Anatomy & Physiology · Chapter 22

The Respiratory System

Every cell in your body burns oxygen to make energy. Every one of them also makes carbon dioxide as waste. So 12 to 16 times a minute, your body trades one gas for the other. You never even have to think about it. This chapter follows a breath from the nostril to the tiny air sac where the trade happens. Then it explains the physics that pulls air in. It shows how the gases swap places and how clever chemistry carries them through the blood. Last, it covers the air volumes doctors measure to tell a healthy lung from a sick one. By the end, you will know not just how you breathe but why this whole elegant machine works the way it does.

By the end of this chapter you can

Trace the path of air through the conducting and respiratory zones. Name each part it passes.
Describe how the airway warms, humidifies, and filters the air you breathe in. Explain the mucociliary escalator.
Draw the respiratory membrane. Explain why being thin and having a huge surface make it ideal for gas exchange.
Explain the job of surfactant. Predict what happens when it is missing.
Use Boyle’s law to explain breathing in and breathing out. Name the muscles and pressures involved in each.
Apply partial-pressure gradients to external and internal respiration. Say which way each gas moves and why.
Describe how oxygen and carbon dioxide are carried in the blood. Include hemoglobin and the bicarbonate system.
Define the lung volumes and capacities. Explain how spirometry tells obstructive disease from restrictive disease.

Section 1The respiratory tract

**Reference.** The organs of the respiratory tract, nose to alveoli — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Two zones: a highway and a destination

The respiratory system has one main job. It moves oxygen in and carbon dioxide out. But it does more, too. It warms and moistens the air and filters out dirt. It gives you your sense of smell and makes your voice. It also helps control the pH of your blood, meaning how acidic it is.

To do all this, the airway is split into two working zones. The conducting zone is the plumbing. It is made of the nose, pharynx, larynx, trachea, bronchi, and bronchioles. These tubes carry air, but they never trade gases. The respiratory zone is where oxygen is actually traded for carbon dioxide. It is made of the respiratory bronchioles, alveolar ducts, and alveoli. Think of the conducting zone as the highway and the respiratory zone as the destination. Air only does its real work once it arrives.

Doctors also divide the tract by location. The upper respiratory tract is everything above the larynx. That means the nose, nasal cavity, sinuses, and pharynx. The lower respiratory tract is the larynx and everything below it. That means the trachea, bronchi, and lungs. This is why a "cold" feels so different from bronchitis or pneumonia. A cold is an upper-tract infection. The other two are lower-tract problems.

The journey of a breath

Follow one breath in. Air enters the nostrils and swirls through the nasal cavity. It passes into the pharynx, or throat, which has three parts. Next it slips through the larynx, the voice box. A flap called the epiglottis guards the larynx. It flips down when you swallow, so food goes to the esophagus and not the lungs.

Then air enters the trachea, or windpipe. C-shaped rings of cartilage hold it open at all times. The trachea splits into the right and left primary bronchi. These branch again and again, like an upside-down tree. They form smaller and smaller bronchi and then bronchioles. At last the tubes end in clusters of alveoli, the tiny air sacs. The whole branching pattern is called the bronchial tree.

Structure	Zone	Key feature
Nasal cavity	Conducting	Warms, moistens, and filters air; lined with mucous membrane
Pharynx	Conducting	Shared path for air and food (naso-, oro-, laryngo-)
Larynx	Conducting	Voice box; the epiglottis guards the airway
Trachea	Conducting	C-shaped cartilage rings keep it open
Bronchi	Conducting	Branch into each lung, then into lobes and segments
Bronchioles	Conducting	No cartilage; smooth muscle sets how wide they are
Alveoli	Respiratory	Thin-walled air sacs where gas exchange happens

The air that arrives is conditioned

By the time air reaches the alveoli, it has changed a lot. It has been warmed to body heat. It is full of water vapor, and it has been scrubbed almost clean. The nasal cavity does most of this work. Its rich blood supply gives off heat. Its mucous lining traps dust and germs.

Under that mucus sits a carpet of tiny hairs called cilia. They beat in waves toward the throat. This is the mucociliary escalator. Like a moving stairway, it sweeps trapped dirt up and out. Then you swallow it or cough it away. This tireless cleaning system protects the delicate air sacs from the outside world.

Smoke stalls the escalator. Cigarette smoke stuns the cilia and in time destroys them. Then the mucociliary escalator stops working. Tar and germs stay stuck in the airway instead of being cleared out. That is why smokers get a lasting "morning cough." The lungs have to cough, because their built-in escalator has stopped running.

Walk the whole tract, one part at a time, in the interactive below.

Interactive Follow a breath from the nose down to the tiny air sacs. Tap each part along the way. Open full ↑

Key points

The conducting zone (nose → bronchioles) moves air. The respiratory zone (respiratory bronchioles → alveoli) trades gases.
Air path: nose → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.
Air coming in is warmed, humidified, and filtered. The mucociliary escalator sweeps dirt up and out.

Section 2The alveolus & respiratory membrane

An alveolus and the respiratory membrane — **Reference.** An alveolus wrapped in a capillary, and the layers of the respiratory membrane — OpenStax, CC BY 3.0 (via Wikimedia Commons)

A surface the size of a room

Your lungs hold about 300 million alveoli. Each is a hollow sac, like a bubble, wrapped in a mesh of capillaries. One alone is tiny. But spread out, all their walls would cover about 70 square meters. That is about the floor of a small apartment, packed inside your chest.

This huge area is the whole point. The more surface you spread out, the more gas can cross it at once. Structure fits function. Here, the function is to move oxygen and carbon dioxide as fast and in as large amounts as possible.

The respiratory membrane: three thin layers

Gases are traded across the respiratory membrane. This is the barrier between the air in the alveoli and the blood in the capillaries. It has an important-sounding name, but it is almost nothing at all. It is about 0.5 micrometers thick, which is thinner than a soap bubble.

It has just three layers. First is the alveolar wall, a single flat layer of type I cells. Last is the capillary wall, a single flat layer of cells that line the vessel. Between them are their fused basement membranes, glued together. Oxygen crosses all three and enters the blood in a split second. Carbon dioxide crosses the other way just as fast.

Cell / layer	What it is	Job
Type I alveolar cell	Simple squamous (flat) cell	Makes up most of the wall; thin enough for gases to cross
Type II alveolar cell	Rounded cell scattered in the wall	Makes and releases surfactant
Alveolar macrophage	Roaming immune cell ("dust cell")	Swallows dirt and germs that reach the sac
Capillary endothelium	Flat cells lining the capillary	The blood side of the respiratory membrane

Why does being thin matter so much? Gases cross much more slowly when the barrier gets thicker. Some diseases thicken this membrane. In pulmonary edema, fluid floods it. In pulmonary fibrosis, scar tissue stiffens it. Now oxygen has farther to travel. The patient gets short of breath, even though the air and the blood are both right there.

Surfactant: keeping the sacs from collapsing

A bubble with wet walls tends to collapse. That is because the water molecules lining it pull toward each other. This pull is called surface tension. If nothing stopped it, the smallest alveoli would snap shut every time you breathed out.

To prevent this, the type II alveolar cells release surfactant. It is a soapy mix of fats and proteins that coats the inside of the sacs. It breaks up the water’s pull and lowers surface tension. This keeps the sacs open and easy to fill.

Why premature babies struggle to breathe. A fetus does not make enough surfactant until late in pregnancy, around 28–34 weeks. If a baby is born too early, its alveoli collapse with each breath. This is infant respiratory distress syndrome. So premature newborns may be given man-made surfactant. And if a baby may be born early, the mother may get steroids to help its lungs mature faster.

Zoom into a single sac and its capillary in the interactive below.

Interactive Zoom into one air sac and the capillary hugging it. Tap each part of the paper-thin respiratory membrane. Open full ↑

Key points

About 300 million alveoli give about 70 m² of surface for gas exchange.
The respiratory membrane is about 0.5 µm thick. It has three layers: the alveolar wall, the fused basement membranes, and the capillary wall.
Type II cells make surfactant. It lowers surface tension and keeps alveoli from collapsing.

Section 3Breathing mechanics

**Reference.** How the diaphragm changes chest volume and pressure to move air — OpenStax, CC BY 4.0 (via Wikimedia Commons)

You make room, and air follows (Boyle’s law)

Here is a surprise. You do not actively suck air into your lungs. You make your chest bigger, and air rushes in on its own to fill the extra space.

The physics behind this is Boyle’s law. At a steady temperature, a gas’s pressure goes up when its volume goes down. Its pressure goes down when its volume goes up. Squeeze a container, and the pressure inside climbs. Make it bigger, and the pressure drops. Air always flows from higher pressure to lower pressure. So your muscles only have to change the size of your chest. The pressure difference moves the air for you.

Inspiration: an active pull

Breathing in, or inspiration, is active. It takes muscle work. The diaphragm is a dome of muscle under the lungs. It tightens and flattens downward. At the same time, the external intercostal muscles between the ribs lift the ribs up and out. Both actions enlarge the thoracic cavity, the space inside your chest.

By Boyle’s law, this bigger space lowers the pressure inside the lungs. This is called the intrapulmonary pressure. It drops just below the pressure of the air outside. So air flows in until the two pressures are equal.

The lungs themselves have no muscle. They follow the chest wall. A thin film of fluid holds them to it. So does the intrapleural pressure, which always stays negative. When the chest gets bigger, the lungs get bigger with it.

Expiration: mostly letting go

Breathing out, or expiration, is the opposite. At rest it is easier, because it is usually passive. The diaphragm and external intercostals just relax. The stretched lungs and chest wall recoil, or spring back, like a rubber band you let go. The chest gets smaller. The pressure inside the lungs rises above the pressure outside, so air flows out.

A quiet breath out needs no muscle work at all. Sometimes you need to force air out, like when you blow out candles, cough, or exercise hard. Only then do the internal intercostals and abdominal muscles join in to squeeze the chest.

	Inspiration	Expiration (quiet)
Effort	Active (muscles tighten)	Passive (muscles relax)
Main muscles	Diaphragm, external intercostals	None; the lungs spring back on their own
Thoracic volume	Goes up	Goes down
Intrapulmonary pressure	Falls below the air outside	Rises above the air outside
Air movement	In	Out

A punctured seal: pneumothorax. The negative intrapleural pressure keeps the lung stuck to the chest wall. Sometimes air gets into the pleural space, from a wound or a burst air sac. Then that seal breaks. The pressures even out, and the lung collapses inward. This is a pneumothorax. It is a clear reminder that the lung has no muscle of its own. It fills only because it is held tight against a moving chest wall.

Watch the diaphragm change the chest’s volume and pressure in the interactive below.

Interactive The diaphragm changes the chest’s volume. That changes its pressure (Boyle’s law), and air follows. Watch it breathe. Open full ↑

Key points

Boyle’s law: making the chest bigger lowers lung pressure, so air flows in. Making it smaller raises pressure, so air flows out.
Inspiration is active (diaphragm + external intercostals). Quiet expiration is passive (the lungs spring back).
A negative intrapleural pressure keeps the lungs stuck to the chest wall. Break the seal, and the lung collapses (pneumothorax).

Section 4Gas exchange

Partial pressures that drive gas exchange — **Reference.** Partial-pressure gradients driving external and internal respiration — NIH NIAID, CC BY 4.0 (via Wikimedia Commons)

Partial pressure: each gas pushes on its own

Air is a mix of gases. In a mix, each gas adds its own share to the total pressure. That share is its partial pressure. Each gas acts as if the others were not there.

Oxygen makes up about 21% of air. So at sea level, its partial pressure is about 160 mmHg in the open air. We write this as P_O₂. Inside the alveoli, the oxygen is mixed with other gases, so its P_O₂ falls to about 104 mmHg.

This matters because of how gases spread, or diffuse. A gas moves from where its partial pressure is high to where it is low. It keeps moving until the two sides even out. Every gas exchange in the body is this one rule, used over and over.

External respiration: loading O₂, dumping CO₂

External respiration is the swap that happens in the lungs. It takes place between the air in the alveoli and the blood. Blood coming back from the body is low in oxygen, with a P_O₂ of about 40 mmHg. It is high in carbon dioxide, with a P_CO₂ of about 45 mmHg. The air in the alveoli is the reverse. It is high in oxygen (about 104 mmHg) and low in carbon dioxide (about 40 mmHg).

So oxygen moves down its gradient into the blood. Carbon dioxide moves the other way, out into the alveoli, and you breathe it out. Blood leaves the lungs fully loaded with oxygen. It has given up only a small part of its carbon dioxide. That is just enough to bring P_CO₂ down from about 45 to about 40 mmHg. Most of the blood’s carbon dioxide stays with it, stored in a large bicarbonate pool. You will meet this pool in Section 5. Each trip through the lungs trims the pool. It does not empty it.

Internal respiration: the reverse at the tissues

Internal respiration is the same physics run backward, out in the body’s tissues. Cells always burn oxygen and give off carbon dioxide. So in the tissues, P_O₂ is low, about 40 mmHg. P_CO₂ is high, about 45 mmHg.

The blood arriving in the tissue capillaries is the opposite. So oxygen moves out of the blood into the cells. Carbon dioxide moves into the blood to be carried back to the lungs. It is the same rule in the opposite direction. The cycle repeats with every heartbeat.

Gas	Alveoli (air)	Deoxygenated blood	Tissues
P_O₂	~104 mmHg	~40 mmHg	~40 mmHg
P_CO₂	~40 mmHg	~45 mmHg	~45 mmHg
Net movement	O₂ in, CO₂ out (external)	—	O₂ out, CO₂ in (internal)

Small gradient, big mover. Look at carbon dioxide’s partial-pressure gradient: 45 vs 40 mmHg. It is much smaller than oxygen’s gradient of 104 vs 40. Yet CO₂ still keeps up. That is because it dissolves about 20 times more easily than O₂, so it crosses the membrane easily. This shows that solubility, not just the gradient, sets how fast a gas moves across.

Switch between the lungs and the tissues in the interactive below.

Interactive Gases move from high to low partial pressure. They go one way in the lungs and the opposite way in the tissues. Switch between them. Open full ↑

Key points

Each gas moves from high to low partial pressure. It stops when the pressures are equal.
External respiration (lungs): O₂ goes into the blood, and CO₂ goes out to be breathed out.
Internal respiration (tissues): O₂ goes out to the cells, and CO₂ goes into the blood. It is the same rule in reverse.

Section 5Gas transport in the blood

How oxygen and carbon dioxide travel in the blood — **Reference.** How oxygen rides on hemoglobin and carbon dioxide travels three ways — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Oxygen rides on hemoglobin

Oxygen barely dissolves in water. So blood plasma alone could never carry enough to keep you alive. Only about 1.5% of the oxygen in blood is dissolved. The other 98.5% is bound to hemoglobin. This is an iron-containing protein packed inside red blood cells.

Each hemoglobin molecule can hold up to four oxygen molecules on its iron sites. Hemoglobin loaded with oxygen is called oxyhemoglobin. This is the whole reason red blood cells exist. Hemoglobin lets blood carry about seventy times more oxygen than it could without it.

The oxygen–hemoglobin dissociation curve

Hemoglobin has a wonderful talent. It grabs oxygen tightly where oxygen is plentiful, in the lungs. It lets go of oxygen where oxygen is scarce, in the tissues. You can graph how full of oxygen hemoglobin is against the P_O₂ around it. The result is an S-shaped line called the oxygen–hemoglobin dissociation curve.

Even better, hemoglobin responds to what the tissue needs. A hard-working tissue is warmer and more acidic. It also has more carbon dioxide. There, hemoglobin loosens its grip. It hands off more oxygen right where it is needed most. The shift caused by low pH and high CO₂ is the Bohr effect. Warmth shifts the curve the same way, but through a separate path. This is homeostasis, or keeping things in balance, on a tiny scale. The busiest cells automatically get the most oxygen.

Carbon dioxide travels three ways

Carbon dioxide takes three different rides back to the lungs. A small share, about 7%, just dissolves in plasma. About 23% binds to hemoglobin as carbaminohemoglobin. It sits on a different spot than oxygen does.

Most of it, about 70%, is changed into bicarbonate inside red blood cells. The enzyme carbonic anhydrase joins CO₂ with water to make carbonic acid. The carbonic acid splits into a hydrogen ion and a bicarbonate ion (HCO₃⁻). The bicarbonate slips out into the plasma. At the lungs, the whole reaction runs in reverse. This frees CO₂ so you can breathe it out.

Gas	Form in blood	Share
Oxygen	Bound to hemoglobin (oxyhemoglobin)	~98.5%
Oxygen	Dissolved in plasma	~1.5%
Carbon dioxide	As bicarbonate (HCO₃⁻)	~70%
Carbon dioxide	Bound to hemoglobin (carbaminohemoglobin)	~23%
Carbon dioxide	Dissolved in plasma	~7%

Breathing is also pH control. Most CO₂ rides as bicarbonate, and making bicarbonate frees a hydrogen ion. So how fast you breathe quietly steers your blood pH. Breathe faster, and you blow off CO₂. That pulls the reaction backward and raises pH. Hold CO₂ in, and pH falls. This is why a person breathing too fast from anxiety can make their blood too basic, or alkaline. The breathing system and the blood’s buffer system are two sides of one coin.

Switch between oxygen and carbon dioxide transport in the interactive below.

Interactive Once gases cross into the blood, they must be carried. Oxygen and carbon dioxide travel very differently. Switch between them. Open full ↑

Key points

Oxygen is carried mostly on hemoglobin (about 98.5%). Each molecule binds four O₂.
In the Bohr effect, low pH plus high CO₂ make hemoglobin release more oxygen in hard-working tissues. The warmth of those tissues shifts the curve the same way, by a separate path.
Carbon dioxide travels as bicarbonate (about 70%), on hemoglobin (about 23%), and dissolved (about 7%). This links breathing to blood pH.

Section 6Lung volumes & capacities

A spirometry trace of lung volumes and capacities — **Reference.** A spirometry trace showing the lung volumes and capacities — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Four volumes

A spirometer is a device that records how much air you move as you breathe. Its tracing is split into four basic volumes. Tidal volume (TV) is the small amount you move in a normal, quiet breath. It is about 500 mL.

Inspiratory reserve volume (IRV, about 3000 mL) is the extra air you can pull in with your deepest breath. Expiratory reserve volume (ERV, about 1100 mL) is the extra you can force out after a normal breath out. Last, residual volume (RV, about 1200 mL) is the air that always stays behind. You can never fully empty your lungs. That leftover air keeps the alveoli from collapsing between breaths.

Four capacities (volumes added together)

A capacity is just two or more volumes added together. Each one answers a useful question. Vital capacity (VC) is the total you can move from your deepest breath in to your hardest breath out. It equals TV + IRV + ERV, about 4600 mL. It is the number doctors care about most.

Total lung capacity (TLC) adds the residual volume back in. It is everything the lungs can hold, about 5800 mL. Inspiratory capacity (IC = TV + IRV) is all you can breathe in from rest. Functional residual capacity (FRC = ERV + RV) is what is left after a normal breath out.

Measure	Definition	Typical value
Tidal volume (TV)	Air in one normal, quiet breath	~500 mL
Inspiratory reserve (IRV)	Extra air you can force in	~3000 mL
Expiratory reserve (ERV)	Extra air you can force out	~1100 mL
Residual volume (RV)	Air that cannot be breathed out	~1200 mL
Vital capacity (VC)	TV + IRV + ERV	~4600 mL
Total lung capacity (TLC)	VC + RV (everything)	~5800 mL

Dead space & what spirometry reveals

Not all the air you breathe in reaches the alveoli. About 150 mL stays in the conducting airways, where no gas exchange happens. This is the anatomical dead space. So of a 500 mL breath, only about 350 mL is fresh air that does real work.

Spirometry becomes truly powerful when it is timed. The FEV₁/FVC ratio tells how much of your vital capacity you can blow out in the first second. This ratio separates the two big families of lung disease. In obstructive disease, like asthma and COPD, the airways are narrow. Air comes out slowly, so the ratio drops. In restrictive disease, like fibrosis, the lungs are stiff and small. The volumes shrink, but the air that is there comes out quickly. So the ratio stays normal or high.

You can never fully deflate. The residual volume never leaves the lungs. A simple spirometer only measures air that moves in and out. So it cannot measure RV, FRC, or TLC directly. Those need special methods. This is also why a lung still floats in water, even after your hardest breath out. There is always air trapped inside.

Trace each volume on a spirometer in the interactive below.

Interactive A spirometer traces how much air you move. Quiet breathing uses only a sliver of your lungs’ capacity. Tap each volume to see it. Open full ↑

Key points

There are four volumes: tidal, inspiratory reserve, expiratory reserve, and residual. Capacities are volumes added together (VC, TLC, IC, FRC).
Residual volume always stays behind and keeps alveoli open. A simple spirometer cannot measure it.
A low FEV₁/FVC ratio points to obstructive disease. Small volumes with a normal ratio point to restrictive disease.

The chapter in one breath

The airway has a conducting zone that moves and conditions air. It also has a respiratory zone where gases are actually traded. Air travels nose → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.
Gases are traded across the paper-thin respiratory membrane of about 300 million alveoli. Surfactant from type II cells keeps the sacs from collapsing.
Breathing follows Boyle’s law. Breathing in is active: the diaphragm and external intercostals make the chest bigger. A quiet breath out is passive: the lungs spring back.
Gases move from high to low partial pressure. O₂ goes in and CO₂ goes out at the lungs (external respiration). The reverse happens at the tissues (internal respiration).
Oxygen is carried mostly on hemoglobin. Carbon dioxide travels mostly as bicarbonate, which ties breathing to blood pH.
The lung’s volumes and capacities measure how much air you move. A low FEV₁/FVC ratio points to obstructive disease. Small volumes point to restrictive disease.

ReferenceWords worth knowing

Here is a quick list of the terms in this chapter. Skim it now, or come back whenever a word trips you up.

Alveolus: A tiny, thin-walled air sac (plural: alveoli). Here oxygen and carbon dioxide are traded with the blood.
Bicarbonate (HCO₃⁻): The ion that most carbon dioxide is turned into so it can travel in the blood. It also plays a key part in the body’s acid–base balance.
Bohr effect: Hemoglobin holds oxygen less tightly when H⁺ rises (lower pH) and carbon dioxide rises. So it releases more oxygen in active tissue. Warmth shifts the curve the same way, but by a separate path, not by the Bohr effect.
Boyle’s law: At a steady temperature, a gas’s pressure goes up as its volume goes down, and the reverse. It explains how the chest moves air.
Bronchiole: A small airway of the conducting zone with no cartilage. Smooth muscle sets how wide it is.
Conducting zone: The airways, from the nose through the bronchioles. They carry, warm, moisten, and filter air, but they do not trade gases.
Dead space (anatomical): The roughly 150 mL of air breathed in that stays in the conducting airways. It never reaches the alveoli for exchange.
Diaphragm: The dome-shaped muscle under the lungs. When it tightens, it is the main force behind breathing in.
Expiration: Breathing out. It is passive at rest, as the lungs spring back, and active when forced.
External respiration: Gas exchange in the lungs, between the air in the alveoli and the blood. Oxygen goes in, and carbon dioxide comes out.
Hemoglobin: The iron-containing protein in red blood cells. It binds and carries up to four oxygen molecules.
Inspiration: Breathing in. It is an active process in which the diaphragm and external intercostals make the chest bigger.
Internal respiration: Gas exchange in the tissues, between blood and cells. Oxygen goes out to the cells, and carbon dioxide goes into the blood.
Intrapleural pressure: The pressure in the fluid-filled space between the lung and the chest wall. It stays negative, so the lung stays filled against the wall.
Partial pressure: The share of the total pressure that comes from one gas in a mix. Gases move from high to low partial pressure.
Residual volume: The air (about 1200 mL) left in the lungs after your hardest breath out. You cannot push it out by choice.
Respiratory membrane: The barrier, about 0.5 µm thick, that gases cross. It is made of the alveolar wall, the fused basement membranes, and the capillary wall.
Spirometry: A test that measures how much air a person breathes and how fast. It is used to diagnose lung disease.
Surfactant: A mix of fats and proteins made by type II alveolar cells. It lowers surface tension and keeps alveoli from collapsing.
Tidal volume: The amount of air (about 500 mL) moved in one normal, quiet breath.
Vital capacity: The most air a person can breathe out after their deepest breath in. It equals the tidal, inspiratory reserve, and expiratory reserve volumes added together.

Check yourself

1. Put these in the order air passes through them: trachea, alveolus, larynx, bronchiole, pharynx. Which one is the first part of the respiratory (not conducting) zone?

Pharynx → larynx → trachea → bronchiole → alveolus. The alveolus is the respiratory-zone part. Everything from the nose down through the bronchioles is the conducting zone, which only moves air. Gas exchange starts in the respiratory zone, at the alveoli.

2. A longtime smoker has a lasting morning cough that brings up mucus. Which airway defense has failed? Why does that cause coughing?

The mucociliary escalator. Smoke stuns and destroys the cilia that normally sweep mucus and trapped dirt up and out. So the mess builds up in the airway, and the lungs must cough to clear it.

3. Name the three layers of the respiratory membrane. Why does pulmonary edema cause shortness of breath?

The alveolar wall, the fused basement membranes, and the capillary wall. Edema adds a layer of fluid and makes the barrier thicker. Oxygen must travel farther and more slowly. Less oxygen reaches the blood, and the patient feels short of breath.

4. What do type II alveolar cells release? What does it do? Why are very premature babies at special risk?

Surfactant, which lowers surface tension so alveoli stay open. A fetus makes little surfactant until late in pregnancy. So a very premature baby’s alveoli tend to collapse with each breath. This is infant respiratory distress syndrome.

5. Use Boyle’s law to explain what happens to chest volume, intrapulmonary pressure, and airflow when you breathe in.

Volume goes up, pressure falls below the air outside, and air flows in. The diaphragm and external intercostals make the chest bigger. By Boyle’s law, the bigger space lowers the pressure inside the lungs. Air moves from higher pressure (outside) to lower pressure (inside).

6. Which phase of quiet breathing is passive? What supplies the force for it?

A quiet breath out is passive. The breathing-in muscles just relax. The stretched lungs and chest wall spring back, which shrinks the chest and pushes air out. No muscle work is needed unless the breath out is forced.

7. A stab wound lets air into the pleural space, and the lung collapses. What normal pressure was lost? What does this show about how the lung fills?

The negative intrapleural pressure was lost, causing a pneumothorax. That negative pressure holds the lung to the chest wall. The lung has no muscle of its own. It fills only because it is held against a moving chest wall. Breaking the seal lets it spring shut.

8. In the lungs, alveolar P_O₂ is about 104 mmHg, and the incoming blood’s P_O₂ is about 40 mmHg. Which way does oxygen move? What is this process called?

Oxygen moves from the alveoli into the blood (high to low partial pressure). This is external respiration. At the same time, carbon dioxide moves the other way. It leaves the blood and enters the alveoli to be breathed out.

9. Carbon dioxide’s partial-pressure gradient across the respiratory membrane is much smaller than oxygen’s. Yet it keeps up. Why?

Carbon dioxide dissolves about 20 times more easily than oxygen. How fast a gas crosses depends on how well it dissolves, not just on the pressure gradient. So CO₂ crosses quickly, even with only a small 45-vs-40 mmHg difference.

10. How is most oxygen carried in the blood? What does the Bohr effect do in an exercising muscle?

About 98.5% of oxygen is bound to hemoglobin. The Bohr effect makes hemoglobin release more of it in active tissue. Exercising muscle is more acidic and rich in CO₂. This loosens hemoglobin’s grip, so more oxygen goes exactly where it is needed most. The muscle’s warmth adds to this, by a separate path.

11. Name the three ways carbon dioxide travels in the blood, from most to least. Name the enzyme behind the biggest one.

As bicarbonate (about 70%), bound to hemoglobin (about 23%), and dissolved (about 7%). Carbonic anhydrase drives the bicarbonate route. Inside red blood cells, carbonic anhydrase joins CO₂ and water into carbonic acid. This splits into a hydrogen ion and bicarbonate. The reaction runs in reverse at the lungs.

12. Two patients have abnormal spirometry. One has a low FEV₁/FVC ratio. The other has small volumes but a normal ratio. Which pattern is obstructive, and which is restrictive?

The low FEV₁/FVC ratio is obstructive. Small volumes with a normal ratio is restrictive. In obstruction (asthma, COPD), narrow airways slow the airflow, so less air comes out in the first second. In restriction (fibrosis), stiff lungs shrink the volumes. But the air that is there still comes out quickly.

Previous← The Lymphatic & Immune System NextThe Digestive System →