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WBCS Preliminary (Biology): Circulatory System

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  • Plants take water and mineral nutrients from the soil through the roots and transport it to the leaves. The leaves prepare food for the plant, using water and carbon dioxide during photosynthesis. Plants absorb water and minerals by the roots. The roots have root hair. The root hair increase the surface area of the root for the obsorption of water and mineral nutrients dissolved in water. The root hair is in contact with the water present between the soil particles.
  • Plants have pipe-like vessels to transport water and nutrients from the soil. The vessels are made of special cells, forming the vascular tissue. The vascular tissue for the transport of water and nutrients in the plant is called the xylem. The xylem forms a continuous network of channels that connects roots to the leaves through the stem and branches and thus transports water to the entire. Two types of xylem cells are involved in transport of water - tracheids and vessels. They are dead cells with lignified walls. They are joined end to end forming a capillary system to draw water up the plant.
  • Leaves synthesise food. The food has to be transported to all parts of the plant. This is done by the vascular tissue called the phloem. Phloem is a living tissue. Sieve tubes and companion cells are the phloem cells involved in the transport of food. Other than sucrose, phloem also transports hormones (from the site of synthesis to the site of action) and some of the mineral ions (from the leaves about to fall to the other regions). The transport of soluble substances like the sugars, amino acids and hormones by the phloem is called translocation.
  • Transpiration is the loss of water from the aerial parts of the plant, mainly through the stomata of the leaves. In tall trees transpiration pulls water up the xylem. The rate of transpiration is affected by many factors such as light, temperature, availability of soil water and atmospheric humidity.
  • Animals in general have a higher metabolic rate than the plants and thus require a more efficient transport system. There are two types of blood circulatory systems:
    • Open circulatory system: In open circulatory system the blood vessels are open-ended as they open into the common cavities called the haemocoel. It is seen in insects.
    • Closed circulatory system: In closed circulatory system the blood always remains inside the blood vessels and never comes in direct contact with the cells. It is seen in mammals including man.
  • Blood is an alkaline fluid that consists of the liquid portion called plasma and three types of corpuscles. Plasma is yellow coloured fluid consisting of water (92%), proteins(6-9%) and 1% minerals. Plasma transports red and white blood cells and platelets throughout the body. It also delivers nutrients to cells and picks up cell waste products.
  • RBC: One type of cells are the red blood cells (RBC) which contain a red pigment called haemoglobin. Haemoglobin bind with oxygen and transports it to all the parts of the body and ultimately to all the cells. It will be difficult to provide oxygen efficiently to all the cells of the body without haemoglobin. The presence of haemoglobin makes blood appear red. The red blood cells are synthesised in the bone marrows of ribs, sternum and vertebrae at the rate of 1.2 million cells per second. The life span of the cells is only about 120 days. They are destroyed in the liver. The iron part is retained and the pigment is excreted in the bile juice as bilirubin.
  • WBC: The blood also has white blood cells (WBC) which fight against germs that may enter our body. They are also called leucocytes. They lack haemoglobin and are therefore colourless. They are nucleated and amoeboid. The WBCs are involved in the production of antibodies that either neutralise, kill or poison the germs. The WBCs can be induced to produce antibodies with the help of vaccinations thus preparing the body for an attack.
  • Platelets: The clot is formed because of the presence of another type of cells in the blood, called platelets. They are also called the thrombocytes. They number 250,000 to 400,000 per cubic mm of blood. Their life span is 8 to 14 days.
  • Blood clotting: It is internal mechanisms of animals to prevent blood loss at the time of injury. During the blood clotting, the blood platelets produce an enzyme called thromokinase which forms prothrombin protein in the plasma. It combines with calcium ion to form thrombin. It convert the soluble plasma protein(fibrinogen) to form fibrin threads. corpuscles get entangled in these threads and forms clot.
  • Blood Groups: There are four types of blood among the humans. The differences in human blood are due to the presence or absence of certain protein molecules called antigens and antibodies. The antigens are located on the red blood cells and the antibodies are in the blood plasma. based on this, four blood groups are identified. They are denoted by the letter A,B, AB, O (null). The blood that has antigen A belongs to group A, the blood that has antigen B belongs to group B, the blood that has both A and B belongs to group AB and the blood which has neither belongs to group O. The presence of these antigens is determined genetically. Thus, there are four types of blood groups A, B, AB and O.
  • The blood also contains antibodies which act against these antigens. The antibodies react to the presence of proteins in the foreign bodies. There are two types of antibodies, corresponding to the types of antigens. The two antibodies are : (a) antibody a and (b) antibody b. Anti-body a reacts to the antigen A and antibody b reacts to the antigen b. Thus, the blood of A group that contains antigen A will contain antibody b and not antibody a. The blood of B group that contains antigen B will contain antibody a and not antibody b. The blood of AB group that contains antigens A and B will not contain either antibody a or antibody b. The blood of O group that does not contain either antigen A or antigen B will contain both antibody a and antibody b.
  • Blood transfusion: Transfusion is the replacement of lost blood of a person with the blood of another person. During transfusion, the person receiving blood is called recipient and the person donating blood is called donor. The general rule for blood transfusion is the donors red cells must be compatible with recipients plasma. Anti A plasma agglutinates A red cells, and anti B plasma agglutinates B red cells. People with O blood group can give blood to any group because they do not contain A and B anti bodies. So they are called universal donors. People with AB blood group are called universal recipients because they can accept blood from any group. So they are known as universal receipients.
  • Rhesus Factor: About 85% of people also have a so called Rh factor on the red blood cells surface. This is also an antigen and those who have it are called Rh+. Those who have not are called Rh-. Rh-ve blood can be safely given to Rh+ve person. However, the transfusion of Rh+ve blood to Rh-ve blood needs to be monitored. During the first transfusion, the Rh factor in the Rh+ve blood will induce the production of anti-Rh antibodies in the Rh-ve blood.
  • Lymph: It is a yellowish fluid carried in the lymphatic system. Lymph is like blood but it contains white blood cells and red blood cells are absent. It fights against germs.
  • In human circulatory system the blood is flows in closed blood vessels called the arteries, veins and their capillaries. Arteries and veins have elastic and muscular walls. Capillaries lack muscles.
    • Arteries: carry oxygen-rich blood from the heart to all parts of the body. Since the blood flow is rapid and at a high pressure, the arteries have thick elastic walls. The two main arteries leaving the heart are: Aorta- The branches of which supply blood to different parts of the body and Pulmonary Artery- The artery that takes blood to the lungs.
    • Veins: are the vessels which carry carbon dioxide-rich blood from all parts of the body back to the heart. The veins have thin walls. There are valves present in veins which allow blood to flow only towards the heart.
  • Every beat of the heart involves a sequence of events called the cardiac cycle. This consists of three major stages: the atrial systole, the ventricular systole, and the complete cardiac diastole. The atrial systole consists of the contraction of the atria and the corresponding influx of blood in to the ventricles. Once the blood has fully left the atria, the atrioventricular valves, which are situated between the atria and ventricular chambers, close. This prevents any backflow into the atria. It is the sound of the valves closing which produces the familiar beating sounds of the heart. The ventricular systole consists of the contraction of the ventricles and flow of blood into the circulatory system. Again, once all the blood has left, the pulmonary and aortic semilunar valves close. Finally complete cardiac diastole involves the relaxation of the atria and ventricles in preparation for new blood to enter the heart.
  • Heart: The heart is an organ which beats continuously to act as a pump for the transport of blood, which carries other substances with it. Human heart is four chambered. There are two receiving chambers, the auricles and two pumping chambers, the ventricles. Oxygen-poor blood enters the right atrium of the heart (via veins called the inferior vena cava and the superior vena cava). The blood is then pumped into the right ventricle and then through the pulmonary artery to the lungs, where the blood is enriched with oxygen (and loses carbon dioxide). The oxygen-rich (oxygenated) blood is then carried back to the left atrium of the heart via the pulmonary vein. The blood is then pumped to the left ventricle, then the blood is pumped through the aorta and to the rest of the body. This cycle is then repeated. Every day, the heart pumps about 7,600 liters of blood, beating about 100,000 times.
  • Heart contains valves which prevents mixing of blood in the four chambers. The wall of the heart is very muscular and does not tire. These muscles contract and relax rhythmically. This rhythmic contraction followed by its relaxation constitute a heartbeat.
  • Blood Pressure: The force that blood exerts against the wall of a vessel is called blood pressure. This pressure is much greater in arteries than in veins. The pressure of blood inside the artery during ventricular systole (contraction) is called systolic pressure and pressure in artery during ventricular diastole (relaxation) is called diastolic pressure. The normal systolic pressure is about 120 mm of Hg and diastolic pressure is 80 mm of Hg. Blood pressure is measured with an instrument called sphygmomanometer. High blood pressure is also called hypertension and is caused by the constriction of arterioles, which results in increased resistance to blood flow. It can lead to the rupture of an artery and internal bleeding.
  • Heart Attack: Heart attack is clinically Also called coronary thrombosis. The blood vessel supplying blood to the heart is blocked. This affects/stops the functioning of the heart resulting in heart attacks. If the blocked artery is one of the arteries supplying to the brain, it causes a condition called stroke. Stroke is the inactivation of a certain region of the brain which controls a particular activity. It may be sight, limb movements, etc.

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