Chapter 3 Transport Systems in Organisms PDF

**CHAPTER 3** **TRANSPORT SYSTEMS IN ORGANISMS** **Learning Outcomes** It is expected that students will be able to - explain transport system in plants - investigate the important functions and operation of a transport system as well as their mechanisms in plants - describe the stru...

**CHAPTER 3** **TRANSPORT SYSTEMS IN ORGANISMS** **Learning Outcomes** It is expected that students will be able to - explain transport system in plants - investigate the important functions and operation of a transport system as well as their mechanisms in plants - describe the structure of transport tissues (xylem and phloem) in plants - explain how water is conducted in a plant - define transpiration and explain the factors that are affecting on the transpiration and its benefits - demonstrate how transpiration pull takes place - define translocation and explain its functions in plants - explain transport system in animals - understand why a circulatory system is needed in larger animals - understand the features of mass transport system - differentiate the types of circulatory system in animals - demonstrate the components of cardiovascular system in mammals - understand vividly on how the structures of the heart, arteries, veins and capillaries are related to their functions - achieve the knowledge on the sequential events of the cardiac cycle - know the role of lymphatic system in human **3.1 TRANSPORT SYSTEMS IN PLANTS** Plants make their own organic molecules, using the process of photosynthesis. Carbon dioxide is the source of carbon and light is the source of energy. The main photosynthetic organs are the leaves, which have evolved a large surface area to volume ratio for efficient capturing of carbon dioxide and light. A large surface area to volume ratio means more area for collection of light and carbon dioxide and less distance for carbon dioxide to diffuse into the leaf and for oxygen to diffuse out. As a result, most plants do not have compact bodies like animals, but have extensive branching bodies with leaves above the ground. In order to obtain the water and mineral salts they needed for nutrition, plants have extensive root systems below the ground. The plant body therefore spreads out to obtain the carbon dioxide, light energy, water and inorganic mineral ions from its environment to make organic molecules like sugars and amino acids (Figure 3.1). Transport systems are needed for the following reasons: - - - Figure 3.1 Transport in a plant **Structure of Transport Tissues** Plants can be very large, but they have a branching shape that helps to keep the surface area to volume ratio large. Their energy needs are generally small compared with those of animals, so respiration does not take place quickly. They can rely on diffusion to supply their cells with oxygen and to remove carbon dioxide. Their leaves are very thin and have a large surface area inside them in contact with the air spaces. This means that diffusion is sufficient to supply the mesophyll cells with carbon dioxide for photosynthesis, and to remove oxygen. Plants have two transport systems: - - Xylem tissue contains dead, empty, hollow cells with no end walls. These are composed of xylem vessels, which are arranged in long lines to form a hollow tube. Water moves through these long, hollow tubes by mass flow from the roots to all other parts of the plant. Phloem tissue contains cells called sieve tube. Unlike xylem vessel, these are living cells and contain cytoplasm and a few organelles but no nucleus. Their walls are made of cellulose. A companion cell is associated with each sieve tube (Figure 3.2).  Figure 3.2 Two transport systems of plant: xylem (left) and phloem (right) **3.2 TRANSPORT MECHANISMS IN PLANTS** Within a plant, mineral ions and organic compounds (e.g., sucrose) are transported by being dissolved in water. The dissolved mineral ions are transported in the xylem tissue and the dissolved organic compounds are transported in the phloem tissue. The plant roots are responsible for the uptake of water and mineral ions and can have root hairs to increase the surface area for absorption of the substances. The uptake of water is a passive process and occurs by osmosis (the diffusion of water from a higher water potential to a lower water potential). The uptake of minerals can be passive transport such as diffusion and osmosis, and/or active transport. **Movement of Water in a Plant** In the active absorption, the water first enters the cell sap and passes from one cell to another. This type of movement where protoplasm is involved is called symplast. In passive absorption, water moves through the apoplast of the root. The apoplast path includes the cell wall and intercellular spaces. Two pathways are apoplastic and symplastic that initiate the passage of water along with ions from root hair via root cortex to xylem. These routes may exist either simultaneously or separately having different rates (Figure 3.3).  Figure 3.3 Apoplastic and symplastic pathway **Apoplastic pathway** - Most water travels via the apoplastic pathway (when transpiration rates are high), which is the series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem. - The water moves by diffusion (as it does not pass through a partially permeable membrane). - The water can move from cell wall to cell wall directly or through the intercellular spaces. - The movement of water through the apoplastic pathway occurs more rapidly than the symplastic pathway. - When the water reaches the endodermis, the presence of a thick, waterproof, waxy band of suberin within the cell wall blocks the apoplastic pathway. - This band is called the Casparian strip and forms an impassable barrier for the water. **Symplastic pathway** - A smaller amount of water travels via the symplastic pathway, which is the cytoplasm and plasmodesmata or vacuole of the cells. - The water moves by osmosis into the cell (across the partially permeable cell surface membrane), possibly into the vacuole (through the tonoplast by osmosis) and between cells through the plasmodesmata. - The movement of water in the symplastic pathway is slower than the apoplastic pathway. **3.3 TRANSPIRATION** The cells in the mesophyll layers are not tightly packed and have many spaces around them filled with air. The walls of the mesophyll cells are wet and some of this water evaporates into the air space, so that the air inside the leaf is usually saturated with water vapour. The air in the internal spaces of the leaf has directly contacted with the air outside the leaf, through small pores called stomata. If there is a water potential gradient between the air inside the leaf (higher water potential) and the air outside of the leaf (lower water potential), then water vapour will diffuse out of the leaf down this gradient. Although some of the water in the leaf will be used, for example, in photosynthesis, most eventually evaporates and diffuses out of the leaf by the process of transpiration (Figure 3.4). IMG\_256 Figure 3.4 Transpiration in a plant (cohesion: pulling the same kinds of molecules; adhesion: pulling the different kinds of molecules) **3.3.1 Factors Affecting Transpiration** The environmental factors affecting the rate of transpiration are: - **Humidity**: If the water potential gradient between the air spaces in the leaf and the air outside becomes steeper, the rate of transpiration will increase. In condition of low humidity, the gradient is steep, so transpiration takes place more quickly than in high humidity. - **Wind speed and temperature**: Transpiration rate may also be increased by an increasing in wind speed or rising in temperature. - **Light intensity**: In most plants, stomata open during the day and close at night. Most transpiration takes place through the stomata, so the rate of transpiration is almost zero at night. Stomata must be opened during the day to allow carbon dioxide to diffuse into the leaf for photosynthesis. This inevitably increases the rate of transpiration. Closing at night, when photosynthesis is impossible, reduces unnecessary water loss. - **Very dry conditions**: In especially dry conditions, when the water potential gradient between the internal air spaces and the external air is steep, a plant may have to compromise by partially or completely closing its stomata to prevent its leaves drying out, even if this mean reducing the rate of photosynthesis. In hot conditions, transpiration plays an important role in cooling the leaves. As water evaporates from the cell walls inside the leaf, it absorbs heat energy from these cells, thus reducing their temperature. If the rate at which water vapour is lost by transpiration exceeds the rate at which a plant can take up water from the soil, then the amount of water in its cells decreases. The cells become less turgid and the plant wilts as the soft parts such as leaves loose the support provided turgid cells. In this situation, the plant will also close its stomata. **3.3.2 Benefits of Transpiration** Transpiration is regarded as a beneficial fact to the plants for many reasons: - **Ascent of sap:** Ascent of sap mostly occurs due to transpiration pull exerted by transpiration of water. This pull also helps in absorption of water. - **Removal of excess water:** It has been held that plants absorb far more amount of water than is actually required by them. Transpiration, therefore, removes the excess of water. - **Cooling effect:** Radiant heat falling on the plants increases their temperature that may be dangerous to the plants. Transpiration, by evaporating water, bring down (or lowers) their temperature by 10°-15°C. - **Mechanical tissue:** The development of mechanical tissue, which is essential for providing rigidity and strength to the plant, is favoured by the increase in transpiration. - **Distribution of mineral salts:** Mineral salts are mostly distributed by rising column of sap. - **Increasing concentration of mineral salts:** The sap absorbed from the soil contains low concentration of mineral salts. The loss of water through transpiration increases the concentration of mineral salts in the plant. - **Root system:** Transpiration helps in better development of root system, which is required for support, and absorption of mineral salts. - **Quality of fruits:** The ash and sugar content of the fruit increases with the increase in transpiration. - **Resistance:** Excessive transpiration induces hardening and resistant to moderate drought. - **Turgidity**: Transpiration helps the shape and structure of plant parts by keeping cells turgid. - **Photosynthesis:** Transpiration supplies water for photosynthesis. As water evaporates through the stomata, it results in pulling of water, molecule by molecule into the leaf from the xylem. **3.3.3 Demonstration of Transpiration Pull** A thistle funnel is filled with water and rubber stopper attaches a leafy twig to it. The bottom of the funnel is immersed in a bowl of mercury. After some time, the level of mercury in the thistle funnel rises. Loss of water from the twig (i.e., transpiration) produces a vacuum in the thistle funnel and water will be drawn up. This in turn produces a vacuum at the bottom of the funnel, which is then filled by the rising mercury. This rise of water and mercury is due to transpiration pull (Figure 3.5).  Figure 3.5 Demonstration of transpiration pull **3.4 TRANSLOCATION (ASSIMILATE TRANSPORT)** Translocation is the movement of dissolved substances through a plant. In general, water and dissolved salts from the soil travel upwards through the vessels of the xylem and food synthesized in the leaves passes downwards or upwards in the sieve tubes of the phloem. When leaves photosynthesize, they produce carbohydrates. These carbohydrates are transported out of the leaf in the form of sucrose to the stem. Once in the stem it may travel upwards to actively growing regions or maturing fruits and seeds or downwards to the roots and underground storage organs. Both upward and downward movement may take place at the same time in the phloem. The phloem tissue is a complex tissue. The consisting components are sieve tube, companion cell, phloem parenchyma and phloem fibre. They are living cells except phloem fibre. In the translocation process, sieve tubes or sieve cells which have no nucleus and companion cells possess do work together. They are cylindrical cells with layers of cytoplasm like materials just inside their cell walls. They are called sieve tubes because of the presence of perforations in the cell septate walls known as sieve plates. These are main components through which translocation take place and permit the plant materials to pass from one cell to another. Sieve cells can manage translocation without nuclei because they are kept alive by the nucleated companion cells that always situated adjacent to sieve cells. There profuse cytoplasmic connections between sieve and companion cells. Moreover, phloem tissue is strengthened by fibre cells besides the sieve and companion cells (Figure 3.6). IMG\_256 Figure 3.6 The pressure-flow hypothesis of assimilate transport. The assimilate, which is rich in sucrose, is actively transported from source cells into companion cells and then into the sieve-tube elements. This reduces the water potential, which causes water to enter the phloem from the xylem. The resulting positive pressure forces the assimilate down toward the sink cells, where sucrose is unloaded. At this point, there is no longer a high concentration of sucrose, and water returns to the xylem. The leaf is an example of a source, and the root is an example of a sink. **3.5 TRANSPORT SYSTEMS IN ANIMALS** Large multicellular animals have evolved transport system as organisms get larger and have higher metabolic rates. **3.5.1 Principles of Circulation** In many large animals, including all the vertebrates, circulatory system is in the form of mass transport system. Mass transport system is an arrangement of structures by which substances are transported in the flow of a fluid with a mechanism for moving it around the body. **3.5.2 The Need for Transport** Within any organism, substances need to be moved from one place to another. Single-celled organisms have a high surface area to volume ratio. Their surface membrane has a large enough area to supply all the oxygen and other materials that their volume demands. In large multicellular organisms, the surface area to volume ratio is low. So, there is not enough surface area to supply all their metabolic demands. To overcome this problem, they need a transport system. In multicellular animals, many chemical reactions take place inside every microscopic cell. These cells require a supply of chemical substances such as glucose and oxygen for cellular respiration. These must be transported from outside of a large organism into the cells. Respiration supplies energy for the other reactions of life, but it also produces the toxic waste product carbon dioxide. This and other waste products need to be removed from the cells before they cause damage to them. **3.5.3 Features of Mass Transport Systems** Mass transport systems are very effective for moving substances around the body. Most mass transport systems have certain similar features as follow: i. exchange surfaces to get materials into and out of the transport system. ii. a system of vessels that carry substances; these are usually tubes, sometimes following a very specific route, sometimes widespread and branching. iii. a way of making sure that substances are moved in the right direction (e.g., nutrients in and waste out). iv. a way of moving materials fast enough to supply the needs of the organism; this may involve mechanical methods such as the pumping of the heart or ways of maintaining a concentration gradient so that substances move quickly from one place to another (e.g., using facilitated diffusion and active transport). v. a suitable transport medium (e.g., fluid). vi. in many cases, a way of adapting the rate of transport to the needs of the organism. **3.6 CIRCULATORY SYSTEM** There are two main types of circulatory system; open circulatory system and closed circulatory system. **Open circulatory system** - Blood pumped by the tubular heart leaves the open-ended vessels to a series of blood spaces surrounding the tissues. When the heart relaxes, the blood flows back into the vessels through pores called ostia. This type is found in most arthropods (jointed leg animals such as insects, spiders and crabs) (Figure 3.7).  Figure 3.7 Open circulatory system and closed circulatory system. **Closed circulatory system**- Blood is pumped by the heart into the blood vessels which carry the blood to the tissues of the entire body. Then, the blood from the tissues is carried back to the heart by the blood vessels. The main advantages of a closed system are that (i) the pressure can be increased to make the blood flow more quickly, and (ii) the flow can be directed more precisely to the organs that need the most oxygen and nutrients. There are two types of closed circulatory system; single circulatory system and double circulatory system. - **Single circulatory system** - It is found in fish. The heart pumps deoxygenated blood to the gills, the organs of gas exchange where the blood takes in oxygen (becomes oxygenated) and gives up carbon dioxide at the same time. The blood then travels on around the rest of the body of the fish, giving up oxygen to the body cells before returning to the heart. In this type, blood passes once through the heart (Figure 3.8). - **Double circulatory system** - Birds and mammals have evolved the most complex type of transport system, known as a double circulatory system because it involves two separate circulations. The systemic circulation carries oxygenated blood (oxygen-rich blood) from the heart to the cells of the body where the oxygen is used. It also carries the deoxygenated blood (blood that has given up its oxygen to the body cells) back to the heart. The pulmonary- circulation carries deoxygenated blood from the heart to the lungs to be oxygenated and then carries the oxygenated blood back to the heart. Thus, blood passes twice through the heart (Figure 3.8). Birds and mammals (and also reptiles) need much more oxygen than fish. Not only do they have to move around without the support of water, but they also maintain a constant body temperature that may be higher or lower than their surroundings. This takes a lot of resources, so their cells need plenty of oxygen and glucose and make waste products that need to be removed quickly. In reptiles, they have double transport system, but some deoxygenated and oxygenated blood mixed in their hearts. Figure 3.8 Comparison of open circulatory system (Arthropods/Mollusca) compared to several variations in closed circulatory systems including a single circuit (Fish) and two variations of double circuits (Reptiles and Mammals/Birds). **The advantages of double circulation** i. The separate circuits of a double circulatory system ensure that the oxygenated and deoxygenated blood cannot mix, so the tissues receive as much oxygen as possible. ii. The fully oxygenated blood can be delivered quickly to the body tissues at high pressure. iii. The blood going through the tiny blood vessels in the lungs is at relatively low pressure, so it docs not damage the vessels and allows gas exchange to take place. iv. When the oxygenated blood returns to the heart, it can be pumped hard and sent around the body at high pressure. This means it reaches all the tiny capillaries between the body cells quickly, supplying oxygen for an active way of life. **3.7 CARDIOVASCULAR SYSTEM IN MAMMALS** In mammals, the cardiovascular system delivers the materials needed by the cells of the body, and carries away the waste products of their metabolism. The cardiovascular system is made up of the heart, which acts as a pump to move blood through the vessels, a series of blood vessels that carry blood, and the blood, which acts as a transport medium. The passage of blood through the vessels is called circulation. **3.7.1 The Structure of the Human Heart** The human heart, like other mammalian hearts, is a muscular pump with four chambers. The upper two chambers are called the right atrium and the left atrium. The lower chambers are the right ventricle and the left ventricle. Both sides of the heart work simultaneously. The walls of the atria are thinner than those of the ventricles. The right atrium receives the blood from the superior vena cava which collects deoxygenated blood from the head, neck, arms and chest, and from the inferior vena cava which receives deoxygenated blood from the lower parts of the body. The left atrium receives oxygenated blood from the lungs via pulmonary veins. After the blood enters into both ventricles from the corresponding atrium, the deoxygenated blood in right ventricle enters the lungs through pulmonary artery. The oxygenated blood in the left ventricle enters into aorta and then passes throughout the body. As the two sides are separated by a complete thick, muscular septum, the blood in one side of the heart does not mix with the blood from the other side. The heart is made of a unique type of muscle, known as cardiac muscle, which has special properties, it can carry on contracting regularly without resting or getting fatigued. Cardiac muscle has a good blood supply by the coronary arteries bringing oxygenated blood while coronary veins carry away the deoxygenated blood. It also contains lots of myoglobin, a respiratory pigment which has a stronger affinity for oxygen than haemoglobin. The muscular wall of the left ventricle is much thicker than that of the right. The right ventricle pumps blood to the lungs, which are relatively close to the heart. The delicate capillaries of the lungs need blood delivered at relatively low pressure. The left ventricle must produce sufficient force to move the blood under pressure to all the extremities of the body and overcome the elastic recoil of the arteries (Figure 3.9).  Figure 3.9 The structure of human heart: external view (left) and vertical cross section (right) There are four sets of valves in the heart; two sets of atrioventricular valves and two sets of semilunar valves. The right atrioventricular valve is called tricuspid valves which is made up of three flaps and is located between the right atrium and right ventricle. The function of the tricuspid valve is prevention of backflow of blood from the right ventricle into the right atrium. The left atrioventricular valve is called bicuspid valve (Mitral valve) which is made up of two flaps and is located between the left atrium and left ventricle. Its function is prevention of backflow of blood from the left ventricle into the left atrium. The flaps of both atrioventricular valves are supported by the chordae tendineae (tendinous cords) which prevent the flaps from turning inside out by the pressure exerted when the ventricles contract. Semilunar valves (semi means half and lunar means moon) - Both are pocket-like valves with half-moon shaped like those in veins. One set is located at the base of the pulmonary artery while the other set is at the base of the aorta and their function is prevention of backflow of blood from the pulmonary artery into the right ventricle and from the aorta into the left ventricle, respectively. **3.7.2 The Blood Vessels** The main types of blood vessels---arteries, veins, and capillaries---have very different characteristics. These affect the way the blood flows through the body and what the vessels do in the body. **Arteries** Arteries carry blood away from the heart towards the cells of the body. The structure of an artery is shown in Figure 3.10. Almost all arteries carry oxygenated blood. The exceptions are: the pulmonary artery, which carries deoxygenated blood from the heart to the lungs; and the umbilical artery, which, during pregnancy, carries deoxygenated blood from the foetus to the placenta. The aorta (large artery) leaving the heart branches off into arteries in every direction, and the diameter of the lumen, the central space inside the blood vessel, gets smaller the further away it is from the heart. The very smallest branches of the arterial system, furthest from the heart, are the arterioles. The major arteries close to the heart must withstand pressure surges. Their walls contain a lot of elastic fibers so they can stretch to accommodate the greater volume of blood without being damaged. Between surges, the elastic fibers return to their original length, squeezing the blood to move it along in a continuous flow. The role of the elastic fibers in artery walls is to return to their original length to help maintain the pressure. This is called recoil. The elastic recoil docs not help to increase pressure, it simply helps to maintain the pressure. A thick muscular wall helps control the flow of blood by dilating (widening) or constricting (narrowing) the vessels. **Veins** Veins carry blood back towards the heart. Most veins carry deoxygenated blood. The exceptions are: the pulmonary vein, which carries oxygen-rich blood from the lungs back to the heart for circulation around the body; and the umbilical vein, which carries oxygenated blood from the placenta into the foetus, during pregnancy. Tiny venules lead from the capillary network, combining into larger and larger vessels going back to the heart as vena cava (large vein). Veins can hold a large volume of blood, in fact more than half of the body\'s blood volume is in the veins at any one time. They act as a blood reservoir. The blood pressure in the veins is relatively low and the pressure surges from the heart are eliminated before the blood reaches the capillary system. This blood at low pressure must be returned to the heart and lungs to be oxygenated again and recirculated. The walls of the veins are thinner than those of the arteries and contain less elastic tissue and muscle (Figure 3.10). There are one-way valves at frequent intervals throughout the venous system. These are called semilunar valves because of their half-moon shape. They develop from enfolding\'s of the inner wall of the vein. Blood can pass through towards the heart, but if it starts to flow backwards the valves close, preventing any backflow. The contraction of large muscles encourages blood flow through the veins (Figure 3.11). **Capillaries** Capillaries have a very simple structure which is well adapted to their function (Figure 3.10 and 3.12). Their walls are very thin and contain no elastic fibers, smooth muscle or collagen. This helps them fit between individual cells and allows rapid diffusion of substances between the blood and the cells. The walls consist of just one very thin cell. Oxygen and other molecules, such as digested food molecules and hormones, quickly diffuse out of the blood in the capillaries into the nearby body cells, and carbon dioxide and other waste molecules diffuse into the capillaries. Blood entering the capillary network from the arteries is oxygenated. When it leaves, it carries less oxygen and more carbon dioxide.  Figure 3.10 A. Diagrammatic structures of of artery, vein, and capillary and B. Transverse section of artery and vein Figure 3.11 Valves in the veins make sure blood only flow one direction towards the heart  Figure 3.12 Exchange of materials between capillaries and cells **3.7.3 The Components of the Blood** The blood is composed of the plasma, blood cells, which are erythrocytes (red blood cells) and several types of leucocytes (white blood cells), and platelets. **Plasma** Plasma is the liquid part of the blood. Over 50% of the blood volume in the body is plasma, and it carries all of blood cells and everything else that needs transporting around the body. This includes: i. digested food products (e.g., glucose and amino acids) from the small intestine to the liver and then to all the parts of the body where they are needed either for immediate use or for storage, ii. nutrient molecules from storage areas to the cells that need them. iii. excretory products (e.g., carbon dioxide and urea) from cells to the excretory organs such as the lungs or kidneys, to be excreted from the body, iv. chemical messages (hormones) from where they are made to the target organs in the body. v. carries heat around the system from internal organs (e.g., the gut) or very active tissues (e.g., leg muscles in someone running) to the skin, where it can be lost to the surroundings. vi. also acts as a buffer to regulate pH changes. **Erythrocytes or red blood cells** There ore approximately 5 million erythrocytes per mm^3^ of blood (4-5 million per mm^3^ in women, 5-6 million per mm^3^ in men). They contain haemoglobin, a red pigment that carries oxygen and gives them their colour. They are made in the bone marrow. Mature erythrocytes do not contain a nucleus and have a limited life of about 120 days. The erythrocytes transport oxygen from the lungs to all the cells. They are well adapted for their function. The biconcave disc shape of the cells means that they have a large surface area to volume ratio, so oxygen can diffuse into and out of them rapidly. Having no nucleus leaves much more space inside the cells for the haemoglobin molecules that carry the oxygen. In fact, each red blood cell contains around 250-300 million molecules of haemoglobin and can carry approximately 1000 million molecules of oxygen. Haemoglobin also carries some of the carbon dioxide produced in respiration back to the lungs. The rest is transported in the plasma (Figure 3.13). **Leucocytes or white blood cells** They are much larger than erythrocytes but can also squeeze through tiny blood vessels because they can change their shape. There are around 4,000 - 11,000 per mm^3^ of blood and there are several different types. They are made in the bone marrow, although some mature in the thymus gland. Their main function is to defend the body against infection. Leucocytes are also very important in the inflammatory response of the body when an area of tissue is damaged. They all contain a nucleus and have colourless cytoplasm, although some types contain granules which can be stained (Figure 3.13). **Platelets** Platelets are tiny fragments of large cells called megakaryocytes, which are found in the bone marrow. There are about 150,000 - 400,000 platelets per mm^3^ of blood. They are involved in blood clotting (Figure 3.13). Figure 3.13 The light micrograph showing composition of blood **3.7.4 The Clotting of the Blood** The body has a limited volume of blood. In theory, a minor cut could endanger life as the torn blood vessels allow blood to escape. First, the blood volume will reduce and if too much blood losses, you will die. Second, pathogens can get into the body through an open wound. In normal circumstances, the body protects through the clotting mechanism of the blood. This mechanism seals damaged blood vessels to minimize blood loss and prevent pathogens getting in. **Formation of clot** Plasma, blood cells and platelets flow from a cut vessel. Contact between the platelets and components of the tissue (e.g., collagen fibres in the skin) causes the platelets to break open in large numbers. They release several substances, two of which are particularly important. - Serotonin causes the smooth muscle of the blood vessel to contract. This narrows the blood vessels, cutting off the blood flow to the damaged area. - Thromboplastin is an enzyme that starts a sequence of chemical changes that clot the blood. The blood clotting process **Blood clotting process** The blood clotting process is a very complex sequence of events in which there are many different clotting factors. Vitamin K is important in the production of many of the compounds needed for the blood to clot, including prothrombin. The events in the blood clotting process are as follows: **First**, thromboplastin catalyses the conversion of a large soluble protein called prothrombin found in the plasma into another soluble protein, the enzyme called thrombin. Prothrombin, a precursor of a thrombin, is biologically inactive while thrombin is biologically active. This conversion happens on a large scale at the site of a wound. Calcium ions need to be present in the blood at the right concentration for this reaction to happen. **Second**, thrombin acts on another soluble plasma protein called fibrinogen, converting it to an insoluble substance called fibrin. Again, fibrinogen is the biologically inactive precursor of biologically active fibrin. **Finally**, the fibrin forms a mesh of fibres to cover the wound and trap the red blood cells, forming blood clot (Figure 3.14).  Figure 3.14 A. Flow diagram of clotting process and B. Red blood cells trapped in fibrin network **3.7.5 The Cardiac Cycle** The blood is moved through the heart by a series of continuous contraction and relaxation of the muscles in the walls of the four chambers. These events form the cardiac cycle. The contractions of the heart are called systole. Systole can be divided into atrial systole, the contraction of atria and the ventricular systole, the contraction of ventricles. Ventricular systole happens about 0.13 seconds after atrial systole. Between contractions the heart relaxes and fills with blood. This relaxation stage is called diastole. One cycle of systole and diastole makes up a single heartbeat, which lasts about 0.8 seconds in humans. The main stages are illustrated in Figure 3.15. Figure 3.15 Phases of cardiac cycle **3.7.6 Control of Heartbeat** Cardiac muscle cells are myogenic, which means they contract without any external stimulus. They also have intrinsic rhythmicity. An adult heart removed from the body will continue to contract as long as it is bathed in a suitable oxygen-rich fluid The intrinsic rhythm of the heart is around 60 beats per minute which is slower than the heartbeat most of the time when awake. There may have many different ways of controlling the heart to make sure it delivers the exact amount of blood when it is needed. The intrinsic rhythm of the heart is maintained by a wave of electrical excitation similar to a nerve impulse which spreads through special tissue in the heart muscle. The area of the heart with the fastest intrinsic rhythm is a group of cells in the right atrium known as the sinoatrial node (SAN), and this acts as the heart's own natural pacemaker which keeps the heart beating regularly. The sinoatrial node establishes a wave of electrical excitation (depolarization) which causes the atria to start contracting. This initiates the heartbeat. - Excitation also spreads to another area of similar tissue called the atrioventricular node (AVN). - The AVN is excited as a result of the SAN but it produces a slight delay before the wave of depolarization passes into the bundle of His, a group of conducting fibres in the septum of the heart. This makes sure the atria have stopped contracting before the ventricles start. - The bundle of His splits into two branches and carries the wave of excitation to the Purkinje fibers (or Purkyne tissue). - The Purkyne tissue consists of conducting fibres that penetrate down through the septum, spreading around the ventricles. As the depolarization travels through the tissue, it starts the contraction of the ventricles, starting at the bottom and so squeezing blood out of the heart. The speed at which the excitation spreads through the heart, with the hesitation before the AVN stimulates the bundle of His, ensure that the atria have stopped contracting before the ventricles start. It is these changes in the electrical excitation of the heart that cause the repeating cardiac cycle. These electrical changes can be measured in an electrocardiogram (ECG). Because the heart has its own basic rhythm, no one has to think about it. However, many people have a faster resting heart rate than this basic rhythm; the average is around 70 beats per minute. This is because lots of other factors, including nerve impulses and hormones, constantly affect the heart rate (Figure 3.16).  Figure 3.16 Impulse generation in sinoatrial node and transmission **3.8 LYMPHATIC SYSTEM** Between the capillaries and the cells is a watery liquid called tissue fluid. Most of the water from tissue fluid reenters capillaries by osmosis. Some fluid passes into another system called the lymphatic system. The lymphatic system consists of vessels similar to blood capillaries, which are sometimes called lymphatic capillaries. They transport the fluid (lymph) back to the blood by opening into the subclavian veins. The lacteals that carry fats from villi of the small intestine are part of the lymphatic system. Before lymph passes back into the blood, it is filtered to remove dead cells and bacteria. This takes place in swellings called lymph nodes, which contains white blood cells that are important in destroying harmful bacteria. When a person gets infection, one of the first sign may be swelling of the lymph nodes, often referred to as swollen glands (Figure 3.17 and 3.18).