Chapter 13 - Respiratory System

The Respiratory System

Overview

The respiratory system is responsible for supplying the body with oxygen and removing carbon dioxide. It consists of a series of organs and structures that facilitate the movement and exchange of gases between the external environment and the bloodstream.

• Main organs: Nose, Pharynx, Larynx, Trachea, Bronchi, Lungs (alveoli)

Functional Anatomy of the Respiratory System

Main Organs and Their Functions

• Nose: The only externally visible part of the respiratory system; entry point for air.

• Pharynx: Muscular passageway for air and food; commonly called the throat.

• Larynx: Voice box; routes air and food into proper channels and plays a role in speech.

• Trachea: Windpipe; connects larynx to bronchi, reinforced with C-shaped cartilage rings.

• Bronchi: Main passageways into the lungs; branch into smaller bronchioles.

• Lungs (Alveoli): Site of gas exchange; contain millions of alveoli surrounded by capillaries.

Upper vs. Lower Respiratory Tract

• Upper respiratory tract: Nose to larynx

• Lower respiratory tract: Trachea to alveoli

• Passageways purify, humidify, and warm incoming air before it reaches the lungs.

The Nose

Structure and Function

• Nostrils (nares): Openings through which air enters the nose.

• Nasal cavity: Interior of the nose, lined with respiratory mucosa that moistens air and traps particles.

• Olfactory epithelium: Specialized tissue for smell, located in the nasal cavity.

• Nasal septum: Divides the nasal cavity into left and right sections.

Concept Link: Any area open to the outside of the body, including respiratory passages, is lined with mucous membrane (mucosa), which is a moist membrane.

Additional Structures

• Conchae: Projections from the lateral walls of the nasal cavity that increase surface area and air turbulence, aiding in trapping particles.

• Palate: Separates the nasal cavity from the oral cavity; hard palate is anterior and bony, soft palate is posterior and muscular.

• Paranasal sinuses: Cavities in surrounding bones (frontal, sphenoid, ethmoid, maxillary) that lighten the skull, act as resonance chambers for speech, and produce mucus.

Pharynx

Regions and Functions

• Nasopharynx: Superior region behind the nasal cavity.

• Oropharynx: Middle region behind the mouth.

• Laryngopharynx: Inferior region attached to the larynx.

• Serves as a common passageway for air and food.

• Epiglottis: Routes food to the esophagus and air to the larynx.

• Pharyngotympanic tubes: Open into the nasopharynx to drain the middle ear.

Tonsils

• Pharyngeal tonsil (adenoid): Located in the nasopharynx.

• Palatine tonsils: Located in the oropharynx at the end of the soft palate.

• Lingual tonsil: Found at the base of the tongue.

• Tubal tonsil: Protects the openings of the pharyngotympanic tubes.

Larynx

Structure and Function

• Commonly called the voice box.

• Routes air and food into proper channels and plays a role in speech.

• Located inferior to the pharynx; made of eight rigid hyaline cartilages.

• Thyroid cartilage: Largest cartilage, also known as Adam's apple.

• Epiglottis: Spoon-shaped flap of elastic cartilage that protects the larynx during swallowing.

• Vocal folds (true vocal cords): Vibrate with expelled air to produce sound; the glottis is the opening between the vocal cords.

Trachea

Structure and Function

• Commonly called the windpipe; connects larynx to bronchi.

• Walls reinforced with C-shaped rings of hyaline cartilage to maintain a patent airway.

• Lined with ciliated mucosa; cilia move mucus and trapped particles upward toward the throat.

• Goblet cells produce mucus to trap debris.

Bronchi and Lungs

Main Bronchi

• Formed by the division of the trachea.

• Each bronchus enters a lung at the hilum.

• Right bronchus is wider, shorter, and straighter than the left.

• Bronchi subdivide into smaller branches (bronchioles).

Lungs

• Occupy the thoracic cavity except for the mediastinum.

• Apex is near the clavicle; base rests on the diaphragm.

• Right lung has three lobes; left lung has two lobes.

• Pleura: Double-layered serous membrane; visceral pleura covers lungs, parietal pleura lines thoracic cavity, pleural fluid reduces friction.

Bronchial (Respiratory) Tree

• Network of branching passageways from main bronchi to terminal bronchioles.

• Conducting zone: All passageways except those involved in gas exchange.

• Respiratory zone: Includes respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Alveoli and the Respiratory Membrane

Structure and Function

• Alveoli: Tiny air sacs composed of simple squamous epithelium; site of gas exchange.

• Alveolar pores connect neighboring alveoli.

• Pulmonary capillaries cover external surfaces of alveoli.

• Respiratory membrane (air-blood barrier): Formed by alveolar and capillary walls; site of gas diffusion.

• Alveolar macrophages ("dust cells"): Remove debris and pathogens.

• Surfactant-secreting cells: Produce surfactant to reduce surface tension and prevent alveolar collapse.

Respiratory Physiology

Functions and Events of Respiration

• Supply oxygen to the body and remove carbon dioxide.

• Four events of respiration:

  1. Pulmonary ventilation: Moving air into and out of the lungs (breathing).

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

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

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

Mechanics of Breathing

• Breathing depends on volume changes in the thoracic cavity, leading to pressure changes and air flow.

• Inspiration: Diaphragm and external intercostal muscles contract, increasing thoracic volume and decreasing pressure, causing air to flow in.

• Expiration: Usually passive; lung elasticity decreases thoracic volume, increasing pressure and causing air to flow out.

• Intrapleural pressure: Always negative relative to intrapulmonary pressure; prevents lung collapse.

Respiratory Volumes and Capacities

Key Volumes

Volume/Capacity	Definition	Typical Value (Adult Male)
Tidal Volume (TV)	Air moved in/out during normal breathing	500 ml
Inspiratory Reserve Volume (IRV)	Air forcibly inhaled beyond TV	3,100 ml
Expiratory Reserve Volume (ERV)	Air forcibly exhaled beyond TV	1,200 ml
Residual Volume	Air remaining after forced expiration	1,200 ml
Vital Capacity (VC)	Total exchangeable air (TV + IRV + ERV)	4,800 ml
Dead Space Volume	Air in conducting zone, not reaching alveoli	150 ml
Functional Volume	Air reaching the respiratory zone	350 ml

Measured with a spirometer.

Nonrespiratory Air Movements

• Cough and sneeze: Clear airways of debris.

• Crying and laughing: Emotional responses involving respiratory muscles.

• Hiccup: Sudden inspiration due to diaphragm spasm.

• Yawn: Very deep inspiration, ventilates all alveoli.

Gas Exchange and Transport

External Respiration

• Oxygen diffuses from alveoli to blood; carbon dioxide diffuses from blood to alveoli.

Internal Respiration

• Oxygen diffuses from blood to tissues; carbon dioxide diffuses from tissues to blood.

Gas Transport in Blood

• Oxygen: Mostly transported as oxyhemoglobin ($HbO_2$); small amount dissolved in plasma.

• Carbon dioxide: Mostly transported as bicarbonate ion ($HCO_3^-$); some bound to hemoglobin at different sites than oxygen.

• To release $CO_2$ from bicarbonate:

  1. Bicarbonate ions enter RBCs and combine with $H^+$ to form carbonic acid ($H_2CO_3$).

  2. Carbonic acid splits into water and $CO_2$.

  3. $CO_2$ diffuses into alveoli for exhalation.

Blood pH: Maintained between 7.35 and 7.45 by buffers such as bicarbonate.

Control of Respiration

Neural Regulation

• Respiratory muscles controlled by phrenic and intercostal nerves.

• Medulla sets basic rhythm (ventral respiratory group, VRG); pons smoothes out rate.

• Normal rate (eupnea): 12–15 breaths per minute.

• Hyperpnea: Increased rate due to extra oxygen needs.

Factors Influencing Rate and Depth

• Physical: Temperature, exercise, talking, coughing.

• Volition: Conscious control.

• Emotional: Fear, anger, excitement.

• Chemical: $CO_2$ levels (most important), $O_2$ levels (detected by peripheral chemoreceptors).

Abnormal Breathing Patterns

• Hyperventilation: Rapid, deep breathing to expel $CO_2$ and raise blood pH (alkalosis).

• Hypoventilation: Slow, shallow breathing allows $CO_2$ accumulation (acidosis).

Respiratory Disorders

Chronic Obstructive Pulmonary Disease (COPD)

• Includes chronic bronchitis and emphysema.

• Features: Smoking history, progressive dyspnea, frequent infections, hypoxia, respiratory acidosis, risk of failure.

Chronic Bronchitis

• Inflamed mucosa, excessive mucus, impaired ventilation, cyanosis ("blue bloaters").

Emphysema

• Alveolar walls destroyed, lungs lose elasticity, barrel chest, late cyanosis ("pink puffers").

Lung Cancer

• Leading cause of cancer death; 90% due to smoking.

• Types: Adenocarcinoma, squamous cell carcinoma, small cell carcinoma.

Developmental Aspects

• Lungs inflate fully about 2 weeks after birth; surfactant is crucial for alveolar stability.

• Respiratory rate decreases with age: Newborns (40–80/min), infants (30/min), age 5 (25/min), adults (12–18/min), increases in old age.

• Asthma: Chronic inflammation and hypersensitivity of bronchioles.

• Aging: Decreased lung elasticity, vital capacity, and blood oxygen; increased risk of infection and sleep apnea.