Transport in animals

Common features:

• A suitable medium, such as the blood, to carry materials.
• A closed system of vessels that contains blood an forms a network reaching all the body.
• A pump, to move medium around vessels.
• Valves to maintain flow in one direction.
• A respiratory pigments (except arthropoda) increasing volume of oxygen transported.

Open

• e.g. insects
• long, dorsal heart running length of body
• pumps blood at a very low pressure
• blood is pumped into spaces called the haemocoel which bathes the tissues, and exchange take splace.
• The blood then returns to the head and there is little control over circulation.
• No respiratory pigment, as oxygen is taken directlyto cells by trachea.

Closed

• e.g. mammals
• blood circulates in a continuous system of tubes called blood vessels.
• The heart is a muscular organ which pumps blood at a high pressure.
• Organs are bathed in tissue fluid seeping out through capillaries.
• Contains pigment called haemoglobin.

Single

• e.g. fish
• blood only passes through heart once in one circuit.
• It goes through the gills, where it loses a lot of pressure.

Double

• e.g. mammals
• blood passes through heart twice in one circuit
• The right side pumps deoxygenated blood to the lungs, and is then returned to the left side.
• the left side pumps oxygenated blood around the body.
• It sidesteps the problem of blood losing pressure in the lungs/gills.

• Innermost layer is endothelium, and is very smooth, reducing friction and resistance to movement of the blood.
• Middle layer is made from elastic fibres and smooth muscle. It is thicker in arteries as it helps accomodate change in blood pressure.
• Outer layer is made of collagen fibres which are resistant to stretching and also protect the vessels from organs.

Arteries: Carry blood Away from the heart. They have thick msucular walls that contract to maintain pressure on blood as it moves away from the heart. They split into arterioles which then divide into capillaries.

Capillaries: These are thin walled vessels that form the vast network supplying all cells with substances. They only have an endothelium;this is permeable to water and dissolved substances. They have a small diameter and friction in the walls, to slow the blood and encourage diffusion. They then develop into venules which become veins.

Veins: Carry deoxygenated blood to the heart. Larger diameters and thinner walls as the pressure is reduced. They also have semi-lunar valves to prevent backflow.

The four-chambered muscle is made from cardiac musclem which is said to be myogenic. It contracts and relaxes rythmically off its own accord.

THE CARDIAC CYCLE

Atrial systole

• Right and left ventricles relax.
• Tricuspid and bicuspid valves (atrio-venticular) open.
• Atria contract and blood is forced into the ventricles.

Ventricular systole

• Atrio ventricular valves close, preventing backflow.
• Ventricles contract.
• This forces semi-lunar valves open.
• The pulmonary artery carries blood to lungs; the aorta carries blood to the body.

Diastole

• ventricles relax, pressure falls. semi lunar valves shut due to pressure in arteries.

• Highest pressure occurs in the arteries/aorta.
• Pressure decrease as you move away from the heart due to friction, large cross sectional area in capillary beds, and the loss of fluid through capillary pores.

Control

• Muscle is myogenic
• Within right atrium is specialised tissue forming the sino-atrial node (SAN)
• This node produces a wave of eletrical stimulation, causing both atria to contract at the same time.
• It is prevented from reaching the ventricles by a thin layer of connective tissue. This ensures that the ventricles and atria do not contract at the same time.
• Instead it reachs the atrio-ventricular node (AVN)  which conducts the wave down the bundle of His to the apex.
• The wave then spreads out through Purkinje fibres causing ventricles to contract from bottom-up.

Blood is a tissue which is 45% cells, and 55% fluid plasma.

Red Blood Cells

• Main function is to carry oxygen to cells.
• Biconcave shape- more surface area for oxygen diffusion.
• No nucleus- more room to pack haemoglobin in.

White Blood Cells

• Posess a nucleus and are irregular in shape.
• Granulocytes are phagocytic and have lobed nuclei.
• Agranulocytes produce antibodies and atitoxins, have clear cytoplasm and a spherical nucleus.

Plasama

• 90% water
• Has dissolved food products, hormones, plasma proteins, mineral ions and vitamins.
• Also transports CO2 and distributes heat.

Haemoglobin has to carry out very contradictory roles; to associate iwth oxygen in oxygen rich areas, and dissociate in oxygen lacking areas.

The oxygen dissociation graph shows this phenomenon; the more to the left the curve is, the more easily the haemglobin associates with oxygen, and to the right, more easily is dissociates.

At first, oxygen association is rapid, and slows as oxygen tension increases. Each haemoglobin molecule can carry 4 molecules.

At higher levels of carbon dioxide, the graph shifts to the RIGHT- allowing dissociation. This is known as the Bohr effect.

Foetal haemoglobin

• mother and babies blood flows close together but doesnt mix.
• Foetal haemoglobin differs in two polypeptides of adults
• It has a higher affinity (to the left) for oxygen
• This means it associates with oxygen easier than the mothers.

Other animals

Haemoglobin is often adapted to the habitat of an animal.

• Lugworm lives in sand, and pumps water through its burrow, accessing oxygen.
• Llamas live in low partial pressures of oxygen also.
• Both haemoglobin's curve very much to the left and so have a very high affinity for the limited amount of oxygen.

Myoglobin

Much more stable than haemoglobin and doesnt release o2 unless under very low kPa of O2. Therefore it acts a reserve for high oxygen demand, such as sustained activity.

Carbon dioxide is carried in three forms:

• dissolved in plasma 5%
• carbamino-haemoglobin  10%
• hydrogen carbonate 85%

The Bohr Effect and the chloride shift

• Carbon dioxide dissolves into the RBC.
• It combines with water to make carbonic acid (this is sped up by the enzyme Carbonic anhydrase)
• This acid provides conditions (H+ and HCO3- ions) for oxygen to dissociate from haemoglobin, and release O2.
• H+ ions also form haemoglobonic acid, and HCO3- ions diffuse out and form Sodium hydrogencarbonate.
• To balance the loss of negatively charged ions, chloride ions move into the cell and maintain electrochemical neutrality.

Plasma fluid contains many dissolved substances (glucose, mineral ions, vitamins, plasma proteins), but the fluid can escape through the capillary walls. This forms tissue fluid, bathing cells with glucose, fatty acids, lipids, amino acids and oxygen.

• Blood reaches the arterial end of a capillary and is under high pressure to do heart pumping and resistance. This hyrdostatic pressure forces blood out capillaries.
• The hydrostatic pressure is opposed by reduced water potential of blood created by plasma proteins. But hydrostatic pressure is greater, so net movement of fluid is out.
• There is also a concentration gradient for glucose, oxygen and ions, into the cells, because they are being used up by metabolism.
• At the venuous end, blood pressure is lower, and water moves back into capillary BY OSMOSIS. Low water potential is created by plasma proteins.
• The venous end also picks up some waste products such as CO2. Some is also pumped into lymphatic system, and later returned to venous sytem by thoraric duct, near the heart.