The Respiratory System: Structure and Function

The Respiratory System

The respiratory system is vital for oxygen intake and carbon dioxide removal. It has two main divisions: the Upper Respiratory System (URS) and the Lower Respiratory System (LRS). Functionally, it is divided into the conducting and respiratory portions.

Structural Division

1. Upper Respiratory System (URS):-

   • Nose:- Filters, warms, and humidifies the air.

   • Nasal Cavity:- Lined with cilia and mucus, trapping dust and pathogens.

   • Paranasal Sinuses:- Air-filled spaces that lighten the skull and enhance voice resonance.

   • Pharynx:- A muscular tube for air and food passage.

   • Larynx:- Responsible for sound production and preventing food aspiration.

2. Lower Respiratory System (LRS):-

   • Trachea:- Conducts air to the bronchi.

   • Bronchi:- Main airways branching into bronchioles.

   • Bronchioles:- Small airways leading to the alveoli.

   • Alveoli:- Tiny sacs where gas exchange occurs.

Functional Division:-

1. Conducting Portion:- Moves air and conditions it (nose, pharynx, larynx, trachea, bronchi, bronchioles).

2. Respiratory Portion:- Involves gas exchange (respiratory bronchioles, alveoli).

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Nose

The nose consists of the external nose and the internal nasal cavity. The external nose is structured by bones and cartilage, while the internal cavity has a mucous membrane with cilia and goblet cells for filtration and moisture.

Nasal Conchae:- Three pairs (superior, middle, inferior) filter, warm, and humidify air.

Cell Types in Nasal Cavity:-

• Ciliated Columnar Cells:- Propel mucus.

• Goblet Cells:- Produce mucus.

• Basal Cells:- Stem cells for epithelial regeneration.

• Olfactory Receptor Cells:- Detect odors.

• Supporting Cells:- Support olfactory cells.

Rhinitis:- Inflammation of the nasal mucosa, either allergic, infectious, or non-allergic.

Paranasal Sinuses:- Air-filled cavities (frontal, maxillary, ethmoid, sphenoid) lighten the skull, produce mucus, and affect voice resonance. Sinusitis occurs when they are blocked.

Lacrimal Duct:- Drains tears from the eyes to the nose, causing nasal discharge during crying.

Olfactory Receptors:- Located in the nasal cavity, these neurons detect smells and send signals to the brain.

Choanal Atresia:- A congenital blockage of the nasal passage that can be unilateral or bilateral, often requiring surgery.

Additional Nose Functions:- The nose filters, humidifies, warms air, and contributes to speech resonance.

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The Pharynx

The pharynx, or throat, is a muscular tube that functions in both respiration and digestion. It directs air from the nose and mouth to the larynx and lungs, and food from the mouth to the esophagus. It is divided into three regions: nasopharynx, oropharynx, and laryngopharynx.

Anatomy of the Pharynx

The pharynx is 12-14 cm long, extending from the base of the skull to the sixth cervical vertebra. It sits behind the nasal cavities, mouth, and larynx, with walls made of skeletal muscle and a mucous membrane.

• Nasopharynx:- Located behind the nasal cavity, it serves as an air passageway and contains the pharyngeal tonsil (adenoids) and Eustachian tube openings, which connect to the middle ear for pressure equalization. The soft palate closes the nasopharynx during swallowing.

• Oropharynx:- Behind the oral cavity, it allows air and food to pass. It houses the palatine and lingual tonsils, part of the immune system, and is lined with stratified squamous epithelium for protection.

• Laryngopharynx:- The lowest part, behind the larynx, directs food into the esophagus and air into the larynx. It contains piriform sinuses and the epiglottis, which prevent food from entering the airway during swallowing.

Functions of the Pharynx

• Respiration:- Air passes through the pharynx to the lungs, filtered and warmed.

• Swallowing:- Food is directed from the mouth to the esophagus by coordinated muscle movements.

• Speech:- It acts as a resonating chamber, affecting voice quality and tone.

• Immune Defense:- Tonsils trap pathogens entering through the nose and mouth.

• Pressure Regulation:- The Eustachian tubes help balance air pressure in the middle ear.

Pathological Conditions

• Pharyngitis:- Inflammation of the pharynx, often from infection, causes sore throat and difficulty swallowing.

• Tonsillitis:- Inflammation of the tonsils, potentially leading to tonsil removal.

• Sleep Apnea:- Blocked airflow during sleep due to collapsed pharyngeal muscles.

• Laryngopharyngeal Reflux (LPR):- Stomach acid irritates the pharynx, causing chronic cough and hoarseness.

• Pharyngeal Cancer:- Malignancies in the pharynx linked to smoking, alcohol, and HPV, presenting symptoms like sore throat and difficulty swallowing.

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The Larynx

The larynx, or voice box, is a crucial organ located between the third and sixth cervical vertebrae (C4-C6). It is vital for sound production, airway protection, and breathing. The larynx is composed of nine cartilages, muscles, membranes, and ligaments, working together to perform its functions.

Cartilages of the Larynx

• Thyroid Cartilage:- The largest cartilage, shaped like a shield. It forms the front wall of the larynx and features the Adam’s apple, which is more prominent in males due to testosterone. It connects to the hyoid bone via the thyrohyoid membrane and articulates with the cricoid cartilage below.

• Cricoid Cartilage:- The only complete ring of cartilage, forming the base of the larynx. It connects with the thyroid cartilage and plays a role in voice pitch modulation.

• Epiglottis:- A leaf-shaped flap that covers the laryngeal inlet during swallowing, preventing food and liquids from entering the trachea.

Paired Cartilages

• Arytenoid Cartilages:- Pyramid-shaped and crucial for vocal cord movement. They connect to muscles that control vocal tension.

• Corniculate Cartilages:- Small, horn-like structures on top of the arytenoids, aiding vocal fold stabilization.

• Cuneiform Cartilages:- Rod-like structures that support the aryepiglottic folds and keep the airway open during breathing.

Adam’s Apple

The Adam’s apple is a protrusion of the thyroid cartilage, more visible in males due to hormonal growth during puberty. It is responsible for a deeper male voice by affecting the size and angle of the larynx.

Vocal Cords

• True Vocal Cords:- These mucosal folds are directly involved in sound production. The vibration of the cords, caused by air from the lungs, produces sound. The tension and length of the cords determine pitch, while vibration force affects volume.

• False Vocal Cords:- Located above the true vocal cords, they protect the airway by closing during swallowing but do not contribute to sound production.

Intrinsic Muscles

• Cricothyroid Muscle:- Tenses and lengthens the vocal cords, increasing pitch.

• Thyroarytenoid Muscle:- Relaxes and shortens the cords, lowering the pitch.

• Lateral Cricoarytenoid Muscle:- Narrows the glottis for phonation by bringing vocal cords together.

• Posterior Cricoarytenoid Muscle:- Abducts the vocal cords to open the airway for breathing.

• Transverse and Oblique Arytenoid Muscles:- Close the back of the glottis during phonation and protect the airway during swallowing.

Functions of the Larynx

• Phonation:- Produces sound by vibrating the vocal cords as air passes through.

• Airway Protection:- Prevents food and liquids from entering the lungs during swallowing with the help of the epiglottis and vocal cords.

• Respiration:- Regulates airflow by controlling the glottis size, which is essential for breathing.

• Cough Reflex:- Expels irritants from the airway by trapping air in the lungs and forcefully releasing it.

Vascular Supply 

The larynx receives blood from the superior and inferior thyroid arteries and drains through corresponding veins. It is innervated by the vagus nerve, with branches like the superior and recurrent laryngeal nerves controlling muscle function and sensation. Damage to these nerves can affect voice and breathing.

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The Trachea

The trachea, or windpipe, is a key structure in the respiratory system, enabling air to move between the lungs and the external environment. Located in the lower respiratory tract, it plays an essential role in respiration.

Anatomy of the Trachea

The trachea is a tube about 10-12 cm long and 2.5 cm in diameter, starting at the larynx (C6 vertebra) and ending at the fifth thoracic vertebra (T5). Positioned in front of the esophagus, it descends into the chest.

Structural Features

• Cartilaginous Framework:- It has 16-20 C-shaped hyaline cartilage rings that keep it open and flexible, allowing the esophagus to expand during swallowing.

• Trachealis Muscle:- Connects the open ends of the rings, controlling the trachea’s diameter during coughing or breathing.

• Mucosal Lining:- Pseudostratified ciliated columnar epithelium lines the trachea, with goblet cells producing mucus to trap particles. Cilia move the mucus upwards.

• Submucosa:- A layer beneath the mucosa contains glands, nerves, and blood vessels, producing mucus to humidify air.

• Adventitia:- The outer connective tissue layer supports and anchors the trachea.

• Bifurcation & Carina:- The trachea splits at T5 into two main bronchi. The carina, a sensitive ridge, triggers the cough reflex if irritated.

Functions of the Trachea

• Air Conduction:- It acts as a passage for air between the larynx and bronchi, staying open during breathing.

• Air Filtration & Humidification:- Mucus traps particles, and cilia move them upwards for expulsion, cleaning the air before it enters the lungs.

• Cough Reflex:- When irritated, the trachea triggers a forceful expulsion of air to clear irritants.

• Support & Flexibility:- Cartilage provides structure, while muscle flexibility adjusts its diameter for breathing and coughing.

Clinical Significance

• Tracheal Stenosis:- Narrowing of the trachea, often due to injury or prolonged intubation, can restrict breathing and may need surgery.

• Tracheitis:- Infection-caused inflammation of the trachea, treated with antibiotics or antivirals.

• Tracheomalacia:- Weakening of tracheal cartilage causes collapse during breathing, managed with CPAP or surgery.

• Tracheostomy:- A surgical opening in the trachea for long-term ventilation or to bypass airway obstructions.

• Foreign Body Aspiration:- Objects in the trachea can block airways, requiring immediate medical attention.

Vascular Supply

The trachea is supplied by the inferior thyroid and bronchial arteries, with venous drainage into the brachiocephalic veins. It’s innervated by the vagus nerve, controlling mucus secretion and the cough reflex, and by the sympathetic trunk, which regulates dilation.

Developmental Anatomy

The trachea develops from the laryngotracheal tube during embryogenesis, with cartilage rings forming during fetal development. Abnormalities like tracheoesophageal fistula may require correction after birth.

Role in Respiratory Mechanics

The trachea stays open during inhalation due to its rigid cartilage, ensuring proper airflow and efficient gas exchange during both inhalation and exhalation.

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Detailed Functions of the Lungs 

• Filtration of Blood Clots:- The lungs filter small blood clots that may form in the veins, especially those arising from deep vein thrombosis (DVT). When these clots travel to the lungs, they can become lodged in the pulmonary arteries, leading to pulmonary embolism (PE). While large clots can be dangerous, smaller clots are often broken down within the lungs.

  Metabolism of Bioactive Substances:- The lungs metabolize certain bioactive substances, such as inactivating bradykinin and serotonin. They also convert angiotensin I to angiotensin II via the angiotensin-converting enzyme (ACE), located on the surface of pulmonary capillary endothelial cells.

  Protection Against Inhaled Pathogens and Particles:-

  • Mucociliary Escalator:- This system in the bronchi and bronchioles traps foreign particles in mucus produced by goblet cells. Ciliated epithelial cells then move the mucus toward the pharynx to be swallowed or expelled.

  • Alveolar Macrophages:- Immune cells in the alveoli engulf and digest pathogens and particulates, helping keep the alveoli clean.

  Thermoregulation:- The lungs assist in thermoregulation by adjusting breathing rates. Faster breathing helps dissipate heat, while slower breathing conserves it.

  Cellular Composition of the Lungs

  • Type 1 Alveolar Cells (Pneumocytes):-

    • Structure and Function:- Thin, flat cells covering 95% of the alveolar surface, designed for efficient gas exchange.

    • Proportion:- Although covering most of the surface, they make up only about 40% of the alveolar cell population.

  • Type 2 Alveolar Cells (Pneumocytes):-

    • Structure and Function:- Cuboidal cells produce surfactant, a substance that reduces surface tension to prevent alveolar collapse during exhalation and makes inhalation easier.

    • Proportion:- These cells make up 60% of the alveolar population but only cover 5% of the alveolar surface area. They can differentiate into Type 1 cells for lung repair.

  • Alveolar Macrophages:-

    • Structure and Function:- These immune cells patrol the alveolar spaces, engulfing pathogens and debris.

    • Proportion:- Present in fewer numbers but essential for lung immunity.

  Additional Lung Functions

  • Ventilation-Perfusion (V/Q) Matching:- Efficient gas exchange requires matching ventilation (airflow) and perfusion (blood flow) in the alveoli. Mismatches, as seen in conditions like PE or COPD, impair gas exchange.

  • Surfactant Production and Function:- Surfactant, produced by Type 2 cells, reduces surface tension in the alveoli. Its absence, as in premature infants, can lead to alveolar collapse and respiratory distress.

  • Elastic Recoil and Compliance:- The lungs’ elastic fibers help them return to their original shape after inhalation. Lung compliance refers to how easily the lungs expand; reduced compliance, as in pulmonary fibrosis, can hinder breathing.

  • Oxygen-Hemoglobin Dissociation Curve:- This curve describes how hemoglobin binds and releases oxygen. Factors like pH, temperature, and carbon dioxide levels affect the curve, altering oxygen affinity.

  • Control of Breathing:- The brainstem (medulla oblongata and pons) regulates breathing based on feedback from chemoreceptors that monitor oxygen, carbon dioxide, and pH levels in the blood.

  • Diaphragm and Intercostal Muscles:- The diaphragm, the main respiratory muscle, contracts to increase thoracic volume for inhalation. Intercostal muscles raise the ribs, further expanding the chest, while elastic recoil aids exhalation.

  • Respiratory Volumes and Capacities:-

    • Tidal Volume (TV):- Air inhaled or exhaled during normal breathing, about 500 ml.

    • Inspiratory Reserve Volume (IRV):- Additional air inhaled after normal inhalation, about 3,000 ml.

    • Expiratory Reserve Volume (ERV):- Additional air exhaled after normal exhalation, about 1,200 ml.

    • Residual Volume (RV):- Air remaining after maximal exhalation, about 1,200 ml.

    • Total Lung Capacity (TLC):- Maximum air volume in the lungs, about 6,000 ml.

  Pathophysiology Considerations

  • Airway Resistance:- Resistance is determined by airway diameter. Conditions like asthma or chronic bronchitis can narrow airways, increasing resistance and making breathing difficult.

  • Pulmonary Compliance:- Reduced compliance, as in restrictive lung diseases like pulmonary fibrosis, impairs lung expansion and gas exchange.

  • Hypoxic Pulmonary Vasoconstriction:- When ventilation is poor, this mechanism directs blood away from poorly ventilated areas to optimize gas exchange. Extensive vasoconstriction can lead to pulmonary hypertension.

  • Pulmonary Hypertension:- Chronic lung disease or left heart failure can elevate pressure in the pulmonary arteries, leading to right ventricular hypertrophy and right-sided heart failure (cor pulmonale).

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Respiration

Respiration is the process by which living organisms take in oxygen and release carbon dioxide. This process is vital for energy production within cells and occurs through gas exchange, airflow, and pressure changes between the environment and the lungs.

Phases of Respiration

1. Pulmonary Ventilation:-
   This refers to the movement of air between the environment and the lungs, controlled by muscles and pressure differences. Inhalation brings air into the lungs, and exhalation expels it.

2. Gas Exchange:-
   Oxygen enters the blood from the lungs, and carbon dioxide is removed from the blood and expelled during breathing. This exchange occurs in the lungs and between the blood and body tissues.

3. Oxygen and Carbon Dioxide Transport:-
   Oxygen is carried by red blood cells from the lungs to body cells, while carbon dioxide is transported back to the lungs for exhalation.

Mechanism of Breathing

1. Inhalation:-
   The diaphragm contracts and flattens, expanding the chest cavity. The external intercostal muscles lift the rib cage, increasing lung volume and lowering internal lung pressure, drawing air in.

2. Exhalation:-
   When the diaphragm and intercostal muscles relax, the chest cavity contracts, raising lung pressure and forcing air out. This process is passive during normal breathing but involves extra muscles during forced exhalation.

Primary Inspiratory Muscles

1. Diaphragm:-
   The diaphragm is the most important muscle for inhalation. Its contraction increases the thoracic cavity’s volume, lowering lung pressure and allowing air to enter.

2. External Intercostal Muscles:-
   These muscles help lift the ribs during inhalation, expanding the chest and assisting airflow into the lungs.

Accessory Inspiratory and Expiratory Muscles:-

During deep breathing or exertion, muscles like the sternocleidomastoid ( raised sternum), scalenes( raised first 2 ribs), and pectoralis & trapezius ( raised shoulders) assist in expanding the chest, while internal intercostal and abdominal muscles help in forced exhalation by compressing the lungs.

Pressure and Airflow in Respiration

1. Atmospheric Pressure:-

   • The air pressure outside the body remains constant at sea level (760 mmHg). This pressure drives airflow into and out of the lungs during respiration.

2. Intrapulmonary Pressure (Lung Pressure):-

   • This is the pressure inside the lungs.

   • Inhalation:- The chest cavity expands, causing the intrapulmonary pressure to drop below atmospheric pressure (around 758 mmHg), allowing air to flow into the lungs.

   • Exhalation:- The chest cavity contracts, increasing intrapulmonary pressure (around 762 mmHg), which forces air out of the lungs.

3. Intrapleural Pressure:-

   • This is the pressure within the pleural cavity, always slightly lower than intrapulmonary pressure.

   • During Inhalation:- The intrapleural pressure becomes even more negative (around 754 mmHg), helping keep the lungs expanded.

   • During Exhalation:- The pressure increases slightly (around 758 mmHg) but remains lower than lung pressure, preventing lung collapse.

4. Pressure Gradient:-

   • Air flows from areas of higher pressure (atmosphere) to areas of lower pressure (lungs).

   • The balance between atmospheric, intrapulmonary, and intrapleural pressures regulates the airflow during inhalation and exhalation.

This pressure difference is key to the movement of air in and out of the lungs, ensuring efficient gas exchange.

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Breathing Patterns

1. Eupnea:-

   • Condition:- Normal, unlabored breathing with a rate of 12-20 breaths/min for adults.

   • Assessment:- Steady depth and rhythm, no effort required.

   • Associated Conditions:- Healthy respiratory system.

2. Tachypnea:-

   • Condition:- Rapid, shallow breathing.

   • Assessment:- Respiratory rate above 20 breaths/min, shallow depth.

   • Associated Conditions:- Fever, anxiety, pneumonia, or heart failure.

3. Bradypnea:-

   • Condition:- Abnormally slow breathing.

   • Assessment:- Rate below 12 breaths/min, shallow depth.

   • Associated Conditions:- Drug overdose, brain injury, or hypothyroidism.

4. Cheyne-Stokes Respiration:-

   • Condition:- Alternating deep, fast breaths followed by apnea.

   • Assessment:- Irregular breathing cycles.

   • Associated Conditions:- Heart failure, stroke, brain injury.

5. Kussmaul Respiration:-

   • Condition:- Deep, labored breathing in response to metabolic acidosis.

   • Assessment:- Rapid, deep breathing.

   • Associated Conditions:- Diabetic ketoacidosis, kidney failure.

6. Obstructive Sleep Apnea:-

   • Condition:- Blocked airway during sleep, causing pauses in breathing.

   • Assessment:- Sleep studies reveal interruptions, snoring, and fatigue.

   • Associated Conditions:- Obesity, large tonsils, hypertension.

7. Hyperventilation:-

   • Condition:- Rapid, deep breathing with excessive CO₂ loss.

   • Assessment:- High respiratory rate, anxiety, dizziness.

   • Associated Conditions:- Panic attacks, stress, metabolic conditions.

8. Hypoventilation:-

   • Condition:- Slow, shallow breathing leading to CO₂ retention.

   • Assessment:- Shallow breaths, low respiratory rate.

   • Associated Conditions:- COPD, neuromuscular disorders, opioid overdose.