3.2

• There are several reasons why transport systems are needed:​

  • Size​
  • Surface area to volume ratio​
  • Metabolic rate​
  • Transporting molecules such as food, hormones, enzymes, and waste products of metabolism.​
• Open circulatory system


  Consists of a heart that pumps a fluid called haemolymph through short vessels and into a large open body cavity called the haemocoel
• Haemolymph circulation in open circulatory system

  1. Haemolymph directly bathes organs and tissues in the haemocoel, enabling diffusion of substances

  2. When the heart relaxes, haemolymph is sucked back in via pores called ostia

  3. Haemolymph moves around the haemocoel due to the movement of the organism
• Open circulatory system


  • Steep gradients cannot be maintained for efficient diffusion
  • The amount of haemolymph flowing cannot be varied to meet changing demands
• In a closed circulatory system, blood is fully enclosed within vessels at all times.​
• Single circulatory systems

  • The blood flows through the ​heart once for each circuit ​of the body.​
  • It passes through two sets ​of capillaries – in the gills ​(where gaseous exchange takes place) and in the body tissues before returning to the heart.​
  • This makes the circulatory system relatively inefficient.​
• A double circulatory system has two separate circuits:​

  • A pulmonary circulation to transport blood between the heart and the lungs.​
  • A systemic circulation to transport blood between the heart and the rest of the body.​
• There are 5 types of blood vessel:​

  • Arteries​
  • Arterioles​
  • Capillaries​
  • Venules​
  • Veins​
• • Arteries and arterioles carry blood away from the heart to the body cells.​
  • They carry oxygenated blood, with one exception – the pulmonary artery.
• • Venules and veins carry blood from the body cells to the heart.​
  • The blood they transport is deoxygenated, except for the pulmonary vein.​
• • Arteries lead into arterioles, then into capillaries, where gas exchange occurs.​
  • From here, capillaries move into venules, then into veins and back to the heart.​
• Endothelium​

  • This is the inner lining of all blood vessels.​
  • It is made of a single layer of cells.​
  • They are particularly smooth, in order to reduce friction with the flowing blood.​
• Elastic Fibres​

  • These are made of the protein elastin.​
  • They provide flexibility, enabling the vessel to stretch and then recoil to its original size.​
  • This evens out surges from the pumping of the heart to allow a continuous flow of blood.​
• Smooth Muscle​

  • This provides some strength to withstand high blood pressure.​
  • Its main role is to contract and relax to change the size of the lumen and alter the flow of blood to particular tissues.​
• Collagen​

  • This provides structure and support.​
  • It limits stretch to maintain the shape and volume of the vessel.​
• • Arteries carry blood away from the heart to the various organs of the body.​
  • Their structure enables them to withstand the high pressure generated from the heart forcing out blood with each heartbeat.
  • They have a narrow lumen to maintain a high blood pressure
• Capillaries allow exchange of substances (by diffusion) between blood and surrounding cells.​
• • Capillary walls (endothelium) is one cell thick. This creates a short diffusion distance. The walls are leaky to allow blood plasma and dissolved substances to leave the blood.​
  • The lumen of a capillary is only 8-10μm. Red blood cells are 7-8μm, so have to travel in single file. This increases contact of red blood cells with the capillary walls and reduces the diffusion distance.​
• Vein structure

  • Low blood pressure means that veins need a large lumen to reduce resistance/friction.​
  • Veins have a thin elastic layer as they don’t need to stretch and recoil with pulses.​
  • They have little smooth muscle.​
  • Walls of veins contain a lot of collagen to provide structural support for the large volume of blood.​
  • They have valves to prevent backflow of blood
• Valves act as one-way blood flow system to prevent back-flow.​
• Blood is responsible for the transport of:​

  • Oxygen​
  • Carbon dioxide​
  • Nitrogenous waste products to excretory organs​
  • Food molecules from storage compounds to cells​
  • Digested food from small intestine​
  • Hormones​
  • Platelets to damaged areas​
  • Cells and antibodies involved in immune response​
• • Blood is the main specialised transport medium of the human circulatory system.​
  • It is a type of connective tissue.​
• Blood has functions including:​

  • Defence​
  • Thermoregulation​
  • Maintaining pH of body fluids​
• Plasma is a straw-coloured liquid, consisting of:​

  • Water (~92%)​
  • Mineral ions (e.g. Na+, K+)​
  • Nutrients (e.g. glucose, ​amino acids, fatty acids)​
  • Hormones​
  • Gases (e.g. O2 and CO2)​
  • Waste products (e.g. urea)​
• Large plasma proteins include fibrinogen – important in blood clotting, globulins– involved in the immune system and albumin– important in maintaining the ​osmotic potential of the blood.
• Albumin dissolves in the water of the blood plasma,​ gives the blood in the capillaries a high solute potential (so a low water potential) compared with the surrounding fluid.​ Therefore, water has a tendency to move into the blood by capillaries from the surrounding fluid by osmosis.​ This effect is called oncotic pressure.​
• • Tissue fluid (also known as interstitial fluid) is the fluid which surrounds the cells in the tissues.​
  • This is to supply them with oxygen and nutrients.​
• • Tissue fluid is formed by substances leaking out of blood plasma in capillaries. It is similar to blood plasma – but:​
  • It does not contain most of the cells found in blood.​
  • It does not contain plasma proteins.​
  • Therefore, it is mainly water and small molecules.​
• Oncotic pressure = a type of osmotic pressure exerted by plasma proteins (particularly albumin) within a blood vessel.​ It usually pulls water into the capillary by osmosis.​
• • Hydrostatic pressure = pressure exerted by a fluid.​
  • This pushes blood from the arterial end of the capillary to the venous end.​
  • Because the capillary walls are permeable, it also pushes fluid out of the capillaries, into the extracellular space.
• • At the arterial end of a capillary, the blood is at a relatively high hydrostatic pressure.​
  • The hydrostatic pressure is higher than the oncotic pressure.​
  • As a result, fluid is squeezed out of capillaries by ultrafiltration
  • Proteins and blood cells are too large to fit through the tiny gaps (pores) between the endothelial cells in the capillary wall, so remain in the blood
  • Exchange then occurs into and out of the cells by:​
  • Diffusion​
  • Facilitated diffusion​
  • Active transport​
• • By the time blood has reached the venous end of the capillary, its hydrostatic pressure has decreased.​
  • It is now lower than the oncotic pressure, which has not changed.​
  • This allows some of the tissue fluid to return to the capillary, carrying CO2 and other waste substances into the blood.​
  • About 10% of the tissue fluid does not re-enter the capillaries.​
  • Instead, it is directed into another tubular system called the lymphatic system
  • It is then returned to the blood system via the subclavian ​vein in the chest.​
• • The fluid in the lymphatic system is called lymph.​
  • Lymph is similar to tissue fluid but also contains lymphocytes.​
  • These are produced in the lymph nodes (swellings found at intervals along the lymphatic system).​
• • Lymph also contains lipids.​
  • This is because the villi of contain structures called lacteals, where lipids are absorbed.​
  • Lacteals are part of the lymphatic system
• The lymphatic system is a secondary circulatory system and a major part of the immune system.​

  It consists of:​

  • Lymphatic capillaries and lymph vessels​
  • Lymph nodes​
  • Lymphatic tissue​
• • The heart is a muscular organ which pumps blood continuously around the body.​
  • This is to enable gas exchange.
• The heart is a hollow, muscular organ located between the lungs in the centre of the chest (thorax).
• • The heart is composed mainly of cardiac muscle tissue.​
  • Cardiac muscle contracts involuntarily (like smooth muscle, unlike skeletal muscle).​
  • Cardiac muscle contains lots of mitochondria and myoglobin (oxygen-binding protein located primarily in muscles) molecules.
• • Cardiac muscle cells are separated by intercalated discs. These are regions where the cytoplasm of neighbouring cells is connected by gap junctions.​
  • This enables electrical impulses to pass through the tissue, facilitating synchronised contraction.​
• • The heart is enclosed within the pericardium.​
  • This is a tough and fibrous sac, which protects the heart.​