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[h] iGCSE Biology Notes Gas exchange in humans
[q] Features of gas exchange surface
[a]
○ Large surface area:
■ Allow faster diffusion.
○ Thin walls.
■ to ensure to diffusion distances remain short.
○ Good ventilation with air.
■ Maintain diffusion gradient.
○ Good blood supply.
■ Maintain a high concentration so diffusion occurs faster.
[q] Gas Exchange
[a]
[q] Structure of the Breathing System
Ribs
[a] Description: Bone Structure that protects internal organs such as the lungs.
[q] Structure of the Breathing System
Intercostal muscles
[a] Description: Muscles between the ribs which control their movement causing inhalation and exhalation.
[q] Structure of the Breathing System
Diaphragm
[a] Description: Sheet of connective tissue and muscle at the bottom of the thorax that helps change the volume of the thorax to allow inhalation and exhalation.
[q] Structure of the Breathing System
Trachea
[a] Description: Windpipe that connected the mouth and nose to the lungs.
● Rings of cartilage surround the trachea (and bronchi).
● The function of the cartilage is to support the airways and keep them open during breathing.
● If they were not present then the sides could collapse inwards when the air pressure inside the tubes drop.
[q] Structure of the Breathing System
Larynx
[a] Description: Also known as the voice box, when air passes across here, sound is produced.
[q] Structure of the Breathing System
Bronchi (singular:Bronchus)
[a] Description: Large tubes branching off the trachea with one bronchus.
[q] Structure of the Breathing System
Bronchioles
[a] Description: Bronchi split to from smaller tubes called bronchioles in the lungs connected to alveoli.
[q] Structure of the Breathing System
Alveoli
[a] Description: Tiny air sacs where gas exchange takes place.
[q] The intercostal muscles.
[a]
○ Muscles are only able to pull on bones, not push on them.
○ There are two sets.
■ Internal Intercostal muscles.
■ External intercostal muscles.
[q] Function of cilia & Mucus.
[a]
○ The passages down to the lungs are lined with ciliated epithelial cells.
○ Cilla help to push mucus up the passages towards the nose and throat where it can be removed.
■ Mucus is secreted by goblet cells.
○ The mucus traps particles, pathogens and dust to prevent them getting into the lungs.
[q]
[a]
[q] Ventilation of the lungs
[a]
○ The diaphragm is a thin sheet of muscle that separates the chest cavity from the abdomen; it is ultimately responsible for controlling ventilation in the lungs.
■ When the diaphragm contracts it flattens and this increases the volume of the chest cavity (thorax) which leads to a decrease in air pressure.
■ When the diaphragm relaxes it moves upwards, and this decreases the volume of the chest cavity, which consequently leads to an increase in air pressure inside the lungs relative to outside the body, forcing air out.
[q] The external and internal intercostal muscles work as antagonistic pairs.
[a] During Inhalation the external set of intercostal muscles contract to pull the ribs up and out:
● Increases the volume of the thorax.
● Decreases air pressure.
● These cause air to get drawn in.
During Exhalation, the external set of intercostal relax so the ribs drop down and in:
● Decreases the volume of the thorax.
● Increases the air pressure.
● Causes air to move out.
[q] Inhalation
[a]
■ External intercostal muscles contract.
■ Rib Cage moves up and out.
■ Diaphragm contracts and flattens.
■ Volume of thorax increases.
■ Pressure inside thorax decreases.
■ Air is drawn in.
[q] Exhalation
[a]
■ External intercostal muscles relax.
■ Rib cage move down and in.
■ Diaphragm relaxes (dome shape).
■ Volume of thorax decreases.
■ Pressure inside thorax increases.
■ Air is forced out.
[q] Composition of air
[a]
[q] Effect of exercise on breathing
Exercise increases the frequency and depth of breathing
[a]
■ More oxygen to be delivered to organs to keep up with energy demand.
■ If it cannot me energy demands the body will start to respire anaerobically, producing lactic acid.
■ After exercise the lactic acid needs to be removed so that it doesn’t affect the body.
● This can be done by combining it with oxygen.
■ Only after this lactic acid has been fully removed does the breathing rate go back to normal.
[q] Carbon dioxide concentration & The brain
[a]
○ Carbon dioxide is acidic in nature.
○ It has to be removed quickly before it causes any problems.
○ As blood flows through the brain, increase in carbon dioxide concentration stimulates receptor cells.
○ These cause pulses to be sent to the respiratory organs causing them to contract more often and with increased strength.
○ This causes the frequency and depth of breathing to increase until carbon dioxide concentration has been lowered sufficiently.
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