Structure & Function of the Respiratory System
5-1 STRUCTURE & FUNCTION OF THE RESPIRATORY SYSTEM
FUNCTION OF THE RESPIRATORY SYSTEM
Multicellular organisms require a specialized respiratory organ in coordination with the circulatory system for effective gas exchange.
Functions:
Gas Exchange: Facilitates the exchange of gases (O₂ and CO₂) between the atmosphere and blood.
Homeostatic Regulation: Maintains body pH levels through the balance of CO₂.
Protection: Acts as a barrier to inhaled pathogens and irritants, helping to filter and defend against infectious agents.
Vocalization: Allows for the production of sound as air passes through the vocal cords.
Four Main Processes:
Ventilation:
Inspiration (Inhalation): Air moves into the lungs.
Expiration (Exhalation): Air moves out of the lungs.
Gas Exchange between Lungs and Blood: Exchanges O₂ and CO₂ through diffusion.
Transport of Gases by Blood: Blood carries CO₂ and O₂ throughout the body.
Exchange of Gases between Blood and Tissues: Provides O₂ to tissues and collects CO₂.
Pressure Measurements: ( don’t need to know )
Deoxygenated veins (upper torso):
PCO₂: 46 mm Hg
PO₂: 40 mm Hg
Ambient air:
PCO₂: 0.2 mm Hg
PO₂: 160 mm Hg
Oxygenated arteries (upper torso):
PCO₂: 46 mm Hg
PO₂: 40 mm Hg
Capillaries (venous blood):
PCO₂: 40 mm Hg
PO₂: 100 mm Hg
STRUCTURE OF THE RESPIRATORY SYSTEM
The respiratory system is built of structures that facilitate both ventilation and gas exchange.
A. Conducting System
Composed of passages (airways) connecting the environment to the lungs.
Divided into two anatomical parts:
Upper Respiratory Tract:
Includes the mouth, nasal cavity, pharynx, and larynx.
Lower Respiratory Tract:
Comprises the trachea, bronchi, bronchioles, and lungs.
B. Exchange Surface
Alveoli:
Tiny hollow sacs at the terminal bronchioles, crucial for gas exchange.
Surrounded by a network of capillaries covering 80-90% of the alveolar surface area, facilitating diffusion of gases.
Gas echange occurs by diffusion between alveoli and capillary
Types of Alveolar Cells:
Type I Alveolar Cells:
Thin and large, enabling rapid gas diffusion.
Type II Alveolar Cells:
Smaller and thicker; responsible for synthesizing and secreting surfactant.
Pulmonary Circulation:
blood vessels of the pulmonary circulation allow for exchange with alveoli
low oygen blood exits the right ventricle of the heart
Goes into the pulmonary arteries via the plumonary trunk
Rate of blood flow is high becasue all the outputs of the right ventricle goes to the lungs versus whole body for the blood leaving the left ventricle
Since blood pressure is low relative to the systemi circuit - the right ventricle does not pump as forcefully as the left ventricle.
two pulmonary arteries - one to each lung ( more in-depth process to help visualize)
Allows blood to exchange gases with the alveoli.
Deoxygenated blood exits the right ventricle, moves into the pulmonary arteries via the pulmonary trunk (two arteries, one for each lung).
High blood flow from the right ventricle to the lungs despite low blood pressure compared to the systemic circuit, as the right ventricle pumps less forcefully.
C. Pumping System
Comprised of the bones and muscles of the thorax, essential for ventilation through inspiration and expiration.’
The relationship between the lungs to the chest
the chest (thorax) is a closed compartment
closed at the top of the neck muscle and connective tissue
closed off at the bottom by muscle - the diaphragm
The walls are formed by ribs and intercostal muscles
Both lungs are surrounded by the pleural sac - forms a double membrane around each lung
pleura sac is filled with fulid and acts as a lubricant
5-2 GAS LAWS AND VENTILATION
A. Gas Laws
The air is a mixture of gases, and the total pressure is the sum of the partial pressures of individual gases.
Gases naturally flow from regions of high pressure to low pressure.
Relationship between Volume and Pressure:
Inverse correlation; as the volume increases, the pressure decreases, and vice versa.
Boyle's Law:
Increase in volume leads to decrease in pressure ( extit{PV = k}).
The volume of a gas in a liquid is determined by:
Partial pressure of the gas
Solubility of the gas in the liquid
B. Partial Pressure
The partial pressure of a gas in the atmosphere can be calculated using:
Example of O₂ calculation:
Pressure volume relationship
during inspiration and expiration, the volume of the thoracic cavity changes
causes changes in the alveolar pressure - driving force of air
No muscles are in the lung itself and can not change volumes on its own
The muscles in the chest wall - the contraction changes the thoracic volume
the lungs are passive elastic strucutres ( balloons)
the Volume
The transpulmonary pressure - difference between alveolar pressure and intrapleural pressure
the degree of elasticity of the lungs far
Pressure inside the lungs is the alveolar pressure
Pressure outside the lungs is the pressure in the intrapleural fluid t (a) is your answe
C. Ventilation
Process defined as the exchange of air between the atmosphere and the lungs; involves conditioning the air:
Warming it to 37°C to maintain core body temperature and protect alveoli
Adding water vapor to prevent drying of the epithelial surface.
Filtering out foreign material (via ciliated epithelium and mucus).
Airways are lined with ciliated epithelia that secrete a watery saline solution
cells move Cl from the ECF into the cell via the NKCC - Ci transported to lumen of airways via apircal anion channel
Na moves between cells for the ECF to lumen, gradient of NaCl draws water towards the lumen creating the watery saline solution
Cilia are covered with mucus that is secreted by globlet cells
Mucus contains immune cells (macrphages) that kill invades
Muscus is moved up the pharynx( muscus escalator)
Transferred to the dijestve tract where additional bacteria are destroyed
The Respiratory Cycle
Step 1: Inspiration
somatic motor neurons trigger contration of diaphragm and inspiratory muscles
Thorax expands - thoracic volume increases
alveolar and intrapleural decrease
lungs expand resulting in air flowing into lungs
Step 2: Expiration
impulses from somatic motor neurons stop
diaphragm and thoracic muscles relax which returns thorax to their original positions- volume decreases ( elastic recoil)
During quiet breathing, expiration is a passive process
passive expiration depends on elastic recoil of the thoratic muscles and the lungs
during excersise of heavy breathing expiration is active
active expiration depends on contraction of internal intercostals and abdominal muscles
Intrapleural Pressure
Intrapleural pressure is normally sub-atmospheric - arises during foetal developpement
Pneumothorax
having lower pressure in the pressure in the pleural fluid (outside th elung) than inside the lung (at rest) helps keep the lung expanded and open
If air gets into the pleural cavity - intrapleural pressure increases
Pressure difference is abloished - the lungs collapses
This is a condition called pneumothorax ( or collapsed lung)
Treatment - apply suction to remove the air and seal the hole
Factors of breathing
the work required to breathe depends on twi main factors
compliance (stretchability) of the lungs
The resistance to air flow in the airway
A. Lung Compliance
Definition: The ability of the lung to stretch; higher compliance indicates easier lung expansion during inhalation.
Individuals with low lung compliance may breathe shallowly and rapidly.
high lung compliance indicates that the lungs strectch easily - easier to breathe
B. Lung Elastance
Definition: the ability /speed of return to resting volume after lung is stretched
When the lung elastance is low, the lung does not return to resting volume passively
Expiration must be active not passive
C. Airway Resistance
Resistance is primarily determined by the airway diameter:
Normally the work needed to overcome airway resistance is low relative to work needed to overcome resistance to stretch, but mucus accumulation from allergies or infections can greatly increase resistance of the airways
Bronchiole diameter can be altered by the nervous system, hormones, and paracrines,
Cos causes bronchodilation
Histamine released in response to tissue damage or allergic reactions causes bronchoconstriction
severe allergic reactions can cause difficulty breathing
Neural control of bronchioles
Primarily by parasympatheic neurons that causes bronchoconstriction
Reflex designed to protect lower respirtory tract from inhaled irritants
no significant sympathetic innervation
Hormonal control of bronchioles is done primarly via circulating epinephrine
through b2 receptors is smooth muscle of bronchioles - relax muscles to dilate bronchioles
used as a treatment for asthma
D. Assessment of Pulmonary Function
Pulmonary function assessed using a spirometer, to determine the amount of air a person moves during quiet breathing and maximal breathing effort
helps diganois multiple diseases such as asthma, emphysema, chronic bronchitis
Four lung volumes:
Tidal Volume (TV): Air moved in a single normal inspiration and expiration .
Inspiratory Reserve Volume (IRV): Maximum amount of air that can be inspired abobve tidal volume
Expiratory Reserve Volume (ERV): Maximum of air that can be exhaled after a normal expiration
Residual Volume (RV): The amount of air left in the lungs after maximal expiration
the sum of two or more lung volumes is called a capacity
Lung Capacities:
Vital Capacity (VC): the maximum amount of air that can be voluntarily moved into and out of the respirtory system
(VC = IRV + ERV + TV).
Total Lung Capacity (TLC): The total volume of air the lungs can hold (TLC = VC + RV).
5-4 GAS EXCHANGE IN LUNGS AND GAS TRANSPORT IN BLOOD
A. Gas Exchange in Lungs
Gas exchange relies on diffusion, which is affected by:
Partial pressure gradient
Surface area available
Thickness of the membranes (thicker membranes reduce diffusion efficiency).
Greatest over short distance
Patial pressure gradient influenced by
1. composition of inspired air - affected by altitude
2. Alveolar ventilation
can be affected by change in airways resistance or change in lung compliance
B. Gas Transport in Blood
Gases are transported dissolved in plasma or inside red blood cells (RBCs).
Oxygen Transport:
Low solubility in plasma; primarily transported by binding to hemoglobin (Hb).
Within RBCs O2 is bound by haemogolbin
each hemaglobin molecule can bind up to 4 oxygen molecules
oxygen binds reversily with iron in haeme group
Haemoglobin bound to oxygen = oxyhaemoglobin
Unbound haemoglobin = deoxyhaemoglobin
Percent saturation of haemoglobin: % of avaliable binding sites that are bound to oxygen
Carbon Dioxide Transport:
Transported in three ways:
Dissolved in plasma.
CO2 is more soluble in body fluids than oxygen; however, cells produce more CO2 than can be carried in plasma
Interacts with proteins (bound to proteins including Hb).
Forms carbaminohemoglobin
Deoxy-haemaglobin interacts more readily with CO2 than oxy-haemaglobin
Converted to bicarbonate (HCO₃) via carbonic anhydrase.
The majority of the CO2 entering the blood is converted by reaction catalysed by carbonic anhydrase (present in RBCs) - the H formed by this reaction binds to Hb
The bicarbonate ions are moved out of the RBC by a transporter protein which exchanges HCO3 for Cl in a process is known as the chloride shift
When venous blood reaches lungs the PCO2 of alveoli is lower than blood
CO2 dissoloved in plasma diffused into alveoli and then CO2 in RBC diffuses into plasma
Equilibrium of CO2-bicarbonate reaction is shifted
Bicarbonate ions moves from plasma into RBCs and then bicarbonate and H form carbonic acid and then CO2 - catalysed by carbonic anhydrase
CO2 diffuses out of RBC into the plasma and then alveoli
Efficiency of Breathing
Estimate the effectivness of breathing by measuring the total pulmonary ventilation - minute volume
Mv (mL/min) = Vt (mL/breath) X respitory rate (breath/min)
There is anatomic dead space located in the airways - no gas exchange
air in trachea, bronchi and bronchioles does not participate in gas exchange
Alveolar volume =Vt - dead space
Effectivness of ventailation determined by the rate and depth of breathing
since of the dead space, increase in depth of breathing is most important
Alveolar ventaliation is the amount of air that reaches the alveoli each minute
A more accurate indicator of efficiency of ventaliation
Alveolar ventilation = ventilation rate X alveolar volume
Ventilation is matched to alveolar blood flow - the body attemps to matches air flow and blood to maximize gas exchange in the capillary bed that surrounds the alveoli
alterations in blood flow in the lungs depends primarly local control exerted by o2 levels in the interstitail fluid around the arteriole surrounding the alveoli
Increases in tissue PO2 result in vasodilation in the arteriole
if ventilation of alveoli in an area of the lung decrease, then tissue o2 in that area also decrease
decrease in tissue PO2 results in vasoconstriction in the arteriole
overall, this diverts blood away from the ventilated area - ensures that blood travels to areas of the lungs that would ensure that oxygen is available to be picked up
5-5 CONTROL OF VENTILATION
A. Muscle Control
Diaphragm and intercostal muscles are skeletal muscles, innervated by somatic motor neurons originating from the medulla oblongata, specifically from two nuclei:
Skeletal muscle cannot constract spontaneously
Contraction of the respirtory skeletal muscles is initated in the medulla oblongata
these neurons have intristic rhythmic activity
There are two nuclei in the meulla oblongata associated with respiration
Dorsal Respiratory Group (DRG): Controls external intercostal mucles and disphragm ( muscles of inspiration)
Ventral Respiratory Group (VRG): Controls internal intercostal and abdominal muscles (active expiration)
B. Regulation by Chemoreceptors
modify or adjust the rhythmicity of the central pattern generator neurons
There are two sets of chemoreceptors responsible for this regulation
Peripheral Chemoreceptors: Located in carotid bodies; respond to low arterial PO₂ and pH, or high PCO₂.
Located in cateroid bodies
sense changes PO2 and pH of plasma or increase in PCO2
Decrease PO2 or decreased pH or increased in PCO2
Most circumstances pH and PCO2 are important
Plasma PO2 must change radically before a signal is sent
Central Chemoreceptors: Located in the medulla; primarily respond to high PCO₂ and corresponding changes in carbonic acid levels in cerebrospinal fluid (CSF).
located in medulla oblongata - most important chemical controller of ventilation
PCPO2 in arterial blood
CO2 crosses blood brain barrier into cerebrospinal fluid (CSF) - activates central chemoreceptors via changes in pH caused in b y the production of carbonic acid
CO2 + H2O - H2CO3 ↔ HCO3- + H+
sense changes of H in CSF and not H in arterial blood
C. Regulation by Mechanoreceptors
In some circumstances mechanoreceptors also control ventilation to protext the lungs
there are two types of mechanoreceptors
Irritant Receptors: Located in airway mucosa
Stimulation triggers parasympatheic neurons that innervate bronchiolar smooth muscle
Stretch Receptors: Located in airway smooth muscles, prevent over-inflation of the lungs (Hering-Breuer inflation reflex).
Triggered if lungs are over-inflated
terminate ventilation - Hering - Breuer inflation reflex
Reflex does not happen during quite breathing or mild exertion