Circulatory System

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  • Circulatory System
    • Single-celled organisms such as 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 multi-cellular organisms, diffusion through the membrane is too slow to satisfy the needs of an organism because;
      • They have a low SA:Vol 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
          • This bulk movement of blood is called mass flow
      • A mass flow system has the following features;
        • A suitable medium to carry materials
        • 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 is pumped at low pressure out of a large 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 circulation 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 tracheae
      • Closed circulatory system
        • e.g. Mammals
          • Blood is contained in vessels
            • Blood is pumped by a muscular heart and is under pressure, so had a rapid flow rate
              • Organs are not in direct contact with the blood, but are bathed in tissue fluid seeping out from this walled capillaries
                • The blood contains a respiratory pigment which carries oxygen
        • Earthworms
          • Even though the earthworm is a relatively simple organism, it also has 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 and double circulatory systems
      • Closed circulatory systems are of two types depending on whether blood passes through the heart once or twice
        • Single
          • Fish have  a single circulatory system
            • The heart pumps deoxygenated blood to the gills
              • Oxygenated blood is carried to the tissues. From there, deoxygenated blood returns to the heart
                • Blood only passes through the heart once in one circuit of the body
        • Double
          • In each circuit, blood passes through the heart twice
            • Once through the right side of the heart - pulmonary circulation - consists of all of the vessels concerned with pumping blood between the heart and lungs
              • Once through the left side of the heart - systemic circulation - which consists of all of the vessels concerned with pumping blood between the heart and the body
          • The right side of the heart pumps deoxygenated blood to the lungs
            • Oxygenated blood then returns to the left side of the heart
              • The left side of the heart pumps oxygenated blood to tissue and 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 and deoxygenated blood are kept seperate
            • Increased oxygen distribution, which can maintain a higher metabolic rate
    • Structure of the heart
      • The heart is made of specialised cardiac muscle which has it's own blood supply and which is able to continuously contract and relax on it's own
        • The heart muscle needs a good supply of blood to provide it with the nutrients and oxygen needed for contraction
          • This is achieved by a dense capillary network that revieves blood from coronary arteries. The blood returns to the heart via the coronary vein
      • The heart is, in effect two pumps side by side
        • The left side of the heart receives oxygenated blood from the lungs and pumps it to the head and body, where oxygen is used in the tissues for respiration
          • He right side of the heart recieves deoxygenated blood from the head and body and pumps it to the lungs to pick up oxygen
            • The two sides of the heart are separated by a thick wall called the intervertricular septum
              • The deoxygenated blood never mixes with the oxygenated blood

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