Topic 3.1 SA: Vol and 3.2 Gas Exchange
Effects of Surface area to volume ratio.
For survival an organism must exchange materials effectively with its environment. Examples of materials that
need to be interchanged between an organism and its environment include: respiratory gases (oxygen and carbon
dioxide), heat, nutrients and excretory products.
Exchanges take place at the surface of an organism, and the materials absorbed are used by the cell. The cell or
cells of the organism make up its volume. For exchange to be effective, the surface area of the organism must be
large compared with its volume. However, surface area to volume ratio gets smaller as the object gets larger.
Single celled organisms and some small multicellular organisms have large enough surface area to volume ratios
to meet their gas exchange needs by diffusion across their surface. However, larger organisms (humans, other
mammals, insects and fish) have relatively small surface area to volume ratios. They can not rely on diffusion
across their surface alone to supply sufficient amounts of oxygen to all of their cells. So larger organisms have
developed specialised gas exchange systems, which have adaptations to ensure the rapid diffusion of gases.
Small organisms have a high metabolic rate as they have a large surface area to volume ratio and lose heat faster
than larger organisms. They have a high rate of respiration to maintain their core body temperature.
Diffusion = SA x concentration gradient/length of pathway
Gas exchange in humans
Lung Structure:
Trachea (Wind pipe) - A tube-like structure that
carries air from the mouth to the lungs.
Bronchi - The trachea splits into two bronchi
as it enters the lungs, which allows air to travel
to the left and right lung (singular: Bronchus)
Bronchioles - The Bronchi further divide into smaller branches called bronchioles. These then supply the alveoli
with air.
Alveoli - Small sacks at the end of the bronchioles, that act as the interface between the air in the lungs and the
blood. There are many alveoli and their walls are folded to provide a large surface area. In fact, the total alveolar
surface area in human adults is about 70m².
The alveoli have a rich blood supply, which circulates and when combined with the process of ventilation help to
maintain a large concentration gradient between the gases in the blood and in the alveoli.
Mechanisms of Breathing:
Inhalation
The external intercostal muscles contract moving the ribcage upwards and outwards. The diaphragm also
contracts and flattens. These actions increase the volume of the thorax, reducing the pressure in lungs below that
of atmospheric air. Air moves into the lungs down a pressure gradient below atmospheric pressure.
Exhalation
The external intercostal muscles relax, moving the ribcage downwards and inwards. The diaphragm also relaxes
causing the diaphragm ro return domed position. These actions decrease the volume of the thorax, increasing the
pressure in the lungs above that of atmospheric air. Air moves out of the lungs down the pressure gradient.
,Inspiration is an active process - Muscle contraction requires ATP.
Expiration (normal) is a passive process however forced expiration is an active process.
- External intercostal muscles relax
- Internal intercostal muscles contract
- Ribs move further down and in
How does oxygen get into the blood?
Oxygen passes by diffusion through the epithelial cells of the alveoli. Then through the endothelial cells of the
capillary and then combines with the haemoglobin in the red blood cells.
How does CO2 get out of the blood?
Carbon dioxide moves from the blood into the alveoli as the concentration of CO 2 in the blood from the pulmonary
artery is greater than that in the inhaled air in the alveoli.
Describe the gross structure of the respiratory system (2)
When air is breathed in, it travels through trachea, which is made up of rings of cartilage to prevent the shape from
deflating. Air is then passed through from the bronchi to the bronchioles towards the alveoli.
How are the lungs adapted for efficient gas exchange? (5)
Many alveoli/many branched capillaries increase SA so rate of diffusion is faster. They have thin squamous
epithelial cells/distance between the cell lining the alveoli and the red blood cells is one cell thick so the diffusion
pathway is shorter.
Pulmonary ventilation - The total volume of air that is moved into the lungs in one minute.
Tidal volume - Volume of air normally taken in at each breath when the body is at rest.
Ventilation/breathing rate - The number of breaths taken in one minute.
Pulmonary ventilation(dm3min-1) = Tidal volume(dm3) x Ventilation rate(min-1)
Fibrosis causes damage to and scarring of the lung epithelium. This results in the deposition of fibrous tissue in
the lung epithelium.
This has two effects:
1. Reduces the rate of diffusion (longer diffusion pathway and less permeable)
2. Reduces the elasticity of the lung therefore breathing out is not as efficient,
Ventilation In Fish:
Exhaling
Fish closes its mouth and raises the floor of the mouth. This decreases the volume and increases the pressure of
the buccal cavity, there is increased volume and decreased pressure at the opercular cavity. Water is forced over
the gills; opercular valve opens.
, Inhaling
Fish opens mouth and lowers the floor of the mouth. This increases the volume and decreases the pressure at the
buccal cavity, there is decreased volume and increased pressure at the opercular cavity; opercular valve closes.
A single gill - The gills are the organ by which gases are exchanged between the fish and the water. Gills enable
fish to absorb oxygen and expel carbon dioxide. Like the lungs, the gills have a large surface area for gas
exchange.
Afferent vessel/artery = Carries deoxygenated blood.
Efferent vessel/artery = Carries oxygenated blood.
Each gill filament has gill lamellae. Gill lamellae are positioned at right angles to the filaments. Lamellae contain
capillaries - Thin epithelium (for short distance between water and blood)
Gill filaments and many lamellae provide a large surface area for gas exchange.
All that separates water and the blood is 2 cells so there is a short diffusion pathway.
The position of the filament and lamellae means that blood and water flow happens in opposite directions. This
massively increases the fish’s ability to absorb oxygen from the water as a diffusion/concentration gradient is
always maintained. This is known as the countercurrent flow.
Water always has a higher oxygen concentration than blood.
As temperature increases, less oxygen will be held.
Salt water holds less oxygen than fresh water.
More active fish require more oxygen for more respiration.
The lamellae can differ in number, distance and thickness.
Why does blood and water flow in opposite directions?
Effects of Surface area to volume ratio.
For survival an organism must exchange materials effectively with its environment. Examples of materials that
need to be interchanged between an organism and its environment include: respiratory gases (oxygen and carbon
dioxide), heat, nutrients and excretory products.
Exchanges take place at the surface of an organism, and the materials absorbed are used by the cell. The cell or
cells of the organism make up its volume. For exchange to be effective, the surface area of the organism must be
large compared with its volume. However, surface area to volume ratio gets smaller as the object gets larger.
Single celled organisms and some small multicellular organisms have large enough surface area to volume ratios
to meet their gas exchange needs by diffusion across their surface. However, larger organisms (humans, other
mammals, insects and fish) have relatively small surface area to volume ratios. They can not rely on diffusion
across their surface alone to supply sufficient amounts of oxygen to all of their cells. So larger organisms have
developed specialised gas exchange systems, which have adaptations to ensure the rapid diffusion of gases.
Small organisms have a high metabolic rate as they have a large surface area to volume ratio and lose heat faster
than larger organisms. They have a high rate of respiration to maintain their core body temperature.
Diffusion = SA x concentration gradient/length of pathway
Gas exchange in humans
Lung Structure:
Trachea (Wind pipe) - A tube-like structure that
carries air from the mouth to the lungs.
Bronchi - The trachea splits into two bronchi
as it enters the lungs, which allows air to travel
to the left and right lung (singular: Bronchus)
Bronchioles - The Bronchi further divide into smaller branches called bronchioles. These then supply the alveoli
with air.
Alveoli - Small sacks at the end of the bronchioles, that act as the interface between the air in the lungs and the
blood. There are many alveoli and their walls are folded to provide a large surface area. In fact, the total alveolar
surface area in human adults is about 70m².
The alveoli have a rich blood supply, which circulates and when combined with the process of ventilation help to
maintain a large concentration gradient between the gases in the blood and in the alveoli.
Mechanisms of Breathing:
Inhalation
The external intercostal muscles contract moving the ribcage upwards and outwards. The diaphragm also
contracts and flattens. These actions increase the volume of the thorax, reducing the pressure in lungs below that
of atmospheric air. Air moves into the lungs down a pressure gradient below atmospheric pressure.
Exhalation
The external intercostal muscles relax, moving the ribcage downwards and inwards. The diaphragm also relaxes
causing the diaphragm ro return domed position. These actions decrease the volume of the thorax, increasing the
pressure in the lungs above that of atmospheric air. Air moves out of the lungs down the pressure gradient.
,Inspiration is an active process - Muscle contraction requires ATP.
Expiration (normal) is a passive process however forced expiration is an active process.
- External intercostal muscles relax
- Internal intercostal muscles contract
- Ribs move further down and in
How does oxygen get into the blood?
Oxygen passes by diffusion through the epithelial cells of the alveoli. Then through the endothelial cells of the
capillary and then combines with the haemoglobin in the red blood cells.
How does CO2 get out of the blood?
Carbon dioxide moves from the blood into the alveoli as the concentration of CO 2 in the blood from the pulmonary
artery is greater than that in the inhaled air in the alveoli.
Describe the gross structure of the respiratory system (2)
When air is breathed in, it travels through trachea, which is made up of rings of cartilage to prevent the shape from
deflating. Air is then passed through from the bronchi to the bronchioles towards the alveoli.
How are the lungs adapted for efficient gas exchange? (5)
Many alveoli/many branched capillaries increase SA so rate of diffusion is faster. They have thin squamous
epithelial cells/distance between the cell lining the alveoli and the red blood cells is one cell thick so the diffusion
pathway is shorter.
Pulmonary ventilation - The total volume of air that is moved into the lungs in one minute.
Tidal volume - Volume of air normally taken in at each breath when the body is at rest.
Ventilation/breathing rate - The number of breaths taken in one minute.
Pulmonary ventilation(dm3min-1) = Tidal volume(dm3) x Ventilation rate(min-1)
Fibrosis causes damage to and scarring of the lung epithelium. This results in the deposition of fibrous tissue in
the lung epithelium.
This has two effects:
1. Reduces the rate of diffusion (longer diffusion pathway and less permeable)
2. Reduces the elasticity of the lung therefore breathing out is not as efficient,
Ventilation In Fish:
Exhaling
Fish closes its mouth and raises the floor of the mouth. This decreases the volume and increases the pressure of
the buccal cavity, there is increased volume and decreased pressure at the opercular cavity. Water is forced over
the gills; opercular valve opens.
, Inhaling
Fish opens mouth and lowers the floor of the mouth. This increases the volume and decreases the pressure at the
buccal cavity, there is decreased volume and increased pressure at the opercular cavity; opercular valve closes.
A single gill - The gills are the organ by which gases are exchanged between the fish and the water. Gills enable
fish to absorb oxygen and expel carbon dioxide. Like the lungs, the gills have a large surface area for gas
exchange.
Afferent vessel/artery = Carries deoxygenated blood.
Efferent vessel/artery = Carries oxygenated blood.
Each gill filament has gill lamellae. Gill lamellae are positioned at right angles to the filaments. Lamellae contain
capillaries - Thin epithelium (for short distance between water and blood)
Gill filaments and many lamellae provide a large surface area for gas exchange.
All that separates water and the blood is 2 cells so there is a short diffusion pathway.
The position of the filament and lamellae means that blood and water flow happens in opposite directions. This
massively increases the fish’s ability to absorb oxygen from the water as a diffusion/concentration gradient is
always maintained. This is known as the countercurrent flow.
Water always has a higher oxygen concentration than blood.
As temperature increases, less oxygen will be held.
Salt water holds less oxygen than fresh water.
More active fish require more oxygen for more respiration.
The lamellae can differ in number, distance and thickness.
Why does blood and water flow in opposite directions?
