CHAPTER 21

🫁 Big Picture: What Does the Respiratory System Do?

The respiratory system has several major jobs:

• Brings oxygen (O₂) into the body

• Removes carbon dioxide (CO₂), which is a waste product of cellular metabolism

• Helps maintain blood pH

• Allows us to speak by moving air through the vocal cords

• Helps with the sense of smell

• Helps protect the body by filtering particles and pathogens from incoming air

The main idea to remember:

Air comes in → oxygen enters blood → oxygen reaches cells → cells produce CO₂ → CO₂ enters blood → CO₂ leaves through lungs.

The Four Steps of Respiration

You should memorize these four.

Pulmonary ventilation

Movement of air into and out of the lungs.

• Inhalation = air moves into lungs

• Exhalation = air moves out of lungs

External respiration

Gas exchange between:

Alveoli ↔ blood

• O₂ moves from alveoli → blood

• CO₂ moves from blood → alveoli

Gas transport

O₂ and CO₂ are transported through the blood.

• O₂ mostly travels attached to hemoglobin

• CO₂ mostly travels as bicarbonate ions

Internal respiration

Gas exchange between:

Blood ↔ body tissues

• O₂ moves from blood → tissues

• CO₂ moves from tissues → blood

⭐ Memorize this sequence:

Ventilation → External respiration → Gas transport → Internal respiration

Upper vs. Lower Respiratory System

Upper respiratory system

Think: everything mostly above the larynx

Includes:

• Nose

• Nasal cavity

• Paranasal sinuses

• Pharynx

Pharynx sections

The pharynx has three regions:

• Nasopharynx

• Oropharynx

• Laryngopharynx

Lower respiratory system

Includes:

• Larynx

• Trachea

• Bronchi

• Bronchioles

• Alveolar ducts

• Alveoli

• Lungs

Easy way to remember:

Upper = nose → pharynx

Lower = larynx → lungs

Conducting Zone vs. Respiratory Zone

This is VERY important.

Conducting zone

The conducting zone moves air but does NOT perform gas exchange.

It includes:

• Nose

• Nasal cavity

• Pharynx

• Larynx

• Trachea

• Bronchi

• Bronchioles

• Terminal bronchioles

Main jobs:

• Move air

• Filter air

• Warm air

• Humidify air

Think:

Conducting = CONDUCTS air

Respiratory Zone

The respiratory zone is where actual gas exchange happens.

Includes:

• Respiratory bronchioles

• Alveolar ducts

• Alveoli

Main job:

Gas exchange

• O₂ → blood

• CO₂ → alveoli

⭐ Biggest difference:

Nasal Conchae

The nasal conchae are curved structures inside the nasal cavity.

Their main purpose is to increase surface area and create turbulence in incoming air.

This allows the nasal cavity to:

• Warm air

• Humidify air

• Filter air

Why does turbulence help?

It makes the air stay in contact with the nasal cavity longer.

⭐ Remember:

Conchae = turbulence + more surface area

Why Can a Cold Cause an Ear Infection?

This is because of the auditory/Eustachian tube.

The middle ear connects to the nasopharynx through the auditory tube.

During a cold:

• The respiratory tract becomes inflamed

• Mucus can block the auditory tube

• Bacteria or viruses can become trapped

• Pressure and fluid can build up in the middle ear

This can lead to an ear infection.

⭐ Test idea:

Cold → inflammation/mucus → auditory tube blocked → fluid trapped → ear infection

Larynx

The larynx has two major functions:

Protects the airway

It helps prevent food and liquid from entering the trachea.

Produces sound

Air passing through the vocal folds causes them to vibrate.

⭐ Remember:

Larynx = airway protection + voice

Three Large Cartilages of the Larynx

Know these:

Thyroid cartilage

• Largest

• Forms the anterior portion of the larynx

• Creates the Adam's apple

Cricoid cartilage

• Ring-shaped

• Located below the thyroid cartilage

Epiglottis

• Leaf-shaped

• Covers the opening of the larynx during swallowing

⭐ Memorize:

Thyroid → Cricoid → Epiglottis

Epiglottis

The epiglottis is a flap of elastic cartilage.

During swallowing:

Epiglottis folds down → covers the laryngeal opening → food goes toward esophagus

When breathing:

Epiglottis is open → air can enter the larynx/trachea

⭐ Easy:

Epiglottis = prevents food from entering airway

True Vocal Cords

True vocal folds are responsible for sound production.

When you exhale:

Air passes between vocal folds → vocal folds vibrate → sound is produced

The tension of the vocal folds affects pitch.

• More tension → higher pitch

• Less tension → lower pitch

Trachea vs. Esophagus

This is easy to mix up.

Trachea

• Anterior

• Carries air

• Leads toward bronchi

Esophagus

• Posterior

• Carries food

• Leads to stomach

⭐ Memorize:

Trachea = anterior = air

Esophagus = posterior = food

Respiratory Membrane

The respiratory membrane is the extremely thin barrier where gas exchange occurs between alveoli and blood.

It separates:

Air in alveoli ↔ blood in capillary

It is very thin so gases can diffuse quickly.

Main structures involved:

• Alveolar epithelium

• Basement membranes

• Capillary endothelium

Why is it thin?

Because gases need to travel a very short distance.

⭐ Key idea:

Thin membrane = faster gas exchange

Alveolar Cells

There are three important cell types to know.

Type I alveolar cells

Main job:

Gas exchange

They are extremely thin, allowing O₂ and CO₂ to diffuse easily.

Remember:

Type I = Exchange

Type II alveolar cells

Main job:

Produce surfactant

Surfactant prevents alveoli from collapsing.

Remember:

Type II = Surfactant

Alveolar macrophages

Main job:

Defense

They engulf:

• Dust

• Bacteria

• Debris

• Other particles

Remember:

Macrophages = eat debris

Surfactant

Surfactant is produced by Type II alveolar cells.

Its main purpose is to reduce surface tension inside the alveoli.

Without enough surfactant, alveoli are more likely to collapse.

Why?

The inside of the alveoli has moisture. Water molecules naturally pull toward each other, creating surface tension.

Surfactant reduces that attraction.

⭐ Memorize:

Surfactant → decreases surface tension → prevents alveolar collapse

Muscles of Inspiration

Quiet inspiration

The main muscles are:

Diaphragm

Most important muscle for normal inspiration.

When it contracts:

Diaphragm moves downward → thoracic cavity gets larger

External intercostals

These muscles help expand the rib cage.

⭐ Memorize:

Quiet inspiration = diaphragm + external intercostals

Deep Inspiration

Deep inspiration requires additional muscles.

Important accessory muscles include:

• Sternocleidomastoid

• Scalenes

• Other muscles that help expand the thoracic cavity

Main idea:

More powerful inhalation = additional muscles help expand chest

Forced Expiration

Normal expiration is mostly passive.

But forced expiration requires muscle contraction.

Important muscles:

• Internal intercostals

• Abdominal muscles

⭐ Memorize:

Forced expiration = internal intercostals + abdominals

Pressure and Breathing

This is one of the MOST IMPORTANT concepts in the chapter.

Air moves because of pressure differences.

The basic rule:

Air moves from HIGH pressure → LOW pressure

You need to understand this.

Atmospheric Pressure

Atmospheric pressure is the pressure of the air outside your body.

Think:

Atmosphere = outside

Intra-alveolar Pressure

Also called intrapulmonary pressure.

This is the pressure inside the alveoli.

Air movement depends heavily on the relationship between:

Atmospheric pressure ↔ intra-alveolar pressure

Intrapleural Pressure

This is the pressure inside the pleural cavity, the space between the pleural membranes surrounding the lungs.

It is normally negative relative to atmospheric pressure.

Why is that important?

It helps keep the lungs expanded against the chest wall.

Why Is Intrapleural Pressure Important?

The lungs naturally tend to recoil inward.

The chest wall tends to move outward.

The negative intrapleural pressure helps keep the lungs pulled outward and prevents lung collapse.

⭐ Remember:

Negative intrapleural pressure → keeps lungs expanded

Inspiration: How Does Air Get Into the Lungs?

This sequence is VERY important.

Step 1

Diaphragm contracts.

Step 2

Diaphragm moves downward.

Step 3

Thoracic cavity volume increases.

Step 4

Lung volume increases.

Step 5

Intra-alveolar pressure decreases.

Step 6

Atmospheric pressure is now higher than alveolar pressure.

Step 7

Air moves:

Atmosphere → lungs

⭐ Memorize:

Volume ↑ → Pressure ↓ → Air IN

Expiration

The opposite happens.

Diaphragm relaxes

↓

Thoracic cavity volume decreases

↓

Lung volume decreases

↓

Intra-alveolar pressure increases

↓

Air moves out

⭐ Memorize:

Volume ↓ → Pressure ↑ → Air OUT

The Most Important Pressure Rule

Boyle's Law

Pressure and volume are inversely related.

Volume ↑ → Pressure ↓

Volume ↓ → Pressure ↑

This explains ventilation.

Dalton's Law

Dalton's law deals with partial pressures.

The total pressure of a mixture of gases equals the sum of the pressures contributed by each individual gas.

For example, air contains:

• Oxygen

• Nitrogen

• Carbon dioxide

• Other gases

Each gas contributes its own partial pressure.

⭐ Think:

Dalton = each gas has its own pressure

Henry's Law

Henry's law explains gases dissolving in liquids.

The amount of gas that dissolves in a liquid depends on:

• The partial pressure of the gas

• The solubility of the gas

⭐ Think:

Henry = gas dissolves in liquid

Dalton vs. Henry

⭐ Memorize:

Dalton = pressure

Henry = dissolved gas

External vs. Internal Respiration

External respiration

Occurs between:

Alveoli ↔ pulmonary capillary blood

O₂:

Alveoli → blood

CO₂:

Blood → alveoli

Internal respiration

Occurs between:

Systemic blood ↔ body tissues

O₂:

Blood → tissues

CO₂:

Tissues → blood

⭐ Easy trick:

External = lungs

Internal = tissues

Gas Exchange

Gas exchange occurs through diffusion.

Gases move from areas of:

HIGH partial pressure → LOW partial pressure

They do NOT need ATP.

Alveoli and Capillaries

The alveoli are surrounded by tiny pulmonary capillaries.

This creates a huge surface area for gas exchange.

Oxygen

O₂ concentration/partial pressure is higher in the alveoli than in deoxygenated blood.

Therefore:

O₂ → alveoli → blood

Carbon dioxide

CO₂ partial pressure is higher in the blood than in the alveoli.

Therefore:

CO₂ → blood → alveoli

Then CO₂ is exhaled.

⭐ Memorize This Gas Exchange Diagram

ALVEOLI

O₂ ↓

BLOOD

CO₂ ↑

Meaning:

O₂ goes INTO blood

CO₂ goes INTO alveoli

Partial Pressure

Partial pressure tells you the pressure contributed by a particular gas.

For diffusion:

High partial pressure → Low partial pressure

So if O₂ has:

Higher PO₂ in alveoli

than

PO₂ in blood

then:

O₂ diffuses into blood.

Oxygen + Hemoglobin

Most oxygen is transported in the blood attached to hemoglobin.

Hemoglobin is a protein inside red blood cells.

Hemoglobin contains heme groups.

Each heme contains an iron atom (Fe²⁺).

Oxygen binds to the iron in heme.

Think of the relationship like this:

Red blood cell

↓ contains

Hemoglobin

↓ contains

Heme

↓ contains

Iron

↓ binds

O₂

⭐ Memorize:

RBC → hemoglobin → heme → iron → oxygen

Oxygen-Hemoglobin Binding

Hemoglobin's ability to bind oxygen depends partly on oxygen partial pressure.

High PO₂

Hemoglobin has a greater tendency to bind oxygen.

This happens in the lungs.

High PO₂ → hemoglobin loads O₂

Low PO₂

Hemoglobin releases oxygen.

This happens in tissues.

Low PO₂ → hemoglobin unloads O₂

⭐ Think:

Lungs = LOAD

Tissues = DROP OFF

Oxygen Affinity

Affinity means how strongly hemoglobin wants to bind oxygen.

High affinity

Hemoglobin holds onto oxygen strongly.

Low affinity

Hemoglobin releases oxygen more easily.

What Happens When Temperature Increases?

When temperature increases:

Hemoglobin's affinity for oxygen decreases.

That means hemoglobin releases more oxygen.

This makes sense because active tissues produce heat and need more oxygen.

⭐ Remember:

Temperature ↑ → affinity ↓ → O₂ release ↑

What Happens When pH Decreases?

A decrease in pH means the environment becomes more acidic.

When pH decreases:

Hemoglobin's affinity for oxygen decreases.

So hemoglobin releases more O₂ to the tissues.

⭐ Remember:

pH ↓ → affinity ↓ → O₂ unloading ↑

This is part of the Bohr effect.

Why Would the Body Do This?

Think about exercising muscle.

Active muscles:

• Produce more CO₂

• Become more acidic

• Become warmer

• Need more oxygen

These conditions make hemoglobin release more oxygen.

⭐ Big picture:

Active tissue → CO₂ ↑ → temperature ↑ → pH ↓ → hemoglobin releases O₂

Three Ways Carbon Dioxide Travels in Blood

You need to know these.

Dissolved CO₂

A small amount travels directly dissolved in plasma.

Carbaminohemoglobin

Some CO₂ attaches to hemoglobin.

Bicarbonate ions

Most CO₂ is transported as bicarbonate (HCO₃⁻).

This is the BIG one.

⭐ Memorize:

CO₂ travels as:

Dissolved + Hemoglobin + Bicarbonate

And remember:

Bicarbonate = MOST

CO₂ and Bicarbonate

Inside red blood cells:

CO₂ combines with water.

This produces carbonic acid.

Carbonic acid can break apart into:

H⁺ + HCO₃⁻

The bicarbonate travels in the blood.

When blood reaches the lungs, the reaction can reverse.

Bicarbonate is converted back toward CO₂.

Then:

CO₂ → alveoli → exhaled

Hyperpnea vs. Hyperventilation

These sound similar but are NOT the same.

Hyperpnea

Increased depth and/or rate of breathing that matches the body's metabolic needs.

For example:

Exercise → muscles need more O₂ → breathing increases

This is appropriate for the body's needs.

Hyperventilation

Breathing more than the body needs for its metabolic demand.

This causes excessive CO₂ loss.

Therefore:

CO₂ ↓ → blood pH ↑

The blood becomes more alkaline.

⭐ Difference:

Hyperpnea = appropriate increase in breathing

Hyperventilation = excessive breathing → CO₂ drops

Altitude Sickness

This is another concept you should understand.

At high altitude:

Atmospheric pressure decreases.

Because atmospheric pressure is lower, the partial pressure of oxygen is also lower.

That means there is less pressure pushing oxygen into the blood.

Result:

High altitude

↓

Atmospheric pressure ↓

↓

PO₂ ↓

↓

Less O₂ enters blood

↓

Blood oxygen decreases

↓

Symptoms of altitude sickness

Possible symptoms include:

• Headache

• Dizziness

• Nausea

• Fatigue

• Shortness of breath

🧠 THE BIG TEST CONNECTIONS

These are the connections I would really focus on memorizing.

Breathing mechanics

Thoracic volume ↑

↓

Alveolar pressure ↓

↓

Air moves IN

Thoracic volume ↓

↓

Alveolar pressure ↑

↓

Air moves OUT

Gas exchange

High partial pressure → low partial pressure

Alveoli PO₂ high → O₂ enters blood

Blood PCO₂ high → CO₂ enters alveoli

Oxygen transport

RBC → hemoglobin → heme → iron → O₂

Hemoglobin affinity

Temperature ↑ → affinity ↓ → O₂ release ↑

pH ↓ → affinity ↓ → O₂ release ↑

CO₂ transport

Most CO₂ = bicarbonate

Other forms:

• Dissolved in plasma

• Attached to hemoglobin

Respiratory zones

Conducting zone = moves air

Respiratory zone = exchanges gases

Alveolar cells

Type I = gas exchange

Type II = surfactant

Macrophage = defense

Muscles

Quiet inspiration = diaphragm + external intercostals

Deep inspiration = accessory muscles

Forced expiration = internal intercostals + abdominal muscles

⭐ HIGH-YIELD "IF THEY ASK THIS..." SECTION

If they ask: "Where does gas exchange occur?"

Respiratory zone, especially the alveoli.

If they ask: "What prevents alveoli from collapsing?"

Surfactant.

If they ask: "Who makes surfactant?"

Type II alveolar cells.

If they ask: "Which alveolar cells perform gas exchange?"

Type I alveolar cells.

If they ask: "Which cells remove debris?"

Alveolar macrophages.

If they ask: "What causes air to move into the lungs?"

Alveolar pressure becomes lower than atmospheric pressure.

If they ask: "What happens when thoracic volume increases?"

Pressure decreases.

If they ask: "Which way does gas diffuse?"

From high partial pressure → low partial pressure.

If they ask: "Where does O₂ go?"

Alveoli → blood → tissues.

If they ask: "Where does CO₂ go?"

Tissues → blood → alveoli → outside the body.

If they ask: "What carries most oxygen?"

Hemoglobin.

If they ask: "Where does oxygen bind to hemoglobin?"

The iron in the heme group.

If they ask: "What carries most CO₂?"

Bicarbonate ions.

If they ask: "What happens to hemoglobin when temperature increases?"

Affinity for O₂ decreases, so more O₂ is released.

If they ask: "What happens when pH decreases?"

Hemoglobin affinity for O₂ decreases, so more O₂ is released.

If they ask: "Why does altitude sickness happen?"

Lower atmospheric pressure → lower O₂ partial pressure → less oxygen enters the blood.

🧠 ONE-MINUTE CHAPTER REVIEW

If you can explain these without looking, you are in a very good position for the test:

Respiratory system

→ brings in O₂ and removes CO₂

Ventilation

→ moves air in/out

Conducting zone

→ moves, warms, humidifies, filters air

Respiratory zone

→ gas exchange

Alveoli

→ O₂/CO₂ exchange

Type I

→ gas exchange

Type II

→ surfactant

Macrophages

→ immune defense

Surfactant

→ decreases surface tension and prevents collapse

Inspiration

→ volume ↑, pressure ↓, air enters

Expiration

→ volume ↓, pressure ↑, air leaves

External respiration

→ alveoli ↔ blood

Internal respiration

→ blood ↔ tissues

O₂

→ alveoli → blood → tissues

CO₂

→ tissues → blood → alveoli

Hemoglobin

→ carries most O₂

Heme

→ contains iron that binds O₂

CO₂

→ mostly transported as bicarbonate

Temperature ↑

→ hemoglobin affinity ↓ → O₂ release ↑

pH ↓

→ hemoglobin affinity ↓ → O₂ release ↑

Altitude ↑

→ atmospheric pressure ↓ → PO₂ ↓ → less O₂ enters blood