Respiratory System (grade 11)
Human Gas Exchange
Diaphragm: The muscular plate that separates the thoracic cavity from the
abdominal cavity in humans.
Alveoli: The part of the lungs where the exchange of gases takes place.
Bronchi: The two main branches of the trachea.
Intercostal muscles: The muscles found between the ribs and involved in the
mechanism of breathing.
Epiglottis: The structure that prevents food from entering the trachea during
swallowing.
Ventilation/Breathing System of Humans Consists of:
1)Nasal cavities:
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2 external nostrils lead to 2 nasal cavities, separated by a septum.
Small hairs filter large particles from inhaled air.
Each is divided into 3 passages by 3 curved turbinate bones.
Lined with a mucus membrane consisting of ciliated columnar
epithelial cells.
• Epithelial cells are richly supplied with blood capillaries that warm
incoming air.
• Goblet cells in the epithelium secrete mucus, which moistens incoming
air, traps dust & germs, and is antiseptic.
• Cilia on epithelial cells perform sweeping movements in a direction
away from the lungs to expel dust carrying mucus to the outside.
Pharynx:
• The nasal cavities open into the pharynx.
• Leads to 2 openings: the epiglottis, which leads to the trachea, and the
opening that leads to the oesophagus.
2)Trachea:
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A long, tubular structure that’s in front of the oesophagus.
The larynx with vocal cords is located at the top of the trachea.
The epiglottis (consisting of cartilage) is at the top of the larynx.
Supported and kept open by C-shaped cartilage rings.
Lined with ciliated columnar epithelium with goblet cells, which
secretes mucus
• Mucus moistens incoming air and trap dust particles
• Mucus with trapped foreign particles are moved to the pharynx through
the sweeping actions of the cillia
2)Bronchi and bronchioles:
• Trachea splits into 2 bronchi, one for each lung.
• Bronchi is lined with a mucous membrane and kept open by C-shaped
cartilage rings.
• Bronchi split into smaller bronchioles within the lungs.
• Bronchioles lack cartilage rings and have smooth muscle that can
contract and relax to alter airflow.
• Each bronchiole ends in an infundibulum (a bunch of alveoli).
Alveoli:
• Alveoli increase the gaseous exchange surface area.
• Walls are thin with a single layer of squamous epithelium, which allows
efficient gaseous exchange.
• The inside surface of alveoli is lined with a thin layer of tissue fluid to
stay moist and prevent cells from drying.
• A network of capillaries surrounds alveoli.
• Capillaries transport O₂ to tissues and CO₂ to lungs.
• Capillary walls are thin with a single layer of squamous epithelium,
allowing efficient gaseous exchange.
3)Lungs:
• Located in the chest, protected by 12 pairs of ribs.
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Spongy and elastic
Can expand and contract during breathing.
The right lung has 3 lobes, and the left lung has 2 lobes.
The double membrane (pleura) surrounds the lungs.
Fluid between pleura prevents friction when the lungs expand or shrink.
4)Respiratory muscles:
• The diaphragm is a dome-shaped muscle plate.
• The ribs have external and internal intercostal muscles.
Human Gas Exchange:
Hemoglobin: The compound that contains iron and carries oxygen in the
blood.
Inspiration: Movement of air from the atmosphere into the lungs.
Expiration: Movement of air from the lungs to the atmosphere.
INSPIRATION
EXPIRATION
Active phase of breathing
External intercoastal muscles
contract
Ribs move upwards and outwards
Diaphragm contracts and flattens
Passive phase of breathing
External intercoastal muscles relax
Volume of thoracic cavity increases
Abdominal muscles relax
Pressure in thoracic cavity and lungs
decrease
Elastic lungs expand
Atmospheric pressure is higher
compared to lungs
Oxygen flows into the lungs
Ribs move downwards and inwards
Diaphragm relax and becomes
dome-shaped
Volume of thoracic cavity increases
Abdominal muscles contract
Pressure in thoracic cavity and lungs
increase
Elastic lungs compress
Air pressure in lungs is higher
Carbon dioxide flows out of the lungs
Gaseous exchange in alveoli:
Inhaled air has a higher oxygen concentration than blood in surrounding
blood capillaries
• O₂ dissolves in the thin layer of moisture that lines the alveoli and
diffuses through the thin walls of the alveoli and endothelial walls of the
capillaries, into the blood.
• Blood from tissues has a higher concentration of CO₂.
• CO₂ diffuses from the blood into the walls of the capillaries and the thin
walls of the alveoli, into the air.
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Gaseous Exchange in the Tissues:
Blood in capillaries has a higher oxygen concentration.
• O₂ diffuses through endothelial walls of the capillaries into tissue fluid
(that surrounds the cells) into the cells.
• CO₂ diffuses from the cells into the tissue fluid, then diffuses into the
blood in the capillaries.
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Ways in which oxygen and carbon dioxide are transported:
TRANSPORT OF OXYGEN
Most of the oxygen combines with
haemoglobin in red blood cells
(erythrocytes) to form
oxyhaemoglobin
A small portion of oxygen dissolves
in the blood plasma and is
transported in solution
TRANSPORT OF CARBON
DIOXIDE
Some carbon dioxide combines with
haemoglobin to form
carbaminohaemoglobin. This is then
transported by erythrocytes to the
lungs
The smallest portion of carbon
dioxide dissolves in the blood
plasma and is transported in
solution
Most of the diffused carbon dioxide
is transported as bicarbonate ions
Carbon dioxide combines with water
to form carbonic acid. The carbonic
acid dissociates and forms
bicarbonate ions
COMPONENT OF
AIR
INSPIRED AIR
EXPIRED AIR
Oxygen
Carbon dioxide
Nitrogen
Water vapour
21%
0,04%
78%
Varies according to the
environment conditions
15%
4,0%
78%
Higher than in inspired
air