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Summary 8 Transport in Mammals
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EVERYTHING you need to know about "Transport in Mammals" for A-level Biology CIE
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Transport in Mammals15 April 2021 15:09
Surface Area to Volume Ratio
• Very small organisms, such as yeast, can meet their requirements for exchange of
materials by simple diffusion because they have a high surface area to volume ratio
• Large, multicellular, and very active organisms will require far more oxygen and nutrients
than can be supplied by diffusion
• These organisms have evolved specialist exchange mechanisms and transport systems so
all cells are supplied
• There are 3 main parts to any transport system:
- A pump, the heart
- Plumbing, the blood vessels carry the blood around the body and can be divided into
arteries, veins, and capillaries
- A transport medium, blood transports nutrients to cells and waste from cells
The Circulatory System
• Mammals possess a closed double circulatory system
• A closed system is one in which the blood always remains within the blood vessels
• A double circulatory system means in one circuit around the body, the blood visits the
heart twice: heart -> body -> heart -> lungs
• It can be divided into pulmonary circulation (via lungs) and systemic circulation (via body)
• The advantage of a double circulatory system is that a different pressure can be
maintained in each circulation
• A higher pressure is maintained in the systemic circulation due to greater resistance to
flow (and allows for the formation of tissue fluid)
• The lower pressure in the pulmonary circulation allows for slower flow through the
capillaries surrounding the alveoli, thereby allowing more time for gas exchange
• A low pressure also avoids forcing plasma through the walls of the capillaries into the
alveoli
Structure of Mammalian Transport System
• Blood leaves the heart in arteries
• These large blood vessel branch into smaller arterioles
• Small arterioles end at capillaries, which is where exchange takes place
• The capillaries lead back into venules
• These small blood vessels all lead back into larger veins
• The veins carry blood back to the heart
Structure of Arteries
• Arteries carry blood away from the heart. To withstand the high pressure they must have
a relatively thick wall with a specific composition
• Tunica intima or endothelium, made of squamous epithelium (flat, close fitting cells).
Minimises friction with the moving blood
• Tunica media, contains smooth muscle and elastic fibres, with some collagen
• Tunica externa, consisting of collagen and some elastic fibres
• The lumen is relatively narrow compared to the thickness of the wall to maintain the high
pressure
• The elastic fibres allow the walls of the artery to stretch as a surge of blood passes
through it under high pressure from the heart
• The walls then recoil inwards as pressure drops, increasing the pressure of the blood in
the vessel
• This serves to 'even out' the pressure of the blood in the arteries
Structure of Arterioles
• As arteries reach the tissues, they branch into smaller vessels called arterioles
• Arterioles are similar in structure to arteries, but have a higher proportion of smooth
muscle in their walls
• This muscle can contract or relax, narrowing (constriction) or widening (dilation) the
lumen. This allows control over the amount of blood flowing to different tissues (eg,
reduced to the gut and increased to muscles during exercise)
Structure of Veins
• Veins return blood to the heart at low pressure, so the walls are relatively thin
• They have a thin tunica media (with some muscle and elastic fibres) and a thin tunica
externa (mostly made of collagen). The endothelium is still made of squamous epithelial
cells
• They have a relatively large lumen compared with the thickness of their walls
• Veins possess semilunar valves which close to prevent the backflow of blood
• Squeezing from the skeletal muscles around the veins also causes blood to flow, with
backflow of blood being prevented by the valves
Venules
• Venules collect blood from the capillary beds
• Venules join up to form the larger veins
Fick's Law
Rate of diffusion ∝ (surface area x concentration difference) / thickness of membrane
Structure of a Capillary
, • Venules join up to form the larger veins
Fick's Law
Rate of diffusion ∝ (surface area x concentration difference) / thickness of membrane
Structure of a Capillary
• Capillaries form a network of blood vessels spreading through the tissue, known as a
capillary bed
• Capillaries' function are to take blood close to cells and allow rapid diffusion of materials
between the blood and cells
• Capillaries consist only of an endothelium, which is one cell thick, thus reducing the
diffusion distance of transported substances
• There are small gaps between these cells called fenestrations, which allow plasma (tissue
fluid) to leave the capillaries
• The lumen is very narrow (~7𝜇𝑚), just wide enough for an erythrocyte (red blood cell) to
squeeze through
• This reduces diffusion distance, and slows the passage of the red blood cell, giving more
time for diffusion
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