The Human Breathing Mechanism•The human respiratory system provides an area for gasexchange between the blood and the environment. Itallows oxygen acquisition and the carbon dioxideelimination.•The gas exchange in lungs is called breathing (externalrespiration). This exchange involves the taking in andletting out of air.
Structure of Human Respiratory System•Air enters the respiratory system through the nasalcavity.•As air passes the nasal cavity, it is warmed andmoistened. Dust is trapped by the hairs.•After the air leaves the nasal cavity, it enters thetrachea.•The wall of the trachea contains C – shaped ringsof cartilage which give it support and prevent itfrom collapsing during inhalation.•The trachea branches into two bronchi (singular:bronchus)
•One bronchus enters the right lung, the others entersthe left lung.•Each bronchus branches into smaller tubes calledbronchioles.•The bronchioles end in millions of tiny air sacs calledalveoli (singular: alveolus).• the lungs are made up of alveoli, bronchioles and bloodcapillaries.•The lungs are located in a space called the thoraciccavity.•The lungs are protected by the ribcage which consists ofribs and intercostal muscles.•The sheet of muscle called the diaphragm separates thethoracic cavity from the abdomen.
Adaptation of the alveoli for efficient gas exchange•Large surface area – the lungs have millions of alveoli toprovide a large surface area for gas exchange.•Very thin walls – the wall of each alveolus is one cell –thick to provide a very short distance for the diffusion ofgases.•Moist inner surface – the inner surface of each alveolusis moist so that oxygen can dissolve in the moisturebefore diffusing across the alveolar wall.•Surrounded by numerous blood capillaries – the bloodcapillaries help to transport gases to and from the alveoliquickly.
Mechanism of human breathing•The breathing mechanism consists of two phases. Thetaking in of air is called inhalation. The letting out of theair is called exhalation.Inhalation
• during breathing, the lungs expand and return totheir normal size to pump air into and out of them.•The movements of the ribs and the diaphragmbring about breathing.Exhalation
Comparison of the mechanisms ofinhalation and exhalationInhalation Exhalation•Diaphragm muscles contract.•Diaphragm flattens out•Diaphragm muscles relax.•Diaphragm arches upwards•Intercostal muscles contract. •Intercostal muscles relax.•Ribcage moves upwards andoutwards.•Ribcage moves downwardsand inwards.•Volume of thoracic cavityincreases•Volume of thoracic cavitydecreases.•Air pressure surrounding thelungs decreases.•Air pressure surrounding thelungs increases.•Air flows into the lungs •Air is forced out of the lungs.
Composition of air in respirationContent Inhaled air Exhaled air Alveolar airOxygen 21 % 16 % 14 %Carbondioxide0.04 % 4 % 6 %Nitrogen 79 % 79 % 80 %Water vapour Variable Saturated SaturatedTemperature Variable 34 ° C 37° C
•The changes in the content of air entering the lungsare due to gas exchange in the alveoli.Common mistake-There is more carbon dioxide than oxygen inexhaled air.The Truth- Exhaled air contains 16 % oxygen, 4 % carbondioxide and 79 % nitrogen.