Respiratory System Overview

• Air moves through the nose, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, and finally the alveoli.

Respiration Processes

• Pulmonary ventilation involves breathing, including inspiration and expiration.
• External respiration is the exchange of gases between alveoli and pulmonary capillaries. Oxygen diffuses into the blood, and carbon dioxide diffuses into the alveoli.
• Gas transport involves oxygenated blood traveling to body tissues and carbon dioxide-rich blood returning to the lungs.
• Internal respiration is the exchange of gases between blood in systemic capillaries and body cells. Oxygen diffuses into the tissues, and carbon dioxide diffuses into the blood.
• Diffusion enables the exchange of oxygen and carbon dioxide between the lungs, bloodstream, and body cells.

Larynx

• The larynx is located superior to the trachea and inferior to the laryngopharynx
• The larynx transports air in and out of the trachea and helps prevent particles from entering.
• It houses the vocal cords and is made of muscles and cartilage bound by elastic tissue.
• The upper pair are the false vocal cords, which produce no sound.
• The lower pair are the true vocal cords, which produce sound.
• The glottis includes the true vocal cords and the opening between them.
• The epiglottis stands vertically, allowing air into the larynx but closes off the glottis during swallowing to prevent food from entering.

Lungs

• The right lung has three lobes: upper, middle, and lower.
• The left lung has two lobes: upper and lower.
• The left lung has a cardiac notch to accommodate the heart.

Alveoli

• Alveoli are microscopic air sacs clustered at the ends of alveolar ducts and are the only sites of gas exchange between air and blood.
• They consist of simple squamous epithelium, which is thin for efficient diffusion of oxygen and carbon dioxide.

Respiratory Membrane

• The respiratory membrane consists of simple squamous epithelial cells of the alveolus, endothelial cells of the capillary, and fused basement membranes.
• It facilitates gas diffusion, allowing oxygen to enter the bloodstream and carbon dioxide to diffuse into the alveoli for exhalation.
• This gas exchange maintains oxygen levels and removes carbon dioxide.

Inhalation Steps

• The diaphragm and intercostal muscles contract, expanding the thoracic cavity.
• This expansion decreases air pressure within the lungs compared to atmospheric pressure.
• Air moves from high to low pressure, causing air to rush into the lungs, then the lungs expand.

Exhalation Steps

• The diaphragm and intercostal muscles relax.
• The rib cage returns to its original position, decreasing thoracic cavity volume.
• Lung volume decreases, which increases air pressure within the lungs.
• Air flows out of the lungs, which is generally passive but can be forced using accessory muscles.
• Atmospheric pressure (760 mmHg at sea level) drives breathing.
• Air flows from high to low pressure. With inhalation air rushes in when lung pressure is lower; during exhalation, pressure is higher, and air flows out.
• Intra-alveolar pressure is within the alveoli.
• Intrapleural pressure is the pressure between the lungs and chest wall.
• During inhalation, the expansion of the thoracic cavity decreases intra-alveolar and intrapleural pressures, causing air to enter the lungs.
• During exhalation, the decrease in volume increases intra-alveolar pressure, forcing air out.

Respiratory Centers

• The human respiratory system facilitates gas exchange, moving air to deliver oxygen and remove carbon dioxide.
• The system involves the upper and lower respiratory tracts and organs such as the lungs, trachea, and bronchi, working with the circulatory system to transport oxygen-rich blood.
• Respiratory centers are located in the medulla oblongata and pons of the brainstem.

Surfactant

• Surfactant, a liquid in the lungs, reduces surface tension in the alveoli and prevents their collapse.
• Respiratory distress syndrome (RDS) is a lung disorder in premature infants where alveoli collapse due to insufficient surfactant.

Tidal Volume

• Tidal volume is the amount of air inhaled and exhaled during normal breathing.

Factors Affecting Respiration

• Carbon dioxide (CO2) is the most important chemical in controlling respiratory rate.
• CO2 levels in the blood and cerebrospinal fluid stimulate the medulla oblongata and pons to control respiration.
• Increased CO2 levels cause faster and deeper breathing to remove excess CO2 and maintain blood pH.
• Hyperventilation causes respiratory alkalosis by decreasing CO2 levels, leading to elevated blood pH.
• Hypoventilation increases plasma pCO2, known as hypercapnia, due to insufficient CO2 expulsion and its buildup in the blood.

Carbon Dioxide Transport

• Around 5-7% of carbon dioxide is dissolved directly in the blood plasma.
• Around 10% binds to hemoglobin in red blood cells, forming carbaminohemoglobin.
• Around 85% is transported as bicarbonate ions (HCO3-). Carbon dioxide combines with water to form carbonic acid in red blood cells, then dissociates into hydrogen and bicarbonate ions, which diffuse out of the red blood cells in exchange for chloride ions.
• In the lungs, bicarbonate ions enter red blood cells, combine with hydrogen ions to form carbonic acid, then converted back into carbon dioxide and water to be exhaled.

Carbaminohemoglobin and Carbonic Anhydrase

• Carbaminohemoglobin is a compound formed when carbon dioxide (CO2) binds to hemoglobin.
• Carbonic anhydrase is a zinc-containing enzyme that catalyzes the reversible interconversion of carbon dioxide (CO2) and water (H2O) into carbonic acid (H2CO3), which then dissociates into bicarbonate (HCO3-) and hydrogen ions (H+).

Oxygen Transport

• Over 98% of oxygen is carried bound to hemoglobin in red blood cells as oxyhemoglobin.
• Around 2% is carried dissolved in the plasma.
• Alveolar PO2 is higher than that of blood in alveolar capillaries, so O2 constantly diffuses from alveoli into blood.

Hemoglobin

• Oxyhemoglobin is hemoglobin with oxygen bound to it.
• Deoxyhemoglobin is hemoglobin without oxygen.