Biology 322 – Human Anatomy

Respiratory system

Respiratory system function and anatomy

•In biology, RESPIRATION can have several meanings:

1. Movement of air in/out of the lungs (really known as ventilation)

2. Exchange of O2 for CO2 in the bloodstream

3. Use of O2 during aerobic respiration (ATP generation)

•Main function of respiration is to replenish O2 and expel CO2• Need O2 for aerobic respiration• Need to eliminate CO2 waste

• CO2 + H2O → H2CO3 → HCO3- + H+→ acid build up in blood and tissues•Additional, important functions:• Need to move air for vocalization (speech, laugh, cry, etc…), sense of smell• Respiratory pump aids blood flow back to heart• Breath holding – allows us to survive underwater briefly, aids in urination, defacation,

childbirth

Respiratory system anatomy

1. Nose2. Pharynx

• nasopharynx, oropharynx, laryngopharynx3. Larynx4. Trachea 5. Bronchi – airways within the lung6. Lungs

• Most distal portions end in sacs called ALVEOLI

How we get O2 from atmosphere to blood Essentially a “dead end” circuit – air enters

resp. system, travels to distal parts of lung, then goes back the way it came in

•Two main functional divisions exist:1. Conducting division –

Organs which allow air to get to lungs, but do not exchange O2 and CO2

Nose, pharynx, larynx, trachea, most bronchi

2. Respiratory division – Portion of resp. system that

carries out gas exchange Some smaller distal bronchi

and alveoli

Conducting Division

•Main function is to move air from outside → inside → back outside•Several organs also play other roles and contribute to normal function of respiratory system•Nasal cavity –

Conditions the air as it enters the body Nasal conchae cause air to swirl as it entersNasal cavity lined by MUCOSAL MEMBRANE

• GOBLET CELLS that secrete mucous, pseudostratified ciliated cells that “sweep” material back so that it can be swallowed

Air is warmed and humidified• ERECTILE TISSUE in inferior conchae fills with warm blood (common site of nosebleed)

Dust, allergens, other particles get trapped in nasal hairs VIBRISSAE and in mucous that lines the nasal cavity

OLFACTORY MUCOSA – patch of cells located mostly in superior portion of nasal cavity• Contains nerve fibers (C.N. I) that pass through cribiform

plate

•Pharynx – divided anatomically into 3 regions1. Nasopharynx – posterior to the nasal cavity, slightly superior to oral cavity

Air enters through nose and is directed inferiorly Lined by mucus membrane Lymphoid tissues (tonsils) are well positioned to combat this Eustachian tube (auditory tube) opens here

2. Oropharynx – posterior to the oral cavity, inferior to the soft palate (roof of mouth) Also well supplied with lymphoid tissues

3. Laryngopharynx – inferior to oropharynx Lined by pseudostratified columnar epithelia (like much of the conducting

airway)

Laryngopharynx and oropharynx transmit BOTH food/liquid and air• Lined by non-keratinized, stratified

squamous epitheliaNasopharynx ONLY transmits air

EPIGLOTTIS covers opening to laryngopharynx – prevents choking!

Larynx – a.k.a. the “voicebox”• Obviously hugely important for its social and psychological function• Also very important for keeping food, fluids out of the airways

Epiglottis – larynx is PULLED upwards when we swallow and closes opening of larynxVestibular folds – also close shut when we swallow

• Very well protected by a number of thick cartilage structuresThyroid cartilage – forms “Adam’s Apple” Cricoid cartilage – connects larynx to trachea belowMany smaller cartilages that are important for vocal portion of larynx

•Several INTRINSIC muscles and ligaments allow opening/closing of glottis (opening of larynx) as well as movement of vocal cords•Vestibular fold controls opening of glottis – prevents choking•Vocal fold - fold of tissue overlying the vocal ligaments

Inferior to vestibular foldVibrate as air moves over them Varying tension on vocal cords allow us to create various “rough” sounds

Trachea – rigid tube found ANTERIOR to the esophagus, continuous with inferior part of larynx

•C-shaped rings of hyaline cartilage offer structural supportRings get smaller, thinner as you go deeper into the lung tissue (more distal)

•Interior lined by pseudostratified columnar epithelium and many GOBLET CELLS (mucus cells)Particles get trapped in mucus and cilia on columnar cells sweep upwardsDirty mucus gets moved towards esophagus where it can be swallowedReferred to as the MUCOCILIARY ELEVATOR

Bronchial Tree

•Trachea branches at a point known as the CARINA to form a right and left PRIMARY BRONCHUS•After 2-3 cm the primary bronchi enter the lung tissue and branch into successively smaller SECONDARY and TERTIARY bronchi•Distal to the bronchi are the BRONCHIOLES•Each bronchiole branches into TERMINAL BRONCHIOLES – the end of the conducting system

•Terminal bronchioles branch into RESPIRATORY BRONCHIOLES that have small ALVEOLAR DUCTS branching off•The ends of the alveolar ducts have small, thin sacs known as ALVEOLI budding from them

This is where gas exchange actually occurs

Air Flow

1. Trachea2. Primary bronchi3. Secondary bronchi4. Tertiary bronchi5. Bronchioles6. Terminal bronchioles7. Respiratory bronchioles8. Alveolar ducts9. Alveoli

Supported by cartilage

No cartilage

Mostly pseudostratified columnar, goblet cells

Simple squamous cells

Often cuboidal, with or without cilia

Lung structure

•Inferior is BASE, superior is APEX

•Surfaces :Costal surface – anterior, lateral, posterior

(faces the rib cage)Mediastinal surface – medial surfaceDiaphragmatic surface – portion of the base

that rests on the diaphragm

•There is a large impression on the left lung where the heart projects laterally – CARDIAC IMPRESSION

•Right lung has 3 lobes – superior, middle, inferior•Left lung has 2 lobes – superior and inferior

Lobes are separated by fissures:• Oblique fissure in left lung • One oblique and one horizontal

fissure in right lung

Pleura

•Like the heart, the lungs reside in the thoracic cavity and are surrounded by two layers of serous membranes

VISCERAL PLEURA – lies directly on top of the lung

PARIETAL PLEURA – lines part of the thoracic cavity, mediastinum, and diaphragm

•The PLEURAL CAVITY is space between the pleurae filled with PLEURAL FLUID that has three functions

1. Reduces friction during ventilation2. Creates pressure gradient3. Compartmentalization of organs

Pulmonary Blood Supply

•Pulmonary arteries and veins enter the heart near where the primary bronchi enter – HILUM•Pulmonary vessels follow roughly the same path and branching pattern as the bronchial tree•The more distal parts of the lung (respiratory bronchioles, alveolar ducts, alveoli) receive O2 and nutrients directly from the pulmonary circulation

•The more proximal parts (primary bronchi, secondary bronchi, pleura, etc…) need their own supply

BRONCHIAL ARTERIES branch off of the descending aortaBRONCHIAL VEINS drain blood that eventually reaches the superior vena cava

However, some venous blood from the bronchial veins ends up in the pulmonary veins….

Alveolar Structure

•Most gas exchange occurs near the terminal ends of the bronchial tree in closed sacs called ALVEOLI•Average human lung has about 150 MILLION alveoli with a huge surface area of well over 200 ft2

•Most cells are simple squamous epithelial cells•Each alveolus is surrounded by a capillary bed

Simple squamous cell of alveolus + endothelial cell/basement membrane of capillary =

RESPIRATORY MEMBRANE

Creates a gas exchange barrier of only 0.5 microns!!Gases are easily exchanged

Alveolar Structure, cont…

•Alveoli are composed of two unique cell types:

1. Type 1 alveolar epithelial cells – simple squamous cells• Most numerous• Well suited for gas exchange

2. Type 2 alveolar epithelial cells – more cuboid in shape• Much less prevalent• Main job is to produce PULMONARY SURFACTANT

Type 1 alveolar cell

Type 2 alveolar cell

Alveolar macrophage

Alveoli also contain many MILLIONS of ALVEOLAR MACROPHAGES• Monitor the alveolus for bacteria, debris that didn’t get filtered out• After phagocytosis, macrophages make their way up the mucociliary elevator

Alveolar Structure

•Interior surface of alveolus is lined by a thin layer of fluid called the HYPOPHASE•Polar nature of H2O causes surfaces of alveoli to cling together•Type 2 cells produce a thin layer of PULMONARY SURFACTANT that reduces surface tension

Prevents alveoli from collapsing when we exhale!!

Pulmonary surfactant – • Complex mix of phospholipids and 4 proteins secreted by TYPE 2 ALVEOLAR CELLS• Plays two roles :

1. Reduces alveolar surface tension2. Assists alveolar macrophages in digestion of bacteria/viruses

Alveolar gas exchange

•In order to exchange O2 and CO2, the gasses must cross the respiratory membrane and the hypophase•This occurs via diffusion•O2 and CO2 that are being exchanged are first dissolved in the water part of the hypophase

AIR (alveolus)

WATER (hypophase)

BLOOD

RESPIRATORY MEMBRANE

CO2 O2

Pulmonary Ventilation

Movement of air into and out of the lungs

•RESPIRATORY CYCLE is the repetitive cycle of inhalation (air in) and exhalation (air out)•Movement of air between two spaces requires a pressure gradient

Air moves from an area of high pressure → low pressure Important to remember that an increase in volume → decrease in pressureAs pressure within the alveoli decreases air rushes in from the atmosphere

Respiratory muscles

•Main muscle responsible for ventilation is the DIAPHRAGM

Contraction increases volume of cavity (and decreases pressure)

•Intercostal muscles of ribs are also importantExternal intercostals assist inhalation Internal intercostals assist in FORCED

exhalation

•Organization of the pleurae is critical to ventilation•Visceral pleura – attached to lung •Parietal pleura – attached to thoracic wall•Pleural cavity – fluid filled space between

Normally a slight vacuum exists in this space (~4mm Hg)

•Vacuum within pleural cavity and fluid on surfaces of membranes cause the two layer to “stick” together•As the thoracic cavity enlarges the parietal pleura moves and draw the visceral pleura with it.

•Lung puncture (PNEUMOTHORAX) breaks this vacuum and prevents that lung from expanding

Visceral Pleura

Parietal Pleura

Pleural Cavity

Mechanics of Inhalation

•Air movement is driven by pressure gradients•At rest, there is no pressure gradient between atmosphere and alveoli – called TRANSPULMONARY PRESSURE

•INTRAPLEURAL PRESSURE – Pressure within the two pleural membranes, normally a -4 mm Hg vacuumThis vacuum causes pleural membranes to stick together and move at the same timeCreates a mechanical coupling of thoracic cavity volume to lung volume

•INTRAPULMONARY PRESSURE – Pressure within the alveoli themselvesAs volume of alveoli increases, pressure

drops below that of atmosphere→ air flows INTO the lung

•Eventually, intrapulmonary pressure EQUALS atmospheric pressure and flow stops

•Since inhalation requires muscle contraction, it is considered to be an active process

P

P

AIR

•In VERY simple terms the bronchial tree and alveoli can be thought of as a pipe (conducting airways) and a balloon (alveoli)

“Balloon and Pipe Model of Respiration”

Expansion of thoracic cavity

Decrease in intrapleural pressure

Decrease in intrapulmonary pressure

Air rushes into lung

Mechanics of Exhalation

•As opposed to inhalation, exhalation is normally a passive processHowever, we can force exhalation (blowing up a balloon or blowing out candles)

•Exhalation is primarily caused by three factors:1. Recoil of thoracic cavity 2. Relaxation of diaphragm3. Recoil of elastic fibers that surround the alveoli

• Decrease in volume of thoracic cavity/alveoli → increase in intrapulmonary pressure As intrapulmonary pressure exceeds atmospheric pressure, air is forced out of the lung

• Rate of exhalation is slowed by CONTROLLED relaxation of diaphragm Allows respiratory cycle to occur smoothly Phrenic nerve continues to stimulate diaphragm slightly

• Forced exhalation results from contraction of abdominal muscles (pull downward on rib cage) and internal intercostals (contract ribs) Forced exhalation DOES have its limits……..

Decreases volume of thoracic cavity/lung

P

P

AIR

Contraction of thoracic cavity(Could be passive or forced)

Increase in intrapleural pressure

Increase in intrapulmonary pressure

Air rushes out of lung

Exhalation

Inhalation/Exhalation

Increased AIRWAY RESISTANCE limits rate of forced expiration

Limits to expiratory rate

“Balloon and squishy pipe model”

1. Cartilage decreases moving from 1° bronchi, 2° bronchi, etc…

2. Increases in thoracic pressure can deform some airways (point of diminishing return)

3. Bernoulli’s principle – the faster air flows, the less pressure it exerts• Airways partially collapse when air flows rapidly out of

the lung

Force

Rate

of e

xpira

tion

Airflow resistance

•Air flowing through lungs encounters resistance like blood flowing through arteries and veins•Resistance decreases airflow in/out of the lung•Resistance is influenced by factors:

1. Bronchiole diameter – • ↓ airway diameter leads to ↑resistance to airflow, and vice versa• Resistance decreases airflow in/out of lung• Increase/decrease in airway diameter is known as BRONCHODILATION or

BRONCHOCONSTRICTION• Mainly occurs in distal portions of lung – trachea, larger bronchi don’t really

change diameter that much (lots of supporting cartilage)

•Diameter is affected by contraction/relaxation of smooth muscle surrounding airway as well as the lining of the airway (inflammation of membranes)•Sympathetic stimulation (epinephrine, norepinephrine) cause bronchodilation •Parasympathetic stimulation causes bronchoconstriction

Resistance to airflow…..

2. Pulmonary compliance – ability of the lungs to expand and contract• Alveoli are surrounded by elastic fibers• Damage to fibers can limit lung compliance

Alveoli can expand but don’t want to contract• Accumulation of scar tissue can cause lungs to become stiff (no expansion)

Emphysema is a disease of decreased lung compliance

• Exposure to toxic chemicals, smoke trigger formation of scar tissue and degradation of elastic fibers

Resistance to airflow…..

3. Alveolar surface tension –• Presence of hypophase within alveoli and distal bronchioles increases surface

tension - walls want to stick together • Pulmonary surfactant reduces surface tension – prevents airway and alveolar

collapse• Lack of surfactant in preterm infants causes respiratory distress syndrome

•Water molecules on opposing surfaces want to form hydrogen bonds with themselves•Surfactant prevents formation of these bonds

Alveolar Ventilation

Describes the air that actually enters/exits the alveoli

•During normal breathing we inhale about 500 mL of air (TIDAL VOLUME)

•Some of this air only gets as far as the conducting airways (trachea, bronchi) and not involved in gas exchange

•ANATOMICAL DEADSPACE – portion of lung where air is present but not involved in gas exchange

•PHYSIOLOGICAL DEADSPACE – anatomical deadspace plus non-functional alveoli

Increases with diseases like emphysema

Alveolar ventilation, cont….

•Normal tidal volume is 500 mL, but about 150 mL is deadspace ventilation

About 350 mL of air reaches the alveoli (ALVEOLAR VENTILATION)

•Alveolar ventilation rate = alveolar ventilation (mL/breath) X breaths per minute

•Ex.: 350 mL/ breath X 12 breaths/min. = 4,200 mL/min

•Not all air in lungs is expelled each breath, so goal of A.V. is to “refresh” the air in the lungs at any given time rather than completely replace it

Spirometry - Assessment of pulmonary/respiratory function

Spirometry - Assessment of pulmonary/respiratory function

Some common respiratory capacities: usually the sum of two or more respiratory volumes