Respiration: All living cells of body require oxygen and produce carbon dioxide

Respiration includes four process: Ventilation: Movement of air into and out of lungs

(breathing) External respiration: Movement of O2 from lungs to blood,

CO2 from blood to lungs

Transport of respiratory gases: transport of O2 from lungs to tissues via blood, CO2 from tissue to lungs via blood

Internal respiration: movement of O2 from blood to tissues, CO2 from tissues to blood

Respiratory system assists in gas exchange and perform other functions as well:

Gas exchange: Oxygen enters blood from air and carbon dioxide leaves blood and enter air

Regulation of blood pH: Alter blood pH by changing blood carbon dioxide levels

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

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

Protection: Protect against microorganisms by preventing entry and removing them from respiratory surfaces

Consists of external nose, nasal cavity, pharynx, trachea, bronchi and lungs

Divided into upper respiratory tract and lower respiratory tract:

Upper Respiratory tract: nose, pharynx and associated structures

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

Nose: Consists of : External nose Nasal cavity

External nose:– Visible, prominent feature

of face– Consists of nasal bones,

extensions of frontal and maxillary bones

Nasal cavity– Extends from nares to choanae– Nares (nostrils): external

openings of nasal cavity– Choanae: Are openings into

pharynx– Vestibule: nasal cavity

superior to nares– The nasal cavity is

separated from the oral cavity by the palate

– Anterior hard palate (bone)– Posterior soft palate (muscle)

Nasal cavity– Nasal septum: Divides nasal

cavity into left & right parts – Anterior part of nasal septum is

made of cartilage– Posterior part consists of vomer

bone & perpendicular plate of ethmoid

– Conchae: 3 bony ridges on lateral walls of nasal cavity

– With meatuses (passageway) between

Paranasal Sinuses• Cavities within bones surrounding

the nasal cavity are called sinuses• Sinuses are located in the

following bones– Frontal bone– Sphenoid bone– Ethmoid bone– Maxillary bone

• Function of the sinuses– Lighten the skull– Act as resonance chambers for

speech– Produce mucus that drains into

the nasal cavity

Passageway for air: Open even when mouth is full of food Cleans the air: Vestibule is lined with hair & filter air Nasal septum and nasal conchae increase the surface area of nasal

cavity– Enhance air turbulence and help filter air– Ciliated epithelial cells remove contaminated mucus

Humidifies, warms air: – Humidified by the high water content in the nasal cavity– Warmed by blood flowing through mucous membrane

Smell: Contains olfactory epithelium, sensory organ for smell Speech: Nasal cavity and paranasal sinuses are resonating chambers

for speech

Pharynx (Throat): Common opening for digestive & respiratory systems

Receives air from nasal cavity and food and drink from oral cavity

Connects to nasal cavity and mouth superiorly– Larynx and esophagus inferiorly

Extends from the base of the skull to sixth cervical vertebra

Divided into three regions:– Nasopharynx– Oropharynx– Laryngopharynx

Nasopharynx: Superior region behind nasal cavity,

inferior to the sphenoid, and superior to the level of the soft palate

Strictly an air passageway

Closes during swallowing to prevent food from entering the nasal cavity

Lined with pseudostratified columnar epithelium with goblet cells

Mucous and debris is moved through nasopharynx and swallowed

Openings of two auditory tubes

Posterior surface of nasopharynx contains Pharyngeal tonsil (adenoid)

• Oropharynx:• Middle region behind mouth

• Extends inferiorly from soft palate to the epiglottis

• Oral cavity opens to oropharynx via an archway called the fauces

• Serves as a common passageway for food and air

• Lined with protective stratified squamous epithelium – abrasion

• Palatine and Lingual tonsils, located near the fauces

Laryngopharynx

Inferior region attached to larynx

Serves as a common passageway for food and air

Extends from tip of epiglottis to esophagus

And passes posterior to larynx

Lined with moist stratified squamous epithelium

• Located in the ant. part of throat & extend to the trachea posteriorly

• Passageway for air between pharynx and trachea

• Attaches superiorly to hyoid bone

• And consists of nine cartilages connected by muscles and ligaments

• Unpaired cartilages:– Thyroid: largest, Adam’s apple– Cricoid: most inferior, base of larynx– Epiglottis: attached to thyroid cartilage – Projects superiorly as a flap near base of tongue– Consists of elastic rather than hyaline cartilage

• Paired Cartilages :– Arytenoids: attached to cricoid cartilage– Corniculate: attached to arytenoids– Cuneiform: contained in mucous membrane, anterior to

corniculate cartilages

• Vocal ligaments attach arytenoid cartilages to the thyroid cartilage• True vocal cords or folds

– Composed of elastic fibers that form mucosal folds– The medial opening between them is the glottis– They vibrate to produce sound as air rushes up from the lungs

• False vocal cords or Vestibular folds– Mucosal folds superior to the true vocal cords– Have no part in sound production

Thyroid and cricoid cartilages maintain an open passageway for air movement

Epiglottis and closure of vestibular folds prevent swallowed material from moving into larynx

Vocal folds are primary source of sound production

The pseudostratified ciliated columnar epithelium traps debris, and moves into pharynx, preventing their entry into the lower respiratory tract

• Trachea (wind pipe):• Is membranous tube attached to larynx• Consists of dense regular connective tissue and smooth muscle • Supported by 15-20 C-shaped hyaline cartilage rings • Absent on posterior side• Posterior surface contains elastic ligamentous membrane and bundles of

smooth muscle called the trachealis muscle• Contracts during coughing, causes air to move inside trachea, helps expel

mucus

• Trachea:• Inner lining of trachea contains pseudostratified ciliated

columnar epithelium with goblet cells

• Mucus traps debris, cilia push it superiorly toward larynx and from which they enter pharynx and are swallowed

• Trachea:• Trachea is 10-12 cm long, 12 mm diameter• Extend from larynx to 5th thoracic vertebrae• Divides to form

– Left and right primary bronchi– Carina: cartilage at bifurcation

Lower Respiratory Tract

• Trachea– Tracheal cartilages and

carina– Lined with

pseudostratified ciliated columnar epithelium

– Goblet cells– Horseshoe shaped

hyaline cartilage– Trachealis muscle

posterior

esophagus

trachealis muscle

hyaline cartilag

e

Pseudostratified ciliated columnar

epithelium adventitia

Bronchial Tree• Primary Bronchi

– Rt. Bronchus is shorter, wider and steeper than the left

– Same wall structure as trachea

– Cartilage in plates in smaller passageways

– Branch into secondary and tertiary bronchi

(R) primary

bronchus

(L) primary

bronchus

Bronchial Tree• Trachea (R) and (L) primary

bronchi secondary (lobar) bronchi

tertiary (segmental) bronchi

smaller bronchi bronchioles

terminal bronchioles respiratory

bronchioles alveolar duct

alveolar sac alveolus

trachea

(R) primary

bronchus

(L) primary

bronchus

Bronchial Tree• Bronchioles

– No cartilage– No cilia or mucous

producing cells– Epithelium changes to

simple columnar

• Terminal Bronchioles– Simple cuboidal epithelium– Lead into respiratory

bronchioles

terminal bronchiol

e

Bronchial Tree• Respiratory Bronchioles

– Lead into alveolar ducts

• Alveoli– Simple squamous epithelium– May open into an alveolar sac– Where gas exchange takes

place capillaries

respirato

ry bronchiol

e

alveoli

alveolar sac

Alveoli• Septal Cells (type II alveolar cells)

– Produce pulmonary surfactant

• Dust Cells– Macrophages in alveoli– Phagocytize bacteria, dirt, foreign

particles

• Respiratory Membrane=– Squamous alveoli epithelium + alveolar

basement membrane + endothelium of capillary walls

– Gas on one side, blood on other– Gas diffuses easily

Trachea divides into two primary bronchi

Primary bronchi divide into secondary bronchi (one/lobe)

Right lung - 3 lobes Left lung – 2 lobes Sec. bronchi then divide into

tertiary bronchi Tertiary bronchi subdivide into

smaller and smaller bronchi then into bronchioles (less than 1 mm in diameter), then finally into terminal bronchioles

Approx. 16 generations of branching occur from trachea to terminal bronchioles

• As conducting tubes become smaller, structure of their walls changes:– Cartilage support structures change:– Main bronchi are supported by C-shaped cartilage

– In lobar bronchi, C-shaped cartilages are replaced with cartilage plates

– As bronchi becomes smaller, amount of cartilage decreases, amount of smooth muscle increases

– smooth muscle controls tube diameter, change the volume of air moving through them

– Epithelium types change: – Bronchi are lined with pseudostratified ciliated

columnar epithelium

– Larger Bronchioles are lined with ciliated simple columnar epithelium

– Terminal bronchioles – ciliated simple cuboidal epithelium

– Trachea to terminal bronchioles are ciliated for removal of debris, Traps debris and moves it to larynx

Respiratory Zone: Respiratory Bronchioles to Alveoli

Respiratory zone: site for gas exchange– Respiratory bronchioles branch from

terminal bronchioles– Respiratory bronchioles give rise to

alveolar ducts– Alveolar ducts end as alveolar sacs

composed of alveoli– Alveoli – are small air-filled chambers

where gas exchange between air and blood takes place

• Approximately 300 million alveoli:

– Occupy most of the lungs’ volume – Provide tremendous surface area for gas

exchange

• Alveoli: Elastic fibers surround the alveoli

• Allow alveoli to expand during inspiration

• Recoil during expiration

• Alveolar ducts and alveoli consist of simple squamous epithelium

• Epithelium is not ciliated, but debris from air is removed by macrophages

• Wall of Alveoli is very thin• Two types of cell forms the wall

– Type I pneumocytes: Thin squamous epithelial cells

– forms 90% of surface of alveolus– Gas exchange between alveolar air

and blood takes place through these cells

– Type II pneumocytes: Round to cube-shaped secretory cells. Produce surfactant (mixture of lipoprotein molecules)

– Makes easier for alveoli to expand during inspiration

• Gas exchange takes place between air and blood in respiratory membrane of lungs

• Formed by alveolar walls and pulmonary capillaries

• Respiratory membrane is very thin for diffusion of gases. Consists of:

– Thin layer of fluid lining the alveolus with surfactant

– Alveolar epithelium composed of simple squamous epithelium

– Basement membrane of the alveolar epithelium

– Thin interstitial space– Basement membrane of the capillary

endothelium– Capillary endothelium composed of simple

squamous epithelium

• Two lungs: Principal organs of respiration– Base sits on diaphragm, apex at the top

– hilus on medial surface where bronchi and blood vessels enter or exit the lung

– Right lung: three lobes. Lobes separated by fissures

– Left lung: Two lobes

• Divisions– Lobes (supplied by secondary bronchi)

– Lobes are subdivided into bronchopulmonary segments

– (supplied by tertiary bronchi), separated by connective tissues

– R. Lung – 10 bronchopulmonary segments – L. Lung – 9 bronchopulmonary segments

– Bronchopulmonary segments are subdivided into lobules (supplied by bronchioles)

• Thoracic wall consists of 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• Diaphragm and other skeletal muscles associated with thoracic wall

are responsible for respiration

• Inspiration: Muscles of inspiration include diaphragm, external intercostals, pectoralis minor, scalenes

• Diaphragm: dome-shaped

• Quiet inspiration: Contraction of diaphragm – accounts for 2/3 of increase in

size of thoracic volume.– Other muscles also increase

thoracic volume by elevating ribs– Pressure decreases– Air flows in

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

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

• Causes decrease in thoracic volume

Each lung is surrounded by pleural cavity formed by the pleural membranes Filled with pleural fluid

Visceral pleura: covers the surface of lung

Parietal pleura: covers the internal thoracic wall

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

• Two sources of blood to lungs:– Two blood flow routes to the lungs exist:

– Pulmonary artery brings deoxygenated blood to lungs from right side of heart to be oxygenated

– After oxygenated, Blood leaves from lung via the pulmonary veins and returns to the left side of the heart

– Oxygenated blood flows from thoracic aorta through bronchial arteries to capillaries

– Oxygenated blood travels to the tissues of the bronchi

– Part of this now deoxygenated blood exits through the bronchial veins to the azygous venous system

Lymphatic Supply• Two lymphatic supplies: superficial and deep lymphatic

vessels. Exit from hilus

– Superficial lymphatic vessels : drain lymph from superficial lung tissue & visceral pleura

– Deep lymphatic vessels: drain lymph from bronchi and associated connective tissues

– No lymphatics vessels are present in the walls of alveoli

• Breathing, or pulmonary ventilation, consists of two phases– Inspiration – air flows into the lungs– Expiration – gases exit the lungs

• Air moves from area of higher pressure to area of lower pressure

F = P1 - P2R

• F = airflow (ml/min.) in a tube• P1 = pressure at one point• P2 = pressure at point two• R = resistance to airflow

• For eg. During inspiration, air pressure is greater outside the body than in alveoli

• And air flows through trachea and bronchi to alveoli

Boyle’s Law: P = k/V

P = gas pressure V = volume k = constant at a given temperature

Pressure in a container, such as thoracic cavity or alveolus is inversely proportional to volume

When volume increases, pressure decreases

When volume decreases, pressure increases

• Movement of air into and out of lungs results from changes in thoracic volume which changes alveolar volume

• Changes in alveolar volume produce changes in alveolar pressure

• If barometric air pressure (atmospheric air pressure) is greater than alveolar pressure

(PB – Palv), then air flows into the alveoli

• End of expiration: • PB = Palv• no movement of air into or out of lung

• During Inspiration: • Diaphragm contracts, & increases

thoracic volume• lungs expand, & increases alveolar

volume & decreases alveolar pressure• PB is more than Palv• Air flows into lungs

• End of Inspiration:• Palv = PB

• No air movement takes place

• During expiration:• Diaphragm relaxes, thoracic volume

decreases• Results in decreased alveolar

volume & increased alveolar pressure

• Palv is greater than PB

• Air moves out of the lungs

Two factors can change alveolar volume Lung Recoil Pleural Pressure

Lung Recoil Is decrease in size of an expanded lung Lung size decreases as alveoli size decreases Alveoli decreases in size for two reasons:

– Elastic recoil: caused by elastic fibers in the alveolar walls– Surface tension: surface tension of film of fluid that lines the

alveoli– surface tension occurs between water and air at the boundary– polar water molecules have great attraction for each other than air– tends to form water droplet– Causes alveoli to collapse

Surfactant: Reduces tendency of lungs to collapse by reducing surface tension

Surfactant is a mixture of lipoprotein molecules produced by type II pneumocytes

Forms layer over the surface of the fluid and reduce surface tension

Infant Respiratory distress syndrome (hyaline membrane disease): Common in infants with gestation age of less than 7 months.

Not enough surfactant produced Cortisol is given to pregnant women – stimulate

surfactant synthesis

Pleural pressure (Ppl) is the pressure in the pleural cavity

When pleural pressure is less than (negative pleural pressure) alveolar pressure, alveoli expand

eg. Balloon expands when outside pressure is less than inside pressure

Pneumothorax is an opening between pleural cavity

caused from penetrating trauma by knife, bullet, broken rib etc.

Pleural pressure increases & equal to barometric air pressure

Alveoli do not expand, lungs collapses symptoms: chest pain, shortness of breath

• In respiratory physiology, small pressure is expressed in cm of H2O. Pressure of 1 cm H2O is .74 mm Hg

• Changes during Inspiration:• At the end of normal expiration, pleural

pressure is – 5cm H2O and alveolar pressure = barometric pressure (0 cm H2O)

• During inspiration, pleural pressure decreases to -8 cm H2O , thoracic cavity volume increases, lungs expand, increased lung recoil, alveolar volume increases, Palv is less than PB, air flows into lungs

• By the end of inspiration Palv = PB

• Changes during Expiration:

• Pleural pressure increases, thoracic volume decreases , decreased lung recoil

• Alveolar volume decreases, Palv is more than PB, air flows out of lung

• As air flows out of lung Palv = PB

Compliance: Is a 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)

– Increased resistance to airflow caused by airway obstruction (asthma, bronchitis, lung cancer)

– Deformities of the thoracic wall that reduce the ability of the thoracic volume to increase (scoliosis)

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 (1100 mL at rest)

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

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

reserve volume (approx. 3500 ml at rest)• Functional residual capacity: expiratory reserve volume

plus residual volume (approx. 2300 ml at rest)• Vital capacity: sum of inspiratory reserve volume, tidal

volume, and expiratory reserve volume (approx. 4600 ml)• Total lung capacity: sum of inspiratory and expiratory

reserve volumes plus tidal volume and residual volume (approx. 5800 ml)

Lung Volumes, and Lung Capacities

• Minute ventilation: total air moved into and out of respiratory system each minute

• = tidal volume X respiratory rate• Respiratory rate (respiratory frequency): number of breaths

taken per minute• Tidal volume = 500 ml• Respiratory rate = 12 breaths/min • Minute ventilation = 6 L/min

• Dead space: Part of respiratory system where gas exchange does not takes place

• Anatomic dead space: Measures approx. 150 ml.• Formed by nasal cavity, pharynx, larynx, trachea, bronchi,

bronchioles, and terminal bronchioles

• Physiological dead space: anatomic dead space plus the volume of any alveoli in which gas exchange is less than normal.

• Emphysema and physiological dead space:• Alveolar walls degenerate – patients with emphysema• small alveoli combine – forms large alveoli• Forms alveoli with large volume and less surface area• less gas exchange• increases physiological dead space • Alveolar ventilation (VA): volume of air available for gas

exchange/minute

VA = f (VT – VD), f = respiratory rate

VT = tidal volume, VD = dead space

Ventilation supplies atmospheric air to alveoli Diffusion of gases between alveoli and blood in pulmonary

capillaries takes place Physical Principles of Gas Exchange:

Dalton’s Law of Partial Pressures

Total pressure exerted by a mixture of gases is the sum of the pressures exerted independently by each gas in the mixture

Pressure exerted by each type of gas in a mixture is Partial pressure of that gas

• When a mixture of gases is in contact with a liquid, each gas will dissolve in the liquid in proportion to its partial pressure

• The amount of gas that will dissolve in a liquid also depends upon its solubility

Conc. of dissolved gas = Pressure of gas x solubility coefficient

– Carbon dioxide is the most soluble– Oxygen is 1/20th as soluble as carbon dioxide– Nitrogen is practically insoluble in plasma

Basic Properties of Gases: Henry’s Law

Diffusion of gases through the respiratory membrane depends upon:

Membrane thickness: The thicker, the lower the diffusion rate

Inflammation of lung tissues such as tuberculosis, pneumonia cause fluid accumulation around alveoli

Diffusion coefficient of gas: Is a measure of how easily a gas diffuses through a liquid or tissue

CO2 is 20 times more diffusible than O2

Surface area. Diseases like lung cancer reduce available surface area

Small decrease in surface area affect gas exchange

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

Increase alveolar ventilation increase partial gradient for O2 and CO2, increase gas exchange

Relationship Between Alveolar Ventilation and Pulmonary Capillary Perfusion

Increased ventilation or increased pulmonary capillary blood flow increases gas exchange– Low PO2 causes arterioles to constrict & reduce blood flow – & reroutes blood to higher PO2 area of alveoli – where oxygen pickup is more efficient

– High PO2 causes arterioles to dilate, increase blood flow into pulmonary capillaries

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

Oxygen Diffusion Gradients– Oxygen moves from alveoli into blood

– Because partial pressure oxygen (PO2) of alveoli is 104 mm Hg

– & pulmonary capillary is 40 mm Hg

– O2 diffuse from higher to lower pressure gradient

– And reaches in equilibrium state in less than .25 sec

– PO2 at venous end decreases because of mixing with deoxygenated blood

– Then oxygen moves from tissue capillaries into the tissues

Carbon Dioxide Diffusion Gradients

– CO2 moves from tissues into tissue capillaries

– Because PCO2 in tissue is 46 mm Hg and tissue capillaries is 40 mmHg

– Reaches equilibrium at venous end of capillaries, PCO2 is 45 mm Hg

– Moves from pulmonary capillaries into the alveoli

• Hemoglobin and Oxygen Transport• Molecular oxygen is carried in the blood in two ways:

– 98.5% O2 bound to Hb within red blood cells – 1.5% O2 Dissolved in plasma

• Each Hb molecule binds four oxygen atoms• The hemoglobin-oxygen combination is called oxyhemoglobin (HbO2)• Hemoglobin that has released oxygen is called reduced hemoglobin (HHb),

or deoxyhemoglobin

HHb + O2

Lungs

Tissues

HbO2 + H+

• Saturated hemoglobin: when all four hemes of the molecule are bound to oxygen

• Partially saturated hemoglobin: when one to three hemes are bound to oxygen

• The rate that hemoglobin binds and releases oxygen is regulated by:

– PO2, temperature, blood pH, PCO2, and the concentration of BPG (an organic chemical)

• Effect of PO2

• Hemoglobin saturation plotted against PO2 produces a oxygen-hemoglobin dissociation curve

• At 104 mm Hg PO2, Hb is 98% saturated

• 98% saturated arterial blood contains 20 ml oxygen per 100 ml blood (20 vol %)

• Decrease in PO2 has small effect on Hb saturation

• Hb saturation curve shows that:• Hemoglobin is almost completely saturated at a

PO2 of 70 mm Hg

• Oxygen loading and delivery to tissue is adequate when PO2 is below normal levels

• Only 20–25% of bound oxygen is unloaded during one systemic circulation

• Hb of venous blood is still 75% saturated with O2 after one systemic circulation

• If oxygen levels in tissues drop (vigorous exercise):

– More oxygen dissociates from hemoglobin and is used by cells

Hemoglobin Saturation Curve

Bohr Effect• Effect of pH on oxygen-hemoglobin dissociation

curve – Bohr effect• As pH of blood declines, amount of oxygen bound to

hemoglobin also declines• Because decreased pH yields increase in H+ • H+ combines with protein part of hemoglobin and change

its shape and oxygen cannot bind to hemoglobin• Increase in blood pH – increase in hemoglobin`s ability to

bind oxygen

Effects of CO2 and Temperature• Increase in PCO2 causes decrease in pH• Carbonic anhydrase causes CO2 and water to combine

reversibly and form carbonic acid (H2CO3) which ionizes to H+ and HCO3

-

CO2 + H2O carbonic anhydrase H2CO3 H+ + HCO3-

• Increase temperature: decreases tendency for oxygen to remain bound to hemoglobin

• As metabolism goes up, temp. rises, more oxygen is released to the tissues from Hb

• Less metabolism, low temp., less O2 is released from Hb

Effect of BPG• 2,3-bisphosphoglycerate (BPG): released

by RBCs as they break down glucose for energy

• Binds to hemoglobin • And reduces affinity for oxygen• and increases release of oxygen

Shifting the Curve• In tissues - When Hb affinity for oxygen decreases – O2-Hb

dissociation curve shifted to right and releases more oxygen• eg. pH decrease, PCO2 increase, temp. increase – curve shifts to

Right• But in the lungs, the curve shifts to left because of lower CO2 level,

lower temp.• Hb affinity for oxygen increases and saturated easily

Fetal Hemoglobin• Fetal hemoglobin is very efficient at picking up oxygen from maternal

hemoglobin for several reasons:

• Concentration of fetal hemoglobin is 50% greater than concentration of maternal hemoglobin

• Oxygen-hemoglobin dissociation of fetal hemoglobin is left of maternal; i.e., fetal hemoglobin can hold more oxygen than maternal Hb

• Movement of carbon dioxide out of fetal blood causes the fetal oxygen-hemoglobin dissociation curve to shift to the left

• Simultaneously, movement of carbon dioxide into mother’s blood causes maternal oxygen-hemoglobin dissociation curve to shift to the right

• Carbon dioxide is transported in the blood in three forms– Dissolved in plasma – 7 to 10%

– Chemically bound to hemoglobin – 23% is carried in RBCs as carbaminohemoglobin

– Bicarbonate ion in plasma – 70% is transported as bicarbonate (HCO3

–)

Transport and Exchange of Carbon Dioxide• CO2 exchange in Tissues:• CO2 diffuses from tissues to plasma and then to

blood• In RBCs – CO2 reacts with water – form carbonic

acid• Carbonic acid dissociates & form bicarbonate ion

and H+• Bicarbonate quickly diffuses out from RBCs into

the plasma• Removing bicarbonate ion from RBCs promotes

CO2 transport• The chloride shift – to counterbalance the charge,

Cl– move from the plasma into the RBCs• H+ combines with Hb and releases O2 – Bohr

effect• O2 diffuses from RBCs to plasma to tissues• CO2 combines with Hb• Smaller the amount of O2 bound to Hb, greater the

amount of CO2 bind to Hb – Haldane effect

Transport and Exchange of Carbon Dioxide• Gas exchange in Lungs:• At the lungs, these processes are reversed• CO2 diffuses from RBCs – plasma – alveoli

• As CO2 levels in RBCs decreases• In presence of carbonic anhydrase Carbonic acid is

converted into CO2 & H2O • In response, Bicarbonate ion combines with H+ and

forms Carbonic acid• As HCO3- and H+ conc. decreases, HCO3- diffuses

in RBCs and Cl- diffuses out• O2 diffuses – plasma - RBCs

• O2 binds to Hb – releases H+• Release of H+ increases Hb affinity for oxygen• CO2 releases from Hb• Diffuses out of RBCs – plasma- alveoli

Hb binds to O2, releases more CO2 – Halden effect

Influence of Carbon Dioxide on Blood pH

• The carbonic acid–bicarbonate buffer system resists blood pH changes

• If hydrogen ion concentrations in blood begin to rise, excess H+ is removed by combining with HCO3

–

• If hydrogen ion concentrations begin to drop, carbonic acid dissociates, releasing H+

CO2 + H2O carbonic anhydrase H2CO3 H+ + HCO3-

Basic rhythm of ventilation is controlled by neurons within medulla oblongata in brain

Medullary respiratory center: Consists of

– Two Dorsal respiratory groups stimulate contraction of diaphragm

– Ventral groups stimulate the intercostal and abdominal muscles

• Pontine (pneumotaxic center) respiratory group:– collection of neurons in pons– Involved with switching between

inspiration and expiration– fine-tuning the breathing pattern

Rhythmic Ventilation

• Starting inspiration– Medullary respiratory center neurons are continuously active– Center receives stimulation from receptors that monitor blood gas levels,

blood temp., movement of muscles and joints– Combined input from all sources causes action potentials to stimulate

respiratory muscles

• Increasing inspiration– Once inspiration begins, More and more neurons are activated

• Stopping inspiration– Neurons stimulating muscles of respiration also responsible for stopping

inspiration– Receive input from pontine group and stretch receptors in lungs– Inhibitory neurons activated and relaxation of respiratory muscles

results in expiration

Chemical Control of Ventilation

• Respiratory system maintains blood oxygen, CO2 conc. Blood pH within normal range

• Chemoreceptors: • Are specialized neurons that respond to changes in

chemicals in solution

– Central chemoreceptors: Located in chemosensitive area of the medulla oblongata; and connected to respiratory center

– Peripheral chemoreceptors: Found in carotid and aortic bodies. Connected to respiratory center by cranial nerves IX and X

Chemical Control of Ventilation

• 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 pH changes

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– Hypercapnia: greater-than-normal amount of carbon

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

dioxide• Chemosensitive area in medulla oblongata and

Carotid and aortic bodies respond to changes in CO2 level

• Vital capacity and maximum minute ventilation decreases

• Residual volume and dead space increase

• Ability to remove mucus from respiratory passageways decreases

• Gas exchange across respiratory membrane is reduced