RESPIRATORY SYSTEM
PREPARED BY MR.
ABHIJIT DAS
INTRODUCTION
Oxygen is utilized by the human body to breakdown
nutrient molecules like glucose and to derive energy for performing various
activities. Carbon dioxide, which is a toxic gas, is also released during the
above catabolic reactions.
So oxygen has to be continuously provided to the
cells and carbon dioxide produced by the cells have to be released out.
This process of exchange of oxygen with carbon
dioxide is called breathing.
On an average a healthy human breaths 12-16 times/minute.
HUMAN RESPIRATORY SYSTEM
We have a pair of external nostrils. It leads to a
nasal chamber (or nasal cavity) through the nasal passage.
The nasal cavity opens into the pharynx which is the
common passage for food and air.
The pharynx opens through the larynx region into the
trachea.
Larynx is a box like structure which helps in
production of sound and hence called the sound box.
The opening region of larynx is known as glottis.
The covering of glottis is known as epiglottis.
Epiglottis is present to prevent the entry of food into the larynx.
Thachea is a straight tube extending up to the mid
thoracic cavity which divides at the level of T5 vertebra (5th
thoracic vertebra) into a right and left primary bronchi.
Each bronchi undergoes repeated divisions to form
the secondary and tertiary bronchi.
Tertiary bronchi divide to initial bronchioles.
The trachea, primary bronchi, secondary bronchi,
tertiary bronchi and initial bronchioles are supported by rings of hyaline
cartilages.
Each terminal bronchiole gives rise to a number of thin
vascularised bag-like structures called alveoli.
The branching network of bronchi, bronchioles and
alveoli comprise the lungs.
Humans have two lungs which are covered by a double
layered pleura with pleural fluid between them.
The lungs are anatomically divided into lobes due to
presence of fissures (horizontal fissures and oblique fissures).
The left lung is divided into two lobes by the
oblique fissure. The right lung is divided into three lobes by the oblique
fissure and the horizontal fissure.
The part starting with external nostrils up to the
terminal bronchioles constitute the conducting part where as the alveoli and
their ducts form the exchange part of the respiratory systems.
The conducting part transports the air from the
atmosphere to the alveoli, filters it from foreign particles and brings the air
to body temperature
Exchange part is the site of actual diffusion of
oxygen and carbon dioxide between blood and alveoli.
STEPS OF RESPIRATION
1. Breathing:
inspiration + expiration
2. External
respiration: gaseous exchange between alveoli and blood vessel
3. Internal
respiration: gaseous exchange between blood capillaries and tissue
4. Cellular
respiration: utilization of oxygen and glucose to produce ATP
MECHANISM OF BREATHING
The lungs are situated in the thoracic chamber which
is an air-tight chamber.
The thoracic chamber is dorsally formed by vertebral
column, ventrally by sternum (or breast bone), laterally by ribs and on the
lower side by diaphragm.
Such an arrangement is essential for breathing so
that the pulmonary volume can be altered easily.
Breathing involves two stages:
Inspiration: During which atmospheric air is drawn
in
Expiration: During which the air from lungs is
released out
Inspiration can occur if the pressure within the
lungs is less than the atmospheric pressure.
Similarly, expiration takes place when the pressure
within the lungs is higher than the atmospheric pressure.
The diaphragm and the external intercostal muscle
between the ribs help in generation of such pressure gradients.
Inspiration is initiated by the contraction of
diaphragm which increases the volume of thoracic chamber in the
antero-posterior axis.
Similarly, the contraction of external intercostal
muscle increases the volume of the thoracic chamber in dorso-ventral axis.
Now the overall increase in the thoracic volume
causes an increase in pulmonary volume.
According to Boyle’s law, an increase in pulmonary
volume decreases the pressure inside the lungs to less than the atmospheric
pressure which forces the air from outside to move into the lungs (known as
inspiration).
Relaxation of the diaphragm and the external
intercostal muscles reduce the thoracic volume which reduces the pulmonary
volume and this leads to expiration.
EXCHANGE OF GASES
Oxygen and carbon dioxide are exchanged based on
pressure/concentration gradient.
Pressure contributed by an individual gas in a
mixture of gases is called partial pressure and is represented as pO2
for oxygen and pCO2 for carbon dioxide.
GASEOUS EXCHANGE SURFACE
Exchange surface has three layers.
1. Squamous
epithelium of alveoli
2. Basement
membrane (the basement membrane of alveoli + the basement membrane of capillary)
3. Endothelium
of alveolar capillary
TRANSPORT OF GASES
Haemoglobin is an iron containing pigment present
inside RBCs. Oxygen can bind with haemoglobin to form oxyhaemoglobin.
Each haemoglobin molecule can carry a minimum of
four molecules of O2.
Partial pressure of O2, partial pressure
of CO2, hydrogen ion (H+ ion) concentration and
temperature are the four factors which can interfere with the binding of oxygen
with haemoglobin.
In the alveoli (during external respiration), there
is high pO2, low pCO2, lesser H+ concentration
and lower temperature. These factors are favourable for the formation of
oxyhaemoglobin.
Similarly in the tissues (during internal
respiration), there is low pO2, high pCO2, high H+
concentration and higher temperature. These factors are favourable for
dissociation of oxygen from the oxyhaemoglobin.
So it is clearly seen that oxygen gets bound to
haemoglobin in the lungs and gets dissociated at the tissues.
Every 100ml of oxygenated blood can deliver around
5ml of oxygen to the tissues. Similarly, every 100ml of deoxygenated blood
delivers approximately 4ml of carbon dioxide to the lungs.
RESPIRATORY VOLUMES
TIDAL VOLUME (TV):
Volume of air inspired or expired during a normal breathing. It is
approximately 500ml.
INSPIRATORY RESERVE VOLUME (IRV):
Additional volume of air, a person can inspire by a forcible inspiration. This
volume can be 2500ml – 3000ml.
EXPIRATORY RESERVE VOLUME (ERV):
Additional
volume of air, a person can expire by a forcible expiration. This volume can be
1000ml – 1100ml.
RESIDUAL VOLUME (RV):
Volume
of air remaining inside the lungs even after a forcible expiration. This volume
can be 1100ml – 1200ml.
RESPIRATORY CAPACITIES
INSPIRATORY CAPACITY (IC):
Tidal
Volume + Inspiratory Reserve Volume
EXPIRATORY CAPACITY (EC):
Tidal
Volume + Expiratory Reserve Volume
FUNCTIONAL RESIDUAL CAPACITY (FRC):
Residual Volume + Expiratory Reserve Volume
VITAL CAPACITY:
The
maximum volume of air a person can breathe in after a forced expiration.
This includes Expiratory Reserve Volume + Tidal Volume
+ Inspiratory Reserve Volume
TOTAL LUNG CAPACITY:
Total volume of air present in the lungs after a forced inspiration.
This includes Residual Volume + Expiratory Reserve
Volume + Tidal Volume + Inspiratory Reserve Volume
REGULATION OF RESPIRATION
NEURAL REGULATION
Humans have a wonderful ability to maintain and
adjust the breathing rate to match the demands of the body. This is controlled
by nervous system.
Respiratory rhythm centre (group of neurons
controlling respiration rate), present in the medulla region of the brain, is
primarily responsible for this neural regulation of respiration.
Another centre present in the pons region of the
brain called pneumotaxic centre can adjust the functions of the respiratory
rhythm centre.
Pneumotaxic centre send neural signal to reduce the
duration of inspiration and thereby change the respiratory rate.
Another centre present in the pons called Apneustic
centre send positive signals to respiratory rhythm centre to restart the
process.
CHEMICAL REGULATION
A chemosensitive area, present in the medulla near
the respiratory rhythm centre, is highly sensitive to CO2 and H+
ions. Increase in these substances can activate this centre, which can signal
the respiratory rhythm centre to make necessary adjustments in the respiratory
process so that those substances (CO2 & H+ ions) can
be eliminated.
Similarly, some receptors present in the Aortic arch
and carotid artery also can recognise changes in CO2 and H+ concentration and
send signals to the respiratory rhythm centre for necessary adjustments.
SOME IMPORTANT TERMS
HYPOXIA:
Low oxygen level in blood.
CYANOSIS:
A bluish colour to the skin usually due to lack of oxygen in blood.
DYSPNEA:
Difficulty in breathing.
PERIODIC BREATHING: Some babies can take a pause in their breathing for up to 10 – 30 seconds. This condition is known as periodic breathing.