The force of the strength of the jerk with which blood flows in blood vessels is called blood pressure .The source of this pressure is strong rythemic contraction of the heart . This force is sufficient for moving the blood from heart to organs through arteries and then from organs to heart through veins . The blood pressure in arteries is more than that in veins .The blood pressure is of two types ;systolic blood pressure and diastolic blood pressure .When heart contracts ,the artery has high blood pressure .This blood pressure is called systolic blood pressure But whe heart relaxes , the artery has reduced or less blood pressure called diastolic blood pressure .The blood pressure is measured by an instrument called sphygmomanometer .It depends upon the upon mental and physical conditions and age .Among young men the systolic blood pressure ranges from 100-140 mm Hg and the diastolic blod pressure fromm 55-80 mm Hg .The systolic pressure is is 90 plus age + 10 .Sometimes ,the blood pressure becomes high or low to the normal level .The high blood pressure develops due to unbalanced diet ,or heavy work ,or mental tension [anxiety ,worry an emotion ] or in old age because the diameter of arteries is reduced .Therefore, heart has to use more force for moving blood to organs .This may lead to heart failure . The low blood pressure develops due to fatigue and reduction in blood volume .There fore heart propels blood in arteries with less force. Consequently , body organs donot get nutrients and other substances in adequate quantities .Therfore , body becomes weak and this may lead to heart failure . Control of Body Heat By Blood All the invertebrates and lower vertebrates animals like fish , amphibia and reptiles are poikilothermic,that is , thier body temperature varies with that of thei surroundings .The temperature of such animal is usually a few degrees above that of its environment , but a rise or fall in the temperature of the air or water in which it lives will produce acorresponding change in the animal's body temperature .These animals produce heat but thy have very little capacity for conserving it or for adjusting supply and demand . Since the speed of most chemical changes that takes place in living organisms increased by a rise in temperature , it follow that the rate of activity of a poikilothermic animal will depend to a large extent on the surrounding temperature ; in cold conditions their low body temperature slows down all chemical changes and reduces the organism to a state of inactivity . Insects can be immobilized by a sudden fall of temperature . The quality of homiothermic condition is restricted to birds and mammals and means the ability ok keeping the temperature of the body constant, day and night , year in and year out Take your own example .A certain amount of heat is always produced by oxidation .Human muscles are about 40 % efficient as mechanical machines .This means that about 40% of energy produced by a muscle is available for machanical work and the other 60% appears as heat .the heat produced in cotracting muscle cells warms the blood in the surrounding capilaries .The warm warm blood transfers the heat to cooler tissues through which it passes .This is much like the distribution of heat by a hot water system Birds and mammals produce considerable amount of heat which is well distributed by the rapid circulating blood .This ,plus the mechanism for excreting excess heat, makes it possible for them to maintain a constant temperature of the body.
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