Transport in Animals

TRANSPORT IN ANIMALS

THE NEED FOR A TRANSPORT SYSTEM

• All living animal cells need a supply of oxygen and nutrients to grow and survive
• Also need to remove waste products so they do not build ip and become toxic
• Very small animals don't need a separate transport system as all their cells are surrounded by (or very close to) the environment in which they live. diffusion will be sufficient to supply the oxygen and nutrients the cells need
• A larger animal will have more than two layers of cells
• The diffusion distance becomes too long, and diffusion alone will be too slow to supply all the requirements
• The three main factors that influence the need for a transport system are;
• size
• surface area to volume ratio
• level of metabolic activity

SIZE SA:VOLUME

• Cells inside a large organism are further away from the surface, diffusion pathway increased
• Diffusion rate is reduced
• Diffusion is too slow to supply all the requirements
• The outer layer of cells use up the supplies, less will reach the cells deep inside the body
• Small animals have a large surface area to volume ratio
• This means they have a sufficient area of body surface through which exchange can occur
• Larger animals have a smaller surface area to volume ratio
• This means there's a smaller area of body surface for exchange in relation to the amount of body tissue

LEVEL OF METABOLIC ACTIVITY

• Animals need energy from food, so that they can move around
• Releasing energy from food by aerobic respiration requires oxygen
• If an animal is very active, its cells need good supplies of nutrients and oxygen to supply the energy for movement
• Animals that keep themselves warm need even more energy

FEATURES - GOOD TRANSPORT SYSTEMS

• A fluid or medium to carry nutrients, oxygen and wastes around the body (blood)
• A pump to create pressure that will push the fluid around the body (heart)
• Exchange surfaces that enable substances to enter the blood and leave it again where they are needed (capillaries)
• Tubes or vessels to carry the blood by mass flow
• Two circuits - one to pick up oxygen and another to deliver oxygen to the tissues

SINGLE&DOUBLE CIRCULATORY SYSTEMS

• Fish have a single circulatory system
• The blood flows through the heart once for each circuit of the body
• For a fish, the blood takes the following route
• heart -> gills -> body -> heart
• Mammals have a double circulatory system
• The system has two seepage circuits
• One carries blood to the lungs to pick up oxygen - pulmonary circulation
• The other carries oxygen and nutrients around the body to the tissues - systemic circulation
• Blood flows through the heart twice for each circuit of the body
• The blood takes the following route
• heart -> body -> heart -> lungs -> heart

IN A SINGLE CIRCULATORY SYSTEM;

• (of fish)
• The blood pressure drops as blood passes through the tiny capillaries of the gills
• Blood has a low pressure as it flows towards the body, and will not flow very quickly
• The rate at which oxygen and nutrients are delivered to respiring tissues, and carbon dioxide and urea are removed, is limited
• Fish are not as metabolically active as mammals as they do not maintain their body temperature, therefore they need less energy
• This system is sufficient for their needs

IN THE DOUBLE CIRCULATORY SYSTEM;

• (of mammals)
• The blood pressure must not be too high in the pulmonary circulation, otherwise it may damage the delicate capillaries in lungs
• The heart can increase the pressure of the blood after it has passed through the lungs, so the blood is under higher pressure as it flows to the body and flows more quickly
• The systemic circulation can carry blood at a higher pressure than the pulmonary circulation
• Mammals are active animals and maintain their body temperature
• Supplying the energy for activity and the heat needed to keep the body warm requires energy from food
• The energy is released from food in the process of respiration
• To release a lot of energy, the cells need a good supply of both nutrients and oxygen, as well as the removal of waste products

OPEN CIRCULATORY SYSTEMS

• Many animals, including insects, have an open circulatory system
• This means that the blood is not always held within blood vessels
• Instead, the blood fluid circulates through the body cavity, so that the tissues and cells are bathed directly in blood
• In some animals, movements of the body help to circulate the blood - and without movement the blood stops moving, so that transport of oxygen and nutrients stops
• In other animals, such as insects, there is a muscular pumping organ much like a heart
• This is a long muscular tube that lies just under the dorsal (upper) surface of the body
• Blood from the body enters the heart through pores called ostia
• The heart then pumps the blood towards the head by peristalsis
• At the end of the heart nearest the head, the blood simply pours into the body cavity
• This circulation can continue when the insect is at rest, but the body movements may still affect ciculation

Slide 10

• Some larger and more active insects, such as locusts, have open-ended tubes attached to the heart
• These direct the blood towards active parts of the body, such as the leg and wing muscles
• Open circulatory systems have certain disadvantages
• Blood pressure is low and blood flow is slow
• Circulation of blood may be affected by body movements or lack of body movements

CLOSED CIRCULATORY SYSTEMS

• In larger animals the blood stays entirely inside the vessels - this is a closed circulatory system
• A separate fluid, called tissue fluid, bathes the tissues and cells
• This has certain advantages over the open system;
• High pressure, so that blood flows more quickly
• More rapid delivery of oxygen and nutrients
• More rapid removal of carbon dioxide and other wastes
• Transport is independent of body movements

BLOOD VESSELS

• Blood flows through a series of vessels
• Each is adapted to its particular role in relation to its distance from the heart
• All types of blood vessel have an inner layer or lining, made of a single layer of cells, called the endothelium
• This is a thin layer that is particularly smooth in order to reduce friction with the flowing blood

ARTERIES

• 
• The blood is at high pressure, so the artery wall must be thick in order to withstand that pressure
• The lumen is relatively small in order to maintain high pressure, and the inner wall is folded to allow the lumen to expand as blood flow increases
• The wall consists of three layers
• Inner layer consists of a thin layer of elastic tissue which allows the wall to stretch and then recoil to help maintain blood pressure
• Middle layer consists of a thick layer of smooth muscle
• Outer layer is a relatively thick layer of collagen and elastic tissue. This provides strength to withstand the high pressure, and recoil maintain the pressure

ARTERIOLES

• Small blood vessels that distribute the blood from an artery to the capillaries
• Arteriole walls contain a layer of smooth muscle
• Contraction of this muscle will constrict the diameter of the arteriole
• This increases resistance to flow and reduces the rate of flow of blood
• Constriction of the arteriole walls can be used to divert the flow of blood to regions of the body that are demanding more oxygen

CAPILLARIES

• Have very thin walls
• They allow exchange of materials between the blood and tissue fluid
• The lumen is very narrow, about one blood cell wide. The red blood cells may be squeezed against the walls of of the capillary as they pass along the capillary; this helps the transfer of oxygen, as it reduces the diffusion path to the tissues. It also increases resistance and reduces rate of flow
• The wall consists of a single layer of flattened endothelial cells. This reduces the diffusion distance for the materials being exchanged
• The walls are leaky. They allow blood plasma and dissolved substances to leave the blood
• 

VENULES

• From the capillaries blood flows into small vessels called venues
• These collect the blood from the capillary bed and lead into the veins
• The venue wall consists of a thin layer of muscle and elastic tissue outside the endothelium, and a thin outer layer of collagen

VEINS

• Carry blood back to the heart. The blood is at low pressure and the walls do not need to be thick
• The lumen is relatively large, in order to ease the flow of blood
• The walls have thinner layers of collagen, smooth muscle and elastic tissue than in artery walls. They do not need to stretch and recoil, and are not actively constricted in order reduce blood flow
• The main feature of veins is that they contain valves to help the blood flow back to the heart and prevent it flowing in the opposite direction. As the walls are thin, the vein can be flattened by the action of the surrounding skeletal muscle. Contraction of the surrounding skeletal muscle applies pressure to the blood, forcing the blood to move along in a direction determined by the valves
• 

BLOOD PLASMA AND TISSUE FLUID

• Blood consists of plasma, containing many blood cells
• The plasma contains many dissolves substances, such as oxygen, carbon dioxide, minerals, glucose, amino acids, hormones and plasma proteins
• The cells include the red blood cells (erythrocytes), various white blood cells (leucocytes) and fragments called platelets
• Tissue fluid does not contain most of the cells found in blood or plasma proteins
• It is formed by plasma leaking from the capillaries
• It surround the cells in the tissue, and supplies them with the oxygen and nutrients they require
• As blood plasma leaks from the capillary, it carries all the dissolved substances into the tissue fluid
• This movement is mass flow rather than diffusion
• Waste products from cell metabolism will be carried back into the capillary as some of the tissue fluid returns to the capillary

FORMATION OF TISSUE FLUID

• When an artery reaches the tissues, it branches into smaller arterioles, and then into a network of capillaries. These eventually link up with venues to carry blood back to the veins. Therefore blood flowing into an organ or tissue is contained in the capillaries
• At the arterial end of a capillary, the blood is at relatively high pressure. This pressure tends to push the blood fluid out of the capillaries through the capillary wall. The fluid can leave through the tiny gaps between the cells in the capillary wall
• The fluid that leaves the blood consists of plasma with dissolved nutrients and oxygen. All the red blood cells, platelets and most of the white blood cells remain in the blood, as do the plasma proteins. These are too large to be pushed out through the gaps in the capillary wall
• This tissue fluid surrounds the body cells, so exchange of gases and nutrients can occur across the plasma membranes. The exchange occurs by diffusion, facilitated diffusion and active uptake. Oxygen and nutrients enter the cells; carbon dioxide and other wastes leave the cells

RETURNING TO THE BLOOD

• The blood pressure at the venous end of the capillary is much lower.
• This allows some of the tissue fluid to return to the capillary carrying carbon dioxide and other waste substances to the blood
• Not all the tissue fluid reenters the blood
• Some tissue fluid is directed into another tubular system called the lymph system
• This drains excess tissue fluid out of the tissues and returns it to the blood system in the subclavian vein in the chest
• The fluid in the lymphatic system is called lymph and is similar in composition to the tissue fluid, but will contain more lymphocytes
• Lymph nodes are swellings found at intervals along the lymphatic system, which have an important part to play in the immune response

BLOOD PLASMA

• Hydrostatic pressure - high
• Oncotic pressure - more negative
• Cells - red blood cells, neutrophils, lymphocytes
• Proteins - plasma proteins
• Fats - transported in lipoproteins

TISSUE FLUID

• Hydrostatic pressure - low
• Oncotic pressure - less negative
• Cells - some neutrophils, especially in infected areas
• Proteins - few proteins
• Fats - few fats

LYMPH

• Hydrostatic pressure - low
• Oncotic pressure - less negative
• Cells - lymphocytes
• Proteins - few proteins
• Fats - more fats, especially near digestive system

MOVEMENT OF FLUIDS

• The hydrostatic pressure of the blood is not the only influence on the movement of fluid into and out of the capillary.
• The tissue fluid has its own hydrostatic pressure, and the oncotic pressure of the solutes also has an influence