Respiratory System Anatomy and Function

• The main functions of the respiratory system include exchanging O2 and CO2 between air and blood, keeping body pH stable, protecting from harmful substances, and vocalisation.
• The functional anatomy of the respiratory system consists of the upper and lower respiratory tracts.

Upper Respiratory Tract

• The upper respiratory tract includes the mouth, nasal cavity, pharynx, and larynx which warms, wets, and filters air.
• The particles trapped by the filtering action in the upper respiratory tract are either swallowed, spat out, or expelled through sneezing.

Lower Respiratory Tract

• The lower respiratory tract consists of the trachea, two primary bronchi, branches of bronchi (22 divisions) i.e. bronchioles, and the lungs.
• The thoracic cage, which surrounds the respiratory system, comprises bones and muscles, including the sternum, ribs, diaphragm, and intercostal muscles.

Airways and Alveoli

• The respiratory system includes a conducting system and an exchange surface.
• The conducting system consists of the trachea, primary bronchi, and smaller bronchi, all the way down to the bronchioles.
• The trachea divides into 2 primary bronchi.
• The trachea has a diameter of 15-22 mm, with one of them and a cross-sectional area of 2.5 cm.
• The primary bronchi have a diameter of 10-15 mm and there are 2 of them.
• The smaller bronchi go trhough 22 divisions.
• The exchange surface consists of bronchioles and alveoli.
• The bronchioles have a diameter of 0.5 - 1mm.
• The exchange surface consists of alveoli, where gas exchange with the blood occurs, and there are 3-6 X 10^8 of them.
• The exchange of the alveoli has a a diameter of 0.3 mm and an area of >1 X 10^6 cm.

Alveolar Structures

• Pulmonary venules carry high O2.
• Pulmonary arterioles carry low O2.

Alveolar Cells

• Alveolar fluid lining contains pulmonary surfactant.
• Other structures in the alveoli include alveolar macrophage, alveolar cells type I and II, erythrocytes and pulmonary capillaries

Exchange Surface of the Alveoli

• The alveolar exchange surface contains a thin layer of alveolar epithelium, endothelium, surfactant, and fused basement membranes.
• It's imperative that this layer be thin otherwise gas exchange will be compromised

Pleura

• The pleura consists of a double-walled closed sac separating each lung from the thoracic wall where the parietal pleura is attached to the thoracic cage wall.
• The visceral pleura is attached to the lungs.
• The space between the pleural membranes contains pleural fluid, which keeps the membranes together and provides lubrication.

Gas Laws

• Boyle's Law describes the inverse relationship between pressure and volume of a gas in a system: P1V1 = P2V2
• Dalton’s law of partial pressures explains that an increase in pressure of one gas, will increase total pressure
• In the human body, it can be assumed that 'n' (number of moles) and 'T' (temperature) are constant, therefore V (volume) = 1/P.

Pulmonary Function Test

• Pulmonary function tests use a spirometer to measure lung volumes and capacities.
• Lung volumes and capacities include inspiratory reserve volume (IRV), tidal volume (VT), expiratory reserve volume (ERV), and residual volume.

Lung Capacities

• Total lung capacity measurement in males is 6000 ml, while in females it is 4200 ml.
• Functional residual capacity is 700ml.

Mechanics of Ventilation

• Inspiration and expiration depend on changes in intrapulmonary or alveolar pressure.
• Intrapleural pressure needs to be lower than atmospheric pressure for optimal breathing.
• During inspiration, volume increases, causing air to flow in. During expiration, volume decreases, causing air to flow out.
• During breathing in the chest expands, the ribs go up and the diaphragm contracts, while during breathing out, the chest contracts, the ribs move downward and the diaphragm relaxes.

Pressure Types in Respiration

• During each breath, pressure gradients move 0.5 liter of air into and out of the lungs.
• Intrapulmonary pressure inside the lung decreases during inspiration and increases during expiration.
• Intrapleural pressure in the pleural cavity becomes more negative as the chest wall expands during inspiration.
• Atmospheric pressure needs to be higher than alveolar pressure.

Anatomical Dead Space

• Anatomical dead space must be taken into account to determine determining alveolar ventilation. It is calculated by subtracting the number of breaths per minute from 500–150 ml per breath.

Ventilation

• Ventilation and perfusion are matched to ensure efficient gas exchange.
• Gas exchange occurs via diffusion.
• Air composition, ventilation properties such as resistance, perfusion, and diffusion properties between alveoli and blood influence gas exchange.

Gas Exchange

• Influences on gas exchange between alveoli and blood include air composition, ventilation properties like resistance, perfusion, and the properties of diffusion between alveoli and blood.
• Oxygen enters the blood at the alveolar-capillary interface and is transported in blood dissolved in plasma, or bound to hemoglobin inside RBCs, then oxygen diffuses into cells.
• CO2 diffuses out of cells and is transported dissolved, bound to hemoglobin, or as HCO3, then CO2 enters alveoli at the alveolar-capillary interface.
• In venous blood pO2 levels should be equal to or less than 40 mm Hg. pCO2 levels should be higher or equal to 46 mm Hg.

Pathological Conditions

• Pathological conditions causing hypoxia include asthma (decreased alveolar ventilation), pulmonary edema (fluid in the interstitial space), and emphysema (less surface area due to destruction of alveoli).

Mass Balance and Flow of Oxygen

• Arterial O2 transport minus venous O2 transport equals cell use of O2.
• O2 transport equals cardiac output (CO) X O2 concentration.
• Hence, CO X (Arterial [O2] - Venous [O2]) equals cell use of O2.
• Arterial transport: Hb+O2 > 98%; Dissolved O2 2
• O2 content is less than CO2, even in arterial blood where PaO2 is 100 mm Hg and PaCO2 is 40 mm Hg.
• Normally around 1% of total haemoglobin.
• High methaemoglobin levels in the blood can cause cyanosis

Blood Gas Transport

• Approximately 98% of blood O2 is delivered to tissues bound to haemoglobin.
• CO2 reacts with H2O to form H2CO3 (carbonic acid), then HCO3- (bicarbonate) + H+.
• If CO2 is not removed, pH disturbances (acidosis) occurs, as well as protein denaturation and nervous system dysfunction.
• In venous blood, CO2 70% load turns into bicarbonate.
• At the lungs ,CO2 unbinds and returns to HCO3, and turns into H2O and CO2 to be exhaled.
• Oxygenation of the blood results in CO2 being removed from the blood, and if PaCO2 = 0, there is no CO2 in the blood.

Lung Characteristics

• Factors that might affect lung parameter are Age, Height and Gender
• Diseases that might affect lung function includes COPD, Bronchitis, Asthma, Infections like Pneumonia, Emphysema, Fibrosis, and Silicosis.
• Vital Capacity is the maximal volume that can be expired after a maximal inspiration.
• Inspiratory Reserve Volume (IRV) is the maximal volume which can be inspired from end-tidal inspiration.
• Expiratory Reserve Volume (ERV) is the maximal volume which can be expired from the resting end-expiratory level.
• Inspiratory Capacity is the maximal volume which can be inspired from the resting end-expiratory level.
• Functional Residual Capacity (FRC) is the volume of gas in the lungs at the resting end-expiratory level.
• Residual Volume is the volume of gas in the lungs at the end of maximal expiration.
• Total Lung Capacity is the volume of gas in the lungs at the end of a maximal inspiration.