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Essay on Blood
Essay Contents:
- Essay on the Definition of Blood
- Essay on the Genesis of Blood
- Essay on the Functions of Blood
- Essay on the Composition of Blood
- Essay on the Reaction of Blood and Its Determination
- Essay on the Buffer Systems Operating in Blood
- Essay on the Specific Gravity and Viscosity of Blood
Essay # 1. Definition of Blood:
Blood is essential for human life. The blood volume of normal adults in a developing country is approximately 4 liters. Blood may be described as a specialized connective tissue in which there is liquid intercellular substance known as plasma and formed elements, the red blood cells, the white blood cells and the platelets suspended in the plasma.
The specific gravity of whole blood varies from 1.055 to 1.060. When freshly shed, blood is red, thick, opaque and slightly alkaline. In order to understand the full significance of blood, it is imperative that one should know something about its genesis.
Blood is a highly complex fluid which is composed of two parts—a liquid, called the plasma and different types of cells which remain suspended in the plasma. The cells are called the blood corpuscles. The plasma constitutes about 55 percent, and the cells about 45 percent, of the total volume of human blood.
Essay # 2. Genesis of Blood:
Life evolved first in the sea water. The primitive unicellular animal took its oxygen and nutrition from the ocean and excreted its waste products into it. But with the development of the multicellular forms, this simple arrangement of drainage and supply could no longer hold. The cells in the deeper parts of the body could not come into free contact with surrounding sea water, and would therefore suffer.
To overcome this difficulty, a system of intercommunicating channels developed, pervading the whole animal body and opening upon the exterior. Through these channels the sea water could freely flow in and out, and in this way, the deeper cells could satisfy their needs. This channel system represents the primitive circulatory system and, it is obvious that, in its original form, this system was an open one.
But this open circulatory system had many disadvantages and consequently, with the birth of the higher forms, the open system became a closed one. This change represents a very important landmark in the history of animal evolution. The sea water, which used to remain outside the body, became entrapped within the body.
In the course of evolution, that enclosed marine water has undergone profound modifications and has been transformed into what we call now blood. Although blood has departed a long way from its primitive ancestor, yet in its inorganic composition, it still maintains a close resemblance with sea water. This, in short, is the history of evolution of blood. Not only blood, but other body fluids also, should be regarded as the modifications of the same primitive marine environment.
From the above discussion, it will be evident that, in the multicellular animals, blood serves those purposes, which, for the unicellular forms, are carried out by the surrounding aquatic medium. But in addition to those, blood has to serve many other important functions, because it has to meet the complex requirements of the more specialized cells, in the higher forms.
Essay # 3. Functions of Blood:
1.Transport of Respiratory Gases:
It carries oxygen from the lungs to the tissues and CO2 from the tissues to the lungs.
2. Transport of Nutrition:
It carries digested food material absorbed from the intestine to the tissue cells for utilisation. It also carries nutritive material from one place of the body to the other. For instance, from the storage depots to the tissue cells.
3. It Acts as a Vehicle:
It acts as a vehicle through which the hormones, the vitamins and other essential chemicals are brought to their places of activity.
4. Drainage of Waste Products:
It carries the waste products of cellular activity and brings them to the organ of excretion, viz., kidney, lungs intestine, etc.
5. Maintenance of Water Balance:
Maintenance of water balance: (Vide’ Water Balance’).
6. Maintenance of Acid-Base Equilibrium:
By its efficient buffering power (e.g., plasma proteins, reduced and oxidized haemoglobin, etc.) and with the help of kidney, skin and lungs it helps to maintain a constant reaction of the body.
7. Maintenance of Ion Balance:
Maintenance of ion balance between the cells and the surrounding fluid.
8. Regulation of Body Temperature:
The water content of blood possesses three qualities which make it very suitable for this purpose:
(a) Due to its high specific heat it can absorb a large amount of heat and there by prevent sudden change of body temperature,
(b) High conductivity—the thermal conductivity of water is higher than that of any other ordinary liquid. This helps quick distribution of heat,
(c) High latent heat of evaporation -latent heat of evaporation of water is very high and since water is constantly evaporating from skin and lungs a large amount of heat is lost in this way.
9. Defensive Action:
Blood acts as a great defensive mechanism in two ways:
(a) The white cells due to their phagocytic properties engulf bacteria and foreign particles;
(b) It develops antibodies which combat toxic agents.
10. By the property of coagulation it guards against haemorrhage.
11. The Plasma Proteins of Blood:
The plasma proteins of blood have various functions (vide functions of plasma proteins).
12. Regulation of Blood Pressure:
Regulation of blood pressure, by changes in volume and viscosity (haematocrit value) of blood.
Essay # 4. Composition of Blood:
The general composition of the whole blood is as follows:
1. Whole Blood Cells:
(a) Red blood corpuscles or erythrocytes (R.B.C.).
(b) White blood corpuscles or leucocytes (W.B.C.).
(c) Platelets or thrombocytes.
2. Plasma:
(a) Water, 91 to 92 percent, and
(b) Solids, 8 to 9 percent.
i. Inorganic Constituents:
0.9% sodium, potassium, calcium, magnesium, phosphorus, iron, copper, etc.
ii. Organic Constituents:
a. Proteins:
7.5% serum albumin, serum globulin, fibrinogen, prothrombin, etc.
b. Non-Protein Nitrogenous Substances (NPN):
Urea, uric acid, xanthine, hypoxanthine, creatine, creatinine, ammonia, amino acids, etc.
c. Fats:
Neutral fat, phospholipid, cholesterol, cholesterides, etc.
d. Carbohydrate:
Glucose, etc.
e. Other Substances:
Internal secretions, antibodies and various enzymes (amylases, proteases, lipases, phosphatases, etc.).
f. Colouring Matter:
The yellow colour of plasma is due to small amounts of bilirubin, carotene and xanthophyllin.
Average Figures in Healthy Adult Indians:
Sugar, 85 mgm per 100 ml of whole blood; urea, 25 mgm%; NPN, 23.4 mgm%; uric acid, 4.0 mgm%; creatinine 1.5 mgm%; cholesterol 154 mgm%; chloride (as NaCI), 396 mgm per 100 ml of whole blood; calcium 10.2 mgm per 100 ml of serum; inorganic phosphorus, 3.6 mgm%; and lipid phosphorus, 9.7 mgm per 100 ml of whole blood.
Relative Volume of Corpuscles and Plasma:
In normal human blood (Fig. 4.1), plasma volume is proportionally more than total corpuscular volume. The plasma volume varies between 52% and 55% and the packed corpuscular volume varies from 45% to 48%. The normal packed cell volume for males is about 45% while that in case of females is a bit lower about 40% of whole blood. The ratio of red blood corpuscles to plasma is expressed as the haematocrit value.
This can be estimated by an instrument called Haematocrit. It consists of a specially prepared graduated capillary (Wintrobe’s) tube of uniform bore in which a specimen of blood, treated with an anticoagulant, is taken.
It is centrifuged at a speed of 3,000 revolutions per minute for 30 minutes, until plasma and corpuscles are completely separated, and the sedimented corpuscles show no further shrinkage in volume. From the graduations on the tube the proportion of plasma and corpuscles can be known.
Essay # 5. Reaction of Blood and Its Determination:
The pH of blood varies between 7.36 and 7.45, the average is about 7.4. The pH of venous blood during rest is lower by 0.02 than that of the arterial blood. The red cells are less alkaline than plasma with a pH of about 7.1. As ordinarily used, the term blood pH really means the plasma pH. The pH of blood is maintained at a constant level under physiological conditions.
Even under extreme conditions of acidosis and alkalosis, very little alteration of pH is seen. The limits of pH compatible with life are usually, not higher than 7.8 and not lower than 6.8. Except in the terminal stages, blood reaction does not go beyond this range. The mechanism by which blood reaction is kept constant has been discussed elsewhere.
Methods of Determination of Blood Reaction:
i. Colorimetric Method (Dale and Evans):
A collodion sac holding about 2-3 ml of oxalated blood is made to dialyse for 15 minutes into a solution of 1 ml of normal saline. Contact with air is prevented to stop CO2 loss. A few drops of an indicator (0.02% of neutral red) are added to the clear dialysate. The colour developed is matched against that of a phosphate solution of known pH and containing neutral red in the same proportion as the dialysate.
ii. Electrometric Methods:
(a) By Means of Hydrogen Electrode:
A hydrogen electrode is composed of platinum black saturated with hydrogen gas. One of a pair of hydrogen electrodes is placed in a solution of known pH and the other in the unknown solution. When the two electrodes are connected, a potential difference is setup between the two electrodes and from its magnitude the pH of the unknown solution can be calculated,
(b) By Means of Glass Electrode:
Instead of hydrogen electrode glass electrode can be used in the same way. The glass electrode has the advantage that it is not affected by oxygen, protein content and other substances in the test material.
Essay # 6. Buffer Systems Operating in Blood:
There are 3 important buffer systems operating in blood to maintain the pH of blood around 7.4. They are:
1. (HCO–3)/(H2CO3) — Bicarbonate buffer pair
2. (HCO–4)/(H2PO4) Phosphate buffer pair
3. Protein buffer (includes hemoglobin and plasma proteins)
While discussing the regulation of pH, it is necessary to understand Henderson-Hasselbalch equation, which states that
pH = pK + log Base/Acid
The buffering capacity of a buffer substance depends on its:
1. pK value
2. Quantity of the buffer substance.
Considering the above two, Table 1.10 depicts the details of all the three buffer systems of blood.
Since large amount of proteins are available in blood and the pK value of protein buffer system is nearer to the pH of blood, and as blood buffer it is the most efficient one.
However, as for the entire body is concerned, it is the HCO3 buffer which is more important, because the concentration of buffer pair HCO3/H2CO3 is more easily maintained by the respiratory system (H2CO3) and by the kidney (HCO3). As long as the proportion between the buffer pair is maintained, the pH is also maintained.
For the bicarbonate buffer system, the normal ratio will be:
HCO–3/H2CO3 = (24 mEq/L)/(1.2 mEq/L)
And finally the ratio will be 20:1
So the pH = 6.1 + log 20/1 = 6.1 + 1.3 = 7.4
Reactions involved with the bicarbonate buffer system:
When an H+ or OH is added, NaHCO3 + HCl → H2CO3 + NaCl
Phosphate buffer system:
When an acid is added
NaHPO4 + HCl → NaH2PO4 + NaCl
Strong acid HCl is converted to weak acid NaH2PO4.
When an alkali is added
NaH2PO4 + NaOH → Na2HPO4 + H2O
A strong alkali is converted to a weak base.
Phosphate buffers play an important role in the renal tubules. They buffer the H+ secreted into the tubular fluid and maintain the tubular fluid pH above 4.2.
Isohydric principle:
Whenever the body fluid pH changes, all the buffers react simultaneously to rectify the pH and this is known as isohydric principle.
Essay # 7. Specific Gravity and Viscosity of Blood:
Specific Gravity of Blood:
Specific gravity of venous blood at 15° C (59° F.) as determined by the psychometric method is given 1.048 to 1.066 with averages of 1.052 to 1.063. The clinical average is 1.057 in males and 1.053 in females. There is a normal diurnal variation of about 0.003, it being generally lower in the afternoon and after meals and higher after exercise and during the night.
The specific gravity of blood serum varies from 1.026 to 1.031 and that of erythrocytes is from 1.092 to 1.095. The specific gravity of the foetal blood in full term is highest and that of the mother at the same time is lowest (about 1.050). The specific gravity rapidly falls after birth due to the destruction of the red cells.
1. Rise in Specific Gravity:
Specific gravity rises in the following conditions:
i. When water is lost from the body, such as in excessive sweating, diarrhoea, cholera, etc.
ii. By exudation of fluid into tissues or serous cavities due to inflammation or surgical operation.
iii. When water intake is inadequate.
2. Fall in Specific Gravity:
i. When large quantity of water is taken.
ii. In severe haemorrhage after which fluid is drawn in from the tissue spaces and the blood is diluted.
iii. Injection of saline into the veins causing dilution of blood.
Method of Determination:
The principle adopted, is to add drops of blood to a series of copper sulphate (CuSO4) solutions of known specific gravity and to note in which particular solution the blood drop neither floats nor sinks. The specific gravity of that particular solution indicates the specific gravity of the blood sample. A series of mixtures of benzene and chloroform and mixtures of glycerine and distilled water of known specific gravity are also used.
Viscosity of Blood:
Viscosity of blood is an important factor since; it determines the peripheral resistance of the blood flow through the blood vessels and thus helps to maintain blood pressure. Human blood is five times more viscous than distilled water. The viscosity of the whole blood is mainly due to cells and that of plasma is due to plasma proteins.
The viscosity of plasma is less than that of whole blood. The relative viscosities of water, plasma and whole blood are roughly 1, 1.8 and 4.7 respectively. Viscosity is measured by noting the time of flow of a given volume of blood through a specially prepared tube (Viscosimeter) and comparing the time taken by the same volume of distilled water to pass through the same tube.
Viscosity is affected by various factors. It depends on the amount of plasma proteins, the number and volume of corpuscles and CO2 tension. Viscosity rises in acidosis, hyperglycaemia, hypercalcaemia, polycythemia, cyanosis, icterus, diabetes mellitus, etc., and is reduced in anaemia, fever, exercise, edema, lymphatic leukaemia and malaria. Rise of temperature reduces the viscosity of blood.