Chapter 23: The Respiratory System Flashcards
List the organs of the upper respiratory system
- Nose:
- External Nose
- Nasal Cavity - Paranasal Sinuses
- Pharynx
List the organs of the lower respiratory system
- Larynx
- Trachea
- Bronchus
- Lung
Functions of the Nose
-provides an airway for respiration
-moistens and warms entering air
-filters and cleans inspired air
-serves as resonating chamber for speech
houses olfactory receptors
External Nose
- Areas include root (area between eyebrows); bridge, dorsum nasi (anterior margin), and apex (tip of the nose)
- Nostrils (external nares or nares): bounded laterally by alae
The External Nose is formed by
- nasal and frontal bones superiorly- gorm bridge and root, respectively
- maxillary bones laterally
- plates of hyaline cartilage inferiorly
Nasal Cavity
- from nasal vestibule -> to internal nares
- found within and posterior to external nose
The Nasal Cavity is divided by the
Nasal Septum.
Posterior Nasal Apertures (internal nares or choanae)
opening where nasal cavity turns into nasopharynx.
The roof of the nasal cavity is formed by
ethmoid and sphenoid bones
The floor of the nasal cavity is formed by
hard palate (bone) and soft palate (muscle)
Nasal Vestibule
nasal cavity superior to nostrils.
lined with vibrissae (hairs) that filter coarse particles from inspired air
Nasal Septum
formed anteriorly by septal cartilage and posteriorly by vomer bone and perpendicular plate of ethmoid bone
Olfactory Mucosa
lines superior region of nasal cafity and contains olfactory epithelium
Respiratory Mucosa
Psuedostratified ciliated columnar epithelium that contains goblet cells and rests on lamina propria that contains many seromuccous nasal glands
Nasal conchae
scroll like, mucosa-covered projections that protrude medially from each lateral wall of nasal cavity
-3 sections: superior, middle, and inferior conchae
Function of Conchae
- during inhalation, conchae and nasal mucosa: filter, heat and moisten air
- during exhalation these structures: reclaim heat and moisture
Paranasal Sinuses
- form ring around nasal cavities
- located in frontal, sphenoid, ethmoid and maxillary bones
Function of Paranasal Sinuses
- lightens skull
- secrete mucus
- help to warm and moisten air
- resonating chamber for sounds (speak or sing)
Rhinitis
- inflammation of nasal mucosa
- nasal mucosa is continuous with mucosa of respiratory tracts, so infections spread from nose to throat to chest.
- can also spread to tear ducts and paranasal sinuses
Pharynx
- funnel-shaped muscular tube that runs from base of skull to vertebra C6
- connects nasal cavity and mouth to larynx and esophagus
- composed of skeletal muscle and lined with mucous membranes
3 Regions of Pharynx
- Nasopharynx
- Oropharynx
- Laryngopharynx
Nasopharynx
- lies posterior to the nasal cavity
- serves only as a passageway for air
- lining contains ciliated psuedostratified columnar epithelium
- soft palate and uvula close nasopharynx during swallowing
- Pharyngeal tonsils and Pharygotympanic tubes
Pharygotympanic tubes
drain and equalize pressure in middle ear and open into lateral walls
Oropharynx
- passageway for food, fluids and air from level of soft palate to level hyoid bone
- lined with nonkeratinized stratified squamous epithelium
- isthmus of fauces (opening of oral cavity), palatine tonsils, and lingual tonsil
Laryngopharynx
- passageway for food, fluids and air
- begins from level of hyoid bone to the esophagus
- anteriorly, lies the larynx
- lined with nonkeratinized stratified squamous epithelium
What are the requirements of the Respiratory System?
- Adequate Surface Area for Gas Exchange - lungs have alvelor sacs that increases surface area
- Adequate Transport System - RBC’s
- Adequate Protection - mucous membranes and wandering macrophages throughout the respiratory tract
- Adequate Moisture - surfactant is produced inside lunges (allows expansion and recoil of alveolar sacs)
Function of the Respiratory System
- bring oxygen rich air into the body for cells
- expel waste products (CO2 & H2o) from the body
- produce air flow that makes speech possible
The Lower Respiratory System is divided into two zones:
- Respiratory Zone - site of gas exchange (consists of respiratory bronchioles, alveolar ducts and alveoli)
- Conducting Zone - conduits that transport gas to and from gas exchange sites (cleanses, warms and humidifies air)
Anatomy of the Larynx
- extends from 4th/5th to 6th cervical vertebra and attaches to hyoid bone
- connects the laryngopharynx with the trachea
- consists of 9 cartilages connected by membranes and ligaments
3 Functions of the Larynx
- Provide patent airway
- Routes air and food into proper channels
- Voice Production (houses vocal folds)
What are the 9 cartilages that form the framework of the larynx?
- Thyroid cartilage: (Adam’s Apple)
- Cricoid cartilage: ring-shaped
3 & 4. Paired Arytenoid cartilages
5 & 6. Paired Cuneiform cartilages
7 & 8. Paired Corniculate Cartilages - Epiglottis
True Vocal Cords
- aka Vocal Folds: formed by vocal ligaments
- contain elastic fibers that appear white because lack of blood vessels
- glottis: opening between vocal folds
- folds vibrate to produce sound as air rushes up from lungs
False Vocal Cords
- aka False Vocal Cords
- superior to vocal folds
- no part in sound production
- help to close glottis during swallowing and function in holding breath against pressure in thoracic cavity
Voice Production: Speech
intermittent release of expire air during opening and closing of glottis
Voice Production: Pitch
determined by diameter, length and tension of vocal cords
Voice Production: Loudness
depends upon force of air as it rushes through the vocal cords.
- chambers of pharynx and oral, nasal and sinus cavities amplify and enhance sound quality
- sound is shaped into language by muscles of pharynx, tongue, soft palate and lips
Trachea
- Windpipe
- extends from larynx into mediastinum, where it divides into the left and right primary bronchi
- it is about 4-5 inches long, 1 inch in diameter and flexible
The Trachea wall is composed of 3 layers:
- mucosa: psuedostratified ciliated columnar epithelium with goble cells and underlying layer of lamina propria
- submucosa: areolar connective tissue with seromucous glands
- adventitia: outermost layer made of areolar connective tissue. Reinforced internally by 16-20 C-shaped hyaline cartilage rings prevent collapse of trachea.
Function of the trachea
integral part of the body’s airway and has the vital function of providing air flow to and from the lungs for respiration
Define oxyhemoglobin
hemoglobin combined with oxygen
Define deoxyhemoglobin
hemoglobin that has release O2
When is hemoglobin said to be “partially saturated”?
when only one to three hemes carry O2
When is hemoglobin said to be “saturated”?
all four heme groups carry O2
Haldane effect
- amount of CO2 transported is affected by P O2
- The lower the P O2 and hemoglobin O2 saturation, the more CO2 can be carried in blood
Hypoxia
inadequate O2 delivery to tissues, can result in cyanoses
transpulmonary pressure
- pressure that keeps lung spaces open (keeps lungs from collapsing)
- the greater transpulmonary presssure -> the larger the lungs will be
- (P pul - P ip )
atelectasis
lungs collapse due to:
- plugged bronchioles, which cause collapse of alveoli or
- pneumothorax
Pneumothorax
- air in pleural cavity
- can occur from either wound in parietal pleura or rupture of visceral pleura
- treated by removing air with chest tubes
- when pleurae heal, lung reinflates
pleurisy
inflammation of pleurae that often results from pneumonia
-inflamed pleurae become rough, resulting in friction and stabbing pain with each breath
hyperventilation
increased depth and rate of breathing that exceeds body’s need to remove CO2
- may be caused by anxiety attacks
- leads to decreased blood CO2 levels
- treatment: breathing into paper bag increases CO2 levels being inspired
apnea
breathing cessation that may occur when P CO2 levels drop abnormally low
Air passages undergo 23 orders of branching
Branching referred to as the …
bronchial tree
Bronchial Tree
-the branching system of bronchi and bronchioles conducting air from the windpipe into the lungs.
-From the tips of the bronchial tree:
Conducting zone structures -> Respiratory zone structures
Conducting Zone Structures
- Trachea divides to form right and left main (primary) bronchi
- Each main bronchus enters hilum of one lung
- Each main bronchus then branches into lobar (secondary) bronchi
- Each lobar bronchus branches into segmental (tertiary) bronchi
- Branches become smaller and smaller: Bronchioles and Terminal Bronchioles
right and left main (primary) bronchi
right main bronchus: wider, shorter and more vertical than the left
lobar (secondary) bronchi
- three on the right and two on the left
- each lobar bronchus supplies one lobe
segmental (tertiary) bronchi
- supply specific bronchopulmonary segments within each lobe of lung
- segmental bronchi divide repeatedly
terminal bronchioles
- smallest of all conducting zone structures
- less than 0.5 mm in diameter
In conducting zone, from bronchi to bronchioles, what changes occur?
- cartilage rings become irregular plates and in bronchioles, elastic fibers replace cartilage
- Psuedostratified ciliated columnar changes to simple cuboidal.
- Cilia and goblet cells become more sparse
- Amount of smooth muscle increases
What are the Respiratory zone structures?
- begin where terminal bronchioles feed into respiratory branchioles
- leads to alveolar ducts
- then into alveolar sacs (saccules)
Alveolar Sacs (saccules)
- aka lobules
- contain clusters of alveoli
- Sites of actual gas exchange
- about 300 million alveoli make up mosat of lung volume
respiratory bronchiole
first place where gas exchange can occur
alveoli: structure
- Surrounded by fine elastic fibers and pulmonary capillaries
- Alveolar pores connect adjacent alveoli
alveoli: function
- Equalize air pressure throughout lung
- Provide alternate routes in case of blockages
- Alveolar macrophages keep alveolar surfaces sterile
What is the importance of alveolar capillary beds?
- allows for rapid gas exchange across membrane by simple diffusion
- Blood air barrier
What is the function of chemical surfactant?
- lowers surface tension of alveolar fluid, preventing the collapse of alveoli with each expiration/breath
- allows the expansion and recoil of the alveolar sacs
- produced by type ii alveolar sacs
Structures of the lung include..
- Root
- Costal Surface
- Apex
- Base
- Hilum
Left Lung
- separated into superior and inferior lobes by oblique fissure
- Smaller than right because of position of heart
- Has Cardiac notch
What is the cardiac notch?
concavity in the left lung for the heart to fit into
Right Lung
- separated into superior, middle, and inferior lobes
- Superior and middle lobes separated by horizontal fissure
- Middle and inferior lobes separated by oblique fissure
Root of the Lung
site of vascular and bronchial attachment to mediastinum
Costal Surface of the Lung
anterior, lateral, and posterior surfaces
Apex of the Lung
superior tip, deep to clavicle
Base of the Lung
inferior surface that rests on diaphragm
Hilum
- found on mediastinal surface
- it is the site for entry/exit of blood vessels, bronchi, lymphatic vessels, and nerves
Each lobe of the lung is divided into
Bronchopulmonary Segments
Bronchopulmonary Segments
- Separated by connective tissue septa
- Each segment is served by its own artery, vein, and segmental (tertiary) bronchus
- If one segment is diseased, it can be individually removed
How many Bronchopulmonary Segments are there in the right and left lungs?
-10 on right and 8–10 on left
Functions of the Lungs
Gas exchange Filtration Blood reservoir Metabolic Thermal
The Lung is composed of
- alveoli; the rest consists of stroma, elastic connective tissue
- Makes lungs very elastic and spongy
The Lungs are perfused by 2 circulations:
- Pulmonary Circulation
2. Bronchial Circulation
Pulmonary Circulation
- Pulmonary arteries - deliver systemic venous blood from heart to lungs for oxygenation
- Branch profusely to feed into pulmonary capillary networks
- Pulmonary veins - carry oxygenated blood from respiratory zones back to heart (Low-pressure, high-volume system)
Bronchial Circulation
- provide oxygenated blood to lung tissue
- Arise from aorta and enter lungs at hilum
- Part of systemic circulation, so are high pressure, low volume
- Supply all lung tissue except alveoli
- Bronchial veins anastomose with pulmonary veins
Pleurae
thin, double-layered serosal membrane that divides thoracic cavity into two pleural compartments and mediastinum
Parietal Pleura
membrane on thoracic wall, superior face of diaphragm, around heart, and between lungs
Visceral Pleura
membrane on external lung surface
Pleural Fluids
- fills slit-like pleural cavity between two pleurae
- Provides lubrication and surface tension that assists in expansion and recoil of lungs
Pleurisy
inflammation of pleurae that often results from pneumonia
Pleural effusion
fluid accumulation in pleural cavity
Atmospheric pressure (P atm)
- Pressure exerted by air surrounding the body
- 760 mm Hg at sea level = 1 atmosphere (1 atm)
Negative respiratory pressure is
less than Patm
Positive respiratory pressure is
greater than P atm
Zero respiratory pressure is
equal to P atm
Intrapulmonary Pressure or Intra-alveolar Pressure (P pul)
- Pressure in alveoli
- Fluctuates with breathing
- Always eventually equalizes with P atm
Intrapleural pressure (P ip)
- Pressure in pleural cavity
- Fluctuates with breathing
- Always a negative pressure (<p></p>
What happens if fluid accumulates in the pleural cavity?
If fluid accumulates, positive P ip pressure develops: Lung collapses
Lungs will collapse if:
P ip = P pul
P ip = P atm
Negative ______ must be maintained to keep lungs inflated
Intrapleural Pressure (P ip)
Volume changes leads to
pressure changes
Pressure changes leads to
to flow of gases to equalize pressure
Inspiration involves inspiratory muscles which include
the diaphragm and external intercostals
Action of the diaphragm during inspiration
- when dome-shaped diaphragm contracts, it moves inferiorly and flattens out
- Results in increase in thoracic volume
Action of intercostal muscles during inspiration
- when external intercostals contract, rib cage is lifted up and out
- Results in increase in thoracic volume
Quiet expiration
normally is passive process
Forced expiration
is an active process that uses oblique and transverse abdominal muscles, as well as internal intercostal muscles
What happens during expiration?
- Inspiratory muscles relax, thoracic cavity volume decreases, and lungs recoil
- Volume decrease causes intrapulmonary pressure (Ppul) to increase by +1 mm Hg
- Ppul > Patm so air flows out of lungs down its pressure gradient until Ppul = Patm
- P ip goes from -6 to -4 mm of Hg
What happens during inspiration?
- As thoracic cavity volume increases, lungs are stretched as they are pulled out with thoracic cage
- Causes intrapulmonary pressure to drop by 1 mm Hg (-1mm Hg)
- Ppul
- P ip goes from -4 to -6 mm Hg
Boyle’s Law
- Gases always fill the container they are in
- If amount of gas is the same and container size is reduced, pressure will increase - Pressure (P) varies inversely with volume (V)
What 3 factors influence pulmonary ventilation?
influence the ease of air passage and the amount of energy required for ventilation:
i. Airway resistance
ii. Alveolar surface tension
iii. Lung compliance
Airway resistance
- Relationship between flow (F), pressure (P), and resistance (R)
- As airway resistance rises, breathing movements become more strenuous
- Severe constriction or obstruction of bronchioles
Effect of epinephrine on airway resistance
dilates bronchioles, reduces air resistance
Alveolar Surface Tension
- Surface tension: the attraction of liquid molecules to one another at a gas-liquid interface
- Tends to draw liquid molecules closer together and reduce contact with dissimilar gas molecules
- Resists any force that tends to increase surface area of liquid
Effect of water on alveoli
- Water, which has very high surface tension, coats alveolar walls in a thin film
- Tends to cause alveoli to shrink to smallest size and causes it to collapse
Lung compliance
- measure of change in lung volume that occurs with given change in transpulmonary pressure
- Measure of how much “stretch” the lung has
The lungs have a ____ lung compliance because
- Normally high because of:
- Distensibility of lung tissue
- Surfactant, which decreases alveolar surface tension - Higher lung compliance means it is easier to expand lungs
Lung Compliance can be diminished by
- Nonelastic scar tissue replacing lung tissue (fibrosis: tuberculosis)
- Reduced production of surfactant
- Fluid or mucus buildup in lung or in respiratory passages
- Decreased flexibility of thoracic cage
List the types of respiratory volumes
- Tidal Volume (TV)
- Inspiratory Reserve Volume (IRV)
- Expiratory Reserve Volume (ERV)
- Residual Volume (RV)
Tidal Volume
~ 500 mL
-the amount of air that moves in and out with each breath while at rest
Inspiratory Reserve Volume
2100 – 3200 mL
-the maximum volume of air that can be inhaled forcibly beyond the tidal volume
Expiratory Reserve Volume (ERV)
1000 – 1200 mL
-the maximum volume of air that can be exhaled forcibly beyond the tidal volume
Residual Volume
the amount of air remaining in the lungs after exhalation (keeps the lungs from collapsing in between breaths)
List the types of respiratory capacities
- Inspiratory Capacity (IC)
- Functional Residual Capicity (FRC)
- Vital Capacity (VC)
- Total Lung Capacity (TLC)
Inspiratory Capacity
IC = TV + IRV
Functional Residual Capicity
FRC = RV + ERV
Vital Capacity
VC = TV + IRV + ERV
Total Lung Capacity
TLC = TV + IRV + ERC + RV
Ventilation-perfusion coupling
- Perfusion: blood flow reaching alveoli
- Ventilation: amount of gas reaching alveoli
Ventilation and perfusion rates must be matched for optimal, efficient gas exchange
Both are controlled by local autoregulatory mechanisms
What autoregulatory mechanisms control perfusion and ventilation?
- PO2 controls perfusion by changing arteriolar diameter
- PCO2 controls ventilation by changing bronchiolar diameter
Bohr Effect
At tissues, as more CO2 enters blood, more oxygen dissociates from hemoglobin
Chronic Bronchitis Symptoms
- Symptoms include frequent pulmonary infections
- Risk factors include smoking and environmental pollutants
Chronic Bronchitis
- Inhaled irritants cause chronic excessive mucus
- Mucosae of lower respiratory passageways become inflamed and fibrosed
- Results in obstructed airways that impair lung ventilation and gas exchange
Tuberculosis (TB)
- Infectious disease caused by bacterium Mycobacterium tuberculosis
- Symptoms: fever, night sweats, weight loss, racking cough, coughing up blood
- Treatment: 12-month course of antibiotics
Lung Cancer
Leading cause of cancer deaths in North America
90% of all cases are result of smoking
What are the 3 most common types of lung cancer?
- Adenocarcinoma (~40% of cases)
- Squamous cell carcinoma (20–40% of cases)
- Small cell carcinoma (~20% of cases)
Adenocarcinoma
originates in peripheral lung areas; develops from bronchial glands and alveolar cells
Squamous cell carcinoma
arises in bronchial epithelium
Small cell carcinoma
contains lymphocyte-like cells that originate in primary bronchi and subsequently metastasize
Asthma
- Characterized by coughing, dyspnea, wheezing, and chest tightness
- Active inflammation of airways precedes bronchospasms
- Airways thickened with inflammatory exudate magnify effect of bronchospasms
Emphysema
- Permanent enlargement of alveoli and destruction of alveolar walls result in decreased lung elasticity, with three consequences.
- Hereditary factors for disease include alpha-1 antitrypsin deficiency
Three consequences of emphysema:
- Accessory muscles are necessary for breathing, leading to exhaustion from using 10–15% more energy to breathe than normal
- Trapped air causes hyperinflation, which flattens diaphragm and causes expanded barrel chest, both of which reduces ventilation efficiency
- Damaged pulmonary capillaries lead to enlarged right ventricle
Chronic Obstructive Pulmonary Disease (COPD)
- Exemplified by chronic emphysema and chronic bronchitis
- Key feature is irreversible decrease in ability to force air out of lungs
- Dyspnea: labored breathing (“air hunger”)
- Coughing and frequent pulmonary infections
- Most patients develop hypoventilation accompanied by respiratory acidosis, hypoxemia
decompression sickness
o “The Bends”
o Formation of bubbles in the bloodstream (nitrogen), accumulate in joints
o Caused by rapid decrease in pressure (nitrogen gas, similar to soda)
hypoventilation
A decreased amount of air entering the lungs, resulting in reduced blood oxygen content, increased blood carbon dioxide content, or both
tracheotomy
surgical incision into trachea for purpose of establishing an airway
chloride shift
The plasma membrane of the RBC passively facilitates the diffusion of bicarbonate ions out of the RBC and chloride ions into the RBC
Forced (deep) inspirations
- occur during vigorous exercise or in people with COPD
- Accessory muscles are activated which further increase thoracic cage size, creating a larger pressure gradient so more air is drawn in.
Accessory inspiratory muscles
Only occurs during forced inspirations
- scalenes
- sternocleidomastoid
- pectoralis minor
- serratus anterior
Anatomical Dead Space
does not contribute to gas exchange.
Consists of air that remains in passageways (RV)
Alveolar Dead Space
- space occupied by nonfunctional alveoli
- can be due to collapse or obstruction
Total Dead Space
sum of anatomical and alveolar dead space
Obstructive Pulmonary Disease
- increased airway resistance.
- TLC, FRC, RV may increase because of hyperinflation of lungs
Restrictive Disease
- reduced TLC due to disease (ex TB) or exposure to environment agents
- VC, TLC, FRC, RV decline because lung expansion is compromised
External Respiration
diffusion of gases between blood and lungs
Internal Respiration
diffusion of gases between blood tissues
Both External and Internal Respiration are subject to
- Basic properties of gases
2. Composition of gases
Dalton’s law of partial pressures
total pressure exerted by mixture of gases is equal to sum of pressures exerted by each gas
Partial Pressure
- pressure exerted by each gas in mixture
- directly proportional to its percentage mixture
What percentage of air does nitrogen make up?
~78.6%
Partial pressure of Nitrogen (P N2)
can be calculated: 0.786 x 760 mm Hg (atm) = 597 mm Hg
What percentage of air does oxygen make up?
~20.9%
Partial Pressure of oxygen P O2
0.209 x 760 mm of Hg (atm) = 159 mm Hg
Henry’s law
for gas mixtures in contact with liquids:
- each gas will dissolve in the liquid in proportion to its partial pressure and its solubility
- at equilibrium, partial pressures in two phases will be equal
According to Henry’s law, the amount of each gas that will dissolve depends on
Solubility, Pressure (increase pressure, increase solubility) and Temperature (temp of liquid rises, solubility decreases)
External Respiration (Pulmonary Gas Exchange)
exchange of O2 and CO2 across respiratory membranes
External Respiration (Pulmonary Gas Exchange) is influenced by
- partial pressure gradients and solubilities
- thickness and surface area of respiratory membrane
- Ventilation-perfusion coupling
Alveolar P O2
= 104 mm Hg
- drives oxygen flow into blood
- equilibrium reached across respiratory membrane in ~0.25 sec, but it takes RBC ~0.75 sec to travel from start to end of pulmonary capillary (ensures adequation oxygenation even if blood flow increases 3x)
Pulmonary Arteriole Blood P O2 =
40 mm Hg
Pulmonary Arteriole Blood P CO2 =
45 mm Hg
Alveolar P CO2 =
40 mm Hg
Factors that influence breathing depth
- determined by how actively respiratory center stimulates respiratory muscles
- The greater the stimulation, the greater the number of motor units excited, increasing depth of inspiration
Factors that influence breathing rate
- determined by how long center is active
- Both are modified by changing body demands
Respiratory Centers are affected by
i. Chemical factors
ii. Influence of higher brain centers
iii. Pulmonary irritant reflexes
iv. Inflation reflex
Chemical Factors
- Most important of all factors affecting depth and rate of inspiration
- Changing levels of PCO2, PO2, and pH are most important
Levels of P CO2, P O2, and pH are sensed by
central chemoreceptors and peripheral chemoreceptors
If blood P CO2 levels rise (hypercapnia)
-CO2 accumulates in brain and joins with water to become carbonic acid
-Carbonic acid dissociates, releasing H+, causing a drop in pH (increased acidity)
-Increased H+ stimulates central chemoreceptors & peripheral chemoreceptors
Respiratory centers increase depth and rate of breathing, which act to lower blood PCO2, and pH rises to normal levels
If blood P CO2 levels decrease (hypocapnia)
respiration becomes slow and shallow
Influence of higher brain centers
- Hypothalamic controls: act through limbic system to modify rate and depth of respiration
- Cortical controls: direct signals from cerebral motor cortex that bypass medullary controls
Pulmonary irritant reflexes
- Receptors in bronchioles respond to irritants such as dust, accumulated mucus, or noxious fumes
- Receptors communicate with respiratory centers via vagus nerve (sensory)
- Promote reflexive constriction of air passages
- Same irritant triggers a cough in trachea or bronchi or a sneeze in nasal cavity
Inflation reflex aka Hering-Breuer reflex
- Stretch receptors in pleurae and airways are stimulated by lung inflation
- Send inhibitory signals to medullary respiratory centers to end inhalation and allow expiration
- May act as protective response more than as a normal regulatory mechanism
