Summary USMLE step 1 Physiology of Respiratory system - Study guide - Study smarter not harder - your guide to pass the exam
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Course
Medicine
Institution
Medicine
USMLE step 1 Physiology of Respiratory system - Study guide - Study smarter not harder - your guide to pass the exam - Homeostasis
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Introduction
The main function of the lung is gas exchange (external respiration)
External respiration consists of 4 processes:
(1) Pulmonary ventilation: (inspiration & expiration)
(2) Exchange of 0 2 & C0 2 between lung air spaces & blood by diffusion.
(3) Transport of 0 2 & C02 by the blood
(4) Exchange of 0 2 & C0 2 between blood & tissues by diffusion.
Non respiratory functions of the lungs:
(1) Regulation of acid -base balance.
(2) Activation of certain plasma proteins (Ang.l ~ Ang.ll)
(3) Defense against pathogens & foreign particles in the airways .
(4) Water & heat loss (via the expired air)
(5) Help venous return (through the respiratory pump).
(6) Help vocalization.
Internal (cellular) respiration: the use of 0 2 within mitochondria to generate A TP.
The Human
Respiratory
System
lung
Heart
Ribs
Ep11hehal celts
The airways:
0 Nose (or mouth) ~ pharynx ~ larynx; are (upper airways)
0 Larynx ~ trachea ~ 2 bronchi (1 enter each lung) ~ (> 20) generations of branching
~
narrower, shorter & more numerous bronchioles .
Alveoli 1st appear in respiratory bronchioles & it in the alveolar ducts then end in grape-like clusters
The walls of the trachea & bronchi contain cartilage (for support) & smooth muscles,
But the wall of bronchioles has only smooth muscles.
Parasympathetic (vagus) stimulation (cholinergic receptors) ~ bronchoconstriction.
Sympathetic stimulation (82 adrenergic receptors) ~ bronchodilatation.
Site
(1) The conducting zone _. __
The airways below the larynx are divided into conducting & respiratory zones:
(2) The respiratory zone
From the top of the trachea to the From the respiratory bronchioles on down
beginning of respiratory bronchioles. Contains alveoli (the area of gas
No alveoli (no gas exchange with blood) exchange)
1 -Warming & humidifying inspired air It is the site of gas exchange with blood .
2 - Distributing air to the gas exchange The rate of diffusion of gases across a
Functions areas of the lung. membrane it by (it membrane surface
3 -A part of the body defense system: area & U the membrane thickness). This
(as it has a mucociliary system that is observed in arrangement of structures
prevents microorganisms, dust & of respiratory zone ; respiratory bronchioles
noxious gases from entering alveoli) ~alveolar ducts~ alveoli (gas exchange)
All 3 functions =air conditioning
157 -
,,._ Respiration Concise medical physiology
Numberor '----~~ Respiratory Tubes and Alveoli
branches
(l)Trachea .
~
'F
The epithelial surfaces of the airways to the end of the respiratory bronchioles contain:
(a) Cilia ~ beat towards the pharynx.
(b) Glands & Goblet cells (individual epithelial cells)~ secrete mucus.
Mucus escalator: dust stick to the mucus & slowly moved by the cilia to the pharynx ~ swallowed
~ prevents accumulation of mucus in the airways & clears trapped foreign matter.
(c) Macrophages: ~ engulf inhaled particles & bacteria.
Smoking & air pollutants ~ injury of macrophages & cilia
(d) Secrete a watery fluid ~ helps the mucus to move freely.
Particles > 10 Jlm are trapped on the hairs & mucus in nasal passages.
Particles < 10 Jlm are inhaled into the lungs & removed by a, b & c
Alveolar sacs : are clusters of alveoli (like a bunch of grapes)
Some alveoli originate directly from respiratory bronchioles.
Alveolar structure facilitates diffusion of gases between blood & air.
The alveolar wall consists of
Type I cells iType II cells
(Type I pneumocytes) (Type II pneumocytes)
A single layer of flat epithelial cells secrete pulmonary surfactant.
laying on a basement membrane. Blood llow Blood !low
I
The alveolar walls are surrounded by capillaries consisting of:
One layer of endothelial cells & underlying basement membrane
The respiratory membrane: (0.2 microns thick)
Fused alveolar wall & capillary wall that separates air from blood.
0 In the lungs there are 300 million alveoli cop"'""'
=total surface area 100 mm 2 (size of tennis court) for rapid & great gas exchange.
0 Adjacent alveoli are connected by alveolar pores ~ flow of air between them
~ equilibration of pressure within the lungs .
0 Alveolar macrophages: engulf inhaled foreign particles & pathogens .
Relation of the lungs to the thoracic (chest) wall
The lungs are located within the thoracic cavity .
The chest wall forms a continuous barrier around the lungs
~ the compartment enclosing the lung is airtight.
The pleural sac: a completely closed sac (like fluid-filled balloon)
Each lung is surrounded by a sac like your fist inside a balloon
So, there are 2 layers of pleura: I Pleural
membrane
1- Visceral pleura: attached to the lung
- Plel;lral
2- Parietal pleura: attached to the interior thoracic wall & diaphragm ca vtty
1
- Di aphragm
The intrapleural space: between the 2 layers of pleura filled with
intrapleural fluid "15ml. " mostly water which is incompressible
~ its volume remains constant, even when chest wall expands & contracts.
158
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