SECTION 01: INTRODUCTION TO RESPIRATORY PHYSIOLOGY

By the end of Section 01, you should be able to:

• Describe the purpose of the respiratory system, and the four steps involved in external respiration.

• Describe the role of the respiratory muscles in both inspiration and expiration.

• Describe the purpose of the pleural space

🫁 External Respiration – Overview

• What it is: The process that brings oxygen into the body and removes carbon dioxide.

• Goal: Deliver O₂ to tissues and remove CO₂ from tissues.

• Main system involved: Respiratory system (lungs, airways) + circulatory system (blood transport).

✅ External Respiration: 4 Main Steps 1. Ventilation (Breathing)

• What happens? Air goes in and out of lungs.

• Why? To move fresh air (with O₂) into the alveoli and remove stale air (with CO₂).

✅ External Respiration: 4 Main Steps 2. Gas Exchange between Air and Blood

• Where? In the alveoli (air sacs in lungs) and pulmonary capillaries.

• What happens?

  • O₂ diffuses from alveoli → blood.

  • CO₂ diffuses from blood → alveoli.

✅ External Respiration: 4 Main Steps 3. Transport of Gases in the Blood

• O₂: Carried from lungs → body tissues.

• CO₂: Carried from tissues → lungs (to be exhaled).

• How? Via red blood cells in circulation.

✅ External Respiration: 4 Main Steps - 4. Gas Exchange between Blood and Tissues

• Where? In body capillaries near tissues.

• What happens?

  • O₂ diffuses from blood → tissues.

  • CO₂ diffuses from tissues → blood.

🎯 Key Concept: Internal vs. External Respiration

• Internal respiration = Cell-level usage of O₂ + production of ATP + release of CO₂.

• External respiration = Body-level process of getting O₂ in and CO₂ out.

🫁 Other Functions of the Respiratory System 🗣 1. Speech Production

• How? Air passes over vocal cords in the larynx (voice box).

• Why important? This airflow lets us talk, sing, shout, etc.

🫁 Other Functions of the Respiratory System 🛡 2. Defense Against Foreign Particles

• What does it do? Filters out dust, bacteria, and allergens from air.

• How?

  • Mucus traps particles.

  • Cilia (tiny hairs) push them out of the airways.

🫁 Other Functions of the Respiratory System 💪 3. Helps in Pushing Movements (Parturition & Defecation)

• Parturition = childbirth

• Defecation = bowel movement

• How? The respiratory muscles (like the diaphragm) contract and help increase pressure in the abdomen, which pushes things out (baby or stool).

🫁 Other Functions of the Respiratory System 🩸 4. Lung as a Blood Reservoir

• What? Lungs can hold extra blood.

• Why? Helps balance blood flow between the right and left sides of the heart quickly when needed.

🫁 Other Functions of the Respiratory System ⚖ 5. Acid-Base Balance

• What does it do? Helps control blood pH.

• How? By controlling how much CO₂ is breathed out (less CO₂ = less acid).

• Will be explained more in Module 05.

🫁 Anatomy of the Respiratory System: Overview

• Main Parts:

  • Lungs

  • Chest Wall

  • Pleural Space (space between lungs & chest wall)

• Focus here: Lungs → Upper & Lower Tract

🛤 Airway vs. Alveoli

• Airways = Passages that move air in & out.

• Alveoli = Tiny air sacs where O₂ and CO₂ exchange happens.

🔼 Upper Respiratory Tract (Airway Part) - 3 main part —> 👃 1. Nose & Nasal Cavities

• Entry point for air

• Warms, moistens, and filters the air

🧠 2. Pharynx (Throat)

• Shared tube for both air and food

• Sends air to lungs and food to esophagus

🗣 3. Larynx (Voice Box)

• Located just below the pharynx

• Contains vocal cords (for speech)

• Directs air into the lower airways

💡 Summary:

Air flows:
Nose → Nasal Cavity → Pharynx → Larynx → Lower Tract

🔽 Lower Respiratory Tract (Airway Part) - 5 parts to this —> 🧵 1. Trachea (Windpipe)

• A tube that carries air down from the larynx.

• Divides into two bronchi—one for each lung.

🌳 2. Bronchi

• Right & Left main branches from the trachea.

• Each bronchus supplies one lung.

• Further split into smaller tubes inside each lung.

🌿 3. Bronchioles

• Tiny branches of the bronchi.

• Get narrower the deeper you go.

• No cartilage, just smooth muscle.

🫧 4. Respiratory Bronchioles

• Smallest bronchioles with very thin walls.

• Start to allow some gas exchange.

🫁 5. Alveoli

• Tiny air sacs at the ends of bronchioles.

• Main site of gas exchange (O₂ in, CO₂ out).

• Surrounded by capillaries.

📍 Air Path Summary:

Trachea → Bronchi → Bronchioles → Respiratory Bronchioles → Alveoli

🩸 What Are Capillaries?

• Capillaries are the smallest blood vessels in your body.

• They connect arteries (which carry blood away from the heart) to veins (which return blood to the heart).

• Their walls are super thin—just one cell thick—so that gas exchange (O₂ and CO₂) can happen easily.

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🔄 What Do Capillaries Do in the Lungs?

• In the lungs, capillaries surround the alveoli.

• O₂ diffuses from the alveoli into the blood.

• CO₂ diffuses from the blood into the alveoli to be exhaled.

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❓ Are Capillaries Part of the Lower Respiratory Tract?

• Not exactly.

• They are part of the circulatory system, not the airway.

• But they work very closely with the lower respiratory tract (especially the alveoli) to exchange gases.

🌳 The Respiratory (Tracheobronchial) Tree

🫁 Airway Branching Pathway

1. Trachea

2. Main Bronchus

3. Bronchus

4. Bronchiole

5. Terminal Bronchiole

6. Respiratory Bronchiole

7. Alveolar Duct

8. Alveolar Sac

💨 Types of Air Flow in the Respiratory Tree 🔄 Convective Flow (Needs energy)

• Happens from Trachea → Terminal Bronchioles

• Air is pushed by muscle contractions (like the diaphragm)

• Similar to how you blow air through a straw

🌬 Diffusive Flow (Passive)

• Starts at Respiratory Bronchioles

• Air moves by diffusion (no energy needed)

• Gas spreads naturally into alveoli where gas exchange happens

📏 Why Does Branching Matter?

• Even though the tubes get smaller, the total surface area increases

• This large area slows airflow, making it ideal for diffusion in the alveoli

🧍‍♂ Chest Wall: What Is It?

• Includes everything that helps with breathing:
  Thorax (chest) + Abdomen

• Houses: Lungs & heart

• Protected by: Rib cage

• Key Muscles: Found between ribs and at the base of the chest (diaphragm)

💪 Muscles of the Chest Wall 🩻 Intercostal Muscles (Between Ribs)

• External intercostals = outer layer

• Internal intercostals = deeper layer

• Help move the ribs to change chest size

🧘‍♀ Muscles Used in Inspiration (Breathing In)

1. Diaphragm

   • Moves down to make room for lungs

   • Main muscle for inhaling

2. External Intercostal Muscles

   • Lift the ribs upward and outward

   • Makes chest cavity bigger

💨 Muscles Used in Expiration (Breathing Out)

1. Internal Intercostal Muscles

   • Pull ribs down and in

   • Decrease chest size

2. Abdominal Muscles

   • Push up against diaphragm

   • Help force air out during coughing, sneezing, exercise, etc.

🧠 Note: In normal relaxed breathing, expiration is passive—these muscles are only used when needed!

🫁 What Is the Pleural Space?

• A thin fluid-filled space between the lungs and the chest wall.

• Only about 2 mm wide.

• Helps lungs move smoothly when you breathe.

pleural space - 🧼 Why Is It Important?

• Filled with pleural fluid.

• Reduces friction as the lungs expand and contract.

• Acts like lubricant between the lung and the chest wall.

🩻 Layers of the Pleura (Membranes)

1. Visceral Pleura

• Sticks directly to the surface of the lungs.

2. Parietal Pleura

• Lines the inside of the chest wall (thoracic wall).

🟦 Pleural Space = The tiny gap between the visceral and parietal pleura.

📍Visual Reference from the Image:

• Right/Left Pleural Sacs: Each lung is inside its own sac.

• Pleural Space: Located between the two blue linings (parietal & visceral pleura).

• Diaphragm: Sits underneath both lungs, helps pull lungs down when inhaling.

🫁 External Respiration: Two Main Processes

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1. Pressure-Driven Airflow

• Muscles (like the diaphragm) create a pressure gradient.

• Air flows from high pressure → low pressure.

• Must overcome airway resistance to get into lungs.

2nd processs - 2. Gas Diffusion Across Alveoli

• Oxygen (O₂) moves from alveoli → blood.

• Carbon dioxide (CO₂) moves from blood → alveoli.

• Happens through thin alveolar-capillary barrier by diffusion.

🧮 Key Equation:

equation - 🧠 What This Means:

• Δ Pressure = Difference in pressure between two points (e.g., inside vs. outside lungs)

• Resistance = How hard it is for air (or gas) to move

• More pressure or less resistance = more flow

🎯 Real-Life Analogy:

Think of it like blowing through a straw:

• Harder you blow (more pressure) = more air flows out.

• Thicker the straw (more resistance) = harder to get air through.

🫁 Path of Air Through the Respiratory System

📍 Step-by-Step Airflow:

1. Nares → Nostrils (openings to the nose)

2. Nasal Cavity

   • Cilia = tiny hairs that filter dust

   • Mucous membranes = add moisture

   • Blood vessels = warm the air

3. Pharynx → Throat (shared by food & air)

4. Larynx → Voice box (contains vocal cords)

   • Epiglottis closes during swallowing to block food

5. Trachea → Windpipe (has cartilage rings to keep it open)

6. Bronchus (pl. bronchi) → Left & right branches into lungs

7. Bronchioles → Smaller branches inside the lungs

8. Alveolar Sacs → End of bronchioles

9. Alveoli → Tiny air sacs where gas exchange happens

🌬 Gas Exchange: How It Works

• Happens in the alveoli, which are:

  • Lined with one cell layer (very thin!)

  • Surrounded by blood capillaries

↔ Diffusion Process:

• Oxygen (O₂) moves:
  Alveoli → Capillaries → Blood

• Carbon Dioxide (CO₂) moves:
  Capillaries → Alveoli → Exhaled

🔁 Respiration Summary

• Inhalation path:
  Nose → Pharynx → Larynx → Trachea → Bronchus → Alveoli

• Exhalation path:
  CO₂ follows the reverse path out

Activity

🫁 Inspiration = Inhalation = Breathing In

• What happens?

  • The diaphragm contracts and moves downward.

  • External intercostal muscles lift the rib cage.

  • This increases the space in your chest (thoracic cavity).

  • Air flows into your lungs because the pressure inside is now lower than the outside air.

Activity pt 2

🏋‍♂️ 2. Forced (Active) Expiration

(Occurs during exercise, coughing, sneezing, singing, shouting)

🔽 Muscles That Contract:

Muscle	Action in Expiration
Internal Intercostals	Pull ribs down and inward to shrink chest cavity
Diaphragm (already relaxed)	Passive part — stays relaxed & dome-shaped
Internal Oblique (abdominal muscle)	Compresses abdomen → pushes diaphragm up
External Oblique (also abdominal)	Same as internal oblique — helps push air out
Rectus Abdominis	Pushes diaphragm up by squeezing abdominal contents
Transversus Abdominis	Deepest core muscle — tightens abdominal wall to force air out

(dont need to know the words above tbh) in passive thoMuscles used:

• ❌ No active muscle contraction

• ✅ Diaphragm relaxes (moves up)

• ✅ External intercostals relax