Respiratory System Functions

• Respiratory physiology involves both respiratory and non-respiratory functions.

Respiratory Functions

• External respiration involves the exchange of oxygen and carbon dioxide between the lungs and the atmosphere, known as pulmonary ventilation.
• Oxygen and carbon dioxide diffusion happens through the respiratory membrane.
• Oxygen and carbon dioxide get transported in blood.
• Internal respiration involves the exchange of oxygen and carbon dioxide between the blood and body tissues.

Non-Respiratory Functions

• Immune function is performed by ciliated epithelium, alveolar and interstitial macrophages, BALT, and IgA.
• Defensive respiratory reflexes include coughing, sneezing, and reflex apnea.
• The respiratory system acts as a blood reservoir.
• The respiratory system aids in the removal of small emboli and fibrinolysis.
• The respiratory system regulates pH.
• Phonation occurs in the respiratory system.
• Swallowing, defecation, micturition, vomiting, and sniffing are non-respiratory functions.
• Metabolic and endocrine functions include the production and elimination of biologically active substances.
• Angiotensin-converting enzyme converts ANGI to ANGII.
• Metabolic acidosis (decreased pH) is compensated by increased pulmonary ventilation.
• Decreased pulmonary ventilation could be a symptom of metabolic acidosis.
• Yawning can be caused by tiredness, sleepiness, boredom, or stress.

Basic Terms in Respiratory Physiology

• Anoxia = Absence of oxygen.
• Hypoxia = Deficiency in the amount of oxygen reaching the tissues.
• Hyperoxia = Excess oxygen or higher than normal partial pressure of oxygen.
• Hypocapnia = Reduced carbon dioxide in the blood.
• Hypercapnia = Excessive carbon dioxide in the blood.
• Cyanosis = Bluish discoloration of the skin due to poor circulation or inadequate oxygenation of the blood.
• Eupnea = Normal, good, unlabored breathing.
• Apnea = Temporary cessation of breathing, especially during sleep.
• Tachypnea = Abnormally rapid breathing.
• Bradypnea = Abnormally slow breathing.
• Dyspnea = Difficult or labored breathing.
• Hypopnea = Shallow or slow breathing.
• Hyperpnoea = Increased depth and rate of breathing.
• Normoventilation = Normal rate and depth of breathing.
• Hypoventilation = Ventilation of the lungs that does not fulfill the body’s gas exchange requirements.
• Hyperventilation = Ventilation of the lungs beyond normal body requirements.

Anatomy of the Respiratory System

• Upper respiratory tract consists of the nasal cavity and nasopharynx.
• Lower respiratory tract consists of the larynx, trachea, bronchi, bronchioles, alveolar ducts, and alveoli.

Terminal Respiratory Unit

• The terminal respiratory unit is characterized by small diameter, thin walls, a large number of units, a large surface area, and a high blood supply.
• There are 600 million alveoli in the lungs which have the surface area of a tennis court.
• Capillaries supplying alveoli belong to the pulmonary low-pressure circulation.
• Trachea and bronchi contain cartilage and smooth muscle.
• Bronchioles lack cartilage but contain smooth muscle.
• Ciliated epithelium is found from the trachea up to the terminal bronchioles and secretes mucus.
• Alveoli contain type I and type II alveolar cells (pneumocytes).

Alveolar Cells

• Type I alveolar cells cover 90% of the surface area, are flat, and form part of the respiratory membrane for gas diffusion.
• Type II alveolar cells cover 10% of the surface area, are granular and cuboidal, and produce surfactant.

Bronchiolar Musculature Control

• Nervous control involves parasympathetic innervation (acetylcholine, M-receptors) causing bronchoconstriction.
• Sympathetic innervation (norepinephrine, β2 receptors) causes bronchodilation.
• Non-adrenergic, non-cholinergic innervation include VIP that causes bronchodilation, and substance P that causes bronchoconstriction.
• Hormonal control involves histamine and circulating catecholamines.

Pleura

• The inner pleura (visceral pleura) covers the lungs.
• The outer pleura (parietal pleura) is attached to the chest wall and diaphragm.
• Pleural fluid exists between the two layers.
• Strong adhesion between layers is due to attractive forces among fluid molecules.
• Pleural fluid is transcellular fluid

Pulmonary Pressures

• Palv refers to alveolar pressure.
• Pip refers to pleural pressure.
• Ptp (transpulmonary pressure) is calculated as Palv - Pip.

Forces Affecting Alveolar Wall

• Elastic forces of the chest wall and lung tissue, along with surface tension, contribute to elastic recoil.
• Intrapleural space exists between the chest wall and lungs.
• Pneumothorax is the presence of air or gas in the cavity between the lungs and the chest wall, causing collapse of the lung.

Pulmonary Ventilation

• Pulmonary ventilation is the inflow and outflow of air between the atmosphere and alveoli.
• Airflow is always due to a pressure difference (gradient).
• F = (Patm - Palv) / R, where F is air flow, Patm is atmospheric pressure, Palv is alveolar pressure, and R is airway resistance.
• During inspiration, Palv < Patm.
• During expiration, Palv > Patm.
• When there is no air flow, Palv = Patm.
• Air flow is due to changes in thorax and lung volume.

Boyle's Law

• The absolute pressure of a gas is inversely proportional to the volume it occupies, given a constant temperature and amount of gas within a closed system.
• P x V = constant.
• Pleural pressure (Pip) decreases i.e. becomes more negative with increased thorax volume.

Respiratory Muscles

• Inspiration muscles include the diaphragm (phrenic nerve, C3-C5), external intercostals (intercostal nerves), and auxiliary muscles (scaleni, sternocleidomastoid muscle, serrati, pectorals).
• Expiration muscles, active only during forced expiration, include the abdominal muscles and internal intercostals.

Inspiration

• Inspiration involves contraction of inspiratory muscles and thorax expansion.
• Pip becomes more negative, and Ptp increases.
• Lungs expand and Palv becomes lower than Patm.
• Air flows into the lungs.

Expiration

• Expiration is passive and involves relaxation of inspiratory muscles.
• Active expiration involves contraction of expiratory muscles.
• Thorax volume decreases, Pip becomes less negative, and Ptp decreases.
• Lung volume decreases, and Palv becomes greater than Patm.
• Air flows out of the lungs.

Compliance

• Compliance is the stretchability of tissue, i.e. the extent to which the thorax and lungs expand for each unit increase in transpulmonary pressure.
• C = ΔV / Δp, where C is compliance, ΔV is the volume change, and Δp is the pressure change.
• Compliance of lungs in the thorax is 100 ml/1 cm H2O.
• Compliance of the thorax is given by thorax anatomy.
• Compliance of the lungs which is extensibility (1/3) involves exchange of lung tissue with fibrotic tissue resulting in decreased compliance.
• Alveolar surface tension is approximately (2/3)
• This surface tension occurs on the air-water interface.

Elastic Recoil

• Elastic recoil is also involved in the compliance of lungs.

Surface Tension

• Surface tension is due to cohesive forces between liquid molecules.
• The surface of alveolar cells is moist, so alveoli are air-filled sacs lined with water.
• Surface tension on the air-water interference results in a constant tendency of alveoli to shrink and to resist stretching.

Law of Laplace

• Law of Laplace gives the relationship between radius, surface tension, and transmural pressure in spherical bodies, P = 2T/r.
• Where p is pressure inside, T is surface tension and r is radius.

Surfactant

• Surfactant decreases surface tension.
• Surfactant is produced by type II pneumocytes, and it is a mixture of phospholipids (dipalmitoylphosphatidylcholine), proteins, and ions (Ca2+).
• Phospholipids have hydrophilic and hydrophobic parts as they are amphipathic molecules.
• Surfactant has immune functions.
• Premature birth leads to respiratory distress syndrome in newborns due to lack of surfactant.

Resistance of Respiratory System

• Elastic resistance is due to elastic recoil, elasticity of lung tissue, and alveolar surface tension.
• Non-elastic resistance consists of tissue resistance and airway resistance.
• Tissue resistance is due to the inertia of the lungs and chest wall, and friction.
• F = (Patm - Palv) / R and R = (8 * viscosity * L) / (π * r^4): where R is the resistance of airways.

Work of Breathing

• Work performed by respiratory muscles occurs during inspiration,.
• Work of breathing involves compliance work (elastic recoil), tissue resistance work, and airway resistance work.
• Minute ventilation (Vmin) is calculated as tidal volume x frequency.
• Maximum voluntary ventilation is Vmax
• Breathing reserve (Br) is the ratio of Vmax and Vmin.

Dead Space (DS)

• Anatomical dead space volume is approximately 150 ml.
• This is the volume of air in the conducting zone where no gas exchange takes place.
• Alveolar dead space refers to alveoli without perfusion where no gas exchange can be performed.
• Physiological dead space is the sum of anatomical and alveolar dead spaces.

Alveolar Ventilation (Valv)

• Alveolar Ventilation is calculated as Valv = (VT - DS) x Freq.
• VT is tidal volume, DS is dead space, and Freq is respiratory rate.
• Shallow and fast breathing decreases Valv, while deep and slow breathing increases Valv.

Local Differences in Pulmonary Ventilation

• Apex of lungs have more negative pleural pressure at the beginning of inspiration.
• (Ptp) = Palv - Pip
• Base of lungs have less negative pleural pressure.
• Ventilation is lower at the apex and higher at the base.

Gas Exchange

• Gas exchange occurs between alveoli and blood through respiratory membrane.
• The respiratory membrane consists of: surfactant, pneumocytes, the basal membrane of alveoli, interstitial space, capillary basal membrane, and endothelial layer.
• It is 0.5 μm thick and 70 m² in area.
• The blood flow duration through a capillary is 0.75 s which is approximately the same time for gas exchange through alveolar air.
• Gas exchange occurs via simple diffusion.

Atmospheric and Alveolar Air

• Partial pressure is the partial pressure of individual gas = total pressure of mixture x fraction of individual gas.
• Gases diffuse from a region of higher pressure to a region of lower pressure.
• Gas in contact with a liquid will dissolve into the liquid at a concentration determined by its pressure and solubility in the liquid.
• Total pressure of air at sea level is 760 mmHg.
• Air in the alveoli is saturated with water vapor.
• Oxygen and carbon dioxide undergo permanent diffusion to the blood and the alveoli, respectively.
• Alveolar air is supplied with air from dead space.
• Alv pO2 < atm pO2 and alv pCO2 > atm pCO2.
• The end of resting expiration is = 2.2 I (FRC)
• 0.35 l of air from airways -> exchange of 1/6 FRC -> slow exchange of alveolar air prevents sudden changes in concentrations of respiratory gases and in oxygenation of tissues.

Pulmonary Circulation

• Bronchial vessels and pulmonary vessels are two types of circulation in the lungs.
• In the bronchial vessel (nutritional circulation) with 1-2% of cardiac output:
  • Oxygenated blood travels through the bronchial arteries.
  • Deoxygenated blood travels through bronchial veins to the superior vena cava.
  • Pulmonary veins, arterioarterial, and arteriovenous anastomosis (physiological shunt) occurs.
• Pulmonary vessels provide functional circulation.
• In the pulmonary system, blood goes from the right ventricle to the pulmonary arteries to the pulmonary capillaries to the pulmonary veins to the left atrium.
• Pulmonary arteries are thinner walled, shorter, and more compliant compared to systemic arteries.
• Pulmonary arterioles have less smooth muscle compared to systemic arterioles.
• Pulmonary arteries and vessels have autonomic innervation (S+P).
• Pulmonary capillaries are wide with many anastomosis.
• Pulmonary veins are similar to systemic ones.
• Anastomosis occurs via arterio-arterial and arteriovenous connections.
• Pulmonary vessels/arteries are rich in lymphatic drainage.

Blood Pressure Values

• The left ventricle systolic pressure is 120 mmHg and the diastolic pressure is 0 mmHg.
• The aorta systolic pressure is 120 mmHg and diastolic is 80 mm Hg.
• The right ventricle systolic pressure is 25 mm Hg and the diastolic pressure is 0 mmHg.
• The pulmonal artery systolic pressure is 25 mm Hg and the diastolic pressure is 8 mmHg.

Parameters of Pulmonary Circulation

• Blood flow is the same as in systemic circulation at 5 l/min.
• Pressure is a low-pressure system at 25/10 mm Hg.
• Volume is about 9% of total blood volume (450 ml)
• Pulmonary capillaries pressure is 7 mmHg.
• The oncotic pressure of pulmonary capillaries is 25 mm Hg.
• There is negative pressure in the interstitial space of pulmonary capillaries -
• Blood flow differences when standing include: apex of lungs is above the heart with decreased flow, base of lungs is below the heart with increased flow
• When lying down, these differences are minimal.
• Vasoconstriction of arterials is increased by norepinephrine and angiotensin II.
• Vasodilation of arterials is increased by isoproterenol, and acetylcholine.
• Local HPV regulation is dependent on O2: Hypoxia in hypoventilated area -> vasoconstriction (smooth muscle direct action)

Comparison

• In comparison, systemic has hihgwer resistance, and greater thickness in wall compared to pulmonary circulation which has vasodilatation

Respiration

• Rate of diffusion directly proportional to = diffusion coefficient, pressure gradient and surface area
• Rate of diffusion inversely proportional to = diffusion length diffusion coefficient =solubility divided by the square root of the molecular weight
• Diffusion coefficient of carbon dioxide id 20x diffusion coefficient of O2 - Atmospheric air is air we take in and it consists pO2: 160 and pCO2: 0.3. - Alveolar air on onthe other hand has a pO2: 101 and pCO2: 40. - Diffusion, depends of respiratory -Length: membrane and thickness ciracd 0.5 micro meters and respiration is lipid soluable - Alveolar-capillary block occurs

Ventilation perfusion:

• Va/Q which has (Va) alveolar ventilation that is Q in the blood flow profusion. -Va/Q- lungs means it's less oxygenated resulting in shunting of blood
  • Va/Q that alveoli less that perfurs into leads to alevolar dead space

Partial Pressure values

Partial Pressure is

The pO2 (mm Hg) for artherial blood measures 94 while pCO2 (mm Hg) is 40