Chapter 13: The Respiratory System – Anatomy & Physiology Study Notes

Functional Anatomy of the Respiratory System

Main Organs and Divisions

The respiratory system is responsible for gas exchange between the blood and the external environment, primarily occurring in the alveoli of the lungs. It is divided into upper and lower respiratory tracts, each with distinct anatomical structures and functions.

• Upper respiratory tract: Includes passageways from the nose to the larynx.

• Lower respiratory tract: Includes passageways from the trachea to the alveoli.

• Functions of passageways: Purify, humidify, and warm incoming air.

Major Respiratory Organs

• Nasal cavity

• Pharynx

• Larynx

• Trachea

• Bronchi

• Lungs

• Diaphragm

The Pharynx and Upper Respiratory Tract

Regions of the Pharynx

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

• Nasopharynx: Superior portion, behind the nasal cavity.

• Oropharynx: Middle portion, behind the oral cavity.

• Laryngopharynx: Inferior portion, leading to the larynx and esophagus.

Detailed Anatomy of the Upper Respiratory Tract

• Nasal cavity: Contains nasal conchae and meatuses, lined with mucosa to filter and humidify air.

• Larynx: Contains epiglottis, vocal folds, and cartilages (thyroid, cricoid, corniculate).

• Pharyngeal, palatine, and lingual tonsils: Lymphatic tissue for immune defense.

The Nose and Paranasal Sinuses

Paranasal Sinuses

Paranasal sinuses are air-filled spaces in the frontal, sphenoid, ethmoid, and maxillary bones. They surround the nasal cavities and have several functions:

• Lighten the skull

• Secrete mucus

• Help warm and moisten air

Vocal Folds and the Larynx

Movements of the Vocal Folds

The larynx contains the vocal folds (true and false vocal cords) which are essential for sound production and airway protection.

• Closed glottis: Vocal folds together, prevents passage of air.

• Open glottis: Vocal folds apart, allows air flow.

Trachea and Bronchial Tree

Trachea Structure

The trachea is a tube supported by hyaline cartilage rings, lined with mucosa and submucosa, and contains the trachealis muscle posteriorly.

• Mucosa: Inner lining, produces mucus.

• Submucosa: Contains glands.

• Hyaline cartilage: Maintains airway patency.

• Adventitia: Outer connective tissue.

Lungs and Pleura

Lung Anatomy

The lungs are divided into lobes and are surrounded by pleural membranes.

• Right lung: Superior, middle, and inferior lobes.

• Left lung: Superior and inferior lobes.

• Pleura: Double-layered membrane (parietal and visceral) that reduces friction.

Respiratory Zone Structures and the Respiratory Membrane

Alveoli and Capillaries

Alveoli are the primary sites of gas exchange, composed mainly of simple squamous epithelial cells. Alveolar pores connect neighboring air sacs, and pulmonary capillaries cover their external surfaces.

• Alveolar macrophages: "Dust cells" that remove debris and pathogens.

• Surfactant: Lipid molecule secreted by cuboidal cells, reduces surface tension and prevents alveolar collapse.

Respiratory Membrane

The respiratory membrane (air-blood barrier) is formed by the alveolar and capillary walls. Gas exchange occurs by diffusion:

• Oxygen: Enters the blood.

• Carbon dioxide: Enters the alveoli.

Respiratory Physiology

Functions and Events of Respiration

The respiratory system supplies oxygen and disposes of carbon dioxide. Respiration consists of four events:

• Pulmonary ventilation: Movement of air into and out of the lungs.

• External respiration: Gas exchange between pulmonary blood and alveoli.

• Respiratory gas transport: Transport of gases via the bloodstream.

• Internal respiration: Gas exchange between blood and tissue cells.

Mechanics of Breathing

Pulmonary Ventilation

Breathing is a mechanical process dependent on volume changes in the thoracic cavity, which lead to pressure changes and the flow of gases to equalize pressure.

• Inspiration: Ribs elevate, diaphragm contracts and moves downward, thoracic volume increases, air flows in.

• Expiration: Ribs depress, diaphragm relaxes and moves upward, thoracic volume decreases, air flows out.

Intrapleural Pressure

• Always negative: Prevents lung collapse.

• If equal to atmospheric pressure: Lungs recoil and collapse.

Respiratory Volumes and Capacities

Key Volumes

• Tidal volume (TV): Normal quiet breathing; about 500 ml per breath.

• Inspiratory reserve volume (IRV): Air that can be forcibly inhaled beyond TV; about 3,100 ml.

• Expiratory reserve volume (ERV): Air that can be forcibly exhaled beyond TV; about 1,200 ml.

• Residual volume: Air remaining after maximal exhalation; about 1,200 ml.

Vital Capacity and Dead Space

• Vital capacity: Total exchangeable air; $\text{VC} = \text{TV} + \text{IRV} + \text{ERV}$

• Dead space volume: Air in conducting zone not reaching alveoli; about 150 ml.

Volume	Definition	Typical Value (ml)
Tidal Volume (TV)	Normal breath	500
Inspiratory Reserve Volume (IRV)	Max air inhaled	3,100
Expiratory Reserve Volume (ERV)	Max air exhaled	1,200
Residual Volume	Air left after exhalation	1,200
Vital Capacity	TV + IRV + ERV	4,800 (men)
Dead Space Volume	Conducting zone air	150

Nonrespiratory Air Movements

Nonrespiratory movements are caused by reflexes or voluntary actions and include:

• Cough and sneeze: Clear lungs of debris.

• Crying: Emotionally induced.

• Laughing: Similar to crying.

• Hiccup: Sudden inspiration.

• Yawn: Very deep inspiration.

Respiratory Sounds

Respiratory sounds are monitored with a stethoscope and include:

• Bronchial sounds: Air rushing through large passageways (trachea, bronchi).

• Vesicular breathing sounds: Soft sounds of air filling alveoli.

External Respiration, Gas Transport, and Internal Respiration

Gas Exchange Mechanisms

Gas exchanges occur by diffusion, moving toward areas of lower concentration.

• External respiration: Exchange between alveoli and pulmonary blood.

• Internal respiration: Exchange between blood and tissue cells.

Oxygen Transport

• Most oxygen: Attached to hemoglobin, forming oxyhemoglobin ($\text{HbO}_2$).

• Small amount: Dissolved in plasma.

Carbon Dioxide Transport

• Most carbon dioxide: Transported as bicarbonate ion ($\text{HCO}_3^-$) in plasma.

• Small amount: Bound to hemoglobin at different sites than oxygen.

Key Equations

• Oxygen loading: $\text{Hb} + \text{O}_2 \rightarrow \text{HbO}_2$

• Carbon dioxide transport: $\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \rightarrow \text{HCO}_3^- + \text{H}^+$

Internal Respiration

Internal respiration is the exchange of gases between blood and tissue cells, with opposite reactions to those in the lungs.

• Carbon dioxide: Diffuses out of tissue cells to blood (loading).

• Oxygen: Diffuses from blood into tissue (unloading).

Control of Respiration

Neural Regulation

• Phrenic and intercostal nerves: Transmit activity to and from the brain.

• Medulla: Sets basic rhythm, contains the ventral respiratory group (VRG).

• Pons: Smoothes out respiratory rate.

Normal and Altered Respiratory Rates

• Eupnea: Normal rate (12–15 respirations/min).

• Hyperpnea: Increased rate due to extra oxygen needs.

Non-Neural Factors

• Chemical factors: CO2 levels are the most important stimulus for breathing.

• High CO2: Lowers blood pH, increases rate and depth of breathing.

• Oxygen levels: Detected by chemoreceptors in the aorta and carotid arteries.

Developmental Aspects of the Respiratory System

Lungs do not fully inflate until about two weeks after birth, a process dependent on surfactant production. Surfactant lowers surface tension, preventing alveolar collapse, and is formed late in pregnancy (28–30 weeks).

Additional info: These notes are based on textbook slides and lecture materials for a college-level Anatomy & Physiology course, focusing on the structure and function of the respiratory system.