Respiratory organs and the human respiratory system

Hold your breath

Take a deep breath and hold it. After half a minute you want to breathe again. Your cells use oxygen all the time, and they make carbon dioxide. The body cannot store much oxygen, so it has to bring in fresh air again and again.

The cells use the oxygen to break down food. You met this in the chapter on respiration in plants. Animal cells do the same thing: oxygen is used, and carbon dioxide and energy are released.

Three words are often mixed up, so let us separate them.

• Breathing is the movement of air into the lungs and out of them.
• Exchange of gases is the movement of oxygen into the blood and of carbon dioxide out of it.
• Cellular respiration is the breakdown of food inside the cells.

This chapter is about the first two. The third one happens in every cell.

Respiratory organs in animals

Animals solve the problem in different ways. The organ depends on the size of the animal and on where it lives.

Look at the pattern. A thin animal, or one with all its cells near the surface, can use simple diffusion. A larger animal needs a special organ. In insects the tubes carry the air straight to the tissues. In most other animals the blood carries the gases between the organ and the tissues.

The path of the air in a human

Follow a breath of air from the nose to the lung.

1. External nostrils. A pair of openings above the upper lip. They lead through the nasal passage into the nasal chamber. Hairs and mucus in the chamber catch dust. The air is also moistened and warmed here.
2. Pharynx. The nasal chamber opens into the pharynx. Part of the pharynx is common with the food passage, so food and air both pass here.
3. Larynx. The pharynx opens through the larynx into the trachea. The larynx is a cartilaginous box that makes sound, so it is called the sound box. The opening into it is the glottis. When we swallow, a thin elastic cartilaginous flap, the epiglottis, covers the glottis. This keeps food out of the larynx.
4. Trachea. The trachea, or windpipe, is a straight tube that runs down to the middle of the chest. It divides at the level of the fifth thoracic vertebra into a right and a left primary bronchus.
5. Bronchi and bronchioles. Each primary bronchus divides again and again. It forms secondary and tertiary bronchi, then bronchioles, and last the very thin terminal bronchioles.
6. Alveoli. Each terminal bronchiole gives rise to many alveoli. An alveolus is a very thin, irregular-walled bag with many blood vessels around it.

The trachea, the primary, secondary and tertiary bronchi and the first bronchioles are held open by incomplete cartilaginous rings. The rings keep the tubes from closing.

The branching tubes and the alveoli together make the lungs.

The conducting part and the exchange part

• Conducting part. It runs from the external nostrils up to the terminal bronchioles. It carries air to the alveoli, clears it of foreign particles, humidifies it and brings it to body temperature.
• Exchange (respiratory) part. It is the alveoli and their ducts. Oxygen and carbon dioxide are exchanged with the blood here.

Only the alveoli and their ducts exchange gases. The trachea and the bronchi only carry air.

The lungs and the chest

We have two lungs. Each is covered by two thin layers, the pleura, and a little fluid, the pleural fluid, lies between the layers. The fluid reduces friction on the surface of the lung. The outer pleural membrane touches the lining of the chest. The inner pleural membrane touches the lung.

The lungs lie in the thoracic chamber. It is closed on all sides, so it is anatomically air-tight.

• At the back it is formed by the vertebral column.
• At the front it is formed by the sternum.
• At the sides it is formed by the ribs.
• Below it is closed by the dome-shaped diaphragm.

Because the chamber is air-tight, any change in the volume of the chest shows at once in the lungs. The next lesson uses this idea. In books you will also see that the right lung has three lobes and the left lung has two.

You should find that the surface grows only as the cube root of the number of sacs. The model gives about 95 square metres for 480 million sacs. Measurements on real lungs give 120 to 140 square metres for the blood-gas barrier, so the model is right in size but only rough. Even so, the huge number of alveoli gives an adult a surface far larger than the skin. Older books quote about 70 square metres.

Try it in Python

The program lists the stops of the air and counts the conducting stops and the exchange stops.

Practise with new numbers

Common mistakes

• Putting gas exchange in the trachea or the bronchi. They only carry air. Exchange takes place in the alveoli.
• Calling the alveoli a part of the conducting part. The conducting part ends at the terminal bronchioles.
• Saying that frogs breathe only through lungs. They also breathe through the moist skin.
• Saying that the rings of the trachea are complete. They are incomplete cartilaginous rings.
• Mixing up the epiglottis and the glottis. The glottis is the opening. The epiglottis is the flap that covers it.

Remember these

• Sponges, coelenterates and flatworms: body surface. Earthworms: moist cuticle. Insects: tracheal tubes. Aquatic arthropods, molluscs and fishes: gills. Amphibians: lungs and skin. Reptiles, birds and mammals: lungs.
• Air path: nostrils, nasal chamber, pharynx, larynx, trachea, primary bronchi, secondary and tertiary bronchi, bronchioles, terminal bronchioles, alveoli.
• The trachea divides at the fifth thoracic vertebra. Incomplete cartilaginous rings hold the airway open.
• Conducting part: nostrils to terminal bronchioles. Exchange part: alveoli and their ducts.
• Two pleural layers with fluid between them reduce friction. The thoracic chamber is air-tight.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the respiratory organ or surface of a sponge, an earthworm, an insect and a fish.

2. What stops food from entering the larynx when you swallow?

3. At which vertebra does the trachea divide, and what does it divide into?

4. Which part of the respiratory system is the exchange part?

5. What is the use of the pleural fluid?

References

• NCERT (2023). Biology, Textbook for Class XI, chapter 14, Breathing and Exchange of Gases.
• Ochs M., Nyengaard J. R., Jung A., Knudsen L., Voigt M., Wahlers T., Richter J. and Gundersen H. J. G. (2004). The number of alveoli in the human lung. American Journal of Respiratory and Critical Care Medicine, 169(1), 120-124. DOI 10.1164/rccm.200308-1107OC.
• Knudsen L., Hummel B., Wrede C., Zimmermann R., Perlman C. E. and Smith B. J. (2023). Acinar micromechanics in health and lung injury: what we have learned from quantitative morphology. Frontiers in Physiology, 14, 1142221. DOI 10.3389/fphys.2023.1142221.