Respiratory System and Gas Exchange

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.

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.

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.

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.

More chest volume means less pressure, so air moves in.

Sources

• OpenStax Anatomy and Physiology 2e: Organs and Structures of the Respiratory System
• OpenStax Anatomy and Physiology 2e: The Process of Breathing
• OpenStax Anatomy and Physiology 2e: Gas Exchange