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OCR Biology AS Level (8)
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Chapter 8: Transport in animals8.1: Transport systems in multicellular animals
The need for specialised transport systems in animals
In single-celled organisms, processes such as dilution, osmosis, active transport, endocytosis, and exocytosis
can supply everything the cell needs to import or export.
These processes are also important in multicellular organisms, transporting substances within and between
individual cells.
However, as organisms get bigger, the distances between the cells and the outside of the body get greater.
Diffusion would transport substances into and out of the inner core of the body, but it would be so slow that
the organism would not survive.
Specialised transport systems are needed because:
o the metabolic demands of most multicellular animals are high so diffusion over the long distances is
not enough to supply the quantities needed
o the surface area to volume ratio gets smaller as multicellular organisms get bigger so not only do the
diffusion distances get bigger but the amount of surface area available to absorb or remove
substances becomes relatively smaller
o molecules such as hormones or enzymes may be made in one place but needed in another
o food will be digested in one organ system, but needs to be transported to every cell for use in
respiration and other aspects of cell metabolism
o waste products of metabolism need to be removed from the cells and transported to excretory
organs.
Types of circulatory systems
Most large, multicellular animals have specialised circulatory systems which carry gases such as oxygen and
carbon dioxide, nutrients, waste products and hormones around the body.
Most circulatory systems have features in common:
o They have a liquid transport medium that circulates around the system
o They have vessels that carry the transport medium.
o They have a pumping mechanism to move the fluid around the system.
When substances are transported in a mass of fluid with a mechanism for moving the fluid around the body
it is known as a mass transport system.
Large, multicellular animals usually have either an open circulatory system or a closed circulatory system.
Open circulatory systems
In an open circulatory system, there are very few vessels to contain the transport medium.
It is pumped straight from the heart into the body cavity of the animal.
This open body cavity is called the haemocoel.
In the haemocoel the transport medium is under low pressure.
It comes into direct contact with the tissues and the cells.
This is where exchange takes place between the transport medium and the cells.
The transport medium returns to the heart through an open-ended vessel
These open-ended circulatory systems are found mainly in invertebrate animals, including most insects and
some molluscs.
insects, gas exchange takes place in the tracheal system.
Insect blood is called haemolymph.
It doesn't carry oxygen or carbon dioxide.
It transports food and nitrogenous waste products and the cells involved in defence against disease.
The body cavity is split by a membrane and the heart extend along the length of the thorax and the
abdomen of the insect.
The haemolymph circulates but steep diffusion gradients cannot be maintained for efficient dilution.
The amount of haemolymph flowing to a particular tissue cannot be varied to meet changing demands
Closed circulatory systems
In a closed circulatory system, the blood is enclosed in blood vessels and does not come directly into contact
with the cells of the body.
The heart pumps the blood around the body under pressure and relatively quickly, and the blood returns
directly to the heart.
, Substance’s leave and enter the blood by diffusion through the walls of the blood vessels.
The amount of blood flowing to a particular tissue can be adjusted by widening or narrowing blood vessels.
Most closed circulatory systems contain a blood pigment that carries the respiratory gases.
Closed circulatory systems are found in many different animal phyla, including echinoderms, cephalopod
molluscs including the octopods and squid, annelid worms including the common earthworm, and all the
vertebrate groups, including the mammals.
Single closed circulatory systems
Single closed circulatory systems are found in a number atrium of groups including fish and annelid worms.
In single circulatory systems the blood flows through the heart and is pumped out to travel all around the
body before returning to the heart. In other words, the blood travels only once through the heart for each
complete circulation of the body.
In a single closed circulation, the blood passes through two sets of capillaries before it returns to the heart.
In the first, it exchanges oxygen and carbon dioxide.
In the second set of capillaries, in the different organ systems, substances are exchanged between the blood
and the cells.
As a result of passing through these two sets of very narrow vessels, the blood pressure in the system drops
considerably so the blood returns to the heart quite slowly
This limits the efficiency of the exchange processes so the activity levels of animals with single closed
circulations tends to be relatively low.
Fish are something of an exception.
They have a relatively efficient single circulatory system, which means they can be very active.
They have a counter current gaseous exchange mechanism in their gills that allows them to take a lot of
oxygen from the water.
Their body weight is supported by the water in which they live, and they do not maintain their own body
temperature.
This greatly reduces the metabolic demands on their bodies and, combined with their efficient gaseous
exchange, explains how fish can be so active with a single closed circulatory system.
Double closed circulatory systems
Birds and most mammals are very active land animals that maintain their own body temperature.
This way of life is made possible in part by their double closed circulatory system.
This is the most efficient system for transporting substances around the body.
It involves two separate circulations:
o Blood is pumped from the heart to the lungs to pick up oxygen and unload carbon dioxide, and then
returns to the heart.
o Blood flows through the heart and is pumped out to travel all around the body before returning to
the heart again.
So, in a double circulatory system, the blood travels twice through the heart for each circuit of the body.
Each circuit -10 the lungs and to the body - only passes through one capillary network, which means a
relatively high pressure and fast flow of blood can be maintained.
8.2: Blood vessels
Circulation in humans is typical of a mammalian circulatory system.
There are several different types of blood vessels in the body and their structural composition is closely
related to their function.
Some examples of different components utilised in some blood vessels are:
o Elastic fibres -- these are composed of elastin and can stretch and recoil, providing vessel walls with
flexibility.
o Smooth muscle - contracts or relaxes, which changes the size of the lumen
o Collagen - provides structural support to maintain the shape and volume of the vessel.
Arteries and arterioles
The arteries carry blood away from the heart to the tissues of the body.
They carry oxygenated blood except in the pulmonary artery, which carries deoxygenated blood from the
heart to the lungs, and the umbilical artery, which carries deoxygenated blood from the foetus to the
placenta.
The blood in the arteries is under higher pressure than the blood in the veins.
Artery walls contain elastic fibres, smooth muscle and collagen
The clastic fibres enable them to withstand the force of the blood pumped out of the heart and stretch to
take the larger blood volume.
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