Respiratory System: Functions

Respiratory System Functions

• The respiratory system's primary function is respiration
• The respiratory system also has non-respiratory functions

Respiratory Functions

• External respiration involves the exchange of oxygen and carbon dioxide between the lungs and the atmosphere and is also termed pulmonary ventilation
• Oxygen and carbon dioxide diffuse through the respiratory membrane
• Oxygen and carbon dioxide are transported in the blood
• Internal respiration involves the exchange of oxygen and carbon dioxide between the blood and tissues

Non-Respiratory Functions

• The immune function includes ciliated epithelium, alveolar and interstitial macrophages, BALT, and IgA
• Defensive respiratory reflexes include coughing, sneezing, and reflex apnea
• The respiratory system functions as a blood reservoir
• It removes small emboli and aids fibrinolysis
• pH regulation is a function
• Phonation is a function
• Swallowing, defecation, micturition, vomiting, and sniffing are functions
• Metabolic and endocrine roles consist of production/elimination of biologically active substances and the conversion of Angiotensin I to Angiotensin II via angiotensin-converting enzyme

Normal Breathing

• Eupnea is normal breathing
• Normoventilation is normal ventilation

Terms

• Anoxia is the absence of oxygen
• Hypoxia is a deficiency in the amount of oxygen reaching the tissues
• Hyperoxia is an excess of oxygen
• Hypocapnia is a state of reduced carbon dioxide in the blood
• Hypercapnia is a state of increased carbon dioxide in the blood
• Cyanosis is a bluish discoloration of the skin resulting from poor circulation or inadequate oxygenation of the blood
• Apnea is temporary cessation of breathing, especially during sleep
• Tachypnea is abnormally rapid breathing
• Bradypnea is abnormally slow breathing
• Hypopnea is a breathing that is shallow or slow
• Hyperpnoea is increased depth and rate of breathing
• Hypoventilation is ventilation of the lungs that does not fulfill the body's gas exchange requirements
• Hyperventilation is ventilation of the lungs beyond normal body needs
• Dyspnea is difficult or labored breathing

Yawning

• Yawning can be caused by tiredness, sleepiness, boredom or stress

Respiratory System Anatomy

• The upper respiratory tract consists of the nasal cavity and nasopharynx
• The lower respiratory tract consists of the larynx, trachea, bronchi, bronchioles, alveolar ducts, and alveoli

Terminal Respiratory Unit

• The alveoli have a small diameter
• The alveoli have a thin wall
• There are a large number of alveoli
• The alveoli have a large surface area
• The alveoli have a high blood supply

Alveoli

• There are 600 million alveoli in the lungs
• The alveoli have a surface area of a tennis court

Lung Circulation

• Capillaries supplying the alveoli belong to the pulmonary low-pressure circulation

Respiratory Airways

• The trachea and bronchi contain cartilage and smooth muscle
• Bronchioles lack cartilage and contain smooth muscle
• Ciliated epithelium is present from the trachea up to the terminal bronchioles and is responsible for mucus secretion
• Alveoli contain type I and type II alveolar cells, also known as pneumocytes

Alveolar Cells

• Type I alveolar cells cover 90% of the surface area
• Type I alveolar cells are flat
• Type I alveolar cells are a key part of the respiratory membrane used for gas diffusion
• Type II alveolar cells make up 10% of the surface area
• Type II alveolar cells are granular(cuboidal) and produce surfactant

Nervous Control of Bronchiolar Musculature

• The sympathetic nervous system originates in the thoracic segments of the spinal cord, using Acetylcholine at the ganglionic Nicotinic receptors
• Sympathetic innervation utilizes Norepinephrine and adrenergic β2 receptors to cause bronchodilation
• Parasympathetic nervous system originates in the Nucleus of n.X, using Acetylcholine at the ganglionic Nicotinic receptors
• Parasympathetic innervation utilizes Acetylcholine at the Muscarinic receptors to cause bronchoconstriction

Hormonal Control of Bronchiolar Musculature

• Histamine and circulating catecholamines influence bronchiolar musculature

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 visceral and parietal pleura
• Strong adhesion exists between both pleural layers due to attractive forces among fluid molecules
• Pleural fluid is transcellular fluid

Pressures

• Alveolar pressure is the pressure within the alveoli
• Pleural pressure is the pressure within the pleural cavity
• Transpulmonary pressure is the difference between alveolar pressure and pleural pressure

Forces Affecting the Alveolar Wall

• Elastic forces of the chest wall
• Elastic forces of the lung tissue
• Surface tension

Pulmonary Ventilation

• Pulmonary ventilation is the inflow and outflow of air between the atmosphere and alveoli
• Pulmonary ventilation is always due to a pressure difference (gradient)
• The equation is: F = (P_atm - P_alv) / R, where F is airflow, P_atm is atmospheric pressure, P_alv is alveolar pressure, and R is airway resistance
• Flow is affected by the pressure gradient and resistance

Pressure Gradient

• Inspiration occurs when Palv < Patm
• Expiration occurs when Palv > Patm
• When Palv = Patm, there is no air flow
• Pressure gradients are due to changes in thorax and lung volume

Boyle's Law

• For a fixed amount of gas at a constant temperature, volume and pressure are inversely proportional (P x V = constant)

Pleural Pressure Change

• Pleural pressure decreases as thorax volume increases

Respiratory Muscles

• The diaphragm (phrenic nerve, C3-C5) contracts during inspiration
• The external intercostals (intercostal nerves) contract during inspiration
• Auxiliary muscles (scaleni, sternocleidomastoid muscle, serrati, pectorals) contract during inspiration
• Expiration involves the abdominal muscles and internal intercostals

Inspiration

• Inspiration includes the contraction of inspiratory muscles
• The thorax expands during inspiration
• There is an ↑ in negative Pip during inspiration
• There is an ↑ in Ptp during inspiration
• The lungs expand during inspiration
• There is a Palv < Patm during inspiration
• Air flows into the lungs during inspiration

Expiration

• Muscle relaxation causes passive expiration
• Abdominal and Intercostal muscle contraction causes active expiration
• During expiration, the thorax volume decreases
• During expiration, negative Pip decreases
• During expiration, Ptp decreases
• During expiration, lung volume decreases
• Palv > Patm, during expiration
• Air flows out of the lungs during expiration

Compliance

• Compliance is the stretchability of tissue
• Compliance is the extent to which thorax/lungs expand for each unit increase in transpulmonary pressure
• C = ΔV / Δp, where C is compliance, ΔV is volume change, and Δp is pressure change
• Lung compliance in the thorax is 100 ml/1 cm H2O

Compliance of Lungs

• Elasticity of lung tissue (1/3) affects lung compliance
• Alveolar surface tension (2/3) affects lung compliance

Surface Tension

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

Law of Laplace

• Laplace's Law states; p = 2T / r, where p is pressure inside, T is surface tension, and r is radius in spherical bodies

Surfactant

• Surfactant decreases surface tension
• Surfactant is produced by type II pneumocytes
• Surfactant contains a mixture of phospholipids (dipalmitoylphosphatidylcholine), proteins, and ions (Ca2+)
• Phospholipids in surfactant have hydrophilic and hydrophobic parts (amphipathic molecule)
• Surfactant has immune functions
• Premature birth can cause respiratory distress syndrome of newborns due to lack of surfactant production

Resistance of Respiratory System

• Elastic resistance includes elastic recoil, elasticity of lung tissue and alveolar surface tension
• Non-elastic resistance includes tissue resistance and airway resistance

Non-Elastic Resistances

• Tissue resistance is caused by the inertia of the lungs and chest wall, as well as friction
• Airway resistance can be found by: F = (P_atm - P_alv) / R and R = (8 * η * L) / (π * r^4), where F is airflow, Patm is atmospheric pressure, Palv is alveolar pressure, R is airway resistance, η is viscosity, L is length of airway, and r is radius of the airway

Work of Breathing

• Work performed by respiratory muscles during resting breathing is only during inspiration
• Compliance work (elastic recoil) is part of the work of breathing
• Tissue resistance work is part of the work of breathing
• Airway resistance work is part of the work of breathing

Pulmonary Ventilation

• Vmin is minute ventilation, calculated by multiplying tidal volume x frequency
• Vmax is maximum voluntary ventilation
• Br is breathing reserve is the ratio of Vmax and Vmin

Dead Space

• Anatomical dead space is approximately 150 ml, It is the volume of air in the conducting zone where no gas exchange occurs
• Alveolar dead space consists of alveoli without perfusion, where no air can be used for gas exchange
• Physiological dead space is the sum of anatomical and alveolar dead space

Alveolar Ventilation

• Alveolar ventilation formula is Valv = (VT - DS) x Freq, where VT is tidal volume, DS is dead space, and Freq is respiratory rate

Local Differences in Pulmonary Ventilation

• At the apex of the lungs, there is more negative pleural pressure at the beginning of inspiration which decreases ventilation
• At the base of the lungs, there is less negative pleural pressure which increases ventilation
• Apex has decreased ventilation
• Base has increased ventilation

Respiratory Membrane

• The respiratory membrane allows gas exchange with simple diffusion
• Surfactant lines the respiratory membrane, then pneumocytes, basal membrane of alveoli, interstitial space, capillary basal membrane, and finally the endothelial layer
• All of this makes the membrane approximately 0.5 μm thick and 70 m² in area
• Red blood cells spend 0.75 s flowing through the alveolar capillaries

Partial Pressure

• In a mixture of non-reacting gases, the total pressure of the mixture is equal to the sum of the partial pressures of the individual gases (Dalton's law)
• The partial pressure of individual gas = total pressure of mixture x fraction of individual gas
• Gases diffuse from regions of higher pressure to regions of lower pressure
• Gas molecules enter a liquid and dissolve there, the concentration of dissolved gas is determined by its pressure and by the solubility in the liquid

Atmospheric Air

• Total pressure of air at sea level is 760 mmHg
• The fraction of Oxygen is 0.21, thus the partial pressure is 160 mmHg
• The fraction of Carbon Dioxide is 0.0004, thus the partial pressure is 0.3 mmHg
• The fraction of Nitrogen 0.78, thus the partial pressure is 599.7 mmHg

Alveolar Air

• Alveolar air is saturated with water vapor
• Permanent diffusion of oxygen occurs to the blood
• Permanent diffusion of carbon dioxide occurs to the alveoli
• Air is supplied from dead space
• alv pO2 < atm pO2 , the partial pressure of oxygen in atmospheric air is higher than the partial pressure of oxygen in the alveoli
• alv pCO2 > atm pCO2 , the partial pressure of carbon dioxide in the alveoli is higher than the partial pressure of carbon dioxide in the atmosphere
• The end of resting expiration equals to 2.2 L (FRC)
• 0.35 l of air from airways exchanges 1/6 FRC and so prevents sudden changes in concentrations of respiratory gases and in oxygenation of tissues

Pulmonary Circulation

• Bronchial vessels provide nutritional circulation supporting tissues of lungs, bronchi and visceral pleura, comprising about 1-2% of cardiac output
• Oxygenated blood is carried by bronchial arteries
• Deoxygenated blood circulates through bronchial veins back to the superior vena cava
• Anastomoses occur between pulmonary veins and arterioarterial and arteriovenous connections, forming a physiological shunt
• Pulmonary vessels provide a functional circulation

Pulmonary System

• The right ventricle pumps blood to the pulmonary arteries
• The blood goes in pulmonary capillaries, then to the pulmonary veins
• After the pulmonary veins, the blood goes to the left atrium

Pulmonary System Features

• Pulmonary arteries have thinner walls, shorter length, and are more compliant than systemic circulation
• Pulmonary arterioles have less smooth muscle
• The pulmonary system has autonomic innervation S+P
• Pulmonary capillaries are wide and have many anastomosis
• Pulmonary veins are similar to systemic ones
• The pulmonary system has anastomosis: arterio-arterial and arteriovenous
• Rich in lymphatic drainage

Blood Pressure Features

• Left Ventricle measurement is 120/0 mmHg (systolic/diastolic)
• Aorta measurement is 120/80 mmHg (systolic/diastolic)
• Right Ventricle measurement is 25/0 mmHg (systolic/diastolic)
• Pulmonal Artery measurement is 25/8 mmHg (systolic/diastolic)

Parameters of Pulmonary Circulation

• Blood flow is the same as systemic circulation at 5 l/min
• Pressure is a low-pressure system at 25/10 mm Hg
• Volume is about 9% of the total blood volume, which is 450 ml

Capillaries

• Capillary pressure is 7 mm Hg
• Oncotic pressure is 25 mm Hg
• There is negative pressure in the interstitial space
• Fluid is drawn into the vessel lumen to maintain "dry alveoli"
• Failure of the left ventricle can lead to pulmonary edema

Blood Flow

• In the standing position, the apex of the lungs, being above the heart, experiences decreased blood flow
• In the standing position, the base of the lungs, being below the heart, experiences increased blood flow
• In the lying position, there are minimum differences across the lungs

Regulation of Pulmonary Blood Flow

• Norepinephrine and angiotensin II cause constriction of pulmonary arterioles
• Isoproterenol and acetylcholine cause dilation
• Physical activity leads to increased cardiac output, increased pressure in the pulmonary artery, recruitment of nonperfused capillaries, increased blood flow through lungs, increased passage of red blood cells, increased O2 delivery to systemic circulation
• Local hypoxia in hypoventilated area leads to vasoconstriction (probably a direct action of O2 on smooth muscle)

Systemic vs. Pulmonary Circulation

• Blood flow is 5 l/min in both systemic and pulmonary circulation
• Blood pressure measures 120/80 mmHg in systemic circulation and lower 25/10 mm Hg in pulmonary circulation
• Resistance is higher in systemic circulation and lower in pulmonary circulation
• Capillary pressure is higher in systemic is lower in pulmonary
• Wall thickness is ↑ in systemic circulation and ↓in pulmonary
• Compliance is ↓ in systemic circulation and ↑ in pulmonary
• Nervous regulation is important in systemic and less important in pulmonary
• Local hypoxia causes vasodilatation in the systemic circulation and vasoconstriction in the pulmonary circulation

Diffusion Coefficient

• CO2 solubility 23x times higher than O2 solubility
• MW of CO2 = 44, MW of O2 = 32
• diffusion coefficient of O2 = 1/√32 = 0.17
• diffusion coefficient of CO2 = 23/√44 = 3.4
• 3.4/0.17 = 20 → diffusion coefficient of CO2 is 20x higher than diffusion coefficient of O2

Ventilation-Perfusion Quotient

• Ventilation-perfusion quotient equals Valv / Q which equals 4,2l / min / 5l / min which equals 0,8
• Ventilation-perfusion mismatching occurs when V/Q decreases, resulting in shunt
• Ventilation-perfusion mismatching occurs when V/Q increases, resulting in alveolar dead space

Diffusion Length

• Distance across the respiratory membrane effects diffusion – including plasma, membrane of red blood cell & fluid inside it
• The membrane thickness is about 0.5 μm (min. 0.2 μm)
• Respiratory gases are lipid-soluble

Diffusing Capacity of Respiratory Membrane

• Gas volume that will diffuse across the membrane each minute per 1 mm Hg pressure difference
• Oxygen: 21 ml/min/1 mmHg
• Carbon dioxide is assumed to be 20x higher than for O2