Chapter 23: The Respiratory System - HCC Learning Web

Copyright 2009, John Wiley & Sons, Inc.

Chapter 23:

The Respiratory System


Respiratory System Anatomy

Structurally Upper respiratory system

Nose, pharynx and associated structures

Lower respiratory system

Larynx, trachea, bronchi and lungs

Functionally Conducting zone – conducts air to lungs

Nose, pharynx, larynx, trachea, bronchi, bronchioles and terminal bronchioles

Respiratory zone – main site of gas exchange

Respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli


Structures of the Respiratory System


Nose

External nose – portion visible on face

Internal nose – large cavity beyond nasal

vestibule

Internal nares or choanae

Ducts from paranasal sinuses and nasolacrimal

ducts open into internal nose

Nasal cavity divided by nasal septum

Nasal conchae subdivide cavity into meatuses

Increase surface are and prevents dehydration

Olfactory receptors in olfactory epithelium



Pharynx

Starts at internal nares and extends to cricoid cartilage of

larynx

Contraction of skeletal muscles assists in deglutition

Functions

Passageway for air and food

Resonating chamber

Houses tonsils

3 anatomical regions

Nasopharynx

Oropharynx

Laryngopharynx


Larynx

Short passageway connecting laryngopharynx with trachea

Composed of 9 pieces of cartilage Thyroid cartilage or Adam’s apple

Cricoid cartilage hallmark for tracheotomy

Epiglottis closes off glottis during swallowing

Glottis – pair of folds of mucous membranes, vocal folds (true vocal cords, and rima glottidis (space)

Cilia in upper respiratory tract move mucous and trapped particles down toward pharynx

Cilia in lower respiratory tract move them up toward pharynx


Larynx


Structures of Voice Production

Mucous membrane of larynx forms

Ventricular folds (false vocal cords) – superior pair

Function in holding breath against pressure in thoracic

cavity

Vocal folds (true vocal cords) – inferior pair

Muscle contraction pulls elastic ligaments which stretch

vocal folds out into airway

Vibrate and produce sound with air

Folds can move apart or together, elongate or shorten,

tighter or looser

Androgens make folds thicker and longer – slower

vibration and lower pitch



Trachea

Extends from larynx to superior border of T5

Divides into right and left primary bronchi

4 layers Mucosa

Submucosa

Hyaline cartilage

Adventitia

16-20 C-shaped rings of hyaline cartilage Open part faces esophagus


Location of Trachea


Bronchi

Right and left primary bronchus goes to right lung

Carina – internal ridge Most sensitive area for triggering cough reflex

Divide to form bronchial tree Secondary lobar bronchi (one for each lobe), tertiary

(segmental) bronchi, bronchioles, terminal bronchioles

Structural changes with branching Mucous membrane changes

Incomplete rings become plates and then disappear

As cartilage decreases, smooth muscle increases Sympathetic ANS – relaxation/ dilation

Parasympathetic ANS – contraction/ constriction



Lungs

Separated from each other by the heart and other

structures in the mediastinum

Each lung enclosed by double-layered pleural membrane

Parietal pleura – lines wall of thoracic cavity

Visceral pleura – covers lungs themselves

Pleural cavity is space between layers

Pleural fluid reduces friction, produces surface tension (stick together)

Cardiac notch – heart makes left lung 10% smaller than right


Relationship of the Pleural Membranes to

Lungs



Anatomy of Lungs

Lobes – each lung divides by 1 or 2 fissures

Each lobe receives it own secondary (lobar) bronchus that

branch into tertiary (segmental) bronchi

Lobules wrapped in elastic connective tissue and

contains a lymphatic vessel, arteriole, venule and

branch from terminal bronchiole

Terminal bronchioles branch into respiratory

bronchioles which divide into alveolar ducts

About 25 orders of branching


Microscopic Anatomy of Lobule of Lungs


Alveoli

Cup-shaped outpouching

Alveolar sac – 2 or more alveoli sharing a common opening

2 types of alveolar epithelial cells Type I alveolar cells – form nearly continuous lining,

more numerous than type II, main site of gas exchange

Type II alveolar cells (septal cells) – free surfaces contain microvilli, secrete alveolar fluid (surfactant reduces tendency to collapse)


Alveolus

Respiratory membrane Alveolar wall – type I and type II alveolar cells

Epithelial basement membrane

Capillary basement membrane

Capillary endothelium

Very thin – only 0.5 µm thick to allow rapid diffusion of gases

Lungs receive blood from Pulmonary artery - deoxygenated blood

Bronchial arteries – oxygenated blood to perfuse muscular walls of bronchi and bronchioles


Components of Alveolus


Pulmonary ventilation

Respiration (gas exchange) steps

1. Pulmonary ventilation/ breathing Inhalation and exhalation

Exchange of air between atmosphere and alveoli

2. External (pulmonary) respiration Exchange of gases between alveoli and blood

3. Internal (tissue) respiration Exchange of gases between systemic capillaries and

tissue cells

Supplies cellular respiration (makes ATP)


Inhalation/ inspiration

Pressure inside alveoli lust become lower than

atmospheric pressure for air to flow into lungs

760 millimeters of mercury (mmHg) or 1 atmosphere (1 atm)

Achieved by increasing size of lungs

Boyle’s Law – pressure of a gas in a closed container is inversely proportional to the volume of the container

Inhalation – lungs must expand, increasing lung volume, decreasing pressure below atmospheric pressure


Boyle’s Law


Inhalation

Inhalation is active – Contraction of

Diaphragm – most important muscle of inhalation

Flattens, lowering dome when contracted

Responsible for 75% of air entering lungs during normal quiet breathing

External intercostals

Contraction elevates ribs

25% of air entering lungs during normal quiet breathing

Accessory muscles for deep, forceful inhalation

When thorax expands, parietal and visceral pleurae adhere tightly due to subatmospheric pressure and surface tension – pulled along with expanding thorax

As lung volume increases, alveolar (intrapulmonic) pressure drops



Exhalation/ expiration

Pressure in lungs greater than atmospheric pressure

Normally passive – muscle relax instead of contract

Based on elastic recoil of chest wall and lungs from elastic

fibers and surface tension of alveolar fluid

Diaphragm relaxes and become dome shaped

External intercostals relax and ribs drop down

Exhalation only active during forceful breathing




Airflow

Air pressure differences drive airflow

3 other factors affect rate of airflow and ease of pulmonary ventilation Surface tension of alveolar fluid

Causes alveoli to assume smallest possible diameter

Accounts for 2/3 of lung elastic recoil

Prevents collapse of alveoli at exhalation

Lung compliance High compliance means lungs and chest wall expand easily

Related to elasticity and surface tension

Airway resistance Larger diameter airway has less resistance

Regulated by diameter of bronchioles & smooth muscle tone


Lung volumes and capacities

Minute ventilation (MV) = total volume of air

inhaled and exhaled each minute

Normal healthy adult averages 12 breaths

per minute

moving about 500 ml of air in and out of lungs

(tidal volume)

MV = 12 breaths/min x 500 ml/ breath

= 6 liters/ min


Spirogram of Lung Volumes and

Capacities


Lung Volumes

Only about 70% of tidal volume reaches respiratory zone

Other 30% remains in conducting zone

Anatomic (respiratory) dead space – conducting airways with air that does not undergo respiratory gas exchange

Alveolar ventilation rate – volume of air per minute that actually reaches respiratory zone

Inspiratory reserve volume – taking a very deep breath


Lung Volumes

Expiratory reserve volume – inhale normally

and exhale forcefully

Residual volume – air remaining after

expiratory reserve volume exhaled

Vital capacity = inspiratory reserve volume +

tidal volume + expiratory reserve volume

Total lung capacity = vital capacity + residual

volume


Exchange of Oxygen and Carbon Dioxide

Dalton’s Law

Each gas in a mixture of gases exerts its own

pressure as if no other gases were present

Pressure of a specific gas is partial pressure Px

Total pressure is the sum of all the partial pressures

Atmospheric pressure (760 mmHg) = PN2 + PO2 + PH2O

+ PCO2 + Pother gases

Each gas diffuses across a permeable membrane

from the are where its partial pressure is greater to the

area where its partial pressure is less

The greater the difference, the faster the rate of

diffusion


Partial Pressures of Gases in Inhaled Air

PN2 =0.786 x 760mm Hg = 597.4 mmHg

PO2 =0.209 x 760mm Hg = 158.8 mmHg

PH2O =0.004 x 760mm Hg = 3.0 mmHg

PCO2 =0.0004 x 760mm Hg = 0.3 mmHg

Pother gases =0.0006 x 760mm Hg = 0.5 mmHg

TOTAL = 760.0 mmHg


Henry’s law

Quantity of a gas that will dissolve in a liquid is

proportional to the partial pressures of the gas and its solubility

Higher partial pressure of a gas over a liquid and higher solubility, more of the gas will stay in solution

Much more CO2 is dissolved in blood than O2 because CO2 is 24 times more soluble

Even though the air we breathe is mostly N2, very little dissolves in blood due to low solubility

Decompression sickness (bends)


External Respiration in Lungs

Oxygen

Oxygen diffuses from alveolar air (PO2 105 mmHg) into blood of

pulmonary capillaries (PO2 40 mmHg)

Diffusion continues until PO2 of pulmonary capillary blood

matches PO2 of alveolar air

Small amount of mixing with blood from conducting portion of

respiratory system drops PO2 of blood in pulmonary veins to 100

mmHg

Carbon dioxide

Carbon dioxide diffuses from deoxygenated blood in pulmonary

capillaries (PCO2 45 mmHg) into alveolar air (PCO2 40 mmHg)

Continues until of PCO2 blood reaches 40 mmHg


Internal Respiration

Internal respiration – in tissues throughout body Oxygen

Oxygen diffuses from systemic capillary blood (PO2 100 mmHg) into tissue cells (PO2 40 mmHg) – cells constantly use oxygen to make ATP

Blood drops to 40 mmHg by the time blood exits the systemic capillaries

Carbon dioxide

Carbon dioxide diffuses from tissue cells (PCO2 45 mmHg) into systemic capillaries (PCO2 40 mmHg) – cells constantly make carbon dioxide

PCO2 blood reaches 45 mmHg

At rest, only about 25% of the available oxygen is used

Deoxygenated blood would retain 75% of its oxygen capacity



Rate of Pulmonary and Systemic Gas Exchange

Depends on

Partial pressures of gases

Alveolar PO2 must be higher than blood PO2 for diffusion to

occur – problem with increasing altitude

Surface area available for gas exchange

Diffusion distance

Molecular weight and solubility of gases

O2 has a lower molecular weight and should diffuse faster

than CO2 except for its low solubility - when diffusion is slow,

hypoxia occurs before hypercapnia


Transport of Oxygen and Carbon Dioxide

Oxygen transport

Only about 1.5% dissolved in plasma

98.5% bound to hemoglobin in red blood cells

Heme portion of hemoglobin contains 4 iron atoms –

each can bind one O2 molecule

Oxyhemoglobin

Only dissolved portion can diffuse out of blood into cells

Oxygen must be able to bind and dissociate from heme



Relationship between Hemoglobin and

Oxygen Partial Pressure

Higher the PO2, More O2 combines with Hb

Fully saturated – completely converted to oxyhemoglobin

Percent saturation expresses average saturation of

hemoglobin with oxygen

Oxygen-hemoglobin dissociation curve

In pulmonary capillaries, O2 loads onto Hb

In tissues, O2 is not held and unloaded

75% may still remain in deoxygenated blood (reserve)


Oxygen-hemoglobin Dissociation Curve


Hemoglobin and Oxygen

Other factors affecting affinity of Hemoglobin for oxygen

Each makes sense if you keep in mind that metabolically active tissues need O2, and produce acids, CO2, and heat as wastes

Acidity

PCO2

Temperature


Bohr Effect

As acidity increases (pH

decreases), affinity of Hb

for O2 decreases

Increasing acidity

enhances unloading

Shifts curve to right

PCO2

Also shifts curve to right

As PCO2 rises, Hb unloads

oxygen more easily

Low blood pH can result

from high PCO2


Temperature Changes

Within limits, as temperature increases, more oxygen is released from Hb

During hypothermia, more oxygen remains bound

2,3-bisphosphoglycerate

BPG formed by red blood cells during glycolysis

Helps unload oxygen by binding with Hb


Fetal and Maternal Hemoglobin

Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin

Hb-F can carry up to 30% more oxygen

Maternal blood’s oxygen readily transferred to fetal blood


Carbon Dioxide Transport

Dissolved CO2

Smallest amount, about 7%

Carbamino compounds

About 23% combines with amino acids including those in Hb

Carbaminohemoglobin

Bicarbonate ions

70% transported in plasma as HCO3-

Enzyme carbonic anhydrase forms carbonic acid (H2CO3)

which dissociates into H+ and HCO3-


Chloride shift

HCO3- accumulates inside RBCs as they pick up

carbon dioxide

Some diffuses out into plasma

To balance the loss of negative ions, chloride (Cl-)

moves into RBCs from plasma

Reverse happens in lungs – Cl- moves out as

moves back into RBCs

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-



End of Chapter 23

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Chapter 23: The Respiratory System - HCC Learning Web