Blood pressure, structure, functions & Mechanism of blood clotting
Blood is the principal medium in the transport process , Its colour is red viscous liquid , PH equals 7.4 ( weak alkaline ) , Volume is 5 – 6 liters of blood on average , The structure of blood : connective tissue consists of Plasma , Red blood cells , White blood cells and Blood platelets .
Plasma is the tissue fluid of blood , It represents 54 % of the blood volume , It contains 90 % water , 1 % inorganic salts such as Ca++, Na+, Cl− and ( HCO3 )− , 7% proteins as albumin , globulin and fibrinogen , 2 % other components as the absorbed food ( sugars and amino acids ) , hormones , enzymes , antibodies and wastes ( urea ) .
RBCs – Red blood cells ( Erythrocytes )
They are the most abundant blood cells , they are nearly about 4 : 5 million cell/mm³ in males , 4 : 4.5 million cell/mm³ in females , Their shape is round corpuscles and biconcave , They produced in the bone marrow of backbone .
Each cell is destroyed after 120 days , They circulate about 172,000 circulations , They are destroyed in the liver , spleen and bone marrow , They are enucleated cells contain haemoglobin ( protein + iron ) which gives the blood its red colour .
- Transporting oxygen from the two lungs to all the body parts , where in the two lungs , the haemoglobin combines with oxygen to form a pale red oxyhaemoglobin , The oxyhaemoglobin carries the oxygen to the different parts of body , where it leaves the oxygen and changes into haemoglobin .
- Transporting CO2 from all the body parts to the two lungs , where the haemoglobin combines with CO2 inside the body cells to form a dark red carbo-aminohaemoglobin , Carbo-aminohaemoglobin carries CO2 to the two lungs , where it leaves CO2 and changes into haemoglobin .
WBCs – White blood cells ( Leucocytes )
The number of white blood cells is 7000 cell/mm³ , this number increases during diseases , Their shape is colourless and nucleated with many shapes , They are formed in the bone marrow , spleen and lymphatic system , some of their kinds live for 13 – 20 days .
Functions : They are produced in many types , each type with a specific function , but the main function is the protection of body against the infectious diseases through the following :
They circulate continuously in the blood vessels , attack the foreign particles , destroy and engulf them , Some of them produce antibodies .
The number of blood platelets is 250 , 000 platelet /mm³ , Their shape is non-cellular and very small in size , Their size is one fourth of the RBCs , They are produced from the bone marrow , They live for about 10 days , They play a role in blood clot after the injury .
Factors of coagulation ( clotting ) of blood :
Mechanism of blood clotting
In case of the presence of blood clotting factors , the steps are shown as the following :
The blood platelets together with the destroyed cells from a protein called thromboplastin .
Platelets + Destroyed cells → Thromboplastin in presence of ( clotting factors in blood )
In the presence of calcium ions ( Ca++ ) and blood clotting factors in the plasma , thromboplastin activates the conversion of prothrombin into thrombin .
Prothrombin → Thrombin ( Active enzyme ) , in the presence of Thromboplastin , Ca++ , clotting factors
Where Prothrombin is a protein that is secreted in the liver with the help of vitamin K and is passed directly into the blood .
Thrombin catalyzes the conversion of fibrinogen ( soluble protein in plasma ) into fibrin ( insoluble protein )
Fibrinogen ( Soluble protein ) → Fibrin ( Insoluble protein ) in the presence of Thrombin
Fibrin precipitates as a network of microscopic interlacing fibers where the blood cells are aggregated , forming a clot which blocks the cut in the damaged blood vessels .
Reasons of the non-clotting of blood inside the blood vessels
- Blood runs in a normal fashion inside the blood vessels without slowing down .
- Platelets also slide easily and smoothly inside the blood vessels in order not to be broken .
- The presence of heparin ( it is secreted by the liver ) which prevents the conversion of prothrombin into active thrombin .
Functions of blood
Transportation : It transports the digested food substances , waste nitrogenous compounds , hormones and some enzymes ( active or inactive ) through the plasma , It transports O2 and CO2 through RBCs .
Controlling : It controls the processes of metabolism , It keeps the body temperature at 37° C , It regulates the internal environment ( homeostasis ) such as osmotic potential , amount of water and pH in the tissues .
Protection : It protects the body against the microbes and pathogenic organisms through the immunity involving the lymphatic system ( WBCs ) , It protects the blood itself against the bleeding by the formation of blood clot .
The blood is a viscous liquid which circulates within the arteries and veins smoothly by the process of heartbeats , but to pass within the microscopic blood capillaries , it needs pressure .
The maximum blood pressure is measured as the ventricles contract and the largest blood pressure is measured in the arteries nearer to the heart .
The minimum blood pressure is measured as the ventricles relax and the blood pressure in the venules is very low ( about 10 mm Hg ) , This pressure is not sufficient to move the blood back to the heart , So , the returning of blood to the heart depends on :
- The skeletal muscles near the veins : when these muscles contract , they put a pressure on the collapsible walls of veins and the blood contained in these vessels .
- Valves of veins : that prevent the backward flow of blood .
Measurement of blood pressure
The blood pressure is measured by means of mercuric instruments sphygmomanometers , There readings consist of two numbers :
- Maximum : measured during the ventricular contraction which represents the maximum blood pressure .
- Minimum : measured during the ventricular relaxation which represents the minimum blood pressure .
Example : 120/80 mm Hg is the normal value of blood pressure in youth , so , the number of 120 mm Hg represents the ventricular contraction ( cystolic ) and 80 mm Hg represents the ventricular relaxation ( diastolic ) .
Structure : a mercuric tube and a scale board .
Idea of working : blood pressure can be measured according to the elevation of mercury level inside the tube and it is represented by a number on the scale board .
Method of measurement
The values of blood pressure are determined by listening to the heartbeats , and also between one beat and another , as the following :
on hearing the sound of heartbeat , the doctor can determine the maximum value of blood pressure , referring to the ventricles contraction ( cystolic ) .
When the sound disappears , the doctor can determine the minimum value of blood pressure , referring to the ventricles relaxation ( diastolic ) .
The blood pressure increases gradually by aging and it must be under medical control to avoid its harmful effects , There are some digital instruments to measure the blood pressure , but they are not accurate as the mercuric instruments .
The blood pressure increases in arteries gradually by aging , leading to the increase of resistance against the passage of blood through them .