Respiratory System: Gaseous Exchange

• The lecture discusses the gaseous exchange of the respiratory system, including essential structures, ventilation mechanics, gaseous exchange processes, and respiratory control.

Respiratory Tract Functional Organization

• The conducting portion is responsible for moving air into and out of the lungs, including structures like the pharynx, larynx, trachea, bronchi, and larger bronchioles.
• The respiratory portion facilitates gaseous exchange in the lungs via delicate bronchioles and alveoli.

Four Processes of Respiration

• Pulmonary ventilation: The process of breathing
• External respiration: Gas exchange between the lungs and the blood
• Transport of respiratory gases: The movement of oxygen and CO2
• Internal respiration: Gas exchange between the blood and the tissues

Respiratory Mucosa

• Mucous membrane lines the conducting portion of the respiratory system.
• Ciliated columnar epithelium is present.
• Goblet cells secrete mucus.
• A loose connective tissue layer is formed by these cells.
• Functions include filtering, warming, humidifying the air, and protecting alveoli.

Bronchial Tree

• The bronchial tree divides into:
• With cartilages:
  • Primary (extrapulmonary) bronchi
  • Secondary (lobar) bronchi
  • Tertiary (segmental) bronchi.
• Without cartilages:
  • Bronchioles and terminal bronchioles.
• Autonomic nervous control (involuntary/unconscious)
• Sympathetic activation leads to bronchodilation
• Parasympathetic activation leads to bronchoconstriction.

Bronchioles, Alveolar Ducts, and Alveoli

• Each terminal bronchiole branches into respiratory bronchioles which connect to alveoli or alveolar ducts, ending in alveolar sacs.
• Alveolar sacs (~150 million) facilitate gas exchange with an extensive capillary network.

Alveolar Epithelium Characteristics

• Surrounded by pulmonary capillaries for gaseous exchange.
• Type I alveolar cells: squamous, thin for gas diffusion.
• Type II alveolar cells: Secrete pulmonary surfactants.
• Alveolar macrophages: eliminate infectious, toxic, or allergic particles.

Pulmonary Surfactants

• Fetal lungs produce pulmonary surfactants to reduce surface tension in the alveolar lining.
• Surfactants prevent alveolar collapse upon expiration after birth.
• Composed of lecithin (L) & sphingomyelin (S).
• Maturation of fetal lungs occurs (30th week to childhood).
• Alveolar type II cells produce surfactants.
• Phosphatidylcholine (lecithin) is stored/secreted by lamellar bodies.

Gestation and Surfactants

• Before 30th week fetal lung primarily synthesizes sphingomyelin.
• Around 30 weeks of gestation lamellar bodies increase, increasing surfactant.
• Later there is a sharp increase in lecithin.
• A lecithin to sphingomyelin (L/S) ratio of 2:1 or greater is characteristic of mature fetal lungs.

Infants Respiratory Distress Syndrome

• Infants lacking pulmonary surfactants in alveoli may have pale or bluish skin.
• Oxygen or surfactant administration can help.
• Pregnant women may receive glucocorticoid treatment to stimulate surfactant production before delivery.
• Glucocorticoid can stimulate lecithin synthesis
• Pulmonary surfactants are composed of lipids and proteins.

Ventilation and Gaseous Exchange

• Pulmonary ventilation (breathing) allows the refreshment of gases inside the lungs.
• External respiration (pulmonary gaseous exchange) involves oxygen and carbon dioxide exchange between alveolar air space and alveolar capillaries.
• Transport of respiratory gases includes oxygen and carbon dioxide transport in alveolar capillaries from the lungs to tissue cells.
• Internal respiration involves oxygen and carbon dioxide exchange between blood and tissue cells.

Pulmonary Ventilation

• Involves inspiration and expiration cycling.
• Air movement relies on pressure differences between the atmosphere and inside the lungs.
• According to Boyle's Law:
• Higher atmoshpere > lungs pressure, air flows into the lungs
• Lower atmosphere > lungs pressure, air flows out of the lungs

Structures Involved in Pulmonary Ventilation

• Lungs: Have parietal and visceral pleura but not directly connected to the rib cage.
• Diaphragm: Dome-shaped structure controlled by phrenic nerves.
• Rib cage: Composed of multiple pairs of ribs.
• Intercostal muscles: Composed of external, internal, and innermost layers.

Pressure and Air Movement

• Movement of the ribcage and diaphragm can change the thoracic cavity and lung volume .
• At rest, no net air movement because atmospheric pressure equals intrapulmonary pressure (760 mmHg at sea level).
• Inspiration involves diaphragm contraction and external intercostal muscles.
• This increases thoracic cavity volume.
• Lungs expand, and alveolar pressure decreases, causing air to flow in.
• Expiration involves relaxation of the diaphragm and contraction of internal intercostal muscles returning the ribcage to its original position and decreasing the thoracic cavity volume.
  • Alveolar pressure rises above atmospheric pressure.

Transpulmonary Pressure (PTP)

• Pressures within the lungs and pleural cavity are different.
• Ptp the difference between these two pressures.
• The pressure inside the pleural cavity is always lower than that inside the lungs.
• Lungs always recoil and collapse easily when breathing out so this pressure difference prevents lungs collapsing after every breath.

Gaseous Exchange

• External respiration occurs between alveolar space and capillaries.
• Partial pressure gradients allow rapid O2 and CO2 diffusion.
• Ventilation-perfusion matching is coupling of gases' amount of gases .
• Thin respiratory membrane to shorten distance of exchange.

Transport of Respiratory Gases

• O2 transport: bound to hemoglobin (Hb).
• Equation in the Lungs: Deoxy-Hb + O2 <> HbO2
• Equation in the Tissues: Deoxy-Hb + O2 <> HbO2
• CO2 transport:
• Dissolved in plasma (7-10%).
• Chemically bound to hemoglobin (~20%): CO2 + Hb <> HbCO2
• As bicarbonate ion in plasma (~70%): CO2 + H2O <> H2CO3 <> H+ + HCO3-. Enzyme involved: carbonic anhydrase

Internal Respiration

• A simple diffusion driven by the partial pressure gradient of O2 and CO2 on the opposite sides of exchange membranes.
• Tissue cells continuously consume O2 and produce CO2.
• Lowering partial pressure of O2 and increasing partial pressure of CO2 in blood.

Alveolus Gas Equation

• PAO₂ = FiO₂ x (PB - PH₂O) - PACO₂ / R
• PAO2: partial pressure of oxygen in alveolar gas.
• Fi02: fraction of inspired oxygen.
• PB: barometric pressure. PH₂O: water vapor pressure.
• PACO2: partial pressure of carbon dioxide in arterial blood.
• R: respiratory quotient.
• If PB drops(e.g. high altitude), PAO2 will be decreased (i.e. mountain sickness).
• If PACO2 rises, (e.g. poor pulmonary ventilation), PAO2 will be decreased (i.e. asthma).

Bicarbonate Buffer System

• A buffer is an aqueous solution that resists changes in pH when acids/bases are added to it.
• Kidneys are also important in this buffer system by reabsorbing filtered bicarbonates.
• The effective buffer near pH 7.4 because the H₂CO₃ of blood plasma is balanced with a reserve of CO2 gas in the lungs.

Arterial Blood Gas Analysis

• The radiometer measures:
• Blood pH: Acidosis vs. alkalosis
• pO2 (arterial oxygen level): Hypoxemia
• pCO2 (arterial carbon dioxide level): Hypocapnia vs. hypercapnia
• HCO3- (bicarbonate level in blood): Renal vs. respiratory problems (e.g. metabolic acidosis or respiratory acidosis)

Blood Sampling

• Arterial blood from the radial artery (forearm).
• Feel for the pulse.

Case Study 1 - Type 1 Diabetes

• 40 year old man with type 1 diabetes and high keto-acids, without severe respiratory problems.
• Blood pH is lower than normal (metabolic acidosis) due to keto-acids
• Bicarbonate is lower than normal:
• Acute factor: consume bicarbonate ions to bind
• Chronid factors: Impaired renal acid reabsorption
• pCO2 is lower than normal to stimulate faster ventilation and remove cardin dioxide gas molecules.

Case Study 2

• 80-year-old after surgery injects morphine for analgesia, but in overdose.
• Morphine suppresses.
• Blood pH lower indicates morphine suppresses the sensitivity to CO2.
• O2is lower + CO2 is higher indicates hypoventilation.
• Bicarbonate level higher indicates compensatory responses through the kidneys to correct respiratory acidosis conditions.

Respiration Control

• Respiratory centers of the brain:
• Involuntary(medulla oblongata and pons): sensory info from the lungs or respiratory trac _ Voluntary(cerebral cortex). Generates signals to control respiratory muscles.
• Chemoreceptor Reflex.
• The reflex is associated with changes in blood and cerebrospinal fluid.
• The depth and rate of respiration is controlled.
• Peripheral are at carolid and aortic bodies.
• Arteries are very sensitive to pO2 Pco2 levels.
• Sensitive to pH, and o2 levels.
• Located at medulla oblongata are cental recpetors .

Summary

• The respiratory system is divided into conduction and respiration sections.
• Ventilation + gas processes such as resp gases and external & internal respiration.
• Layered epithelium. Surfactant produced is reduce the alvelolar tension.
• Lungs shrink due to elastic structure. and creates a negative pressure to facilitate alveolar cavities.
• Oxygen bound to haemoglobin & CO2 dissolving. Also presents as Bicarbonate inside plasma
• Chemoreceptor controls depths and speeds up respiration