Breathing and Exchange of Gases: Class 11 Biology · Chapter 14

Unit 5 turns from plants to your own body. Think about how a cricket side is studied. One analyst looks at each player alone: strike rate, economy, catches taken. Another watches the whole team: how a partnership is built, how fielders cover for one another. You need both views, because a team can do things that no single player can.

This unit applies both views to the human body. Its first four chapters take major processes (exchange of gases in this chapter, then body fluids and circulation, excretion, and locomotion and movement) right down to cells and molecules. Its last two, on neural and chemical coordination, step back to show how the whole organism keeps those processes in step.

Why insist on both? Take an example from this chapter. On its own, one of haemoglobin's four protein chains holds oxygen tightly and shows no S-shaped behaviour; four chains working together in one molecule load oxygen in the lungs and release it in the tissues far better (Section 14.4.1). That behaviour belongs to the assembly, not to any single part: an emergent property. Systems biology studies exactly this, how interactions create new properties at every level, from networks of molecules and large molecular assemblies through cells, tissues and organisms to populations and communities.

Twentieth-century physiology leaned the other way. Its great advances came from isolating parts: slices of tissue kept alive in a dish, cell-free extracts, and the instruments of physics and chemistry. That reductionist programme produced molecular biology and made molecular physiology almost indistinguishable from biochemistry and biophysics. Biologists now accept that neither the purely molecular view nor the purely whole-organism view is enough by itself.

Before you read on, sit still for a minute and count your breaths. Most people at rest manage somewhere between 12 and 16. You did nothing to start them, and they will carry on while you sleep. Why does the body never take a break from breathing?

The answer lies in your cells. Each one runs a slow, controlled burn: glucose, fatty acids and amino acids are broken down in a long chain of reactions, and oxygen is needed at the end of that chain, so its role in releasing energy is indirect. The same reactions give off carbon dioxide, which forms an acid in water and becomes harmful if it piles up. Two deliveries therefore have to run nonstop: oxygen in, carbon dioxide out.

Swapping atmospheric oxygen for the carbon dioxide made by cells is called breathing (श्वसन), and in everyday speech it is also called respiration. This chapter first compares how different animals manage it, then follows air into your own lungs, watches gases cross into the blood, ride in it and leave it, and ends with how the brain sets the pace and what goes wrong in some diseases.

14.1 Respiratory Organs

Oxygen reaches cells by diffusion, and diffusion is quick only over tiny distances, a fraction of a millimetre. So the way an animal exchanges gases depends on two things: where it lives (water or air) and how its body is organised, from a thin sheet of cells to a large body with organ systems (Fig. 14.1).

Start with the animals you know best. Land vertebrates (amphibians, reptiles, birds and mammals) breathe with lungs, internal air sacs with a rich blood supply; this is pulmonary respiration. Fishes use gills instead, and so do most aquatic arthropods such as prawns and crabs and most aquatic molluscs; gas exchange through gills is branchial respiration. Frogs keep a second route open, because their thin, moist skin also exchanges gases (cutaneous respiration); that is how a frog buried in mud for weeks stays alive.

Move down the animal kingdom and the special organs disappear. Insects pipe air through a branching network of tracheal tubes that open through pores along the body and carry atmospheric air right up to the tissues. An earthworm has no lungs or gills at all: gases dissolve in its moist cuticle and pass through to blood vessels just beneath. In sponges, coelenterates such as Hydra, and flatworms, every cell lies close to the surface or to water flowing through the body, so oxygen and carbon dioxide simply diffuse across the whole body surface.

14.1.1 Human Respiratory System

Follow one breath from outside. Air enters through the pair of external nostrils just above the upper lip, runs along the nasal passage into the nasal chamber and drops into the pharynx, a passage at the back of the throat that food and air share. Below the pharynx it passes through the larynx into the trachea, the windpipe.

The larynx is a box of cartilage that produces your voice, which is why it is called the sound box. Its opening, the glottis, sits right beside the entrance to the food pipe. Each time you swallow, a thin flap of elastic cartilage, the epiglottis, folds over the glottis and steers food into the oesophagus instead of the windpipe (Fig. 14.2). Laugh with your mouth full and the timing can slip; the cough that follows clears the crumb.

The trachea runs straight down into the middle of the thorax. Level with the fifth thoracic vertebra it forks into a right and a left primary bronchus (plural bronchi). Inside each lung the bronchi branch again and again, into secondary and then tertiary bronchi, then into narrower bronchioles, and last into very thin terminal bronchioles (Fig. 14.3). Incomplete rings of cartilage support the trachea, the primary, secondary and tertiary bronchi and the first bronchioles, so these tubes stay open instead of collapsing when pressure inside them drops.22In the trachea the rings are C-shaped, open at the back where the trachea rests against the oesophagus.

Each terminal bronchiole ends in a bunch of tiny, thin-walled, irregular sacs, the alveoli, each wrapped in a net of blood capillaries. A lung is simply this whole branching system of bronchi, bronchioles and alveoli.

Each lung is wrapped in the pleura, a double membrane. The outer layer lines the inside of the chest wall, the inner layer clings to the surface of the lung, and a thin film of pleural fluid lies between them. The fluid lets the lung glide against the chest wall with every breath, so friction stays low (Fig. 14.4).

Functionally the system has two parts. The conducting part runs from the external nostrils to the terminal bronchioles. No gas exchange happens along it, but it delivers air to the alveoli and prepares it on the way: the air is warmed to body temperature, picks up moisture, and loses dust and microbes to the sticky lining. Only beyond it, in the alveolar ducts and alveoli that make up the respiratory (or exchange) part, do oxygen and carbon dioxide actually diffuse between air and blood.

Here is a puzzle: lungs contain no muscle, so they cannot make themselves bigger or smaller. Breathing works only because of where they sit. Picture the chest as a sealed, air-tight box, the thoracic chamber. Its floor is the dome-shaped diaphragm, the ribs form its sides, the sternum closes it at the front (ventrally) and the vertebral column at the back (dorsally). The pleura holds each lung against the inside of this box, so the lungs follow every change in its size: make the box bigger and the lungs are stretched bigger with it. We cannot alter lung volume directly, only through the chest.

With the parts in place, here is the whole job of respiration in order. The rest of the chapter takes the stages one at a time.

14.2 Mechanism of Breathing

Seal the nozzle of a syringe with a fingertip and pull the plunger: the trapped air expands, its pressure drops, and the finger is sucked against the nozzle. Lift the finger and air rushes in until the pressure inside matches the room. Breathing works the same way. At a fixed temperature a gas's pressure falls as its volume grows (Boyle's law); the chest is the syringe, the diaphragm and ribs are the plunger, and the airway is the nozzle.

The air pressure inside the lungs is the intra-pulmonary pressure. Air enters only while it is lower than atmospheric pressure, a negative pressure compared with the outside air, and leaves only while it is higher. A breath therefore has two stages: inspiration (अंतःश्वसन) draws atmospheric air in and expiration (उच्छ्वसन) sends alveolar air out. Muscles create the pressure gradient each time: the diaphragm, and two sets of intercostal muscles between the ribs, the external and the internal intercostals.

Inspiration. Two movements enlarge the chest at once. The diaphragm contracts and its dome flattens downwards, lengthening the chamber along the antero-posterior axis.33In zoology the antero-posterior axis runs from head to tail, so in a person standing upright it is the vertical axis. The external intercostal muscles contract and raise the ribs and sternum, deepening it along the dorso-ventral (back-to-front) axis. The lungs, sealed to the chest wall, grow with it, so their pressure drops below atmospheric and air flows in until the pressures match (Fig. 14.5).

Expiration. At rest this stage needs no muscular work. The diaphragm and intercostals simply relax: the diaphragm domes up, the sternum and ribs drop back, chest and lungs get smaller, and the intra-pulmonary pressure rises slightly above atmospheric, pushing air out. When you need more, in a sprint or while blowing up a balloon, extra muscles, including those of the abdomen, make both stages more forceful.

A spirometer, the instrument doctors use to check how well the lungs work, records how much air moves with each breath. At rest, a healthy adult takes 12 to 16 breaths a minute.

14.2.1 Respiratory Volumes and Capacities

A spirometer traces how much air is in the lungs over time. Quiet breathing draws small waves; a deep breath in or a forceful breath out draws big swings (Fig. 14.6). From such a trace we read four basic volumes. Each is a separate slice of the space in the lungs, and no two slices overlap.

The tidal volume sets how much air you move at rest. At about 500 mL a breath and 12–16 breaths a minute, a healthy adult shifts roughly 6000–8000 mL, that is 6–8 litres, of air every minute.

Doctors often need a combined figure rather than one slice. Adding neighbouring volumes gives the lung capacities, which are used in clinical diagnosis. The one quoted most often is the vital capacity, the biggest single breath a person can move.

14.3 Exchange of Gases

Gas exchange needs no pump. Oxygen and carbon dioxide cross by simple diffusion, with no carrier and no energy spent, each moving from where its pressure, or concentration, is higher to where it is lower. To follow them, we first need a way to state the pressure of one gas inside a mixture.

In a mixture such as air, each gas contributes its own share of the total pressure, its partial pressure (आंशिक दाब). We write it $p\mathrm{O}{\phantom{X}}_2$ for oxygen and $p\mathrm{C}\mathrm{O}{\phantom{X}}_2$ for carbon dioxide. A gas's share of the pressure equals its share of the mixture, so partial pressures are easy to work out.

With that tool, look at the two exchange sites: the alveoli, where most of the exchange happens, and the tissues all over the body. Typical partial pressures at both sites, and along the way, are collected in Table 14.4 and drawn as a loop in Fig. 14.7.

Read the gradients from the table. Oxygen runs downhill all the way: from alveolar air (104) into the blood arriving at the lungs (40), and later from oxygenated blood (95) into the tissues (40). Carbon dioxide runs downhill the opposite way: from the tissues (45) into the blood (40), and from deoxygenated blood (45) into alveolar air (40).

How fast a gas crosses depends on three dials: the size of its pressure gradient, how soluble it is in the watery membrane, and how thick the membrane is. Compare the gradients first. Oxygen crosses into the lung capillaries down $104-40=64$ mm Hg, carbon dioxide out of them down only $45-40=5$ mm Hg, yet both exchanges finish in the fraction of a second a red cell spends in a lung capillary. Solubility makes up the difference: $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ is 20–25 times more soluble than $\mathrm{O}{\phantom{X}}_2$, so for every mm Hg of difference far more $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ diffuses across.

Think of the barrier as a sandwich (Fig. 14.8). On the air side lies one flattened, squamous epithelial cell of the alveolar wall; on the blood side, one endothelial cell of the capillary wall. The filling is the basement substance: a thin basement membrane under the epithelium and another around the single layer of endothelial cells. Stack all three and the total is still far less than a millimetre; in most places it is under a thousandth of a millimetre. Steep gradients, the high solubility of $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ and this very thin barrier together make diffusion run exactly the way the body needs: oxygen from alveoli towards tissues, carbon dioxide from tissues towards alveoli.

14.4 Transport of Gases

Blood carries both gases, but in very different ways. About 97 per cent of the oxygen rides on haemoglobin inside red blood cells (RBCs); only about 3 per cent is dissolved in the plasma, because oxygen dissolves poorly in water. Carbon dioxide has three routes: about 70 per cent travels as bicarbonate, 20–25 per cent is carried by RBCs bound to haemoglobin, and about 7 per cent is simply dissolved in the plasma.

14.4.1 Transport of Oxygen

A red blood cell passes through the lungs and then the tissues about once a minute, and its haemoglobin must grab oxygen in one place and let go of it in the other. Haemoglobin, the red, iron-containing pigment packed into RBCs, can do this because its binding is reversible: oxygen attaches to form oxyhaemoglobin and detaches again, depending on the surroundings. Each molecule is built from four polypeptide chains, each with one iron-containing haem group, so it holds at most four $\mathrm{O}{\phantom{X}}_2$ molecules.

The surrounding $p\mathrm{O}{\phantom{X}}_2$ matters most. A graph of the percentage saturation of haemoglobin with oxygen against $p\mathrm{O}{\phantom{X}}_2$, the oxygen dissociation curve (ऑक्सीजन वियोजन वक्र), comes out S-shaped, or sigmoid, rather than straight (Fig. 14.9). The S-shape comes from teamwork inside the molecule: once the first $\mathrm{O}{\phantom{X}}_2$ binds, haemoglobin changes shape so that the next ones bind more easily, and the reverse happens as oxygen leaves. This is the emergent property promised at the start of the unit.

Three other conditions interfere with the binding and move the whole curve: $p\mathrm{C}\mathrm{O}{\phantom{X}}_2$, the concentration of hydrogen ions ($\mathrm{H}{\phantom{X}}^{+}$) and temperature. That makes the curve the standard tool for studying their effects.

Now compare the two ends of the journey (Table 14.5). Every factor pushes the same way at each site, so oxygen is picked up at the lung surface and let go in the tissues.

The shape of the curve has a purpose. Its flat top is a safety margin: even if alveolar $p\mathrm{O}{\phantom{X}}_2$ dropped from about 100 to 70 mm Hg, haemoglobin would still be more than 90 per cent loaded. Its steep middle is where the tissues work: a small fall in $p\mathrm{O}{\phantom{X}}_2$ there releases a lot of oxygen. Under normal resting conditions every 100 mL of oxygenated blood delivers about 5 mL of oxygen to the tissues; in a hard-working muscle it delivers far more.

14.4.2 Transport of Carbon dioxide

As bicarbonate. Most carbon dioxide, about 70 per cent, travels as bicarbonate. The enzyme carbonic anhydrase is packed into RBCs in huge amounts, and plasma holds traces of it. The enzyme speeds up the reaction of $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ with water in both directions; the carbonic acid formed splits into a bicarbonate ion and a hydrogen ion.

At the tissues, catabolism keeps $p\mathrm{C}\mathrm{O}{\phantom{X}}_2$ high. $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ diffuses into the blood, both into RBCs and into plasma, and the reactions of (14.3) run forwards, from top to bottom, trapping it as $\mathrm{H}\mathrm{C}\mathrm{O}{\phantom{X}}_3{\phantom{X}}^{-}$ and $\mathrm{H}{\phantom{X}}^{+}$. At the alveoli, $p\mathrm{C}\mathrm{O}{\phantom{X}}_2$ is low, so the same reactions run backwards, from bottom to top: bicarbonate and hydrogen ions recombine, carbonic acid gives back $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ and water, and the $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ diffuses into alveolar air to be breathed out (Fig. 14.11). Each 100 mL of deoxygenated blood hands about 4 mL of $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ to the alveoli.

As carbamino-haemoglobin. The second route skips the enzyme. About 20–25 per cent of the carbon dioxide attaches straight to haemoglobin, forming carbamino-haemoglobin. The amount bound rises with $p\mathrm{C}\mathrm{O}{\phantom{X}}_2$, and $p\mathrm{O}{\phantom{X}}_2$ has a strong effect too, because haemoglobin that has given up its oxygen takes on $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ more readily. In the tissues, with plenty of $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ and little $\mathrm{O}{\phantom{X}}_2$, loading is favoured; in the alveoli, with little $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ and plenty of $\mathrm{O}{\phantom{X}}_2$, the $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ comes off and is delivered for breathing out. It is the mirror image of what haemoglobin does with oxygen.

14.5 Regulation of Respiration

You breathe faster on a staircase and slower in your sleep, without deciding to. The neural system runs this rather like a thermostat: sensors check the blood, a control centre sets the rhythm to suit the tissues' demands, and the breathing muscles carry it out (Fig. 14.12).

The quantity being watched is mainly carbon dioxide, not oxygen. Beside the rhythm centre in the medulla lies a chemosensitive area that responds strongly to $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ and hydrogen ions, and receptors on the arch of the aorta and on the carotid arteries pick up changes in the same two substances. Oxygen's part in setting the normal rhythm is insignificant.

All these signals reach the respiratory rhythm centre in the medulla of the brain, the centre chiefly responsible for regulating breathing; its impulses make the diaphragm and intercostal muscles contract. When $\mathrm{C}\mathrm{O}{\phantom{X}}_2$ and $\mathrm{H}{\phantom{X}}^{+}$ rise, it adjusts the rhythm until the excess has been removed. A second centre in the pons, the pneumotaxic centre, can moderate the rhythm centre: its signals shorten each inspiration and so alter the breathing rate.

14.6 Disorders of Respiratory System

Damage to any part of the system shows up as poorer gas exchange. Three disorders are in your syllabus (Fig. 14.13).

Asthma. The walls of the bronchi and bronchioles become inflamed and the airways narrow. Breathing becomes difficult, and air squeezing through the narrowed tubes makes a whistling sound called wheezing. Attacks are often set off by triggers such as dust, pollen, smoke or cold air.

Emphysema (वातस्फीति). In this chronic, long-lasting disease the walls of the alveoli are damaged and break down. Many small alveoli merge into fewer, larger sacs, so the respiratory surface shrinks and each breath delivers less oxygen to the blood. Smoking cigarettes is its best-known cause.

Occupational respiratory disorders. Stone-crushing units, quarries and grinding workshops throw up clouds of fine dust. The lungs' cleaning systems can cope with a little, but not with this much, day after day. The trapped dust keeps the lungs inflamed, and long-term inflammation ends in fibrosis: fibrous tissue multiplies, the lung thickens and stiffens, and the damage is serious. Protective masks are the first line of defence for such workers.