🦴 Anatomy introductory Lesson 5 of 8 4 min read

The Respiratory System

The airways and lungs move air to the alveoli, where oxygen enters the blood and carbon dioxide leaves it, all under automatic control from the brainstem.

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What you'll learn

  • Trace the path of air from the nose to the alveoli through the upper and lower airways.
  • Describe how the diaphragm and rib muscles drive inhalation and exhalation.
  • Explain how gas exchange occurs across the alveolar-capillary membrane.
  • Identify how breathing is controlled and adjusted by the brainstem and chemoreceptors.
  • Connect respiratory function to the cardiovascular system.

Overview

The respiratory system brings oxygen into the body and removes carbon dioxide, the waste gas of metabolism. It is often divided into a conducting zone (airways that carry air) and a respiratory zone (alveoli where gas exchange happens). It works in close partnership with the cardiovascular system: the lungs load oxygen onto blood, and the heart distributes it.

The Airways

Air travels through a branching series of passages, conventionally split into upper and lower airways.

RegionStructuresFunction
Upper airwayNose, nasal cavity, pharynx, larynxWarms, humidifies, and filters air; contains vocal cords
Lower airwayTrachea, bronchi, bronchiolesConduct air deep into the lungs
Respiratory zoneAlveolar ducts and alveoliSite of gas exchange

The trachea splits into right and left bronchi, which branch repeatedly into ever-smaller bronchioles, ending in clusters of alveoli. The lungs contain roughly 300 million alveoli, giving a gas-exchange surface about the size of a tennis court. A thin film of surfactant keeps these delicate sacs from collapsing. The upper airway also protects the lungs: the epiglottis covers the airway during swallowing so food goes into the esophagus, and tiny hair-like cilia lining the airways sweep trapped dust and mucus upward to be cleared.

The Lungs and Pleura

The two lungs sit in the chest on either side of the heart. The right lung has three lobes and the left has two, leaving room for the heart. Each lung is wrapped in a double membrane called the pleura; the thin, slippery fluid between its layers lets the lungs glide against the chest wall and, through surface tension, keeps them expanded. If air or fluid enters this space (a pneumothorax or pleural effusion), the lung can partially collapse and gas exchange suffers.

Mechanics of Breathing

Breathing is driven by muscles that change the volume of the chest:

  • Inhalation (active): the diaphragm contracts and flattens while the external intercostal muscles lift the ribs. The chest expands, pressure inside the lungs falls below atmospheric pressure, and air flows in.
  • Exhalation (usually passive at rest): these muscles relax, the elastic lungs recoil, chest volume shrinks, pressure rises, and air flows out. Forced exhalation adds the internal intercostals and abdominal muscles.

This is a direct application of the muscular system to a life-sustaining task.

Gas Exchange

At the alveoli, only two thin cell layers separate air from blood. Gases move by diffusion down their concentration gradients:

  • Oxygen diffuses from alveolar air (high O₂) into capillary blood (low O₂), where it binds hemoglobin in red blood cells.
  • Carbon dioxide diffuses from blood (high CO₂) into the alveoli to be exhaled.

Oxygen-rich blood then returns to the heart and is pumped to the entire body, while the tissues perform the reverse exchange at their own capillaries. Most oxygen does not simply dissolve in blood; it is carried bound to hemoglobin inside red blood cells, which lets blood hold far more oxygen than plasma alone could. Carbon dioxide travels back to the lungs mostly as dissolved bicarbonate, a form that also helps buffer the blood’s pH. A typical adult breathes about 12 to 20 times per minute at rest, moving roughly half a liter of air with each quiet breath.

Control of Respiration

Breathing is automatic. Respiratory centers in the medulla oblongata and pons set the rhythm. Chemoreceptors constantly monitor the blood: a rise in carbon dioxide (and the associated fall in pH) is the strongest signal to breathe faster and deeper. Low oxygen also stimulates breathing, but is a weaker everyday driver. You can override this system voluntarily for a while, but the automatic drive always reasserts itself.

Clinical Relevance

Asthma is a chronic condition in which airways become inflamed and the smooth muscle around the bronchioles tightens (bronchoconstriction), causing wheezing and shortness of breath; inhaled bronchodilators relax that muscle to reopen the airways. Chronic obstructive pulmonary disease (COPD), usually from smoking, permanently limits airflow. Pneumonia fills alveoli with fluid and pus, impairing gas exchange. Because oxygen delivery depends on both breathing and circulation, respiratory failure and heart failure often occur together and are central concerns in emergency care.

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The oxyhemoglobin dissociation curve and the Bohr effect

Hemoglobin's sigmoidal O₂ binding curve reflects cooperative binding: the flat upper portion ensures near-complete saturation across a range of alveolar pressures, while the steep lower portion allows large offloading of O₂ to tissues for small drops in PO₂. A rightward shift, promoted by increased CO₂, H+ (lower pH), temperature, and 2,3-BPG, lowers hemoglobin's affinity and enhances unloading in metabolically active tissue, the Bohr effect. A leftward shift (as with fetal hemoglobin or CO poisoning) raises affinity and impairs tissue delivery.

V/Q matching, compliance, and surfactant

Efficient gas exchange depends on matching alveolar ventilation (V) to capillary perfusion (Q); the ideal ratio is about 0.8 overall, but it varies from apex to base of the upright lung. A high V/Q approaches dead space (ventilation without perfusion, as in pulmonary embolism), while a low V/Q approaches shunt (perfusion without ventilation, as in pneumonia or atelectasis). Lung compliance, the change in volume per change in pressure, is preserved by surfactant from type II pneumocytes, which lowers alveolar surface tension; its deficiency causes neonatal respiratory distress syndrome.

Key terms

Alveolus
A tiny air sac in the lungs where oxygen and carbon dioxide are exchanged with the blood.
Trachea
The windpipe, the main airway that carries air from the larynx toward the lungs.
Bronchi
The two large airways branching from the trachea into the right and left lungs.
Diaphragm
The dome-shaped muscle beneath the lungs whose contraction draws air in during inhalation.
Gas exchange
The diffusion of oxygen into the blood and carbon dioxide out of it across the alveolar wall.
Surfactant
A lipid-protein substance that lowers surface tension in alveoli and keeps them from collapsing.
Ventilation
The mechanical movement of air into and out of the lungs, i.e., breathing.
Medulla oblongata
The brainstem region that sets the basic automatic rhythm of breathing.

Check your understanding

5 questions · answers reveal instantly.

  1. 1.Where does gas exchange between air and blood actually take place?
  2. 2.During normal inhalation, the diaphragm:
  3. 3.Oxygen moves from the alveoli into the blood primarily by:
  4. 4.Which brainstem region sets the basic rhythm of breathing?
  5. 5.The strongest normal stimulus to increase breathing rate is a rise in blood levels of:

Citations & References

Links open publicly available educational and peer-reviewed sources.

  1. OpenStax. Anatomy and Physiology 2e. ↗
  2. National Heart, Lung, and Blood Institute (NHLBI). ↗
  3. MedlinePlus, U.S. National Library of Medicine. ↗