🫀Anatomy and Physiology II Unit 5 – Respiratory System: Gas Exchange & Transport

Key Structures and Functions

• Nose and nasal cavity filter, warm, and humidify inhaled air before it reaches the lungs
• Pharynx is a passageway for both air and food, connects the nasal and oral cavities to the larynx and esophagus
• Larynx, also known as the voice box, contains the vocal cords and prevents food and liquid from entering the trachea during swallowing
• Trachea, or windpipe, is a tubular structure that connects the larynx to the bronchi and allows for the passage of air
  • Lined with ciliated pseudostratified columnar epithelium and mucus-secreting goblet cells to trap and remove debris
• Bronchi are two main branches of the trachea that lead to the left and right lungs
  • Further divide into smaller bronchioles, which terminate in clusters of alveoli
• Lungs are the primary organs of the respiratory system, responsible for gas exchange between the air and the blood
  • Contain millions of alveoli, tiny air sacs surrounded by capillaries, where gas exchange occurs
• Diaphragm is a dome-shaped muscular structure that separates the thoracic cavity from the abdominal cavity and plays a crucial role in breathing

Mechanics of Breathing

• Breathing, or ventilation, involves the movement of air in and out of the lungs
• Inhalation (inspiration) occurs when the diaphragm and external intercostal muscles contract
  • Diaphragm flattens and moves downward, increasing the volume of the thoracic cavity
  • External intercostal muscles lift the ribs upward and outward, further expanding the chest cavity
• Increased volume of the thoracic cavity decreases the pressure inside the lungs relative to the atmospheric pressure
  • Air flows from an area of higher pressure to lower pressure, causing air to rush into the lungs
• Exhalation (expiration) is typically a passive process involving the relaxation of the diaphragm and external intercostal muscles
  • Elastic recoil of the lungs and chest wall causes the thoracic cavity to decrease in volume
  • Increased pressure inside the lungs relative to the atmospheric pressure forces air out of the lungs
• During forced exhalation, internal intercostal muscles and abdominal muscles contract to push the diaphragm upward and compress the lungs, expelling air more forcefully

Gas Exchange Process

• Gas exchange occurs between the alveoli and the blood in the surrounding capillaries
• Alveoli are tiny, thin-walled air sacs that provide a large surface area for efficient gas exchange
  • Alveolar walls are composed of simple squamous epithelium, which allows for rapid diffusion of gases
• Oxygen diffuses from the alveoli, where its partial pressure is high, into the blood, where its partial pressure is low
  • Oxygen binds to hemoglobin in red blood cells, forming oxyhemoglobin
• Carbon dioxide diffuses from the blood, where its partial pressure is high, into the alveoli, where its partial pressure is low
  • Carbon dioxide is produced by cellular respiration in the tissues and transported to the lungs for removal
• Diffusion of gases occurs rapidly due to the large surface area of the alveoli, thin alveolar-capillary membrane, and steep concentration gradients
• Ventilation-perfusion ratio (V/Q ratio) is the relationship between the amount of air reaching the alveoli (ventilation) and the amount of blood flowing through the capillaries (perfusion)
  • Optimal gas exchange occurs when the V/Q ratio is balanced, ensuring that well-ventilated alveoli are matched with well-perfused capillaries

Oxygen Transport in Blood

• Oxygen is primarily transported in the blood by binding to hemoglobin in red blood cells
• Hemoglobin is a protein consisting of four subunits, each containing an iron-containing heme group that can bind one oxygen molecule
  • When oxygen binds to hemoglobin, it forms oxyhemoglobin, giving oxygenated blood its bright red color
• Oxygen-hemoglobin dissociation curve describes the relationship between the partial pressure of oxygen and the percentage of hemoglobin saturated with oxygen
  • At high partial pressures of oxygen (in the lungs), hemoglobin becomes fully saturated with oxygen
  • At lower partial pressures of oxygen (in the tissues), oxygen dissociates from hemoglobin and is released to the cells
• Factors affecting the oxygen-hemoglobin dissociation curve include temperature, pH, and the presence of 2,3-bisphosphoglycerate (2,3-BPG)
  • Higher temperature, lower pH, and increased 2,3-BPG levels shift the curve to the right, promoting the release of oxygen to the tissues
• A small amount of oxygen is also dissolved directly in the plasma, but this accounts for only a minor portion of the total oxygen content in the blood

Carbon Dioxide Transport

• Carbon dioxide is transported in the blood in three main forms: dissolved in plasma, bound to hemoglobin, and as bicarbonate ions
• Dissolved carbon dioxide: A small amount of carbon dioxide (about 10%) is dissolved directly in the plasma
• Carbaminohemoglobin: Carbon dioxide binds to the amino groups of hemoglobin, forming carbaminohemoglobin (about 20%)
  • This binding is reversible and helps to facilitate the transport of carbon dioxide from the tissues to the lungs
• Bicarbonate ions: The majority of carbon dioxide (about 70%) is transported as bicarbonate ions in the plasma
  • In the tissues, carbon dioxide diffuses into red blood cells, where it is converted to carbonic acid by the enzyme carbonic anhydrase
  • Carbonic acid dissociates into hydrogen ions and bicarbonate ions
  • Bicarbonate ions are then transported out of the red blood cells into the plasma in exchange for chloride ions (chloride shift)
• In the lungs, these processes are reversed, allowing carbon dioxide to be released from the blood and exhaled
• The transport of carbon dioxide as bicarbonate helps to maintain the blood's pH by buffering the hydrogen ions produced during cellular respiration

Regulation of Breathing

• Breathing is regulated by the respiratory center in the brainstem, which consists of the medulla oblongata and the pons
• The medulla oblongata contains the dorsal respiratory group (DRG) and the ventral respiratory group (VRG)
  • DRG is primarily responsible for generating the basic rhythm of breathing
  • VRG controls the depth and rate of breathing in response to sensory input
• The pons contains the pneumotaxic center and the apneustic center, which modulate the activity of the medulla oblongata
  • Pneumotaxic center helps to regulate the transition between inhalation and exhalation
  • Apneustic center prolongs inhalation and prevents the lungs from overinflating
• Chemoreceptors detect changes in blood chemistry and send signals to the respiratory center to adjust breathing
  • Central chemoreceptors in the medulla oblongata detect changes in cerebrospinal fluid pH
  • Peripheral chemoreceptors in the carotid and aortic bodies detect changes in blood pH, oxygen, and carbon dioxide levels
• Stretch receptors in the lungs and chest wall provide feedback to the respiratory center about lung inflation and prevent overinflation
• Higher brain centers, such as the cerebral cortex and hypothalamus, can also influence breathing in response to emotions, pain, and temperature changes

Common Respiratory Disorders

• Asthma is a chronic inflammatory disorder characterized by airway hyperresponsiveness and reversible airflow obstruction
  • Triggered by allergens, irritants, or exercise, leading to bronchoconstriction, mucus production, and airway inflammation
• Chronic obstructive pulmonary disease (COPD) is a progressive lung disease that encompasses chronic bronchitis and emphysema
  • Characterized by persistent airflow limitation, inflammation, and destruction of lung tissue
  • Primarily caused by long-term exposure to irritants, such as cigarette smoke or air pollution
• Pneumonia is an infection of the lungs caused by bacteria, viruses, or fungi
  • Leads to inflammation and fluid accumulation in the alveoli, impairing gas exchange
• Lung cancer is a malignant tumor that originates in the lungs and can metastasize to other parts of the body
  • Risk factors include smoking, exposure to radon or asbestos, and family history
• Sleep apnea is a disorder characterized by repeated pauses in breathing during sleep
  • Can be obstructive (due to airway collapse) or central (due to impaired signaling from the brain to the respiratory muscles)
• Acute respiratory distress syndrome (ARDS) is a severe, life-threatening condition characterized by widespread inflammation and fluid accumulation in the lungs
  • Often occurs as a complication of sepsis, severe pneumonia, or trauma

Clinical Applications

• Pulmonary function tests (PFTs) are a group of tests that assess lung function and help diagnose respiratory disorders
  • Spirometry measures the volume and flow of air during inhalation and exhalation
  • Lung volume measurements determine the amount of air in the lungs at different points during the breathing cycle
• Pulse oximetry is a non-invasive method of measuring the oxygen saturation of the blood using a sensor placed on the finger or earlobe
  • Useful for monitoring patients with respiratory disorders or those undergoing anesthesia
• Arterial blood gas (ABG) analysis measures the levels of oxygen, carbon dioxide, and pH in the blood
  • Helps to assess the effectiveness of gas exchange and the acid-base balance of the body
• Mechanical ventilation is used to support or replace spontaneous breathing in patients with respiratory failure
  • Invasive ventilation involves the use of an endotracheal tube or tracheostomy to deliver air directly to the lungs
  • Non-invasive ventilation, such as continuous positive airway pressure (CPAP), delivers air through a mask or nasal prongs
• Oxygen therapy is used to treat hypoxemia (low blood oxygen levels) in patients with respiratory disorders
  • Can be delivered through nasal cannulas, face masks, or high-flow nasal oxygen systems
• Pulmonary rehabilitation is a comprehensive program designed to improve the quality of life for patients with chronic respiratory disorders
  • Includes exercise training, education, nutritional counseling, and psychological support to help patients manage their symptoms and improve their functional capacity