Mass Transport Animals

• Double circulatory system


  One in which the blood flows through the heart twice for each circuit of the body
• Single circulatory system


  One in which the blood flows through the heart once for each circuit of the body
• Transport


  The movement of substances such as oxygen, nutrients, hormones, waste and heat around the body
• All living animal cells need a supply of oxygen and nutrients to grow and survive. They also need to remove waste products so that these do not build up and become toxic.
• Very small animals do not need a separate transport system, because all their cells are surrounded by (or very close to) the environment in which they live. Diffusion will supply enough oxygen and nutrients to keep the cell alive.
• A larger animal with a complex anatomy 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.
• Factors that influence the need for a transport system

  • Size
  • Surface area to volume ratio
  • Level of metabolic activity
• Size


  The cells inside a large organism are further from its surface — the diffusion pathway is increased. The diffusion rate is reduced, and diffusion is too slow to supply all the requirements. Also, the outer layers of cells use up the supplies, so that less will reach the cells deep inside the body.
• Surface area to volume ratio

  Small animals have a large surface area to volume ratio. This means that for each gram of tissue in their body they have a sufficient area of the body surface through which exchange can occur. However, larger animals have a smaller surface area to volume ratio. This means that each gram of tissue has a smaller area of the body surface for exchange.
• 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, such as mammals, need even more energy.
• Features of a good transport system

  • A fluid or medium to carry nutrients, oxygen and wastes around the body — this is the blood
  • A pump to create pressure that will push the fluid around the body — this is the heart
  • Exchange surfaces that enable substances to enter the blood and leave it again where they are needed — these are the capillaries
• Features of an efficient transport system

  • 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 circulatory system


  The blood flows through the heart once for each circuit of the body. The blood takes the following route: heart->gills-> body->heart
• Double circulatory system


  The system has two separate circuits. One circuit carries blood to the lungs to pick up oxygen. This is pulmonary circulation. The other circuit carries the oxygen and nutrients around the body to the tissues. This is 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
• Advantages of a double circulation

  • An efficient circulatory system will deliver oxygen and nutrients quickly to the parts of the body where they are needed. The blood can be made to flow more quickly by increasing the blood pressure created by the heart.
• In the 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.
• 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 the 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.
• Blood vessels


  • Arteries
  • Arterioles
  • Capillaries
  • Venules
  • Veins
• Arteries


  Vessels that carry blood away from the heart
• Arterioles


  Small blood vessels that distribute blood from an artery to the capillaries
• Capillaries


  Very small vessels with very thin walls
• Closed circulatory system


  One in which blood is held in vessels
• Open circulatory system


  One in which blood is not held in vessels
• Veins


  Vessels that carry blood back to the heart
• Venules


  Small blood vessels that collect blood from capillaries and lead into the veins
• 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 forward end of the heart (nearest the head), the blood simply pours out into the body cavity.
• 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 vessels. A separate fluid, called tissue fluid, bathes the tissues and cells. This has certain advantages over the open system: Higher 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.
• Layers of artery wall

  • Inner layer (tunica intima)
  • Middle layer (tunica media)
  • Outer layer (tunica adventitia)
• Arteries


  Carry blood away from the heart. 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.
• 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 — its diameter is about the same as that of a red blood cell. The walls consist of a single layer of flattened endothelial cells. 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 venules. These collect the blood from the capillary bed and lead into the veins. The venule wall consists of thin layers 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 to reduce blood flow. The main feature of veins is that they contain valves to help the blood flow back to the heart and to prevent it flowing in the opposite direction.
• Blood


  The fluid used to transport materials around the body
• Hydrostatic pressure


  The pressure that a fluid exerts when pushing against the sides of a vessel or container