The Respiratory System: Structure and Function

Overview

I. Conducting Portion

1. Nasal Cavities

2. Pharynx

• Nasopharynx - posterior to the nasal cavity, contains the pharyngeal tonsil (adenoids) and the openings of the Eustachian tubes
• Oropharynx - posterior to the oral cavity, lined with stratified squamous epithelium (reflecting mechanical stress from food)
• Laryngopharynx - connects to the larynx anteriorly and the esophagus posteriorly

3. Larynx

4. Trachea

• Mucosa: pseudostratified ciliated columnar epithelium + lamina propria with elastic fibers
• Submucosa: loose connective tissue with mucoserous glands
• Cartilaginous rings
• Adventitia: outer connective tissue

5. Bronchi

• Secondary (lobar) bronchi - one per lobe (3 right, 2 left)
• Tertiary (segmental) bronchi - one per bronchopulmonary segment (10 right, 8-10 left)

6. Bronchioles and Terminal Bronchioles

• Secrete surfactant-like proteins and immunoglobulins
• Produce detoxifying enzymes
• Serve as stem cells for bronchiolar epithelial renewal
• Produce CC16 (club cell secretory protein), which has anti-inflammatory functions

II. Respiratory Portion

7. Respiratory Bronchioles

8. Alveolar Ducts and Sacs

9. Alveoli - The Gas Exchange Units

1. Alveolar epithelium (Type I pneumocyte)
2. Fused basement membranes
3. Pulmonary capillary endothelium

III. The Lungs

• Visceral pleura - adheres directly to the lung
• Parietal pleura - lines the thoracic wall and diaphragm
• Between them is the pleural cavity filled with a thin film of serous fluid (~5-15 mL) that reduces friction and maintains lung adhesion to the chest wall.

IV. Mechanics of Breathing (Ventilation)

• The diaphragm contracts and flattens (the primary muscle - accounts for ~70% of tidal volume)
• External intercostal muscles elevate the ribs and expand the thoracic cage
• Thoracic volume increases → intrapleural pressure falls → lungs expand → alveolar pressure drops below atmospheric → air flows in

• Respiratory muscles relax
• Elastic recoil of lung tissue and chest wall restores volume
• Alveolar pressure rises above atmospheric → air flows out

V. Gas Exchange and Transport

Pulmonary Gas Exchange

Oxygen Transport in Blood

• ~98.5% carried bound to hemoglobin as oxyhemoglobin (HbO2)
• ~1.5% dissolved in plasma

CO2 Transport in Blood

• ~70% as bicarbonate (HCO3⁻) in plasma (via carbonic anhydrase in RBCs)
• ~23% bound to hemoglobin as carbaminohemoglobin
• ~7% dissolved in plasma

VI. Control of Breathing

• Dorsal respiratory group (medulla) - drives inspiration
• Ventral respiratory group (medulla) - drives forced expiration
• Pneumotaxic center (pons) - limits inspiration duration
• Apneustic center (pons) - prolongs inspiration

• Central chemoreceptors in the medulla respond to changes in cerebrospinal fluid pH (driven by CO2 levels) - the primary driver of minute-to-minute ventilation
• Peripheral chemoreceptors (carotid and aortic bodies) respond to falling arterial PO2 (<60 mmHg), rising PCO2, and falling pH

VII. Additional Functions

1. Immune defense - mucociliary escalator traps particles; IgA, alveolar macrophages, and BALT provide immune surveillance
2. Phonation - vocal cord vibration creates voice
3. Acid-base regulation - adjusting ventilation rate changes CO2/bicarbonate balance
4. Metabolic functions - pulmonary endothelium converts angiotensin I → angiotensin II (via ACE); inactivates bradykinin, serotonin
5. Olfaction - olfactory epithelium mediates sense of smell

• Guyton and Hall Textbook of Medical Physiology, 14e
• Junqueira's Basic Histology: Text and Atlas, 17e
• Histology: A Text and Atlas with Correlated Cell and Molecular Biology (Ross & Pawlina)

The Respiratory System - Simple Notes

What does the Respiratory System do?

1. Move air in and out of the body
2. Clean and warm the incoming air
3. Exchange gases - take in Oxygen (O2) from air and release Carbon Dioxide (CO2) from blood

Two Main Parts

Part 1 - The Path Air Takes (Conducting Part)

1. Nose (Nasal Cavity)

• Air enters through the nostrils
• Inside the nose there are small hairs (vibrissae) that block dust and large particles
• The walls of the nose are full of blood vessels - these warm and moisten the cold, dry air
• Three shelf-like bones called conchae (turbinates) create swirling air so it contacts the warm walls longer
• The top part of the nose has smell receptors (olfactory cells) - these detect odors

Think of the nose as an air filter + heater + humidifier all in one.

2. Paranasal Sinuses

• Air-filled spaces inside the skull bones (forehead, cheeks)
• They also help humidify air and make the skull lighter
• When they get infected = sinusitis (blocked, painful face)

3. Pharynx (Throat)

• A shared passage for both food and air
• 3 parts: Nasopharynx (behind the nose), Oropharynx (behind the mouth), Laryngopharynx (lower throat)
• The adenoids and tonsils here help fight germs

4. Larynx (Voice Box)

• Made of several pieces of cartilage (firm but flexible tissue)
• Has the vocal cords - two bands that vibrate when air passes through, making sound/voice
• Has the epiglottis - a flap that covers the airway when you swallow, so food doesn't go into the lungs
• When food "goes down the wrong way," the epiglottis didn't close in time

5. Trachea (Windpipe)

• A tube about 10-12 cm long connecting the voice box to the lungs
• Held open by 16-20 C-shaped rings of cartilage - like the rings of a vacuum cleaner hose
• Without these rings, the tube would collapse when you breathe in
• The back of the trachea has a muscle (trachealis) that can slightly narrow the tube
• Inside is lined with cells that have tiny hair-like structures called cilia - these sweep dust and mucus upward toward the throat (like a conveyor belt). This is called the mucociliary escalator

6. Bronchi (Air Tubes Inside the Lungs)

• The trachea splits into 2 main bronchi (one for each lung) at a point called the carina
• Right bronchus is wider and more vertical - that's why swallowed objects usually get stuck on the right side
• Inside each lung, the bronchi keep branching like a tree - getting smaller and smaller:
  • Main bronchus → Lobar bronchus (one per lobe) → Segmental bronchus → smaller bronchi
• The large bronchi still have cartilage rings to stay open
• They are lined with mucus-producing cells and cilia to trap and remove germs

7. Bronchioles (Tiny Air Tubes)

• When the tubes get very small (less than 1 mm wide), they become bronchioles
• No cartilage here - instead, they have smooth muscle in their walls
• This smooth muscle can squeeze (constrict) or relax (dilate) to control airflow
• Asthma happens when this muscle squeezes too much, narrowing the tubes and making breathing hard
• The last purely conducting tubes are called terminal bronchioles
• Special cells here called club cells (Clara cells) produce protective secretions and can repair damaged airway cells

Part 2 - Where Gas Exchange Happens (Respiratory Part)

8. Respiratory Bronchioles

• Bronchioles that now have tiny air sacs (alveoli) budding off their walls
• Gas exchange begins here - they both conduct air AND do some exchange

9. Alveolar Ducts and Alveolar Sacs

• Tubes whose walls are almost entirely made of alveoli
• Alveolar sacs = clusters of alveoli at the very end, like a bunch of grapes

10. Alveoli - The Most Important Part ⭐

• Tiny, thin-walled, balloon-like air sacs
• The human lung has about 300-500 million alveoli
• Total surface area = 70-80 square meters (size of a tennis court!) - all packed inside your chest
• Each alveolus is wrapped in a net of tiny blood vessels called pulmonary capillaries

1. Alveolar wall cell (Type I pneumocyte)
2. A thin glue layer (basement membrane)
3. Capillary wall cell (endothelium)

The Lungs

• Two spongy, cone-shaped organs sitting in the chest
• Right lung - 3 lobes (upper, middle, lower)
• Left lung - 2 lobes (upper, lower) - smaller because the heart takes up space on the left side
• Each lung is covered by a pleura - a double plastic-wrap like membrane with fluid in between
  • Inner layer sticks to the lung (visceral pleura)
  • Outer layer lines the chest wall (parietal pleura)
  • The fluid between them acts like oil, reducing friction during breathing
  • When this space fills with air = pneumothorax (collapsed lung); with blood = hemothorax

How We Breathe (Mechanics)

Breathing In (Inspiration)

1. The diaphragm (a dome-shaped muscle below the lungs) contracts and moves downward
2. The rib muscles (intercostals) pull the ribs upward and outward
3. The chest gets bigger → pressure inside drops → air rushes in (like a bellows)
4. The diaphragm does about 70% of the work of breathing

Breathing Out (Expiration) - at rest, this is passive

1. The diaphragm and rib muscles relax
2. The lungs spring back (elastic recoil) like a stretched rubber band
3. Chest gets smaller → pressure rises → air is pushed out

Lung Volumes (Good to memorize!)

Gas Exchange - Simple Explanation

• Oxygen moves from the alveoli into the blood (because there's more O2 in air than in blood)
• Carbon dioxide moves from the blood into the alveoli (because there's more CO2 in blood than in air)
• Both move by diffusion - always moving from where there is MORE to where there is LESS

• 98.5% bound to hemoglobin (in red blood cells) as oxyhemoglobin
• 1.5% dissolved directly in blood plasma

• 70% as bicarbonate (HCO3⁻) - converted by an enzyme called carbonic anhydrase inside red blood cells
• 23% attached to hemoglobin as carbaminohemoglobin
• 7% dissolved in plasma

Control of Breathing

• Rising CO2 in blood is the main signal - detected by receptors in the brain (central chemoreceptors)
• Falling O2 (below 60 mmHg) is detected by receptors in neck arteries (carotid bodies) - called peripheral chemoreceptors
• Falling blood pH also triggers faster breathing

Key point: It's rising CO2, NOT falling O2, that mainly drives you to breathe faster.

Quick Summary Table