Process of Respiration

BiologyBreathing and Exchange of GasesFor NEET aspirants

The process of respiration in humans covers how air is breathed in and out, how oxygen and carbon dioxide are exchanged at the alveoli and the tissues, how blood carries both gases, and how the brain controls the rhythm of breathing. This page explains inspiration and expiration, respiratory volumes and capacities, partial pressures, the oxygen dissociation curve, carbonic anhydrase and the respiratory centres. It covers every point of the NCERT text on these topics. NEET often asks the volumes and capacities, the partial pressure values and the percentages in which each gas is carried.

On this page1Mechanism of breathing2Volumes and capacities3Exchange of gases4Transport of gases5Regulation6Exam essentials7Quick revision8Solved examples9Practice

Key Points at a Glance

★ Must learn Inspiration: intra-pulmonary pressure below atmospheric; expiration: intra-pulmonary pressure above atmospheric.
Diaphragm contraction enlarges the thorax in the antero-posterior axis; external intercostals in the dorso-ventral axis.
A healthy human breathes 12-16 times per minute; a spirometer measures the volumes of air.
★ Must learn TV about 500 mL; IRV 2500-3000 mL; ERV 1000-1100 mL; RV 1100-1200 mL.
IC = TV + IRV; EC = TV + ERV; FRC = ERV + RV.
★ Must learn VC = IRV + TV + ERV and TLC = VC + RV.
★ Must learn $p O_{2}$ (mm Hg): air 159, alveoli 104, oxygenated blood 95, deoxygenated blood 40, tissues 40.
$pC O_{2}$ (mm Hg): air 0.3, alveoli 40, oxygenated blood 40, deoxygenated blood 45, tissues 45. $C O_{2}$ is 20-25 times more soluble than $O_{2}$ .
★ Must learn $O_{2}$ : 97% by RBCs, 3% dissolved in plasma. $C O_{2}$ : 70% as bicarbonate, 20-25% by RBCs, 7% dissolved in plasma.
Each haemoglobin carries up to 4 $O_{2}$ ; 100 mL oxygenated blood gives about 5 mL $O_{2}$ to tissues; 100 mL deoxygenated blood gives about 4 mL $C O_{2}$ to alveoli.
Rhythm centre in the medulla; pneumotaxic centre in the pons; the role of $O_{2}$ in regulation is insignificant.

1. Mechanism of Breathing

1.1 Two stages and the pressure gradient

Breathing has two stages: inspiration, in which atmospheric air is drawn in, and expiration, in which alveolar air is released out.
Air moves into and out of the lungs because of a pressure gradient between the lungs and the atmosphere.
Intra-pulmonary pressure: the pressure within the lungs.
★ Exam imp Inspiration occurs when the intra-pulmonary pressure is less than the atmospheric pressure, that is, a negative pressure in the lungs.
Expiration occurs when the intra-pulmonary pressure is higher than the atmospheric pressure.
The diaphragm and a specialised set of muscles, the external and internal intercostals between the ribs, create these gradients.

★ Very important Air always flows from higher pressure to lower pressure. Breathing works by changing the lung volume, which changes the intra-pulmonary pressure relative to the atmosphere.

1.2 Inspiration

The diaphragm contracts. This increases the volume of the thoracic chamber in the antero-posterior axis.
The external intercostal muscles contract. They lift the ribs and sternum, increasing the thoracic volume in the dorso-ventral axis.
The overall increase in thoracic volume causes a similar increase in pulmonary volume.
The larger pulmonary volume lowers the intra-pulmonary pressure below the atmospheric pressure.
Air from outside is forced into the lungs. This is inspiration.

1.3 Expiration

The diaphragm and the intercostal muscles relax.
The diaphragm and sternum return to their normal positions.
The thoracic volume, and so the pulmonary volume, decreases.
The intra-pulmonary pressure rises slightly above the atmospheric pressure.
Air is expelled from the lungs. This is expiration.

Feature	Inspiration	Expiration
Diaphragm	Contracts	Relaxes, returns to its normal position
External intercostal muscles	Contract; ribs and sternum lifted	Relax; ribs and sternum return
Thoracic and pulmonary volume	Increase	Decrease
Intra-pulmonary pressure	Less than atmospheric	Slightly more than atmospheric
Air movement	Air enters the lungs	Air is expelled from the lungs

Test yourself: hide labels

Figure 1: Mechanism of breathing showing (a) inspiration and (b) expiration. Contraction of the diaphragm and raising of the ribs and sternum enlarge the thorax; relaxation reduces it again.

Quick Recall: Figure 1 labels

In Figure 1(a), what has happened to the ribs and sternum?

They are raised.

In Figure 1(a), what is the state of the diaphragm?

Contracted.

In Figure 1(b), how is the diaphragm described?

Relaxed and arched upwards.

In Figure 1(b), what has happened to the volume of the thorax?

It has decreased.

1.4 Forced breathing, breathing rate and the spirometer

We can increase the strength of inspiration and expiration with the help of additional muscles in the abdomen.
★ Exam imp On average, a healthy human breathes 12-16 times per minute.
Spirometer: an instrument that estimates the volume of air involved in breathing movements.
It helps in the clinical assessment of pulmonary functions.

Memory Trick

Contract to inhale, relax to exhale. In normal breathing, inspiration needs muscle contraction; normal expiration happens when the same muscles relax.

NEET Focus

Diaphragm: antero-posterior axis. External intercostals: dorso-ventral axis. Questions often swap them.
During expiration, the intra-pulmonary pressure is only slightly above atmospheric.
Inspiration needs a negative pressure in the lungs relative to the atmosphere.

Key idea

Muscles change the volume of the air-tight thorax; the lungs follow, and the pressure change moves air in or out.

2. Respiratory Volumes and Capacities

2.1 Respiratory volumes

Volume	Meaning	Average value
Tidal volume (TV)	Volume of air inspired or expired during a normal respiration	About 500 mL
Inspiratory reserve volume (IRV)	Additional volume of air a person can inspire by a forcible inspiration	2500-3000 mL
Expiratory reserve volume (ERV)	Additional volume of air a person can expire by a forcible expiration	1000-1100 mL
Residual volume (RV)	Volume of air remaining in the lungs even after a forcible expiration	1100-1200 mL

★ Exam imp With a tidal volume of about 500 mL, a healthy person inspires or expires about 6000-8000 mL of air per minute.
This follows from 500 mL per breath at 12-16 breaths per minute.

Memory Trick

Tide 500; inspiratory reserve the biggest (2500-3000); expiratory reserve and residual are near twins (about 1000-1200). ERV 1000-1100 is just below RV 1100-1200.

2.2 Respiratory capacities

Adding two or more respiratory volumes gives the pulmonary capacities.
Capacities are used in clinical diagnosis.

Capacity	Meaning	Volumes included	Range from the average volumes
Inspiratory capacity (IC)	Total air a person can inspire after a normal expiration	TV + IRV	3000-3500 mL
Expiratory capacity (EC)	Total air a person can expire after a normal inspiration	TV + ERV	1500-1600 mL
Functional residual capacity (FRC)	Air that remains in the lungs after a normal expiration	ERV + RV	2100-2300 mL
Vital capacity (VC)	Maximum air a person can breathe in after a forced expiration, or breathe out after a forced inspiration	ERV + TV + IRV	4000-4600 mL
Total lung capacity (TLC)	Total air the lungs hold at the end of a forced inspiration	RV + ERV + TV + IRV, or VC + RV	5100-5800 mL

★ Very important Vital capacity = ERV + TV + IRV: the maximum volume that can be breathed in after a forced expiration, or out after a forced inspiration. Total lung capacity = VC + RV.

Tips and Tricks

A volume is one non-overlapping slice of lung air; a capacity is a sum of two or more volumes. Look at the starting point in the definition: "after a normal expiration" gives IC (air you can still take in) or FRC (air still left inside). "After a forced breath" points to VC or TLC. RV appears only in FRC and TLC, because it can never be breathed out.

Memory Trick

Read the names literally: Inspiratory capacity adds the Inspiratory reserve, Expiratory capacity adds the Expiratory reserve, and both include TV. Vital = all you can move; Total = Vital + Residual.

Key idea

Four volumes, five capacities: every capacity is a sum of volumes, and only FRC and TLC contain residual volume.

3. Exchange of Gases

★ Exam imp Alveoli are the primary sites of exchange of gases.
Gases are also exchanged between blood and tissues.
$O_{2}$ and $C O_{2}$ are exchanged at these sites by simple diffusion, mainly based on the pressure or concentration gradient.
Two other factors affect the rate of diffusion: the solubility of the gases and the thickness of the membranes involved.

★ Very important Partial pressure: the pressure contributed by an individual gas in a mixture of gases. It is written as $p O_{2}$ for oxygen and $pC O_{2}$ for carbon dioxide.

Partial pressures (in mm Hg) at the sites of diffusion, compared with atmospheric air

Respiratory gas	Atmospheric air	Alveoli	Blood (deoxygenated)	Blood (oxygenated)	Tissues
$O_{2}$	159	104	40	95	40
$C O_{2}$	0.3	40	45	40	45

★ Exam imp There is a gradient for $O_{2}$ from alveoli to blood and from blood to tissues.
The gradient for $C O_{2}$ runs in the opposite direction: from tissues to blood, and from blood to alveoli.

Memory Trick

Deoxygenated blood matches the tissues (40 and 45); oxygenated blood nearly matches the alveoli (95 against 104 for $O_{2}$ , 40 for $C O_{2}$ ). Blood leaving a site takes on that site's partial pressures.

Test yourself: hide labels

Figure 2: Exchange of gases at the alveolus and at the body tissues, and transport of oxygen and carbon dioxide by blood. Each gas moves from its higher partial pressure to its lower one: oxygen from alveolus to blood to tissues, carbon dioxide the opposite way.

3.1 Why carbon dioxide diffuses easily

★ Exam imp The solubility of $C O_{2}$ is 20-25 times higher than that of $O_{2}$ .
So, for each unit difference in partial pressure, much more $C O_{2}$ than $O_{2}$ diffuses through the membrane.

3.2 The diffusion membrane

The diffusion membrane has three major layers:

Thin squamous epithelium of the alveoli.
Endothelium of the alveolar capillaries.
Basement substance between them: a thin basement membrane supporting the squamous epithelium, and the basement membrane around the single layer of endothelial cells of the capillaries.

Its total thickness is much less than a millimetre.
Therefore all the factors in the body favour diffusion of $O_{2}$ from the alveoli to the tissues, and of $C O_{2}$ from the tissues to the alveoli.

Memory Trick

Air to blood, the layers are Squamous, Basement, Endothelium (S-B-E): two thin cell layers with the basement substance between them.

Test yourself: hide labels

Figure 3: Section of an alveolus with a pulmonary capillary. Air in the alveolar cavity is separated from the blood only by the very thin diffusion membrane.

Quick Recall: name the parts in Figure 3

Name the one-celled-thick lining of the alveolar wall.

Squamous epithelium of the alveolar wall.

Name the layer between the alveolar epithelium and the capillary endothelium.

Basement substance.

Name the lining of the blood capillary.

Endothelium of the blood capillary.

Name the cells seen inside the blood capillary.

Red blood cells.

Key idea

Steep partial pressure gradients, high

C O_{2}

solubility and a membrane far thinner than a millimetre make diffusion fast in both directions.

4. Transport of Gases

Blood is the medium of transport for $O_{2}$ and $C O_{2}$ .

Gas	How it is carried	Share
$O_{2}$	By RBCs (as oxyhaemoglobin)	About 97%
$O_{2}$	Dissolved in plasma	About 3%
$C O_{2}$	As bicarbonate	About 70%
$C O_{2}$	By RBCs (as carbamino-haemoglobin)	Nearly 20-25%
$C O_{2}$	Dissolved in plasma	About 7%

Memory Trick

Oxygen: 97 in cells, 3 in plasma. Carbon dioxide: 70 as Bicarbonate, 20-25 Bound to haemoglobin, 7 Bare in plasma.

4.1 Transport of oxygen

Haemoglobin: a red-coloured, iron-containing pigment present in the RBCs.
★ Exam imp $O_{2}$ binds with haemoglobin in a reversible manner to form oxyhaemoglobin.
★ Exam imp Each haemoglobin molecule can carry a maximum of four molecules of $O_{2}$ .
Binding of $O_{2}$ with haemoglobin depends primarily on $p O_{2}$ .
$pC O_{2}$ , hydrogen ion concentration and temperature can also interfere with this binding.

★ Very important Oxygen dissociation curve: the sigmoid curve obtained by plotting the percentage saturation of haemoglobin with $O_{2}$ against $p O_{2}$ . It is very useful for studying how factors like $pC O_{2}$ and $H^{+}$ concentration affect the binding of $O_{2}$ with haemoglobin.

Test yourself: hide labels

Figure 4: Oxygen dissociation curve: percentage saturation of haemoglobin with oxygen plotted against

p O_{2}

. The sigmoid shape means haemoglobin loads oxygen almost fully in the alveoli and unloads it readily in the tissues.

Condition	In the alveoli	In the tissues
$p O_{2}$	High	Low
$pC O_{2}$	Low	High
$H^{+}$ concentration	Lesser	High
Temperature	Lower	Higher
Result	Oxyhaemoglobin forms ( $O_{2}$ binds)	Oxygen dissociates from oxyhaemoglobin

So $O_{2}$ gets bound to haemoglobin at the lung surface and is released at the tissues.
★ Exam imp Under normal physiological conditions, every 100 mL of oxygenated blood delivers about 5 mL of $O_{2}$ to the tissues.

Memory Trick

Lungs load: one high, three low. High $p O_{2}$ ; low $pC O_{2}$ , low $H^{+}$ , low temperature. Tissues unload: one low, three high.

4.2 Transport of carbon dioxide

★ Exam imp About 20-25% of $C O_{2}$ is carried by haemoglobin as carbamino-haemoglobin.
This binding is related to $pC O_{2}$ ; $p O_{2}$ is a major factor that can affect it.
In the tissues (high $pC O_{2}$ , low $p O_{2}$ ), more $C O_{2}$ binds to haemoglobin.
In the alveoli (low $pC O_{2}$ , high $p O_{2}$ ), carbamino-haemoglobin dissociates, so $C O_{2}$ picked up in the tissues is delivered at the alveoli.
★ Exam imp RBCs contain a very high concentration of the enzyme carbonic anhydrase; plasma has only minute quantities.
Carbonic anhydrase speeds up the following reaction in both directions:

C O_{2} + H_{2} O ⇌ Carbonic anhydrase H_{2} C O_{3} ⇌ Carbonic anhydrase HC O_{3}^{-} + H^{+}

At the tissues, $pC O_{2}$ is high because of catabolism, so $C O_{2}$ diffuses into the blood (RBCs and plasma).
There it forms $HC O_{3}^{-}$ and $H^{+}$ ; $C O_{2}$ is now trapped as bicarbonate.
The blood carries the bicarbonate to the alveoli.
At the alveoli, $pC O_{2}$ is low, so the reaction runs in the opposite direction, forming $C O_{2}$ and $H_{2} O$ .
The $C O_{2}$ diffuses into the alveoli and is breathed out.

★ Exam imp Every 100 mL of deoxygenated blood delivers about 4 mL of $C O_{2}$ to the alveoli.

Oxygen

Mainly as oxyhaemoglobin (97%). Binds at high $p O_{2}$ in the alveoli; released at the tissues. About 5 mL per 100 mL blood delivered.

Carbon dioxide

Mainly as bicarbonate (70%), then carbamino-haemoglobin (20-25%). Bound at the tissues; released at the alveoli. About 4 mL per 100 mL blood delivered.

Key idea

Haemoglobin loads oxygen where

p O_{2}

is high and unloads it where

p O_{2}

is low; carbon dioxide travels mostly as bicarbonate.

5. Regulation of Respiration

Humans can maintain and moderate the respiratory rhythm to suit the demands of the body tissues.
This is done by the neural system.

Centre or receptor	Location	Role
Respiratory rhythm centre	Medulla region of the brain	Primarily responsible for regulating the respiratory rhythm
Pneumotaxic centre	Pons region of the brain	Moderates the rhythm centre; its signal can reduce the duration of inspiration and so alter the respiratory rate
Chemosensitive area	Medulla, adjacent to the rhythm centre	Highly sensitive to $C O_{2}$ and $H^{+}$ ; when they rise, it signals the rhythm centre to adjust breathing so they are eliminated
Receptors of the aortic arch and carotid artery	Aortic arch and carotid artery	Recognise changes in $C O_{2}$ and $H^{+}$ concentration and signal the rhythm centre for remedial action

★ Exam imp The role of oxygen in the regulation of the respiratory rhythm is quite insignificant.

Memory Trick

Medulla makes the rhythm; pons (pneumotaxic) puts the brakes on inspiration. Both names in the pons pair start with P.

NEET Focus

The chemosensitive area responds to $C O_{2}$ and $H^{+}$ , not to $O_{2}$ .
The pneumotaxic centre reduces the duration of inspiration; it does not start the rhythm.
Aortic arch and carotid artery receptors also sense $C O_{2}$ and $H^{+}$ and report to the rhythm centre.

Key idea

Breathing is driven mainly by rising

C O_{2}

and

H^{+}

, sensed in the medulla and in the aortic and carotid receptors; oxygen plays little role.

6. Exam Essentials

Pairs to Match

List I	List II
Diaphragm contraction	Thoracic volume rises in the antero-posterior axis
External intercostal contraction	Ribs and sternum lifted; dorso-ventral axis
Tidal volume	About 500 mL
Inspiratory reserve volume	2500-3000 mL
Expiratory reserve volume	1000-1100 mL
Residual volume	1100-1200 mL
Functional residual capacity	ERV + RV
Vital capacity	ERV + TV + IRV
Total lung capacity	VC + RV
Carbamino-haemoglobin	20-25% of $C O_{2}$
Bicarbonate	About 70% of $C O_{2}$
Carbonic anhydrase	Very high concentration in RBCs
Respiratory rhythm centre	Medulla
Pneumotaxic centre	Pons
Chemosensitive area	Medulla, beside the rhythm centre; senses $C O_{2}$ and $H^{+}$

Exceptions

Residual volume cannot be expired even by a forcible expiration; it is not part of vital capacity.
$O_{2}$ is carried mostly by RBCs, but $C O_{2}$ is carried mostly as bicarbonate, not by haemoglobin.
$C O_{2}$ has the smaller partial pressure gradient, yet diffuses readily because it is 20-25 times more soluble.
The role of $O_{2}$ in regulating breathing is insignificant, unlike $C O_{2}$ and $H^{+}$ .
Carbonic anhydrase is present in plasma only in minute quantities; most of it is in RBCs.
In normal expiration, the intra-pulmonary pressure is only slightly above atmospheric.

Numbers to Remember

Breathing rate: 12-16 per minute; air moved: 6000-8000 mL per minute.
TV 500 mL; IRV 2500-3000 mL; ERV 1000-1100 mL; RV 1100-1200 mL.
$p O_{2}$ (mm Hg): 159 air, 104 alveoli, 95 oxygenated blood, 40 deoxygenated blood, 40 tissues.
$pC O_{2}$ (mm Hg): 0.3 air, 40 alveoli, 40 oxygenated blood, 45 deoxygenated blood, 45 tissues.
$C O_{2}$ solubility: 20-25 times that of $O_{2}$ . Diffusion membrane: 3 layers, much less than 1 mm thick.
$O_{2}$ : 97% RBCs, 3% plasma. $C O_{2}$ : 70% bicarbonate, 20-25% RBCs, 7% plasma.
Haemoglobin: up to 4 $O_{2}$ per molecule. Delivery per 100 mL blood: 5 mL $O_{2}$ , 4 mL $C O_{2}$ .

7. Quick Revision

Inspiration: intra-pulmonary pressure below atmospheric; expiration: slightly above atmospheric.
Diaphragm contraction (antero-posterior axis) and external intercostal contraction (dorso-ventral axis) enlarge the thorax.
Relaxation of these muscles reduces thoracic and pulmonary volume, causing expiration.
Abdominal muscles add strength to breathing; healthy rate 12-16 per minute; spirometer measures volumes.
TV 500, IRV 2500-3000, ERV 1000-1100, RV 1100-1200 mL.
IC = TV + IRV; EC = TV + ERV; FRC = ERV + RV; VC = ERV + TV + IRV; TLC = VC + RV.
Gas exchange is by simple diffusion; rate depends on the partial pressure gradient, solubility and membrane thickness.
$O_{2}$ gradient: alveoli to blood to tissues; $C O_{2}$ gradient: the reverse.
$C O_{2}$ is 20-25 times more soluble than $O_{2}$ .
Diffusion membrane: squamous epithelium, basement substance, capillary endothelium; much thinner than 1 mm.
$O_{2}$ : 97% as oxyhaemoglobin in RBCs, 3% in plasma; each haemoglobin binds up to 4 $O_{2}$ .
Oxygen dissociation curve is sigmoid; $O_{2}$ binds in alveoli and dissociates in tissues; 5 mL $O_{2}$ per 100 mL blood.
$C O_{2}$ : 70% bicarbonate, 20-25% carbamino-haemoglobin, 7% plasma; 4 mL $C O_{2}$ per 100 mL blood.
Carbonic anhydrase (mainly in RBCs) forms bicarbonate at tissues and releases $C O_{2}$ at alveoli.
Rhythm centre in medulla; pneumotaxic centre in pons; chemosensitive area senses $C O_{2}$ and $H^{+}$ ; $O_{2}$ role insignificant.

8. Solved Examples

Solved Example 1

Match List I with List II.
List I: A. Tidal volume, B. Inspiratory reserve volume, C. Expiratory reserve volume, D. Residual volume
List II: I. 1100-1200 mL, II. About 500 mL, III. 1000-1100 mL, IV. 2500-3000 mL
Choose the correct answer:
(A) A-II, B-III, C-IV, D-I
(B) A-IV, B-II, C-III, D-I
(C) A-II, B-IV, C-I, D-III
(D) A-II, B-IV, C-III, D-I

Solution:

Answer: (D). TV about 500 mL (II); IRV 2500-3000 mL (IV); ERV 1000-1100 mL (III); RV 1100-1200 mL (I).

Solved Example 2

Using the average values, find the vital capacity and the total lung capacity of a healthy adult.

Solution:

Answer: VC = IRV + TV + ERV = (2500 to 3000) + 500 + (1000 to 1100) = 4000-4600 mL. TLC = VC + RV = (4000 to 4600) + (1100 to 1200) = 5100-5800 mL.

Solved Example 3

Read the statements about transport of gases.
A. About 97% of

O_{2}

is carried by RBCs.
B. About 70% of

C O_{2}

is carried as carbamino-haemoglobin.
C. Each haemoglobin molecule can carry a maximum of four

O_{2}

molecules.
D. Carbonic anhydrase is present in very high concentration in RBCs.
E. Low

p O_{2}

and high

pC O_{2}

favour formation of oxyhaemoglobin.
Choose the correct answer:
(A) A, B and C only
(B) B, D and E only
(C) A, C and D only
(D) A, C and E only

Solution:

Answer: (C). B is wrong: 70% of $C O_{2}$ travels as bicarbonate; carbamino-haemoglobin carries 20-25%. E is wrong: high $p O_{2}$ and low $pC O_{2}$ , as in the alveoli, favour oxyhaemoglobin formation.

Solved Example 4

Arrange the events of a normal inspiration in the correct order.
A. Intra-pulmonary pressure falls below atmospheric pressure
B. Diaphragm and external intercostal muscles contract
C. Air moves into the lungs
D. Pulmonary volume increases
E. Thoracic volume increases
Choose the correct answer:
(A) B, D, E, A, C
(B) B, E, D, A, C
(C) E, B, D, A, C
(D) B, E, A, D, C

Solution:

Answer: (B). Muscles contract, the thorax enlarges, the lungs follow, pressure falls, and air flows in.

Solved Example 5

Which of the following is NOT correct about the regulation of respiration?
(A) The respiratory rhythm centre lies in the medulla.
(B) The pneumotaxic centre lies in the pons.
(C) The chemosensitive area is highly sensitive to

O_{2}

.
(D) Aortic arch and carotid artery receptors sense

C O_{2}

and

H^{+}

Solution:

Answer: (C). The chemosensitive area responds to $C O_{2}$ and $H^{+}$ ; the role of $O_{2}$ in regulation is quite insignificant.

Solved Example 6

Statement I:

C O_{2}

diffuses across the alveolar membrane more readily than

O_{2}

for the same difference in partial pressure.
Statement II: The solubility of

C O_{2}

is 20-25 times higher than that of

O_{2}

.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrect

Solution:

Answer: (A). Higher solubility lets more $C O_{2}$ cross the membrane per unit difference in partial pressure.

9. Practice Questions

Practice Questions

Match List I with List II.
List I: A. Inspiratory capacity, B. Expiratory capacity, C. Functional residual capacity, D. Total lung capacity
List II: I. ERV + RV, II. TV + IRV, III. VC + RV, IV. TV + ERV
Choose the correct answer:
(A) A-IV, B-II, C-I, D-III
(B) A-II, B-IV, C-III, D-I
(C) A-II, B-IV, C-I, D-III
(D) A-II, B-I, C-IV, D-IIIAnswer: (C). IC = TV + IRV; EC = TV + ERV; FRC = ERV + RV; TLC = VC + RV.
What are $p O_{2}$ and $pC O_{2}$ in deoxygenated blood?
(A) 40 and 45 mm Hg
(B) 95 and 40 mm Hg
(C) 104 and 40 mm Hg
(D) 159 and 0.3 mm HgAnswer: (A). Deoxygenated blood has the same values as the tissues.
Read the statements about the diffusion membrane.
A. It has three major layers.
B. Its total thickness is about 1 cm.
C. It includes the squamous epithelium of the alveoli.
D. It includes the endothelium of the alveolar capillaries.
Choose the correct answer:
(A) A, B and C only
(B) B, C and D only
(C) A and B only
(D) A, C and D onlyAnswer: (D). Its total thickness is much less than a millimetre.
Arrange the steps of $C O_{2}$ transport from tissues to lungs.
A. Bicarbonate is carried to the alveoli
B. $C O_{2}$ diffuses from the tissues into the blood
C. $C O_{2}$ and $H_{2} O$ form again at low $pC O_{2}$
D. $HC O_{3}^{-}$ and $H^{+}$ form
Choose the correct answer:
(A) D, B, C, A
(B) B, D, A, C
(C) B, A, C, D
(D) A, B, D, CAnswer: (B). Diffusion into blood, bicarbonate formation, transport, reversal at the alveoli.
Which of the following does NOT favour the dissociation of oxygen from oxyhaemoglobin?
(A) Low $p O_{2}$
(B) High $pC O_{2}$
(C) Low temperature
(D) High $H^{+}$ concentrationAnswer: (C). Higher temperature, as in the tissues, favours dissociation; lower temperature favours binding.
Statement I: The pneumotaxic centre can reduce the duration of inspiration.
Statement II: The respiratory rhythm centre is present in the pons.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrectAnswer: (B). The rhythm centre is in the medulla; the pneumotaxic centre is in the pons.
About how much $O_{2}$ does 100 mL of oxygenated blood deliver to the tissues under normal conditions?
(A) 4 mL
(B) 20 mL
(C) 97 mL
(D) 5 mLAnswer: (D). About 5 mL of $O_{2}$ ; deoxygenated blood delivers about 4 mL of $C O_{2}$ to the alveoli.

Short-Answer Practice

Define vital capacity. What is its significance?Answer: Vital capacity is the maximum volume of air a person can breathe in after a forced expiration (or out after a forced inspiration); it equals ERV + TV + IRV, about 4000-4600 mL. Like other capacities, it is used in clinical diagnosis to assess how well the lungs work.
State the volume of air remaining in the lungs after a normal breathing.Answer: This is the functional residual capacity, FRC = ERV + RV = (1000 to 1100) + (1100 to 1200) = about 2100-2300 mL.
What are the major transport mechanisms for $C O_{2}$ ? Explain.Answer: About 70% travels as bicarbonate, formed by carbonic anhydrase in RBCs at the tissues and reversed at the alveoli; 20-25% binds haemoglobin as carbamino-haemoglobin (more binding at high $pC O_{2}$ and low $p O_{2}$ ); about 7% is dissolved in plasma.
What will be the $p O_{2}$ and $pC O_{2}$ in the atmospheric air compared to those in the alveolar air?
(A) $p O_{2}$ lesser, $pC O_{2}$ higher
(B) $p O_{2}$ higher, $pC O_{2}$ lesser
(C) $p O_{2}$ higher, $pC O_{2}$ higher
(D) $p O_{2}$ lesser, $pC O_{2}$ lesserAnswer: (B). Air: $p O_{2}$ 159 and $pC O_{2}$ 0.3 mm Hg; alveoli: 104 and 40 mm Hg.
Explain the process of inspiration under normal conditions.Answer: The diaphragm and external intercostal muscles contract. The thorax enlarges in the antero-posterior and dorso-ventral axes, so the pulmonary volume rises and the intra-pulmonary pressure falls below atmospheric. Air then flows into the lungs.
How is respiration regulated?Answer: By the neural system: the rhythm centre in the medulla sets the rhythm; the pneumotaxic centre in the pons can shorten inspiration; the chemosensitive area beside the rhythm centre and receptors in the aortic arch and carotid artery respond to rising $C O_{2}$ and $H^{+}$ . Oxygen plays an insignificant role.
What is the effect of $pC O_{2}$ on oxygen transport?Answer: High $pC O_{2}$ , as in the tissues, favours dissociation of oxygen from oxyhaemoglobin; low $pC O_{2}$ , as in the alveoli, favours its formation. So $C O_{2}$ helps haemoglobin release oxygen where it is needed.
What happens to the respiratory process in a man going up a hill?Answer: Breathing becomes faster and deeper. Extra Depth: with height, atmospheric pressure and so the $p O_{2}$ of air fall, so less oxyhaemoglobin forms in the alveoli; receptors in the carotid and aortic bodies sense the low $p O_{2}$ and raise the breathing rate (and heart rate). Climbing is also exercise: muscles produce more $C O_{2}$ and $H^{+}$ , which activate the chemosensitive area and the aortic and carotid receptors, so the rhythm centre increases breathing further.
Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?Answer: It is the curve of percentage saturation of haemoglobin with $O_{2}$ plotted against $p O_{2}$ . Extra Depth: it is sigmoid because each haemoglobin binds four $O_{2}$ , and binding of the first makes binding of the next ones easier.
Distinguish between (a) IRV and ERV, (b) inspiratory capacity and expiratory capacity, (c) vital capacity and total lung capacity.Answer: (a) IRV is extra air inspired by forcible inspiration (2500-3000 mL); ERV is extra air expired by forcible expiration (1000-1100 mL). (b) IC = TV + IRV, air inspired after a normal expiration; EC = TV + ERV, air expired after a normal inspiration. (c) VC = ERV + TV + IRV; TLC = VC + RV, the total air in the lungs after a forced inspiration.
What is tidal volume? Find the tidal volume (approximate value) for a healthy human in an hour.Answer: Tidal volume is the air inspired or expired in a normal breath, about 500 mL. At 12-16 breaths per minute: 500 $\times$ 12 $\times$ 60 = 360,000 mL and 500 $\times$ 16 $\times$ 60 = 480,000 mL, so about 360-480 litres per hour.

Common Mistakes to Avoid

Watch out

Swapping the axes: the diaphragm enlarges the thorax in the antero-posterior axis; the external intercostals in the dorso-ventral axis.
Writing that expiration happens when intra-pulmonary pressure is below atmospheric. It is slightly above atmospheric.
Including RV in vital capacity. VC = ERV + TV + IRV only; TLC = VC + RV.
Mixing up ERV (1000-1100 mL) and RV (1100-1200 mL). RV is the slightly larger one.
Using 104 mm Hg for oxygenated blood. Oxygenated blood has $p O_{2}$ 95; 104 is alveolar air.
Saying most $C O_{2}$ travels as carbamino-haemoglobin. Most (about 70%) travels as bicarbonate.
Placing the rhythm centre in the pons. The rhythm centre is in the medulla; the pneumotaxic centre is in the pons.
Writing that the chemosensitive area responds strongly to $O_{2}$ . It responds to $C O_{2}$ and $H^{+}$ .

Frequently Asked Questions

How does inspiration take place in humans?

The diaphragm and the external intercostal muscles contract. This enlarges the thorax in the antero-posterior and dorso-ventral axes, so the lungs expand. The intra-pulmonary pressure falls below the atmospheric pressure, and air flows into the lungs until the pressures are equal.

What is the difference between respiratory volumes and capacities?

Respiratory volumes are separate, non-overlapping amounts of air: tidal volume, inspiratory reserve, expiratory reserve and residual volume. Capacities are sums of two or more volumes, such as vital capacity (ERV + TV + IRV) and total lung capacity (VC + RV). Capacities are used in clinical diagnosis.

What is vital capacity and its normal value?

Vital capacity is the maximum volume of air a person can breathe in after a forced expiration, or breathe out after a forced inspiration. It equals ERV + TV + IRV. Using the average volumes, it works out to about 4000 to 4600 mL in a healthy adult.

Why is carbon dioxide exchanged easily even though its partial pressure gradient is small?

Carbon dioxide is 20 to 25 times more soluble than oxygen. So for each unit difference in partial pressure, much more carbon dioxide diffuses across the membrane. The diffusion membrane is also much thinner than a millimetre, which favours diffusion of both gases.

What is the oxygen dissociation curve and why is it useful?

It is the sigmoid curve obtained by plotting the percentage saturation of haemoglobin with oxygen against $p O_{2}$ . It is used to study how factors such as $pC O_{2}$ , hydrogen ion concentration and temperature affect the binding of oxygen. It shows that haemoglobin binds oxygen at the high $p O_{2}$ of the alveoli and releases it at the low $p O_{2}$ of the tissues.

How is carbon dioxide transported in blood?

About 70 per cent is carried as bicarbonate, formed with the help of carbonic anhydrase in the RBCs. Nearly 20 to 25 per cent binds haemoglobin as carbamino-haemoglobin, and about 7 per cent is dissolved in plasma. At the alveoli, these reactions reverse and carbon dioxide is released.

Which parts of the brain regulate breathing?

The respiratory rhythm centre in the medulla is mainly responsible. The pneumotaxic centre in the pons can shorten inspiration and alter the rate. A chemosensitive area beside the rhythm centre, and receptors in the aortic arch and carotid artery, respond to rising carbon dioxide and hydrogen ions.

Which numbers from this topic must be memorised for NEET?

Learn the breathing rate of 12 to 16 per minute, the four respiratory volumes, all partial pressures in the table, the 20 to 25 times higher solubility of carbon dioxide, the transport percentages of both gases, four oxygen molecules per haemoglobin, and the 5 mL oxygen and 4 mL carbon dioxide delivered per 100 mL blood.

Previous year questions on Process of Respiration

9 questions from past papers, each with a step-by-step solution.

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