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 .


  1. 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 .
  2. 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 .

Blood platelets  

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 .

Blood clot

Blood clot occurs when a blood vessel is cut , The importance of clotting : Blood forms a clot to prevent the bleeding before it leads to shock followed by death .

Factors of coagulation ( clotting ) of blood :

  1. Exposure of blood to air .
  2. Friction of blood with rough surfaces as destroyed cells & tissues .

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 cellsThromboplastin            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 
  1. Blood runs in a normal fashion inside the blood vessels without slowing down .
  2. Platelets also slide easily and smoothly inside the blood vessels in order not to be broken .
  3. 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 .

Blood Pressure  

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 :

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 .

Human Transport system , Structure of human circulatory system ( Heart , blood vessels and blood )

Lymphatic system , Blood circulation ( Pulmonary circulation , Systemic circulation & Hepatic portal circulation )

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