Transport Systems

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  • Transport Systems
    • Single Celled Organisms e.g. Amoeba can obtain nutrients and excrete waste simply by diffusion across the cell membrane. They can do this because;
      • They have a short diffusion pathway, so the rate of diffusion will satisfy demand
      • They have a slow metabolic rate so demand is low
    • In multicellular organisms, diffusion through the membrane is too slow to satisfy the needs of the organism because;
      • They have a low SA:Volume ratio
      • They have a high metabolic rate
      • Some cells are deep within the body, creating a long diffusion pathway
      • They have a tough outer surface so gases cannot diffuse through their skin
    • Mass Flow
      • Multicellular organisms have developed a specialised transport system to carry materials from their specialised exchange organs to their body cells
      • In mammals this is the circulatory system. It uses blood to carry gases, nutrients, hormones and waste products around the body
      • The bulk movement of blood is called mass flow
      • A mass flow system has the following properties;
        • A suitable medium to carry materials (blood)
        • A closed system of vessels that contain blood and form a branching network to distribute it to all parts of the body
        • A pump, e.g. the heart for moving blood within the vessels
        • Valves to maintain the flow in one direction
        • A respiratory pigment which increases the volume of oxygen that can be transported
    • Types of Circulatory System
      • Open Circulatory System e.g. Insects and Snails
        • Blood doesn't flow through vessels
          • It flows at low pressure out of a long dorsal tube shaped heart running the length of the body into a fluid filled body cavity (haemocoel)
        • Blood bathes the tissues directly and exchange of materials takes place. There is little control over the direction of the blood
          • Blood then returns to the heart slowly. In the heart, valves and waves of contraction of the muscle move the blood forward to the head region where the open circulation is started again
        • There is no respiratory pigment in the blood as it does not transport oxygen. This is transported directly to the tissue via the trachae
      • Closed Circulatory System e.g. Mammals
        • Blood contained in vessels
        • Blood is pumped by a muscular heart and is under pressure so has a rapid flow rate
        • Organs not in direct contact with the blood, but are bathed in tissue fluid seeping out from thin walled capillaries
        • Blood contains a respiratory pigment which carries oxygen
        • Earthworms have a closed circulatory system, It has dorsal and ventral vessels running the length of the body and is connected by 5 pseudohearts
          • Blood moves through the vessels by the pumping action of the pseudohearts
        • Single Circulatory System
          • Fish have a single circulatory system
          • The heart pumps deoxygenated blood to the gills
          • Oxygenated blood is carried to the tissues and from there deoxygenated blood returns to the heart
          • Blood only passes through the heart once in one circuit of the body
        • Double Circulatory System
          • Mammals have a double circulatory system
          • In each circuit, blood passes through the heart twice
            • Once through the right side - pulmonary circulation - consists of all the vessels concerned with pumping blood between the heart and lungs
              • Once through the left side - systemic circulation - which consists of all the vessels concerned with pumping blood between the heart and body
          • Right side: Pumps deoxygenated blood to the lungs. Oxygenated blood then returns to the left side of the heart
          • Left Side: Pumps oxygenated blood to the tissues. Deoxygenated blood returns to the right side of the heart
          • Advantages;
            • Sustained high blood pressure in the systemic circulation
            • Circulation is faster in the systemic circulation
            • Oxygenated blood and deoxygenated blood is kept seperate
            • Increased oxygen distribution which can maintain a higher metabolic rate


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