Human Respiratory System and Gas Exchange
The Air Passageway — From Nostrils to Bronchioles
The human respiratory system begins at the nostrils, the external openings through which air enters the body. Air then passes into the nasal cavity, a chamber subdivided into three passage ways by bony projections from the internal nose walls. The nasal cavity is lined with a mucous membrane of ciliated epithelium that traps larger dust particles with hair and mucus. As air travels through the nasal cavity, it becomes moist, warm, and filtered of smaller foreign particles. From each nasal cavity, two internal openings lead into the pharynx (throat) — a muscular passage also lined with mucous membrane that channels air toward the larynx.
Filtration: Hair and mucus in the nostrils trap large dust particles before they reach deeper airways
Warming and Moistening: Blood-rich mucous membranes heat and humidify incoming air, protecting delicate lung tissue
Ciliated Epithelium: Tiny hair-like cilia sweep trapped particles and mucus toward the pharynx for removal
The larynx (voice box) is a complex cartilaginous structure surrounding the upper end of the trachea. The epiglottis — a cartilage with hinge-like muscular control — acts as a lid that automatically covers the laryngeal opening during swallowing, preventing food or liquids from entering the airway. The opening of the larynx is called the glottis, lined with mucous membrane stretched into two thin-edged fibrous bands called vocal cords. When air vibrates these vocal cords, sound is produced. Below the larynx, the trachea (windpipe) extends into the thorax as a tubular structure lying ventral to the oesophagus. C-shaped cartilage rings in the tracheal wall prevent collapse and maintain an open air passage.
Epiglottis Mechanism: During swallowing, the tongue moves backward forcing the epiglottis horizontal, while the larynx moves upward under it — together these divert food safely toward the oesophagus
Cartilage Rings: C-shaped rings keep the trachea open while allowing flexibility; the open part of the 'C' faces the oesophagus so it can expand during swallowing
Vocal Cords: Stretched across the glottis, these fibrous bands vibrate when air passes between them, producing voice
In the thorax, the trachea divides into right and left bronchi. Each bronchus enters a lung and progressively divides into smaller and smaller bronchi. When the diameter reaches 1 mm or less, they are called bronchioles. Unlike the trachea and larger bronchi which contain cartilage rings (progressively replaced by irregular cartilage plates), the bronchioles are composed mainly of circular smooth muscle — this muscle can constrict or dilate to regulate airflow. The bronchioles continue dividing deep into the lungs and finally open into a large number of alveolar sacs.
Progressive Branching: Trachea → primary bronchi → secondary bronchi → smaller bronchi → bronchioles → terminal bronchioles → alveolar ducts → alveolar sacs
Cartilage Transition: Trachea has complete C-rings, larger bronchi have irregular cartilage plates, and bronchioles have no cartilage at all
Smooth Muscle Control: Bronchioles are surrounded by circular smooth muscle that adjusts airway diameter — important in conditions like asthma where bronchioles constrict
Structural Changes Along the Air Passageway
1
Trachea — C-shaped cartilage rings, ciliated epithelium
2
Bronchi — progressively smaller cartilage plates, ciliated epithelium
3
Bronchioles (≤1 mm diameter) — no cartilage, circular smooth muscle, thin epithelium
4
Terminal bronchioles — lead into alveolar ducts and alveolar sacs
Lungs and Alveoli — The Gas Exchange Surfaces
The lungs are paired, spongy organs placed in the thoracic cavity, which is bounded by ribs and intercostal muscles on the sides and floored by the diaphragm — a sheet of skeletal muscle. Lungs are closed sacs connected to the outside via the trachea and nostrils (or mouth). Their spongy texture comes from the millions of tiny alveoli packed within them. Each lung is covered with a double-layered thin membranous sac called the pleura. The inner layer (visceral pleura) adheres tightly to the lung surface, while the outer layer (parietal pleura) lines the thoracic cavity wall. The space between these layers is the pleural cavity, filled with a thin film of fluid that reduces friction during breathing movements.
Diaphragm Function: The diaphragm is a dome-shaped skeletal muscle sheet separating the thoracic cavity from the abdominal cavity — its contraction and relaxation drive the pressure changes that move air in and out of the lungs
Pleural Fluid: The thin fluid between pleural layers lubricates lung movement and creates surface tension that helps keep the lungs inflated against the chest wall
Spongy Texture: The presence of approximately 300 million alveoli gives the lungs their light, spongy appearance while providing an enormous surface area for gas exchange
Each alveolar sac consists of several microscopic single-layered structures called alveoli. Overlying each alveolus is a rich network of blood capillaries, creating an excellent site for gas exchange. The wall of each alveolus is extremely thin — just one cell thick (simple squamous epithelium) — and the capillary walls are equally thin. This means the total barrier between air in the alveolus and blood in the capillary is only about two cells thick. The enormous number of alveoli provides a combined surface area of roughly 70 square metres for gas exchange.
Single-Cell Wall: Alveolar walls are composed of simple squamous epithelium — a single layer of flat cells that minimizes the diffusion distance
Capillary Network: Dense capillary beds surround each alveolus, ensuring that a large volume of blood is always exposed to the alveolar air
Functional Unit: The alveolus is the functional unit of the lung — it is here that oxygen enters the blood and carbon dioxide leaves it
Mechanism of Gas Exchange in Alveoli
The exchange of gases between alveolar air and blood occurs by simple diffusion, driven by differences in partial pressure. Partial pressure is the individual pressure that each gas in a mixture exerts — proportional to its concentration. In the alveoli, the partial pressure of oxygen (PO₂) is higher than in the deoxygenated blood arriving from the tissues, so oxygen diffuses from the alveolar air into the blood. Conversely, the partial pressure of carbon dioxide (PCO₂) is higher in the blood arriving from tissues than in the alveolar air, so carbon dioxide diffuses from the blood into the alveoli.
$$P_{\text{gas}} = \text{Fraction of gas} \times P_{\text{total}}$$
The partial pressure of any gas in a mixture equals its fractional concentration multiplied by the total pressure of the gas mixture
$P_{\text{gas}}$=Partial pressure of the individual gas(mmHg)
$P_{\text{total}}$=Total pressure of the gas mixture (atmospheric pressure = 760 mmHg at sea level)(mmHg)
Oxygen Gradient: PO₂ in alveoli (~100 mmHg) is higher than in deoxygenated blood (~40 mmHg), driving oxygen into the blood
Carbon Dioxide Gradient: PCO₂ in deoxygenated blood (~45 mmHg) is higher than in alveolar air (~40 mmHg), driving CO₂ out of the blood
Diffusion Rate: The rate of gas exchange depends on the partial pressure difference, the surface area available, and the thickness of the diffusion barrier
Three structural features make the alveoli highly efficient for gas exchange. First, blood is distributed in extremely thin layers within the dense capillary network surrounding each alveolus, exposing a large volume of blood to the alveolar surface. Second, the blood in the lung capillaries is separated from the alveolar air by extremely thin membranes — the capillary wall and the alveolar wall together measure only about 0.5 micrometres. Third, the enormous total surface area of all alveoli combined (approximately 70 m²) provides a vast interface for diffusion.
Thin Capillary Layers: Blood flows through capillaries that are only one red blood cell wide, ensuring every cell is close to the alveolar air
Minimal Barrier Thickness: The combined alveolar-capillary membrane is just two cells thick, offering minimal resistance to diffusion
Large Surface Area: Approximately 300 million alveoli provide a combined gas exchange surface of about 70 m² — roughly the area of a tennis court
The composition of air changes as it passes through the respiratory system. Inhaled air is approximately 21% oxygen and 0.04% carbon dioxide. Exhaled air has a reduced oxygen content of about 16% and an increased carbon dioxide content of about 4%. Nitrogen remains unchanged at 79%. Water vapour, which is variable in inhaled air, becomes saturated in exhaled air as moisture is picked up from the respiratory surfaces.
Oxygen Decrease: About 5% of the inhaled oxygen is absorbed into the blood (21% → 16%)
CO₂ Increase: Carbon dioxide rises from 0.04% to 4% as metabolic waste gas is released from blood into exhaled air
Nitrogen Unchanged: Nitrogen is biologically inert and passes through unchanged at 79%
Composition of Inhaled vs Exhaled Air
•
Oxygen: Inhaled 21% → Exhaled 16%
•
Carbon dioxide: Inhaled 0.04% → Exhaled 4%
•
Nitrogen: Inhaled 79% → Exhaled 79%
•
Water vapour: Inhaled variable → Exhaled saturated
Transport of Respiratory Gases in Blood
Oxygen is transported in the blood primarily bound to haemoglobin, the respiratory pigment contained within red blood corpuscles. Haemoglobin combines reversibly with oxygen to form bright red oxyhaemoglobin. This reaction is reversible — in the lungs where oxygen partial pressure is high (about 100 mmHg), haemoglobin binds oxygen. In the tissues where oxygen partial pressure is low (about 40 mmHg or less), oxyhaemoglobin dissociates, releasing oxygen. A small proportion of oxygen (about 1.5%) also dissolves directly in blood plasma. The enzyme carbonic anhydrase in red blood cells facilitates these gas exchange reactions.
$$Hb + O_2 \rightleftharpoons HbO_2$$
Haemoglobin binds oxygen in the lungs (high PO₂) and releases it in tissues (low PO₂) in a reversible reaction
$Hb$=Deoxygenated haemoglobin (purple-red)(—)
$O_2$=Molecular oxygen(—)
$HbO_2$=Oxyhaemoglobin (bright red, oxygenated form)(—)
$PO_2 \approx 100 \text{ mmHg}$ (in alveoli)
→Haemoglobin is ~98% saturated, carrying about 19.6 ml O₂ per 100 ml blood
$PO_2 < 60 \text{ mmHg}$ (in tissues)
→Oxygen saturation decreases sharply, releasing large quantities of oxygen to tissues
Maximum Oxygen Capacity: Fully oxygenated blood can carry about 20 ml of oxygen per 100 ml of blood at sea level
Normal Oxygenation: Under normal conditions, alveolar blood is about 98% saturated at an oxygen tension of 115 mmHg, containing 19.6 ml O₂ per 100 ml blood
Sharp Dissociation Below 60 mmHg: When oxygen pressure falls below 60 mmHg in tissues, oxyhaemoglobin dissociates rapidly, releasing oxygen where it is needed most
Plasma Oxygen: About 1.5% of oxygen is carried dissolved in plasma — the majority is bound to haemoglobin
Three important factors affect the capacity of haemoglobin to bind oxygen. First, increased carbon dioxide pressure decreases the oxygen-carrying capacity — this is known as the Bohr effect. When tissues produce more CO₂, the increased CO₂ tension causes haemoglobin to release more oxygen precisely where it is needed. Second, a rise in temperature reduces oxygen binding, which is significant during increased muscular activity when muscles generate heat. Third, a decrease in blood pH (increase in H⁺ ions) reduces oxygen binding because hydrogen ions combine with the protein part of haemoglobin, weakening its ability to hold oxygen.
Carbon Dioxide Pressure (Bohr Effect): Higher PCO₂ shifts the oxygen-haemoglobin dissociation curve to the right — haemoglobin releases oxygen more readily in tissues with high metabolic activity
Temperature: Increased body temperature (as during exercise) reduces haemoglobin's oxygen affinity, promoting oxygen release to active tissues
pH: Lower pH (more acidic blood) decreases oxygen binding — H⁺ ions attach to haemoglobin's protein chains, reducing its ability to carry O₂; higher pH has the opposite effect
Carbon dioxide is transported in the blood in four different ways, with bicarbonate ions being the dominant form. Carbon dioxide is more soluble than oxygen and dissolves freely in tissue fluid. From tissue fluid, dissolved CO₂ passes into blood plasma. About 70% of CO₂ is carried as bicarbonate ions (HCO₃⁻) combined with sodium in the plasma. When CO₂ enters the capillaries from tissues, it reacts with water (catalysed by carbonic anhydrase) to form carbonic acid, which quickly ionizes into hydrogen ions and bicarbonate ions. About 20% of CO₂ is carried as carboxyhaemoglobin (combined with amino groups of haemoglobin), about 5% is carried by other plasma proteins, and a small amount combines with potassium in red blood cells.
$$CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$$
Carbon dioxide enters blood, reacts with water to form carbonic acid, which dissociates into hydrogen and bicarbonate ions — the major CO₂ transport mechanism
$CO_2$=Carbon dioxide from tissue respiration(—)
$H_2CO_3$=Carbonic acid (unstable, quickly dissociates)(—)
$HCO_3^-$=Bicarbonate ion — the main form of CO₂ transport in blood(—)
$H^+$=Hydrogen ion — combines with haemoglobin, contributing to the Bohr effect(—)
In tissues (high CO₂)
→Reaction proceeds forward: CO₂ → H₂CO₃ → H⁺ + HCO₃⁻ (carbonic anhydrase in RBCs catalyses this)
In lungs (low CO₂)
→Reaction reverses: HCO₃⁻ + H⁺ → H₂CO₃ → CO₂ + H₂O (CO₂ diffuses out into alveolar air)
Bicarbonate Ion (70%): The dominant CO₂ transport form — bicarbonate ions combine with sodium in the plasma and are carried to the lungs where the reaction reverses
Carboxyhaemoglobin (20%): CO₂ binds to amino groups of haemoglobin — this is different from oxyhaemoglobin where O₂ binds to the iron in haem groups
Plasma Protein Transport (5%): Other plasma proteins carry a small fraction of CO₂ from tissues to lung capillaries
Potassium Combination: A small amount of CO₂ is also carried by red blood cells combined with potassium
Forms of CO₂ Transport in Blood
1
Bicarbonate ions (HCO₃⁻) in plasma — approximately 70%
2
Carboxyhaemoglobin (CO₂ bound to haemoglobin amino groups) — approximately 20%
3
Transport by other plasma proteins — approximately 5%
4
Dissolved in plasma and combined with potassium in RBCs — small amount
There is a measurable difference in carbon dioxide concentration between arterial and venous blood. Arterial blood contains about 50 ml of CO₂ per 100 ml of blood, while venous blood contains about 54 ml of CO₂ per 100 ml of blood. This means each 100 ml of blood picks up approximately 4 ml of CO₂ as it passes through the tissues and releases approximately 4 ml of CO₂ as it passes through the lungs.
Arterial CO₂: About 50 ml CO₂ per 100 ml blood — blood that has just passed through the lungs and released CO₂
Venous CO₂: About 54 ml CO₂ per 100 ml blood — blood that has just passed through tissues and picked up CO₂
Net CO₂ Exchange: Only 4 ml of CO₂ is exchanged per 100 ml of blood in each cycle through tissues or lungs
Lung Capacities and Breathing Rate
In an adult human, when the lungs are fully inflated, the total lung capacity is about 5 litres. During normal rest or sleep, only about 0.5 litres of air is exchanged per breath (tidal volume). During exercise, the volume of air moved in and out rises to about 3.5 litres. Even during vigorous exercise, a residual volume of about 1.5 litres remains in the lungs and cannot be expelled — this ensures that some air always remains in the alveoli for continuous gas exchange between breaths.
Total Lung Capacity: Approximately 5 litres when fully inflated in an adult
Tidal Volume (Rest): About 0.5 litres — the volume of air inhaled and exhaled in a normal breath at rest
Exercise Volume: About 3.5 litres — the volume exchanged during physical activity
Residual Volume: About 1.5 litres — air that cannot be expelled even during maximum exhalation, keeping alveoli partially inflated for continuous gas exchange
The normal breathing rate at rest is 15 to 20 breaths per minute. During exercise, the breathing rate can rise to about 30 breaths per minute. This increased rate combined with deeper breathing allows more oxygen to dissolve in the blood and be supplied to active muscles. The extra carbon dioxide produced by active muscles is removed by this faster and deeper breathing.
Resting Rate: 15–20 breaths per minute — adequate for the body's oxygen demand at rest
Exercise Rate: Up to about 30 breaths per minute — matches the increased metabolic demand of active muscles
Dual Response: Both breathing rate (frequency) and depth (tidal volume) increase during exercise to maximise gas exchange
Application Strategy
Worked Examples
Problem Statement
If fully oxygenated blood carries 20 ml of oxygen per 100 ml of blood, and haemoglobin is 98% saturated in the lungs at an oxygen tension of 115 mmHg, how much oxygen is actually carried per 100 ml of blood under these normal alveolar conditions?
oxygen transporthaemoglobin saturation
Strategic Toolkit
$$\text{O}_2 \text{ carried} = \text{Maximum capacity} \times \text{Saturation fraction}$$
Step 1
Identify the maximum oxygen capacity
Fully oxygenated blood at sea level carries a maximum of 20 ml of oxygen per 100 ml of blood.
$$\text{Maximum capacity} = 20 \text{ ml O}_2 \text{/ 100 ml blood}$$
Step 2
Apply the saturation percentage
At 115 mmHg oxygen tension in the alveoli, haemoglobin is 98% saturated.
$$\text{O}_2 \text{ carried} = 20 \times 0.98$$
Step 3
Calculate the result
Multiply to find the actual oxygen content in normally oxygenated arterial blood.
$$\text{O}_2 \text{ carried} = 19.6 \text{ ml O}_2 \text{/ 100 ml blood}$$
Problem Statement
Arterial blood contains 50 ml of CO₂ per 100 ml of blood and venous blood contains 54 ml of CO₂ per 100 ml of blood. How much CO₂ does each 100 ml of blood pick up from the tissues, and how much does it release in the lungs?
carbon dioxide transportarterial vs venous blood
Strategic Toolkit
$$\Delta CO_2 = CO_{2(\text{venous})} - CO_{2(\text{arterial})}$$
Step 1
Calculate CO₂ picked up in tissues
As blood passes through tissues, it gains CO₂. The difference between venous and arterial CO₂ gives the amount picked up.
$$\text{CO}_2 \text{ picked up} = 54 - 50 = 4 \text{ ml/100 ml blood}$$
Step 2
State CO₂ released in lungs
As blood passes through the lungs, it releases the same 4 ml of CO₂ per 100 ml, returning to the arterial level of 50 ml.
$$\text{CO}_2 \text{ released} = 4 \text{ ml/100 ml blood}$$
Step 3
Interpret the result
Each 100 ml of blood picks up and releases approximately 4 ml of CO₂ in each complete circuit through the body. This relatively small difference reflects the efficiency of the bicarbonate transport system.
$$\text{Net CO}_2 \text{ exchange per cycle} = 4 \text{ ml/100 ml blood}$$
Problem Statement
Atmospheric pressure at sea level is 760 mmHg. If oxygen makes up 21% of atmospheric air, what is the partial pressure of oxygen in inhaled air?
partial pressuregas exchange
Strategic Toolkit
$$PO_2 = \text{Fraction of } O_2 \times P_{\text{total}}$$
Step 1
Identify the fraction and total pressure
Oxygen constitutes 21% (0.21 fraction) of atmospheric air, and total atmospheric pressure at sea level is 760 mmHg.
$$\text{Fraction of } O_2 = 0.21, \quad P_{\text{total}} = 760 \text{ mmHg}$$
Step 2
Apply the partial pressure formula
Multiply the fractional concentration of oxygen by the total atmospheric pressure.
$$PO_2 = 0.21 \times 760$$
Step 3
Calculate the result
The partial pressure of oxygen in inhaled air at sea level is approximately 160 mmHg. In the alveoli, after some oxygen has been absorbed, PO₂ is about 100 mmHg.
$$PO_2 \approx 160 \text{ mmHg}$$
Problem Statement
If inhaled air contains 21% oxygen and exhaled air contains 16% oxygen, by what percentage has the oxygen concentration decreased? Express this decrease as a percentage of the original inhaled oxygen.
gas exchange in alveoliinhaled vs exhaled air
Strategic Toolkit
$$\text{Decrease} = \frac{O_{2(\text{inhaled})} - O_{2(\text{exhaled})}}{O_{2(\text{inhaled})}} \times 100$$
Step 1
Identify the oxygen values
Inhaled air has 21% oxygen and exhaled air has 16% oxygen.
$$O_{2(\text{inhaled})} = 21\%, \quad O_{2(\text{exhaled})} = 16\%$$
Step 2
Calculate the percentage decrease
Subtract the exhaled value from the inhaled value and divide by the inhaled value.
$$\text{Decrease} = \frac{21 - 16}{21} \times 100 = \frac{5}{21} \times 100$$
Step 3
Compute the final percentage
The oxygen concentration decreases by approximately 23.8% of its original value as air passes through the respiratory system.
$$\text{Decrease} \approx 23.8\%$$
Problem Statement
At 100 mmHg oxygen pressure in the lungs, haemoglobin is approximately 98% saturated with oxygen, carrying 19.6 ml O₂ per 100 ml blood. If oxygen pressure falls to 40 mmHg in active tissues and saturation drops to 75%, how much oxygen is released per 100 ml of blood?
oxygen transporthaemoglobin dissociation
Strategic Toolkit
$$\text{O}_2 \text{ released} = \text{O}_2 \text{ carried at } PO_2(\text{lungs}) - \text{O}_2 \text{ carried at } PO_2(\text{tissues})$$
Step 1
Determine oxygen in arterial blood
In the lungs at PO₂ = 100 mmHg, haemoglobin is 98% saturated. With a maximum capacity of 20 ml/100 ml, the oxygen carried is 19.6 ml/100 ml.
$$\text{O}_2 \text{ in arterial blood} = 20 \times 0.98 = 19.6 \text{ ml/100 ml}$$
Step 2
Determine oxygen remaining in tissue blood
In tissues at PO₂ = 40 mmHg, haemoglobin is 75% saturated.
$$\text{O}_2 \text{ remaining} = 20 \times 0.75 = 15 \text{ ml/100 ml}$$
Step 3
Calculate oxygen released
Subtract the oxygen remaining in blood at tissue level from the oxygen carried in arterial blood.
$$\text{O}_2 \text{ released} = 19.6 - 15 = 4.6 \text{ ml/100 ml blood}$$
Memorize formulas
Oxyhaemoglobin Formation and Dissociation
Haemoglobin binds oxygen in the lungs (high PO₂) forming oxyhaemoglobin; releases oxygen in tissues (low PO₂). Fully oxygenated blood carries 20 ml O₂ per 100 ml; normally ~19.6 ml at 98% saturation.
Formula
$$Hb + O_2 \rightleftharpoons HbO_2$$
Carbon Dioxide to Carbonic Acid
CO₂ reacts with water (catalysed by carbonic anhydrase in RBCs) to form carbonic acid, which quickly dissociates. This is the first step of the dominant CO₂ transport pathway (70% as bicarbonate).
Formula
$$CO_2 + H_2O \rightleftharpoons H_2CO_3$$
Carbonic Acid Dissociation to Bicarbonate
Carbonic acid (H₂CO₃) is unstable and dissociates into H⁺ and HCO₃⁻. The bicarbonate ion is the primary CO₂ transport form. In the lungs, the reverse reaction releases CO₂ for exhalation.
Formula
$$H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$$
Partial Pressure of a Gas
Partial pressure = fractional concentration × total pressure. Used to calculate PO₂ (oxygen partial pressure) and PCO₂ (carbon dioxide partial pressure) that drive diffusion of gases across the alveolar membrane. Total atmospheric pressure = 760 mmHg at sea level.
Formula
$$P_{\text{gas}} = \text{Fraction of gas} \times P_{\text{total}}$$
Review
1
Which structure acts as a flap that covers the glottis during swallowing to prevent food from entering the trachea?
Ans
Epiglottis
Review
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Identify the Structure Guarding the Tracheal Opening
The glottis is the opening of the larynx that leads into the trachea. A protective flap must cover it during swallowing to prevent choking.
The epiglottis covers the glottis during swallowing
Eliminate Incorrect Options
The vocal cords are involved in sound production, not airway protection. The pharynx is a shared passageway, not a protective flap. The larynx is the voice box containing the glottis, but the epiglottis is the specific structure that covers it.
Only the epiglottis serves this protective function
2
Which of the following correctly describes the pathway of air from the nostrils to the bronchi?
Ans
Nostrils → Nasal cavity → Pharynx → Larynx → Trachea → Bronchi
Review
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Trace the Air Passageway Step by Step
Starting from the external nares (nostrils), air enters the nasal cavity — a large chamber lined with mucous membranes and cilia that filter, warm, and humidify the air.
Nostrils → Nasal cavity → ...
Follow Air Through the Pharynx and Larynx
From the nasal cavity, air moves into the pharynx (a shared passage for air and food), then through the larynx (voice box) where the vocal cords are located.
...→ Pharynx → Larynx → ...
Continue to the Trachea and Bronchi
Below the larynx, air enters the trachea, which is reinforced by C-shaped cartilage rings. The trachea divides at the carina into the left and right bronchi, each leading into a lung.
...→ Trachea → Bronchi
3
Which structural feature of alveoli most directly enables efficient gas exchange between air and blood?
Ans
Their walls are one cell thick (simple squamous epithelium) and they provide a combined surface area of about 70 m²
Review
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Recall Alveolar Structure
Alveoli are the terminal sacs of the respiratory tree, surrounded by dense networks of pulmonary capillaries. Their walls are composed of simple squamous epithelium — a single cell layer.
Alveolar walls are one cell thick
Apply Fick's Law of Diffusion
The rate of diffusion is directly proportional to surface area and inversely proportional to membrane thickness. Thin walls (short diffusion distance) and a large total surface area (~70 m²) together maximize the diffusion rate of oxygen and carbon dioxide.
$$\text{Diffusion rate} \propto \frac{\text{Surface Area}}{\text{Thickness}}$$
4
A double membrane that encloses each lung and reduces friction during breathing movements is called the:
Ans
Pleura
Review
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Identify the Correct Anatomical Membrane
Each lung is enclosed in a double-walled sac. The outer wall adheres to the thoracic cavity and the inner wall clings to the lung surface, with a thin fluid-filled space between them.
This membrane is the pleura
Distinguish from Other Body Membranes
The pericardium protects the heart, the peritoneum lines the abdominal cavity and covers abdominal organs, and the endothelium lines blood vessels and the heart chambers.
Only the pleura is associated with the lungs and thoracic cavity
5
Surfactant is secreted by type II alveolar cells. What would happen if surfactant were absent in the lungs?
Ans
Alveoli would collapse during exhalation due to high surface tension of the fluid lining their walls
Review
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Function of Surfactant
Surfactant molecules intersperse among water molecules at the air-water interface inside alveoli, disrupting hydrogen bonding between water molecules and thereby lowering surface tension.
Surfactant reduces surface tension within alveoli
Predict the Consequence of Absence
Without surfactant, water molecules at the alveolar surface would form strong hydrogen bonds with each other, creating high surface tension. This inward-pulling force would cause small alveoli to collapse (atelectasis), especially during exhalation when alveolar radius decreases.
Alveoli would collapse during exhalation
6
Oxygen diffuses from alveolar air into the pulmonary capillary blood because:
Ans
The partial pressure of O₂ is higher in alveolar air (~104 mmHg) than in deoxygenated blood arriving at the lungs (~40 mmHg)
Review
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Compare Partial Pressures of O₂
In alveolar air, the partial pressure of oxygen ($P_{O_2}$) is about 104 mmHg. In deoxygenated blood reaching the pulmonary capillaries, $P_{O_2}$ is about 40 mmHg.
$$P_{O_2} \text{(alveoli)} = 104 \text{ mmHg} \quad > \quad P_{O_2} \text{(venous blood)} = 40 \text{ mmHg}$$
Apply the Principle of Diffusion
Gases diffuse from regions of higher partial pressure to regions of lower partial pressure. The pressure difference ($104 - 40 = 64$ mmHg) provides a strong driving force for oxygen to move into the blood.
Oxygen diffuses from alveolar air → pulmonary capillary blood
7
The combined surface area of all alveoli in both human lungs is approximately:
Ans
70 m²
Review
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Alveolar Count and Individual Size
There are approximately 300 million alveoli in both lungs. Each alveolus has a diameter of about 100–300 μm.
~300 million alveoli total
Calculate Combined Surface Area
The total surface area is the sum of all individual alveolar surfaces. Despite each being microscopic, their massive number yields a combined area of approximately 70 m².
$$A_{\text{total}} \approx 70 \text{ m}^2$$
8
The reversible reaction $Hb + O_2 \rightleftharpoons HbO_2$ represents which process in the body?
Ans
Formation of oxyhaemoglobin in the lungs and its dissociation in the tissues
Review
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Identify the Components in the Equation
Hb is haemoglobin, O₂ is molecular oxygen, and HbO₂ is oxyhaemoglobin. The double arrow indicates a reversible equilibrium.
$$Hb + O_2 \rightleftharpoons HbO_2$$
Determine Where Each Direction Dominates
In the lungs (high $P_{O_2}$), the reaction shifts right: O₂ binds to Hb forming $HbO_2$. In tissues (low $P_{O_2}$), the reaction shifts left: $HbO_2$ releases O₂.
Forward in lungs, reverse in tissues — this is the oxyhaemoglobin dissociation cycle
9
Which enzyme catalyses the rapid conversion of carbon dioxide and water into carbonic acid inside red blood cells?
Ans
Carbonic anhydrase
Review
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Write the Reaction for CO₂ Conversion
Carbon dioxide entering red blood cells reacts with water to form carbonic acid, which then dissociates into hydrogen ions and bicarbonate.
$$CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$$
Identify the Enzyme
The enzyme carbonic anhydrase, present in high concentrations in red blood cells, catalyses the first step of this reaction, increasing its rate enormously to match the body's CO₂ transport demands.
Carbonic anhydrase catalyses this conversion
10
Approximately what percentage of carbon dioxide transported in the blood is carried as bicarbonate ions ($HCO_3^-$)?
Ans
About 70%, produced inside red blood cells via the carbonic anhydrase reaction
Review
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Break Down CO₂ Transport Mechanisms
Carbon dioxide is transported in blood by three mechanisms: (1) dissolved in plasma (~7%), (2) bound to haemoglobin as carbamino compounds (~23%), and (3) as bicarbonate ions (~70%).
Bicarbonate transport is the dominant mechanism (~70%)
Trace the Bicarbonate Formation Pathway
CO₂ diffuses into red blood cells, where carbonic anhydrase converts it to $H_2CO_3$, which dissociates into $H^+$ and $HCO_3^-$. The $HCO_3^-$ exits into plasma, and $H^+$ is buffered by haemoglobin.
$$CO_2 \xrightarrow{\text{carbonic anhydrase}} H_2CO_3 \rightarrow H^+ + HCO_3^-$$
11
Carboxyhaemoglobin is formed when haemoglobin binds to which gas, and why is this binding dangerous?
Ans
Carbon monoxide — it binds to haemoglobin with an affinity approximately 200–250 times greater than oxygen, severely reducing O₂ delivery
Review
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Identify the Gas That Forms Carboxyhaemoglobin
Carboxyhaemoglobin is specifically the compound formed when carbon monoxide binds to the iron (Fe²⁺) in haemoglobin's haem groups.
$$Hb + CO \rightarrow HbCO \text{ (carboxyhaemoglobin)}$$
Explain the Danger — Relative Affinity
The affinity of CO for haemoglobin is approximately 200–250 times that of oxygen. This means that at equal partial pressures, CO will occupy far more binding sites than O₂ would, blocking oxygen transport and causing oxygen starvation in tissues.
CO's high affinity severely limits O₂ transport, causing tissue hypoxia
12
Atmospheric air contains 21% oxygen. If the total atmospheric pressure is 760 mmHg, what is the partial pressure of oxygen in inspired air?
Ans
159.6 mmHg
Review
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State Dalton's Law of Partial Pressures
The partial pressure of a gas in a mixture equals the fraction of that gas multiplied by the total pressure of the mixture.
$$P_{\text{gas}} = \text{Fraction of gas} \times P_{\text{total}}$$
Substitute Values and Calculate
Fraction of O₂ = 0.21, Total pressure = 760 mmHg.
$$P_{O_2} = 0.21 \times 760 = 159.6 \text{ mmHg}$$
13
During forceful inhalation, which combination of muscle actions increases the volume of the thoracic cavity?
Ans
Diaphragm contracts and flattens downward, while external intercostal muscles lift the ribs upward and outward
Review
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Identify Muscles Active During Inhalation
The diaphragm is the primary muscle of inspiration. The external intercostal muscles are the secondary muscles that assist by expanding the rib cage.
Diaphragm + external intercostal muscles contract during inhalation
Determine the Effect on Thoracic Volume
Diaphragm contraction pulls it downward (increases vertical dimension). External intercostal contraction lifts ribs up and out (increases AP and lateral dimensions). Both actions increase thoracic volume, lowering intrapulmonary pressure and drawing air in.
Increased thoracic volume → decreased pressure → air flows in
14
The normal resting breathing rate in a healthy adult human is approximately:
Ans
12–20 breaths per minute
Review
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Recall Normal Respiratory Parameters
At rest, a healthy adult takes 12–20 breaths per minute. Each breath moves approximately 500 mL of air (tidal volume).
Normal breathing rate = 12–20 breaths/min
Calculate Minute Ventilation
Minute ventilation = breathing rate × tidal volume = (12–20) × 500 mL = 6,000–10,000 mL/min (6–10 L/min).
$$\text{Minute ventilation} \approx 6\text{–}10 \text{ L/min}$$
15
Compared to inhaled air, exhaled air contains significantly more carbon dioxide and significantly less oxygen. What are the approximate percentages of O₂ and CO₂ in exhaled air?
Ans
O₂: 16%, CO₂: 4%
Review
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State Inhaled Air Composition
Inhaled (atmospheric) air contains approximately 21% O₂ and 0.04% CO₂.
Inhaled: O₂ ≈ 21%, CO₂ ≈ 0.04%
Determine Exhaled Air Composition
During gas exchange, oxygen diffuses into the blood and carbon dioxide diffuses out. Exhaled air contains approximately 16% O₂ and 4% CO₂ — the change represents the net gas exchange across the alveolar membrane.
Exhaled: O₂ ≈ 16%, CO₂ ≈ 4%
Question 1 of 15
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1
Which structure acts as a flap that covers the glottis during swallowing to prevent food from entering the trachea?