23-1

Chapter 23

Respiratory System

23-2

Respiration

• Ventilation: Movement of air into and out of lungs

• External respiration: Gas exchange between air in lungs and blood

• Transport of oxygen and carbon dioxide in the blood

• Internal respiration: Gas exchange between the blood and tissues

23-3

Respiratory System Functions • Gas exchange: Oxygen enters blood and carbon

dioxide leaves• Regulation of blood pH: Altered by changing

blood carbon dioxide levels (increase CO2 = decrease pH)

• Voice production: Movement of air past vocal folds makes sound and speech

• Olfaction: Smell occurs when airborne molecules are drawn into nasal cavity

• Protection: Against microorganisms by preventing entry and removing them from respiratory surfaces.

23-4

Respiratory System Divisions

• Upper tract: nose, pharynx and associated structures

• Lower tract: larynx, trachea, bronchi, lungs and the tubing within the lungs

23-5

Nose (Nasus) and Nasal Cavities

• External nose (visible part – includes hyaline cartilage plates & nasal bones )

• Nasal cavity

– From nares (nostrils) to choanae (openings into the pharynx)

– Vestibule: just inside nares – lined with stratified squamous epithelium – continuous with skin

– Hard palate: floor of nasal cavity – separates nasal cavity from oral cavity – covered by mucous membrane

– Nasal septum: partition dividing cavity. Anterior cartilage; posterior vomer and perpendicular plate of ethmoid (divides nasal cavity into right & left parts)

– Choanae: bony ridges on lateral walls with meatuses (passageways) between. Openings to paranasal sinuses and to nasolacrimal duct

23-6

Functions of Nasal Cavity:• Passageway for air (open even if mouth full of food)

• Cleans the air [vestibule lined with hair & this traps particles / mucous

membrane consists of pseudostratified ciliated columnar epithelium with goblet cells (mucus)]

• Humidifies( moisture from mucous membranes & from excess tears that drains

into nasal cavity through nasolacrimal duct), warms air ( warm blood flowing

through mucous membranes - this prevents damage to respiratory passages caused by cold air)

• Smell [superior part of nasal cavity consists of olfactory epithelium (sensory receptors)]

• Along with paranasal sinuses are resonating chambers for speech

23-7

Pharynx:• Common opening for digestive and respiratory

systems (connected to respiratory at larynx & to digestive at esophagus)

• Three regions– Nasopharynx:

a. Pseudostratified columnar epithelium with goblet cells. b. Mucous and debris from nasal cavity is swallowed. c. Openings of Eustachian (auditory) tubes – air that passes through them to equalize air pressure between atmosphere & middle ear.d. Floor is soft palate (separates nasopharynx from oropharynx), uvula is posterior extension of the soft palate – prevents swallowed materials from entering nasopharynx & nasal cavity

–Oropharynx: shared with digestive system (extends from soft palate to epiglottis). Lined with moist stratified squamous epithelium – air, food, & drink passes through.–Laryngopharynx: epiglottis to esophagus. Lined with moist stratified squamous epithelium – food & drink pass through here to esophagus (very little air passes / too much air = gas)

23-8

Larynx

23-9

Larynx - base of tongue to trachea / passageway for air

• Unpaired cartilages– Thyroid: largest, Adam’s apple– Cricoid: most inferior, base of larynx (other cartilages rest here)

– Epiglottis: attached to thyroid and has a flap near base of tongue. Elastic rather than hyaline cartilage

• Paired– Arytenoids: attached to cricoid– Corniculate: attached to arytenoids– Cuneiform: contained in mucous membrane

• Ligaments extend from arytenoids to thyroid cartilage– Vestibular folds or false vocal folds– True vocal cords or vocal folds: sound production. Opening

between is glottis - laryngitis is an inflammation of mucosal epithelium of vocal folds

23-10

Functions of Larynx• Maintain an open passageway for air movement: thyroid and cricoid

cartilages

• Epiglottis and vestibular folds prevent swallowed material from moving into larynx – during swallowing, epiglottis covers the opening of larynx so, food & liquid slide over epiglottis toward esophagus. Also, closure of vestibular folds can also prevent the passage of air----when person holds breath.

• Vocal folds are primary source of sound production. Greater the amplitude of vibration, louder the sound (force of air moving past vocal cords determines amplitude). - Frequency of vibration determines pitch. Also, length of vibrating segments of vocal folds affect-------ex: when only anterior parts of folds vibrate, higher pitched tones are produced & when longer sections of vibrate, lower tones result.

- Arytenoid cartilages and skeletal muscles determine length of vocal folds and also abduct the folds when not speaking (only

breathing) to pull them out of the way making glottis larger (allows greater

movement of air).

• The pseudostratified ciliated columnar epithelium (lines larynx) traps debris, preventing their entry into the lower respiratory tract.

23-11

Vocal Folds

23-12

Trachea - windpipe

• Membranous tube of dense regular connective tissue and smooth muscle; supported by 15-20 hyaline cartilage C-shaped rings (protects & maintains open passageway for air) . Posterior surface is devoid of cartilage & contains elastic ligamentous membrane and bundles of smooth muscle called the trachealis.

Contracts during coughing-----this causes air to move more rapidly through trachea, which helps expel mucus & foreign objects.

• Inner lining: pseudostratified ciliated columnar epithelium with goblet cells. Mucus traps debris, cilia push it superiorly toward larynx and pharynx. Divides to form– Left and right primary bronchi (each extends to a lung)

– Carina: cartilage at bifurcation (forms ridge). Membrane of carina especially sensitive to irritation and inhaled objects initiate the cough reflex

23-13

Tracheobronchial Tree and Conducting Zone

• Trachea to terminal bronchioles which is ciliated for removal of debris. – Trachea divides into two primary

bronchi. (right is larger in diameter & more in line with trachea than left)

– Primary bronchi divide into secondary (lobar) bronchi (one/lobe) which then divide into tertiary (segmental) bronchi.

– Bronchopulmonary segments: defined by tertiary bronchi.

–Tertiary bronchi further subdivide into smaller and smaller bronchi then into bronchioles (less than 1 mm in diameter), then finally into terminal bronchioles.

• Cartilage: holds tube system open; smooth muscle controls tube diameter-----ex: during exercise, diameter increases, decreases resistance to airflow, increases volume of air moved during asthma attack, diameter decreases, increases resistance to airflow, decreases volume of air flow

• As tubes become smaller, amount of cartilage decreases, amount of smooth muscle increases------ex: terminal bronchioles have no cartilage & only have smooth muscle.

23-14

Respiratory Zone: Respiratory Bronchioles to Alveoli

• Respiratory zone: site for gas exchange

– Respiratory bronchioles branch from terminal bronchioles. Respiratory bronchioles have very few alveoli (small, air filled chambers where gas exchange between air &

blood takes place). Give rise to alveolar ducts which have more alveoli. Alveolar ducts end as alveolar sacs that have 2 or 3 alveoli at their terminus.

– Tissue surrounding alveoli contains elastic fibers (alveoli expand during inspiration & recoil during expiration)

– No cilia, but debris removed by macrophages. Macrophages then move into nearby lymphatics or into terminal bronchioles.

23-15

The Respiratory Membrane • Three types of cells in membrane.

– Type I pneumocytes. Thin squamous epithelial cells, form 90% of surface of alveolus. Gas exchange.

– Type II pneumocytes. Round to cube-shaped secretory cells. Produce surfactant (makes it easier for alveoli to expand during inspiration).

– Dust cells (phagocytes)• Layers of the respiratory membrane

– Thin layer of fluid lining the alveolus– Alveolar epithelium (simple squamous

epithelium– Basement membrane of the alveolar

epithelium– Thin interstitial space– Basement membrane of the capillary

endothelium– C apillary endothelium composed of

simple squamous epithelium• Tissue surrounding alveoli contains elastic

fibers that contribute to recoil.

23-16

Lungs• Two lungs: Principal organs of respiration

– Base sits on diaphragm, apex at the top, hilus (hilum) on medial surface where bronchi and blood vessels enter the lung. All the structures in hilus called root of the lung.

– Right lung: three lobes. Lobes separated by fissures (deep & prominent)

– Left lung: Two lobes– Right lung is larger & heavier than left

• Divisions– Lobes (supplied by secondary bronchi), each lobe is subdivided into

bronchopulmonary segments (supplied by tertiary bronchi and separated from one another by connective tissue partitions), bronchopulmonary segments are subdivided into lobules (supplied by bronchioles and separated by incomplete partitions).

– Note: 9 bronchopulmonary segments present in left lung & 10 present right lung– Note: Individual diseased bronchopulmonary segments can be surgically removed, leaving the rest of lung

intact, because major blood vessels & bronchi do not cross connective tissue partitions.

23-17

Thoracic Wall and Muscles of Respiration

23-18

Thoracic Wall

• Thoracic vertebrae, ribs, costal cartilages, sternum and associated muscles

• Thoracic cavity: space enclosed by thoracic wall and diaphragm

• Diaphragm separates thoracic cavity from abdominal cavity

23-19

Inspiration and Expiration

• Inspiration: diaphragm, external intercostals, pectoralis minor, scalenes– Diaphragm: dome-shaped with base of dome attached to inner

circumference of inferior thoracic cage. Central tendon: top of dome which is a flat sheet of connective tissue.

• Quiet inspiration: accounts for 2/3 of increase in size of thoracic volume. Inferior movement of central tendon and flattening of dome. Abdominal muscles relax

– Other muscles: elevate ribs and costal cartilages allow lateral rib movement

• Expiration: muscles that depress the ribs and sternum: such as the abdominal muscles and internal intercostals.

• Quiet expiration: relaxation of diaphragm and external intercostals with contraction of abdominal muscles

• Labored breathing: all inspiratory muscles are active and contract more forcefully. Expiration is rapid

23-20

Effect of Rib and Sternum

23-21

Pleura• Pleural cavity surrounds

each lung and is formed by the pleural membranes. Filled with pleural fluid.

• Visceral pleura: adherent to lung. Simple squamous epithelium, serous.

• Parietal pleura: adherent to internal thoracic wall.

• Pleural fluid: acts as a lubricant and helps hold the two membranes close together (adhesion).

• Mediastinum: central region, contains contents of thoracic cavity except for lungs.

23-22

Blood and Lymphatic Supply• Two sources of blood to lungs: Pulmonary & Bronchial

– Pulmonary artery brings deoxygenated blood to lungs from right side of heart to be oxygenated in capillary beds that surround the alveoli. Blood leaves via the pulmonary veins and returns to the left side of the heart.

– Bronchial arteries provide oxygenated systemic blood to lung tissue. They arise from the aorta & run along the branching bronchi. Part of this now deoxygenated blood exits through the bronchial veins to the azygous (drains chest muscles); part merges with blood of alveolar capillaries and returns to left side of heart.

– Blood going to left side of heart via pulmonary veins carries primarily oxygenated blood, but also some deoxygenated blood from the supply of the walls of the conducting and respiratory zone.

• Two lymphatic supplies: superficial and deep lymphatic vessels. Exit from hilus– Superficial drain superficial lung tissue and visceral pleura– Deep drain bronchi and associated C.T.– No lymphatics drain alveoli– Phagocytic cells within lungs phagocytize carbon particles & other

debris from inspired air & move them to lymphatic vessels– Older people & smokers lungs appear gray to black because accumulation of these particles– Cancer cells from lungs can spread to other parts of body through lymphatic vessels.

23-23

Ventilation

• Movement of air into and out of lungs

• Air moves from area of higher pressure to area of lower pressure (requires a pressure gradient)

• If barometric pressure (atmospheric pressure) is greater than alveolar pressure, then air flows into the alveoli.

• Boyle’s Law : P = k/V, where P = gas pressure,V = volume, k = constant at a given temperature

• If diaphragm contracts, then size of alveoli increases. Remember P is inversely proportionate to V; so as V gets larger (when diaphragm contracts), then P in alveoli gets smaller.

23-24

Alveolar Pressure Changes: (Note: Barometric air pressure is

always assigned a value of zero)

23-25

Changing Alveolar Volume: Lung Recoil( Lung recoil & changes in pleural pressure cause changes in alveolar volume which results in changes in

pressure )

• Causes alveoli to collapse resulting from – Elastic recoil: elastic fibers in the alveolar walls– Surface tension: film of fluid lines the alveoli. Where

water interfaces with air, polar water molecules have great attraction for each other with a net pull in toward other water molecules. Tends to make alveoli collapse.(attracted molecules of fluid = surface tension = draws alveoli to their smallest possible dimension)

• Surfactant: Reduces tendency of lungs to collapse by reducing surface tension. Produced by type II pneumocytes.

• Respiratory distress syndrome (hyaline membrane disease). Common in infants with gestation age of less than 7 months. Not enough surfactant produced.

23-26

Pleural Pressure ( pressure in pleural cavity) :

• Negative pressure can cause alveoli to expand

• Alveoli expand when pleural pressure is low enough to overcome lung recoil

• Pneumothorax is an opening between pleural cavity and air that causes an increase of pleural pressure (air gets into pleural cavity by an opening in the thoracic wall or lung---------can be caused by penetrating trauma ex: knife, bullet, broken rib or by non-penetrating trauma ex: blow to chest, medical procedure (inserting catheter to withdraw pleural fluid), infections. Causes part or all of the lung to collapse.

23-27

Normal Breathing Cycle: (Inspiration: pleural pressure decreases =

alveolar volume increases = alveolar pressure decreases below barometric pressure = air flow into lungs.

23-28

Compliance• Measure of the ease with which lungs and thorax

expand– The greater the compliance, the easier it is for a change in

pressure to cause expansion– A lower-than-normal compliance means the lungs and thorax

are harder to expand• Conditions that decrease compliance

– Pulmonary fibrosis: deposition of inelastic fibers in lung (emphysema)

– Pulmonary edema (the alveoli fill with fluid instead of air, preventing oxygen from being absorbed into your

bloodstream)

– Respiratory distress syndrome– Increased resistance to airflow caused by airway obstruction

(asthma, bronchitis, lung cancer)– Deformities of the thoracic wall (kyphosis (hunchback), scoliosis)

23-29

Pulmonary Volumes and Capacities• Spirometry: measures volumes of air that move into and

out of respiratory system. Uses a spirometer• Tidal volume: amount of air inspired or expired with each

breath. At rest: 500 mL• Inspiratory reserve volume: amount that can be inspired

forcefully after inspiration of the tidal volume (3000 mL at rest)

• Expiratory reserve volume: amount that can be forcefully expired after expiration of the tidal volume (100 mL at rest)

• Residual volume: volume still remaining in respiratory passages and lungs after most forceful expiration (1200 mL)

23-30

Pulmonary Capacities• The sum of two or more pulmonary volumes• Inspiratory capacity: tidal volume plus

inspiratory reserve volume• Functional residual capacity: expiratory reserve

volume plus residual volume• Vital capacity: sum of inspiratory reserve

volume, tidal volume, and expiratory reserve volume

• Total lung capacity: sum of inspiratory and expiratory reserve volumes plus tidal volume and residual volume.

* Factors such as sex, age, body size, and physical conditioning cause variations in respiration & capacities from one individual to another. Ex: males, younger people, thin people, tall people, athletes------------have greater vital capacities.

23-31

Spirometer, Lung Volumes, and Lung Capacities

23-32

Minute Ventilation and Alveolar Ventilation

• Minute ventilation: total air moved into and out of respiratory system each minute; tidal volume X respiratory rate

• Respiratory rate (respiratory frequency) (f): number of breaths taken per minute

• Anatomic dead space: formed by nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles (part of respiratory system where gas exchange does NOT take place)

• Physiological dead space: anatomic dead space plus the volume of any alveoli in which gas exchange is less than normal. (these are nonfunctional alveoli--------few exist in healthy individual)

• Alveolar ventilation (VA): volume of air available for gas exchange/minute VA = f ( VT – VD)

VT = tidal volume VD = dead space

23-33

Physical Principles of Gas Exchange• Partial pressure

– The pressure exerted by each type of gas in a mixture ex: atmospheric pressure = 760 mmHg (contains: nitrogen 79% & oxygen 21%)

– Dalton’s law: in a mixture of gases, the percentage of each gas is proportionate to its partial pressure

N2 = 79% =79/100 = 0.79 ----partial pressure = 0.79 x 760 mmHg = 600mmHg

partial pressure is denoted---- PN2

– Water vapor pressure: pressure exerted by gaseous water in a mixture of gases (water evaporated into air)

– Air in the respiratory system contains humidity because of mucus lining system

• Diffusion of gases through liquids (gas molecules move from air into liquid, or from a liquid into air, because of partial pressure gradient----ex: partial pressure of gas in the air is greater than in the liquid, movement of gas molecules into the liquid)– Henry’s Law: Concentration of a gas in a liquid is determined by its

partial pressure and its solubility coefficient (solubility coefficient is a measure of how easily the gas dissolves in the liquid. Ex: solubility coefficient for oxygen is 0.024; carbon dioxide is 0.57-----CO2 is 24 times more soluble than O2 )

23-34

Physical Principles of Gas Exchange• Diffusion of gases through the respiratory membrane

depends upon three things1. Membrane thickness. The thicker, the lower the

diffusion rate (diseases can cause an increase in thickness)

2. Diffusion coefficient of gas (measure of how easily a gas diffuses through a liquid or tissue). This takes into account the solubility of the gases & size of gas molecules (molecular weight). CO2 is 20 times more diffusible than O2

3. Surface area. Diseases like emphysema and lung cancer reduce available surface area

4. Partial pressure differences. Gas moves from area of higher partial pressure to area of lower partial pressure. Normally, partial pressure of oxygen is higher in alveoli than in blood. Opposite is usually true for carbon dioxide

23-35

Relationship Between Alveolar Ventilation and Pulmonary Capillary Perfusion

• Increased ventilation or increased pulmonary capillary blood flow increases gas exchange

• Shunted blood: blood that is not completely oxygenated• Physiologic shunt is deoxygenated blood returning from lungs.

Two sources:– Blood returning from bronchi bronchioles– Blood from capillaries around alveoli* 1% - 2% of cardiac output makes up the physiological shunt

• Regional distribution of blood flow determined primarily by gravity, but can also be determined by alveolar PO2. – Low PO2 causes arterioles to constrict so that blood is

shunted to a region of the lung where the alveoli are better ventilated. Ex: when bronchus becomes partially blocked

– In other tissues of the body, low PO2 causes arterioles to dilate to deliver more blood to the tissues.

23-36

Oxygen and Carbon Dioxide Diffusion Gradients

• Oxygen– Moves from alveoli into

blood. Blood is almost completely saturated with oxygen when it leaves the capillary

– PO2 in blood decreases because of mixing with deoxygenated blood (because blood from pulmonary capillaries mixes with deoxygenated blood from bronchial veins)

– Oxygen moves from tissue capillaries into the tissues

• Carbon dioxide– Moves from tissues

into tissue capillaries

– Moves from pulmonary capillaries into the alveoli

23-37

Gas Exchange

23-38

Hemoglobin and Oxygen Transport• Oxygen is transported by

hemoglobin (98.5%) and is dissolved in plasma (1.5%)

• Oxygen-hemoglobin dissociation curve: describes the percentage of hemoglobin saturated with oxygen at any given PO2

• Oxygen-hemoglobin dissociation curve at rest shows that hemoglobin is almost completely saturated when PO2 is 80 mm Hg or above. At lower partial pressures, the hemoglobin releases oxygen.

• Thus, as tissues use more oxygen, hemoglobin releases more oxygen to those tissues.

23-39

Bohr Effect

• Effect of pH on oxygen-hemoglobin dissociation curve: as pH of blood declines, amount of oxygen bound to hemoglobin at any given PO2 also declines

• Occurs because decreased pH yields increase in H + that combines with hemoglobin changing its shape and oxygen cannot bind to hemoglobin

23-40

Effects of CO2 and Temperature

• Increase in PCO2 causes decrease in p H• Carbonic anhydrase causes CO2 and water to

combine reversibly and form H2CO3 (carbonic acid)

which ionizes to H + and HCO3- (bicarbonate ion)

• Increase temperature: decreases tendency for oxygen to remain bound to hemoglobin, so as metabolism goes up, more oxygen is released to the tissues.

23-41

Effect of BPG

• 2,3-bisphosphoglycerate (BPG): released by RBCs as they break down glucose for energy

• Binds to hemoglobin and increases release of oxygen (reduces its affinity for oxygen)

• Ex: High altitudes = decrease barometric pressure = partial pressure of oxygen in alveoli decreased = % saturation of blood with oxygen in pulmonary capillaries decreased = less oxygen in blood to be

delivered to tissues

BPG helps increase oxygen delivery to tissues because increased levels of BPG increase the release of oxygen in tissues.

23-42

Shifting the Curve

23-43

Transport of Carbon Dioxide• Carbon dioxide is transported as bicarbonate ions

(70%) in combination with blood proteins (23%: primarily alpha & beta globin chains of hemoglobin) and in solution with plasma (7%)

• Hemoglobin that has released oxygen binds more readily to carbon dioxide than hemoglobin that has oxygen bound to it ( Haldane effect)

• In tissue capillaries, carbon dioxide combines with water inside RBCs to form carbonic acid which dissociates to form bicarbonate ions and hydrogen ions

23-44

Carbon Dioxide Transport and Chloride Movement

(a) Tissue capillaries: as C O2 enters red blood cells, reacts with water to form bicarbonate and hydrogen ions. C hloride ions enter the RB C and bicarbonate ions leave: chloride shift. H ydrogen ions combine with hemoglobin. (pH of RBC does not decrease bec. hemoglobin is a buffer ) Lowering the concentration of bicarbonate and hydrogen ions inside red blood cells promotes the conversion of C O2 to bicarbonate ion.

(b) Pulmonary capillaries: C O2 leaves red blood cells, resulting in the formation of additional C O2 from carbonic acid. The bicarbonate ions are exchanged for chloride ions, and the hydrogen ions are released from hemoglobin.

• Increased plasma carbon dioxide lowers blood p H. The respiratory system regulates blood p H by regulating plasma carbon dioxide levels

23-45

Respiratory Areas in the Brainstem

• Medullary respiratory center– Dorsal groups stimulate the

diaphragm– Ventral groups stimulate the

intercostal and abdominal muscles

– This section is especially sensitive during infancy, and the neurons can be destroyed if the infant is dropped and/or shaken violently. The result can be death due to "shaken baby syndrome”

• Pontine (pneumotaxic) respiratory group– Involved with switching between

inspiration and expiration (fine tunes the breathing pattern-----there is a connection with medullary resp. center but precise function unknown)

23-46

Rhythmic Ventilation• Starting inspiration

– Medullary respiratory center neurons are continuously active– Center receives stimulation from receptors (that monitor blood gas levels)

and simulation from parts of brain concerned with voluntary respiratory movements and emotion

– Combined input from all sources causes action potentials to stimulate respiratory muscles

• Increasing inspiration– More and more neurons are activated (to stimulate respiratory muscles)

• Stopping inspiration– Neurons stimulating the muscles of respiration also stimulate the

neurons in the medullary respiratory center that are responsible stopping inspiration. They also receive input from pontine group and stretch receptors in lungs. Inhibitory neurons activated and relaxation of respiratory muscles results in expiration.

– Note: although the medullary neurons establish the basic rate & depth of breathing, their activities can be influenced by input from other parts of the brain & by input from peripherally located receptors.

23-47

Rhythmic Ventilation

• Apnea. Cessation of breathing. Can be conscious decision, but eventually PCO2 levels increase to point that respiratory center overrides

• Hyperventilation. Causes decrease in blood PCO2 level, which causes respiratory alkalosis (high blood pH). Fainting, leads to changes in the nervous system fires and leads to the paresthesia (pins & needles)

• Cerebral (cerebral cortex)and limbic system. Respiration can be voluntarily controlled and modified by emotions (ex: strong emotions can cause hyperventilation or produce the sobs & gasps of crying)

• Chemical control– Carbon dioxide is major

regulator, but indirectly through p H change

• Increase or decrease in pH can stimulate chemo-sensitive area, causing a greater rate and depth of respiration

– Oxygen levels in blood affect respiration when a 50% or greater decrease from normal levels exists

• CO2. – Hypercapnia: too much CO2

– Hypocapnia: lower than normal CO2

23-48

Modifying Respiration

23-49

Chemical Control of Ventilation• Chemoreceptors: specialized neurons that respond

to changes in chemicals in solution– Central chemoreceptors: chemosensitive area of the

medulla oblongata; connected to respiratory center– Peripheral chemoreceptors: carotid and aortic

bodies. Connected to respiratory center by cranial nerves IX and X (9 & 10)

• Effect of pH : chemosensitive area of medulla oblongata and carotid and aortic bodies respond to blood pH changes– Chemosensitive areas respond indirectly through

changes in carbon dioxide– Carotid and aortic bodies respond directly to p H

changes

23-50

Chemical Control of Ventilation• Effect of carbon dioxide: small change in carbon

dioxide in blood triggers a large increase in rate and depth of respiration - ex: an increase PCO2 of 5 mm Hg causes an increase in ventilation of 100%.– Hypercapnia: greater-than-normal amount of carbon

dioxide– Hypocapnia: lower-than-normal amount of carbon

dioxide• Chemosensitive area in medulla oblongata is more

important for regulation of PCO2 and pH than the carotid & aortic bodies (responsible for 15% - 20% of response)

• During intense exercise, carotid & aortic bodies respond more rapidly to changes in blood pH than does the chemosensitive area of medulla

23-51

Chemical Control of Ventilation

• Effect of oxygen: carotid and aortic body chemoreceptors respond to decreased PO2 by increased stimulation of respiratory center to keep it active despite decreasing oxygen levels (50% or greater decrease----------bec. of oxygen-hemoglobin dissociation curve-------at any PO2 above 80 mm Hg nearly all of hemoglobin is saturated with oxygen)

• Hypoxia: decrease in oxygen levels below normal values

23-52

Regulation of Blood pH and Gases

23-53

Hering-Breuer Reflex• Limits the degree of inspiration and prevents

overinflation of the lungs • Depends on stretch receptors in the walls of

bronchi & bronchioles of the lung. • It is an inhibitory influence on the respiratory

center & results in expiration. (as expiration proceeds, stretch receptors no longer stimulated)

– Infants• Reflex plays a role in regulating basic rhythm of

breathing and preventing overinflation of lungs

– Adults• Reflex important only when tidal volume large as in

exercise

23-54

Effect of Exercise on Ventilation• Ventilation increases abruptly

– At onset of exercise– Movement of limbs has strong influence (body movements

stimulate proprioceptors in joints of the limbs)

– Learned component (after a period of training, the brain “learns” to match ventilation with the intensity of exercise)

• Ventilation increases gradually– After immediate increase, gradual increase occurs (4-6

minutes it levels off)

– Anaerobic threshold: highest level of exercise without causing significant change in blood pH. If exercise intensity is high enough to exceeded anaerobic threshold, lactic acid produced by skeletal muscles

23-55

Other Modifications of Ventilation

• Activation of touch, thermal and pain receptors affect respiratory center

• Sneeze reflex (initiated by irritants in the nasal cavity), cough reflex (initiated by irritants in the lungs)

• Increase in body temperature yields increase in ventilation

23-56

Respiratory Adaptations to Exercise

• Athletic training– Vital capacity increases slightly; residual volume

decreases slightly– At maximal exercise, tidal volume and minute

ventilation increases– Gas exchange between alveoli and blood increases at

maximal exercise– Alveolar ventilation increases– Increased cardiovascular efficiency leads to greater

blood flow through the lungs

23-57

Effects of Aging

• Vital capacity and maximum minute ventilation decrease (these changes are related to weakening of respiratory muscles & decreased compliance of thoracic cage caused by stiffening of cartilage & ribs)

• Residual volume and dead space increase

• Ability to remove mucus from respiratory passageways decreases

• Gas exchange across respiratory membrane is reduced