Respiratory system diagram

1. 1

   Step 1: Nose and nasal cavity conducting zone

   Air enters through the nares. Hairs, mucus and cilia catch debris, and the air is warmed and humidified on its way through.

2. 2

   Step 2: Pharynx conducting zone

   The throat: nasopharynx, oropharynx, then laryngopharynx. The oropharynx and laryngopharynx are shared with food.

3. 3

   Step 3: Larynx conducting zone

   The voice box, built of cartilages. The glottis, the vocal cords and the space between them, sits inside it.

4. 4

   Step 4: Trachea conducting zone

   The windpipe, held open by 16 to 20 C-shaped rings of hyaline cartilage.

5. 5

   Step 5: Primary bronchi conducting zone

   The trachea divides at the carina into the right and left primary bronchi, which enter the lungs at the hilum.

6. 6

   Step 6: Bronchial tree and bronchioles conducting zone

   The bronchi branch again and again. Bronchioles are about 1 mm across and have no cartilage; the last ones are the terminal bronchioles.

7. 7

   Step 7: Respiratory bronchioles respiratory zone

   Where terminal bronchioles join respiratory bronchioles, the respiratory zone, and gas exchange, begins.

8. 8

   Step 8: Alveolar ducts and sacs respiratory zone

   Alveolar ducts are tubes of smooth muscle and connective tissue that open into clusters of alveoli, the alveolar sacs.

9. 9

   Step 9: Alveoli respiratory zone

   About 200 micrometres across, with thin elastic walls. Oxygen and carbon dioxide cross the respiratory membrane here by simple diffusion.

Nasal cavity

Nasal septum

Divides the cavity. In front it is septal cartilage; behind, the perpendicular plate of the ethmoid bone and the vomer.

Conchae

Three on each side wall: superior, middle and inferior. They add surface area and make the air swirl against the lining. The inferior conchae are separate bones; the superior and middle are parts of the ethmoid bone.

Floor

The hard palate (bone) at the front and the soft palate (muscle) behind it. Air leaves the cavity through the internal nares into the pharynx.

Paranasal sinuses

Frontal, maxillary, sphenoidal and ethmoidal. They warm and humidify air, make mucus and lighten the skull.

Lining

Pseudostratified ciliated columnar epithelium with goblet cells; the cilia sweep mucus toward the throat. Lysozyme and defensins in the mucus help against microbes. The olfactory epithelium, for smell, lies deeper in the cavity.

Pharynx: three regions

Nasopharynx

An airway only. Holds the pharyngeal tonsil (adenoid) and the uvula, and the auditory (Eustachian) tubes open into it.

Oropharynx

Shared by air and food. Joins the mouth at the fauces. Holds the palatine and lingual tonsils. Lined with stratified squamous epithelium.

Laryngopharynx

Below the oropharynx and behind the larynx. Opens into the larynx in front and the esophagus behind.

Larynx

Thyroid cartilage

The largest. Forms the laryngeal prominence.

Epiglottis

A flexible elastic cartilage at the top. During swallowing the larynx lifts and the epiglottis swings down over the opening of the trachea.

Cricoid cartilage

The ring-shaped cartilage at the bottom.

Arytenoid, corniculate and cuneiform cartilages

Three smaller pairs.

Vestibular folds

The "false vocal cords": folds of mucous membrane.

True vocal cords

White, membranous folds whose free inner edges vibrate to make sound. Differences in their size account for differences in pitch between people.

Trachea

• It is held open by 16 to 20 stacked C-shaped rings of hyaline cartilage joined by dense connective tissue.
• The open part of each C faces back. There, the trachealis muscle and elastic tissue form a fibroelastic membrane that lets the trachea stretch during breathing. The trachealis can contract to help force air out when exhaling.
• Its lining is pseudostratified ciliated columnar epithelium with goblet cells, continuous with the larynx.
• At its lower end the carina marks where it divides into the right and left primary bronchi. Specialized nervous tissue in the carina triggers a strong cough if a foreign body is present.

Bronchi and bronchioles

• The primary bronchi have the same ciliated lining as the trachea and cartilage rings that keep them from collapsing. They enter each lung at the hilum.
• Bronchioles are about 1 mm across, have no cartilage, and have muscular walls that can change their width and so change airflow.
• Respiratory bronchioles lead to alveolar ducts, which open into alveolar sacs made of clusters of alveoli. Alveolar pores link neighbouring alveoli and help even out pressure.

Cells of the alveoli

Type I alveolar cells

Thin simple squamous cells, up to 97 percent of the alveolar surface area, about 25 nm thick and very permeable to gases.

Type II alveolar cells

Scattered among the type I cells. They secrete pulmonary surfactant, a mix of phospholipids and proteins that lowers surface tension in the alveoli.

Alveolar macrophages

Phagocytic immune cells that move along the alveolar wall and clear debris and microbes.

The respiratory membrane is where the alveolar wall meets the capillary wall. It is about 0.5 micrometres thick, and gases cross it by simple diffusion.

Shape and place

Paired, pyramid-shaped organs joined to the trachea by the bronchi. Their lower surfaces rest on the diaphragm, a dome-shaped muscle. Each has an apex (top), a base (bottom), a costal surface facing the ribs and a mediastinal surface facing the midline.

Right lung

Three lobes: superior, middle and inferior. It is shorter and wider than the left.

Left lung

Two lobes: superior and inferior. It has a cardiac notch, an indentation that makes room for the heart.

Fissures

The grooves that separate the lobes.

Bronchopulmonary segment

A division of a lobe that gets air from its own tertiary bronchus and blood from its own artery.

Hilum

Where the pulmonary veins leave the lung and nerves enter, and where the visceral and parietal pleurae meet.

The pleura

Visceral pleura

The serous membrane that covers the lung and lines its fissures.

Parietal pleura

Attaches to the thoracic wall, the mediastinum and the diaphragm.

Pleural cavity

The space between the visceral and parietal layers.

Pleural fluid

Made by mesothelial cells in both layers. It lubricates the surfaces and its surface tension keeps the lungs against the chest wall, so they expand as the wall expands.

Pressures

Boyle’s law

At constant temperature, gas pressure and volume are inversely related (P1V1 = P2V2). A bigger thoracic volume means a lower pressure inside, so air flows in.

Atmospheric pressure

760 mm Hg at sea level (1 atm). The other respiratory pressures are given relative to it.

Intra-alveolar pressure

Equalizes with atmospheric pressure (0 mm Hg relative to it) but rises and falls during the breathing cycle.

Intrapleural pressure

About –4 mm Hg through the breathing cycle, so always negative relative to intra-alveolar pressure.

Surfactant

From type II alveolar cells. It lowers surface tension; without it the alveoli would collapse during expiration.

Muscles

Quiet inspiration

The diaphragm moves down and the external intercostals lift and push out the ribs.

Forced inspiration

The scalene muscles of the neck lift the chest wall further, adding volume.

Quiet expiration

Passive: elastic recoil of the lungs and relaxation of the diaphragm and intercostals shrink the chest volume.

Forced expiration

The abdominal obliques push the abdominal organs up against the diaphragm and the internal intercostals squeeze the rib cage.

Volumes and capacities

Tidal volume (TV)

About 500 mL moved in one quiet breath.

Inspiratory reserve volume (IRV)

The extra air that can be breathed in beyond a tidal breath.

Expiratory reserve volume (ERV)

The extra air that can be breathed out beyond a tidal breath, up to about 1200 mL in males.

Residual volume (RV)

The air left after the biggest possible exhalation; it keeps the alveoli from collapsing.

Vital capacity (VC)

TV + ERV + IRV, between 4000 and 5000 mL.

Inspiratory capacity (IC)

TV + IRV.

Functional residual capacity (FRC)

ERV + RV.

Total lung capacity (TLC)

About 6000 mL in males and about 4200 mL in females.

Breathing rate by age

Under 1 year

30 to 60 breaths a minute

About 10 years

18 to 30 breaths a minute

Adolescents and adults

12 to 18 breaths a minute

Control centers

In the medulla oblongata the ventral respiratory group sets the rhythm by driving the inspiratory muscles and the dorsal respiratory group takes in sensory input. In the pons, the pneumotaxic center inhibits the dorsal group to end inspiration and the apneustic center controls the depth of inspiration. Carbon dioxide, acting through hydrogen ion levels, is the main chemical driver of breathing.

Partial pressures (mm Hg)

• Dalton’s law: each gas in a mixture exerts its own partial pressure, and the total pressure is the sum of them (760 mm Hg for air).
• Henry’s law: the amount of a gas that dissolves in a liquid depends on its partial pressure and its solubility.
• External respiration is gas exchange in the lungs; internal respiration is gas exchange in the tissues. Both are simple diffusion down partial pressure gradients and need no energy.
• The oxygen gradient at the lungs is about 64 mm Hg (104 against about 40); the carbon dioxide gradient is only about 5 mm Hg, but carbon dioxide is roughly 20 times more soluble than oxygen, so similar amounts cross.
• Ventilation-perfusion coupling: if one alveolus is poorly ventilated, the arterioles serving it constrict and those serving well-ventilated alveoli dilate, steering blood to where the oxygen is.

Oxygen on hemoglobin

• Only about 1.5% of oxygen is dissolved in plasma; the rest rides on hemoglobin inside red blood cells.
• Each hemoglobin molecule has four subunits, each with a heme group holding one iron atom, so it can bind up to four O2 molecules and form oxyhemoglobin.
• Binding the first O2 makes binding the next easier, and releasing one makes the next release easier.
• In healthy people with normal hemoglobin, saturation generally runs 95% to 99%.
• The oxygen-hemoglobin dissociation curve plots hemoglobin saturation against the partial pressure of oxygen: the higher the PO2, the more oxygen is bound.

What changes how readily hemoglobin lets go of oxygen

Higher temperature

Promotes oxygen release. Very active tissue gives off heat, which helps release oxygen where it is needed.

Lower pH (the Bohr effect)

Promotes oxygen release.

Higher PCO2

More carbon dioxide makes more hydrogen ions, which lowers pH, so oxygen is released more readily.

2,3-bisphosphoglycerate (BPG)

A by-product of glycolysis in red blood cells that promotes oxygen release.

Fetal hemoglobin

Has a greater affinity for oxygen than adult hemoglobin, which helps the fetus take up oxygen in the placenta.

Carbon dioxide in the blood

Bicarbonate (HCO3–)

About 70%. Carbonic anhydrase speeds CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3– inside red blood cells.

Carbaminohemoglobin

About 20%. CO2 binds amino acid groups on the globin part of hemoglobin, not the iron.

Dissolved in plasma

About 7% to 10%.

• Chloride shift: bicarbonate leaves the red blood cells in exchange for chloride ions, which keeps the electrical charge balanced. It runs in reverse in the lungs.
• Haldane effect: hemoglobin saturated with oxygen binds less CO2, and when oxygen is low hemoglobin binds CO2 more readily.

8 · Label the diagram

Label the schematic

The same schematic as part 2, with the names taken off. Match each number to a structure from the list, then check. Every row is marked, and the labeled version in part 2 is the answer key.

9 · Practice questions

20 practice questions

The airway, the larynx and trachea, the alveoli, the lungs and pleura, breathing pressures and muscles, gas exchange and gas transport, multiple choice, one at a time. Every answer is explained, and your score is free: no sign-up needed.