The Respiratory System: Structure, Function, and Medical Terminology

The Respiratory System

Overview and Functions

The respiratory system is essential for gas exchange, supplying oxygen to the body and removing carbon dioxide. It also plays roles in blood pressure regulation, speech, and maintaining acid-base balance. The system is divided into upper and lower respiratory tracts, each with distinct anatomical structures and functions.

• Key Functions: Gas exchange, blood pressure control (via ACE enzymes), speech (phonation), and acid-base balance.

• Major Structures: Nose, nasal cavity, mouth, pharynx, larynx, trachea, bronchi, bronchioles, alveoli, lungs, and diaphragm.

Anatomical Divisions

Upper and Lower Respiratory Tracts

The respiratory system is divided into two functional regions:

• Upper Respiratory Tract: Nose, nasal cavity, oral cavity, pharynx, larynx, upper trachea.

• Lower Respiratory Tract: Lower trachea, lungs, bronchi, bronchioles, alveoli.

Upper Respiratory Tract

Nose, Nasal Cavity, and Sinuses

The nose is the primary entrance for air. The nasal cavity is lined with hairs and mucus to filter, warm, and humidify air. Sinuses are hollow spaces in the skull that regulate air temperature and humidity.

• Sinuses: Frontal, ethmoid, sphenoid, and maxillary sinuses.

• Air Cleaning: Nasal hairs and mucus trap particles.

• Mouth Breathing: Occurs when nasal passages are blocked or during heavy exercise.

Pharynx

The pharynx is a muscular tube that serves both the respiratory and digestive systems. It is divided into three regions:

• Nasopharynx: Connected to the nasal cavity.

• Oropharynx: Connected to the oral cavity.

• Laryngopharynx: Connected to the larynx and esophagus.

Functions:

• Digestive: Passage for food to the esophagus.

• Respiratory: Passage for air to the larynx and trachea.

• Speech: Resonating chamber for phonation.

Larynx

The larynx, or voice box, protects the lower respiratory tract and houses the vocal cords. The epiglottis prevents food from entering the airway during swallowing.

• Epiglottis: Flexible cartilage acting as a trap door during swallowing.

• Phonation: Vocal cords produce sound.

Lower Respiratory Tract

Trachea

The trachea is a tube supported by C-shaped cartilage rings, preventing collapse. It bifurcates at the carina into left and right primary bronchi.

• Length: 10–12 cm

• Diameter: 2.5 cm

• Function: Conducts air to bronchi

Bronchi and Bronchioles

The bronchi branch into smaller bronchioles, which lead to alveoli. Bronchioles are less than 1 mm in diameter and further subdivide.

• Bronchi: Primary, secondary, tertiary

• Bronchioles: Smallest airways, lead to alveoli

salbutamol inhailour broncho delater

nebs used alot for children

Alveoli and Gas Exchange

Alveoli are tiny air sacs where gas exchange occurs. Oxygen diffuses into the blood, and carbon dioxide diffuses out to be exhaled. Alveoli are lined with surfactant to maintain their shape and prevent collapse.

• Gas Exchange: Oxygen enters blood, CO2 exits

• Surfactant: Reduces surface tension, keeps alveoli open

will not grow back. once gone they are gone ( smoking/ vaping)

300 million alveoli - like a soap bubble

diffuison

Lungs and Pleural Membranes

Lung Anatomy

The lungs are elastic organs in the thoracic cavity, protected by pleural membranes. The right lung has three lobes, the left has two, due to the position of the heart.

• Right Lung: Superior, middle, inferior lobes

• Left Lung: Superior, inferior lobes

• Pleural Membranes: Visceral (covers lung), parietal (lines thoracic cavity)

Mechanics of Breathing

Boyle's Law and Pulmonary Ventilation

Breathing is governed by Boyle's Law: volume is inversely proportional to pressure. Air moves from regions of high pressure to low pressure.

• Inspiration: Diaphragm contracts, thoracic volume increases, pressure decreases, air enters.

• Expiration: Diaphragm relaxes, thoracic volume decreases, pressure increases, air exits.

Boyle's Law Equation:

$P_1 V_1 = P_2 V_2$

Pathway of Air

Air Flow Sequence

Air enters through the nose or mouth, travels down the trachea, through bronchi and bronchioles, and finally reaches the alveoli for gas exchange.

• Pathway: Nose/Mouth → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli

Physiological Measurements

Respiratory Rate

The respiratory rate is the number of breaths per minute. Normal adult rate is 12–20 breaths/minute. Rates vary by age.

• Signs of abnormal breathing: Chest hyperinflation, intercostal/subcostal/ sternal recession, nasal flaring, use of accessory muscles, tracheal tug.

Age	Rate (breaths/min)
Infant	30–53
Toddler	22–37
Preschooler	20–28
School Aged Child	18–25
Adolescent	12–20

Acid-Base Balance and Blood Tests

Blood Gas and pH Balance

Blood gas tests measure pH, oxygen, and carbon dioxide levels. The body maintains pH between 7.35 and 7.45. Acid-base disorders include metabolic and respiratory acidosis/alkalosis, with compensatory mechanisms to restore balance.

• Acidemia: pH < 7.35

• Alkalemia: pH > 7.45

• Disorders: Metabolic acidosis/alkalosis, respiratory acidosis/alkalosis

Example: Metabolic acidemia is compensated by respiratory alkalosis.

Interesting Facts

• Humans lose about 0.5 liters of water per day through breathing (insensible loss).

• Each lung contains about 300 million alveoli.

• When filled with air, lungs are the only organs that can float on water.

References

• OpenStax (2023) Anatomy and Physiology

• Tortora, G.J. and Derrickson, B. (2021) Principles of Anatomy and Physiology

• NHS (2023) How the lungs work

• Guyton, A.C. and Hall, J.E. (2020) Textbook of Medical Physiology

• British Lung Foundation (2022) The Respiratory System