Human Respiratory System - Forensics Digest

In our last post, we studied about human circulatory system. Hope you have understood it well, if you have any doubts fee free to post your query in the comment section below. In this post we will study about human respiratory system.

The process of exchange of O₂from the atmosphere with CO₂ produced by the cells is called breathing.

Breathing is simply defined as intake of fresh air i.e. oxygen and removal of carbon dioxide.

Respiration is defined as an oxidation of food to form CO₂, H₂O and energy.

Lets find out what encompasses human respiratory system

RESPIRATORY PASSAGE: it is the passage that takes air from outside to respiratory surface of lungs. It consists a pair of nostrils, nasal cavity, pharynx, larynx, trachea, bronchi and respiratory surface of lung i.e. alveoli.

EXTERNAL NOSTRILS: The pair of nostril are the holes which leads to nasal chamber of nasal cavity through nasal passage.
NASAL CHAMBERS: the epithelial lining of cavities is known as respiratory epithelium. It has brush border goblet cells which secretes mucus. Mucus traps the dust particles present in the air which is inspired.
NASOPHARYNX: it is a portion of pharynx, only air is present here. It opens into trachea.
LARYNX: it is the voice box which helps in production of sound. It is made up of cartilage, present at upper part of trachea.
TRACHEA: it is known as wind pipe. It is a straight tube extending up to mid thoracic cavity commonly called as chest cavity. This tube divides into right and left primary bronchi.
BRONCHI (plural), bronchus (sing.): Each bronchus undergoes repeated division to form its branches which are secondary, tertiary bronchi and bronchioles. Bronchioles are terminal branching of bronchi, which gives rise to very thin well supplied blood vessels, irregular walled balloon like structures called alveoli.
ALVEOLI: there are 300 millions of alveoli is two lungs. They are richly supplies with blood vessels. Due to close contact of blood vessels with alveoli, the exchange of gases takes place.
LUNGS: Pair of lungs is present in humans, lies in air tight chamber known as thoracic cavity or chest cavity. The cavity is formed dorsally by vertebral column and ventrally by sternum, laterally ribs. It is closed below by the diaphragm, which is dome shaped structure made of muscles. Lungs are made up of network of bronchi, bronchioles and alveoli. Alveoli are the main sites where actual diffusion of gases O₂ and CO₂ takes place between blood and atmospheric oxygen.

STEPS OF RESPIRATION

Breathing: it is the inhalation of atmospheric air and exhalation of CO₂.
Diffusion of gases between alveoli and blood: diffusion of gases takes place across the alveolar membrane to blood capillaries surrounding it.
Transport of gases: blood is the medium for transport of gases O₂ and CO₂ which transports oxygen to body cells from alveoli and carbon dioxide from body cells to alveoli.
Diffusion of gases between tissues and blood: O₂is diffused from blood to tissues and CO₂is diffused fromtissues to blood.
Utilization of oxygen: oxygen is used by the body cells for release of energy. Breakdown of glucose occurs in presence of O₂which produces CO_2,water and energy. This is known as cellular respiration as occurs inside the cells. The CO₂ is eliminated out of the body.

The reaction involved is

MECHANISM OF RESPIRATION

INSPIRATION

The process of intake of atmospheric air is known as inspiration, it is and active process.
There is Expansion in thoracic cavity which leads to increase in volume of thoracic cavity due to which air pressure decreases, inspiration takes place.
Contraction of external intercostal muscles contracts due to which the volume of thoracic cavity increases.
Contraction of diaphragm further increases the size of the thoracic cavity. Therefore, lungs expand.
The pressure inside lungs drop or decreases.
The pressure is equalized by the rushing of atmospheric air into the lungs.

EXPIRATION

The process of exhaling carbon dioxide is called expiration, it is a passive process.
It occurs when size of thoracic cavity decreases and air pressure outside increases.
Now, external intercostal muscles relax. As a result ribs are pulled inward and size of thoracic cavity is reduced.
The diaphragm is relaxed and lungs get compressed.
Consequently, pressure increases and air is forced outside.

FACTORS RESPONSIBLE FOR RATE OF DIFFUSION

SOLUBILITY OF GASES: solubility of carbon dioxide is 20-25 times higher than that of oxygen, the amount of CO₂ diffuses are much higher than that of O₂.
PARTIAL PRESSURE: O₂ is diffused from atmospheric air having partial pressure 159 mmHg to alveoli where pO₂ is less, i.e.104 mm Hg.

THICKNESS OF MEMBRANE:
More the thickness of membrane, less will be rate of diffusion.
More the membrane will be thin more will be rate of diffusion.

*Fig: diagram of alveolus*
Picture Source

TRANSPORT OF OXYGEN

Oxygen is transported in following manner:

In dissolved form: about 3% O, is carried In dissolved state through plasma.
As oxyhaemoglobin: About 97% O₂ is transported by RBCs in blood. Haemoglobin binds with blood in reversible manner to form oxyhaemoglobin (OxyHb) and transports it. Single molecule of Hb can carry maximum of four molecules of oxygen.

Note : There are four different haemoglobin species : oxyhemoglobin (oxy-Hb), deoxyhemoglobin (deoxy-Hb), methemoglobin (met-Hb), and hemichromes.

TRANSPORT OF CO₂

Indissolved form through plasma: about 7% of CO₂is transported by plasma.
As bicarbonate ion: About 70% of CO₂ is converted into HCO₃^–and transported in plasma.

By RBCs as carbaminohaemoglobin (HbCO₂): About 20-25% is transported as carbaminohaemoglobin.

*Diagrammatic representation of exchange of gases*

RESPIRATORY DISORDERS

ASTHMA:

Cause: due to allergic reactions to foreign substances that affects respiratory tract.

Symptoms: spasm of smooth muscles present in walls of bronchioles. Coughing, wheezing, difficulty in breathing.

EMPHYSEMA:

Causes: excessive cigarette smoking.

Symptoms: the walls of alveoli are damaged, loss of elasticity of walls of bronchioles and alveoli. Due to which surface area for exchange is reduced.