Breathing and Exchange of Gases

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 14.

Why can't you hold your breath forever?

Try it now. Hold your breath until you simply have to breathe. You might expect that the urge arrives because oxygen is running out. It doesn't. The urge is triggered mostly by carbon dioxide building up in your blood. Your brain is far more worried about too much CO₂ than too little O₂.

That one fact explains how this chapter is built. Gases move only down a gradient, blood carries each gas in a specific way, and the brain watches CO₂ and H⁺ to decide how fast you breathe. Follow the gradients and nearly every fact in the chapter falls into place.

Breathing is not cellular respiration

Your cells burn glucose, amino acids and fatty acids to release energy, and that needs O₂. The same reactions produce CO₂, which is harmful and must leave. The exchange of O₂ from the atmosphere with the CO₂ made by cells is called breathing (commonly called respiration). The burning inside cells is cellular respiration, which you meet in Respiration in Plants. Breathing is only the delivery service.

Respiration, in this sense, has five steps, and CUET likes the order.

One need, many solutions

A sponge has no lungs because it doesn't need them. Where every cell is near water or air, plain diffusion is enough. The bigger and more active the animal, the more specialised the organ.

The path of air in humans

Air passes through a series of tubes that look like an upside-down tree.

The air path in one line: external nostrils → nasal chamber → pharynx → larynx → trachea → primary bronchi → secondary and tertiary bronchi → bronchioles → terminal bronchioles → alveoli

The pharynx is the common passage for food and air. The larynx is the sound box. When you swallow, the epiglottis, a thin elastic cartilaginous flap, covers the glottis so food doesn't enter the airway. The trachea is a straight tube that divides at the level of the 5th thoracic vertebra into right and left primary bronchi.

Why it works this way. The trachea, the first three orders of bronchi and the initial bronchioles are supported by incomplete cartilaginous rings. The rings keep the tube open so that air can always flow, like the ridges in a vacuum-cleaner hose. They are incomplete (C-shaped) because the gap lies against the oesophagus behind, which has to bulge a little when a lump of food passes.

The tree has two working parts, and the split is a standard exam question.

Alveoli are very thin, irregular-walled, vascularised bag-like structures that arise from the terminal bronchioles. A huge number of them packs a huge surface into a small chest, and each is wrapped in capillaries.

The two lungs sit inside a double-layered pleura. Pleural fluid between the layers cuts friction, so the lung surface slides smoothly against the chest wall as it inflates. The outer layer touches the thoracic lining and the inner layer touches the lung.

How air moves: a pressure story

The thoracic chamber is an air-tight box. Its back is the vertebral column, its front is the sternum, its sides are the ribs, and its floor is the dome-shaped diaphragm. Because the box is air-tight, any change in its volume is passed on to the lungs. You cannot inflate a lung directly. You can only change the volume of the chest and let the lungs follow.

Air moves because of a pressure gradient. When the volume of the lungs increases, the pressure inside (intra-pulmonary pressure) drops below atmospheric pressure and air rushes in. When the volume falls, the pressure rises slightly above atmospheric and air is pushed out.

A healthy person breathes 12–16 times a minute. You can deepen both phases with the help of additional muscles in the abdomen. Normal quiet expiration needs no effort at all because it is the relaxation of the muscles that did the work of inspiration.

Pause and predict: a stab wound lets air into the chest cavity around one lung. Why does that lung collapse?

Check your answer: the lung only follows the chest wall because the pleural space is air-tight and the two pleural layers cling together. Once air enters, that link is broken and the lung's own elastic recoil lets it shrink, however the diaphragm moves.

Counting your breath: volumes and capacities

A spirometer records how much air you move. CUET loves these numbers because they are fixed and easy to turn into questions. A volume is a single compartment. A capacity is a sum of two or more volumes.

At 12–16 breaths a minute, 500 mL per breath gives the 6000–8000 mL of air per minute that a healthy man moves. The capacities are built from these four.

Worked example. Take TV = 500 mL, IRV = 3000 mL, ERV = 1100 mL and RV = 1200 mL. Then IC = 500 + 3000 = 3500 mL. EC = 500 + 1100 = 1600 mL. FRC = 1100 + 1200 = 2300 mL. VC = 1100 + 500 + 3000 = 4600 mL. TLC = 4600 + 1200 = 5800 mL. The common mistake is to put RV inside VC. Vital capacity is the air you can actually move, and RV is the air you can never move.

Pause and predict: which volume cannot be measured directly by a spirometer, and which two capacities are therefore also out of reach?

Check your answer: residual volume, because that air never leaves the lungs, so the spirometer never sees it. Any capacity that includes RV is also out of reach: functional residual capacity and total lung capacity.

Beyond the textbook. For the exam: tidal volume is about 500 mL and all of it counts as ventilation. In reality: about 150 mL of every breath stays in the conducting passages and never reaches an alveolus, because it is just filling the nose, trachea and bronchi (the anatomical dead space). So only about 350 mL of a 500 mL breath reaches the exchange surface. Why it matters: this is why quick shallow breaths are inefficient. If each breath were only 250 mL, barely 100 mL of fresh air would reach the alveoli.

Gas exchange: follow the pressure

Gases cross the alveolar wall and the tissue capillary wall by simple diffusion. The only thing that decides direction is the partial pressure (pO₂ or pCO₂), which is the pressure contributed by one gas in a mixture. A gas always moves from where its partial pressure is higher to where it is lower. Learn this table once, and every question on exchange becomes a comparison of two numbers.

At the alveolus, O₂ goes from 104 into blood at 40, and CO₂ goes from blood at 45 into alveolar air at 40. At the tissues, O₂ goes from blood at 95 into cells at 40, and CO₂ goes from cells at 45 into blood at 40.

Pause and predict: at the alveolus the O₂ gradient is 64 mm Hg (104 − 40), but the CO₂ gradient is only 5 mm Hg (45 − 40). How can such a small gradient still clear all the CO₂?

Check your answer: solubility. CO₂ is 20–25 times more soluble in the diffusion membrane than O₂, so a tiny pressure difference moves a lot of CO₂. Three things set the rate of diffusion: the pressure gradient, the solubility of the gas and the thickness of the membrane.

The membrane is thin enough to make this easy. It has three layers: the thin squamous epithelium of the alveolus, the endothelium of the alveolar capillary, and the basement substance between them. The whole thing is less than a millimetre thick.

Carrying oxygen: haemoglobin

Blood moves O₂ and CO₂ between lungs and tissues. About 97% of O₂ travels bound to haemoglobin inside RBCs, and only 3% is dissolved in plasma. Haemoglobin is a red, iron-containing pigment that binds O₂ reversibly to make oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four O₂ molecules, one on each iron-containing haem.

The binding is mainly set by pO₂, but pCO₂, H⁺ concentration and temperature modify it. If you plot percentage saturation of haemoglobin against pO₂ you get an S-shaped (sigmoid) curve, the oxygen dissociation curve. Its S shape arises because once one O₂ is bound, the next binds more easily.

The conditions in the two places are opposite, and so are the results.

Why it works this way. The tissues that are working hardest make the most CO₂, acid and heat, and these are the very signals that make haemoglobin let go of its O₂. So haemoglobin gives up more O₂ exactly where it is needed. Every 100 mL of oxygenated blood delivers about 5 mL of O₂ to tissues.

Beyond the textbook. For the exam: CO₂, H⁺ and heat help haemoglobin release O₂ at the tissues. In reality: this shift of the curve is called the Bohr effect. A different molecule makes the point from the other side. Carbon monoxide binds haemoglobin more than 200 times more tightly than O₂ does, and it blocks the O₂ sites. That is why CO from a burning charcoal stove in a closed room is so dangerous even though the air still contains plenty of O₂. Why it matters: it shows that carrying O₂ depends on haemoglobin's binding, not on how much O₂ is around.

Carrying carbon dioxide: three routes

CO₂ travels three ways, and the exam usually asks for the percentages.

The bicarbonate route needs an enzyme, carbonic anhydrase, which is very concentrated in RBCs and present in minute traces in plasma.

The reaction in one line: CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺

At tissues, pCO₂ is high, so the reaction runs to the right and CO₂ is converted into bicarbonate. At the alveoli, pCO₂ is low, so the reaction runs to the left and CO₂ is released and breathed out. The same enzyme does both jobs, and only the partial pressure decides which way it goes. Every 100 mL of deoxygenated blood delivers about 4 mL of CO₂ to the alveoli.

The brain's breathing controller

Breathing is automatic but adjustable. You can speed it up for a sprint or slow it for a song, and the brain handles all of it.

Exercise raises CO₂ and H⁺, the chemosensitive area is stimulated, and you breathe faster to blow the CO₂ out. The role of O₂ in the regulation of breathing is insignificant.

Beyond the textbook. For the exam: CO₂ and H⁺ regulate breathing, and O₂ has no significant role. In reality: oxygen is monitored too, by receptors in the carotid and aortic bodies, but they only fire when blood pO₂ falls to a seriously low level. This has a practical side. If someone hyperventilates before a breath-hold dive, they wash CO₂ out of the blood, so the urge to breathe is delayed, while O₂ keeps falling unnoticed, and they can black out. Why it matters: it is the same lesson as the opening question, seen from the risky side.

When the machinery fails

The three are separated by where the damage is: the airways, the alveoli or the lung tissue as a whole.

Exam traps

60-second revision

Where this chapter connects

• Before: Respiration in Plants covers what cells do with O₂ once it arrives (cellular respiration).
• Next: Body Fluids and Circulation follows the blood that carries the gases, including the double circulation that links heart and lungs.
• Control link: Neural Control and Coordination explains the medulla and pons, which hold the breathing centres.
• Comparative link: Animal Kingdom and Structural Organisation in Animals cover the gills, tracheae and lungs of specific animals.
• Class XII link: Human Health and Disease discusses the effects of smoking on the lungs.