli. Mucus traps particles, and cilia move the mucus upward. Cartilage helps keep larger airways open.
The epiglottis helps direct swallowed material away from the airway. The trachea divides into a right and left bronchus. Each bronchus branches into bronchioles. Bronchioles can change diameter, altering resistance to airflow. Alveoli are the exchange surface, not just the end of a tube.
From nose to lungs
Follow the airway from the nose into the lungs and down to the tiny air sacs. The diaphragm is the muscle below the lungs. When it contracts, chest space increases and air moves in. Air travels through airways, not blood vessels.
From nose to lungs — OpenStax College, Anatomy & Physiology (2014 CC BY archive). CC BY 3.0. Original source image reproduced unchanged; explanatory caption added.
Pressure changes move air
Ventilation is air movement. During inhalation, the diaphragm contracts and moves down. Chest volume increases, lung pressure falls below outside pressure, and air moves in. During quiet exhalation, the diaphragm relaxes. Volume decreases, pressure rises, and air moves out. Air always moves from higher pressure to lower pressure.
The lungs follow chest-wall movement because the pleural membranes and thin pleural-fluid layer couple them. If air enters the pleural space, the lung may not expand normally. During forced breathing, rib muscles and abdominal muscles can add to diaphragm action.
Thin membranes and opposing gradients
Alveoli sit beside capillaries. Their shared barrier is thin and moist. Oxygen is higher in alveolar air, so it diffuses into blood and binds hemoglobin, the oxygen-carrying protein in red blood cells. Carbon dioxide is higher in arriving blood, so it diffuses into the alveoli and is exhaled. A thicker barrier or less surface area slows exchange.
Most oxygen travels bound to hemoglobin; effective diffusion alone cannot compensate for inadequate oxygen-carrying hemoglobin.
Keep four processes separate: ventilation is bulk air movement, diffusion is gas movement across the alveolar-capillary membrane, perfusion is blood flow past alveoli, and gas transport carries gases in blood. A person may ventilate an alveolus that receives little blood, or perfuse an alveolus that receives little air. Either mismatch reduces effective exchange even when the other process continues.
Gas exchange follows partial-pressure gradients
At the lungs, oxygen moves from the air sacs into blood. Carbon dioxide moves from blood into the air sacs to be breathed out. Each gas moves down its own pressure difference. The extra chemical labels show how carbon dioxide is carried; focus first on the opposite directions of the two gases.
Gas exchange follows partial-pressure gradients — OpenStax College, Anatomy & Physiology (2014 CC BY archive). CC BY 3.0. Original source image reproduced unchanged; explanatory caption added.
Carbon dioxide links breathing and pH
Cells make carbon dioxide. In blood, much of it becomes bicarbonate and hydrogen ions. More hydrogen ions lower pH. Chemical sensors called chemoreceptors detect this change and signal the brainstem to increase breathing. Faster ventilation removes more carbon dioxide, helping pH return toward its normal range.
Worked example: During exercise, cells make more carbon dioxide. Blood carbon dioxide rises and pH begins to fall. Chemoreceptors signal the brainstem, breathing becomes faster and deeper, and more carbon dioxide is exhaled. Removing the cause of the pH change is negative feedback.
Terms to remember
- alveolus
- A tiny lung air sac where gas exchange occurs.
- ventilation
- Movement of air into and out of the lungs.
- diffusion
- Net movement from higher to lower concentration.
- diaphragm
- The main muscle used for quiet breathing.
- hemoglobin
- A red blood cell protein that carries oxygen.
- carbon dioxide
- A waste gas that also affects blood pH.
Your quick summary
- The diaphragm changes chest volume and air pressure.
- Gas exchange occurs across thin alveolar-capillary walls.
- More carbon dioxide lowers pH and usually increases breathing.
Lesson quiz
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