Blood: Composition and Circulation

In this article we will discuss about Blood:- 1. Blood Composition 2. Blood Circulation 3. Factors of Infection.

Blood Composition:

Plasma:

The fluid portion of the fresh un-coagulated blood is known as plasma which makes up 55 per cent of whole blood. It is yellowish and transparent. It contains proteins, food for the body tissues and waste products from the tissues.

It consists of 91 percent water, 1 per cent minerals (salts of Na, K, Ca, Mg, P), 1 percent various substances uric acid, urea, amino acids, glucose, carbon dioxide cells, white cells and platelets which constitute remaining 45 per cent.

Plasma contains fibrinogen which is involved in the clotting of the blood, which occurs when the soluble fibrinogen is converted into insoluble fibrin. Other proteins (albumins and various globulins) maintain the water balance of the body.

Gamma globulin is of the globulin fraction or component. Blood is a rich source of antibodies. These antibodies can be used for treatment and prophylaxis of certain diseases and play an important role in the antibody mediated or humoral immunity.

Red Blood Corpuscles:

Human erythrocytes (Greek, erythros, red; kyotes, cell) are non- nucleated, biconcave disks. They have a life span of 130 days and are constantly replaced. In adult male, their number is about 5 millions per cubic millimeter and in women, it is about 4.5 million per cubic millimeter. Red blood cells are manufactured in the bone marrow and released into the plasma of the blood.

Haemoglobin (respiratory pigment) forms a loose chemical bond with oxygen when exposed to air in the lung, carries oxygen to the tissues of the body and releases it to them. Haemoglobin is released into the surrounding when the red blood cells rupture. The process is known as hemolysis (Greek, haima, blood, lysis to disrupt or dissolve). In vivo and in vitro, red blood cells can be lysed by certain bacterial toxins.

White Blood Corpuscles:

They are also known as leukocytes (Gr. leukos, white). They are larger and less in number than the red blood cells. They are about 7,000 per cubic millimeter of the blood. They act as scavengers or policemen in the blood.

They engulf any foreign particles (dead tissues or bacteria) entering the blood stream, they ingest these particles directly, they move with the help of pseudopodia and enter between the tissue cells of the body.

If the bacteria enter into the leukocytes through their thin cell membrane, they may be killed and digested by digestive enzymes of the leukocytes. This process is known as phagocytosis. Leukocytes can move out of the blood vessels by their amoeboid movement between the cells lining the blood vessels.

Leukocytes can be attracted by the chemo-taxis to the area of the inflammations caused by foreign particles (bacteria). In the infected spot or boil the white creamy material consists of dead leukocytes (pus cells), dead bacteria, tissue debris, fluid from blood and tissues and is known as “Pus“.

Platelets:

They are irregular and disk shaped particles of varying sizes and smaller than erythrocytes. They are about 300,000 per cubic millimeter of blood and look like fragments of cells. Their origin is yet unclear and they are also responsible for coagulation of blood. Blood with low platelets may not clot quickly and may cause surgical hazard.

Plasma:

1. Yellowish fluid portion of whole blood;

2. It contains fibrinogen, in blood clotting, soluble fibrinogen is converted into insoluble fibrin, the jelly-like substance of the clot.

3. It contains antibodies (antibacterial substances and bacterial toxins);

4. In microbiology (serology), it is used to diagnose some diseases as it may contain antibodies against some infections;

5. It plays a great role in humoral immunity.

Serum:

1. Straw coloured fluid remaining after clotting of blood;

2. It contains no fibrinogen;

3. It contains antibodies and bacterial toxins;

4. It is used in the diagnosis of diseases as it contains antibodies against the diseases;

5.It plays an important role in humoral immunity.

Blood Circulation:

Left ventricle of the heart pushes the blood continuously through the arterial system to the remotest branches-microscopic arterioles, venules or capillary vessels. The wall of the capillary vessels are of one cell thickness.

Nutrients and other substances (hormones, oxygen etc.) filter through these thin membranous walls, and diffuse into the fluid (lymph) which is constantly bathing the tissues cells. The nutrients are brought from the gastrointestinal tract and liver by the blood.

Waste metabolic products (acids, carbon dioxide etc.) diffuse from the tissue cells into the lymph; then from the lymph into the blood stream of capillary venule and venous system which is in continuity with arterioles.

These waste products in the venous (impure) blood are carried to the right auricle of the heart and are pumped to the right ventricle through the lungs when the red blood cells and plasma take up oxygen from the air and give off carbon dioxide.

This purified blood returns to the left auricle and then to the left ventricle which pushes again the purified blood to the tissues. In this way, the blood circulates in the body for the benefits of the body (nutrition, metabolism, growth, protection against diseases etc.).

Blood and Diseases:

When blood passes through the tissues infected with toxin producing pathogenic microorganisms (e.g. diphtheria, tetanus) carries with it toxin (toxaemia) and if bacteria ( Salmonella) enter the blood stream (bacteriaemia).

The toxin and bacteria are carried along with the blood and are distributed to various parts of the body where they set-up many infections e.g. brain abscess, osteomyelitis (inflammation of bone), otitis media (middle ear infection).

Blood borne diseases (malaria, filariasis, yellow fever etc.) can be transmitted by arthropods (mosquitoes) from person to person. The blood carries drugs and antibiotics injected or swallowed to abate the infection.

Blood and Immunity:

Cell and antibody mediated immunities against the infection are performed through the circulating blood, leukocytes in the circulating blood are responsible for cellular immunity, whereas the antibodies for humoral immunity. Antibodies (Gamma globulins) are soluble protein molecules secreted into the blood by lymphoid cells which are stimulated by the foreign proteins (Refer to chapter on immunology).

Infection:

The term infection is used to designate an entry and the presence of organism, whereas the disease to designate the response of the host to the infection. Host-parasite relationship is a part of science ecology where the organisms live together and help mutually.

A host-parasite relationship develops during the evolution of many species of living organisms to thrive with each other. Symbiosis or mutualism is a host parasite relationship; when the organisms are mutually antagonistic they are antibiosis. Parasitism or predation, when some organisms live at the expense of the others. The infectious disease is an example of parasitism relationship.

Commensalism is also an example of host relationship where the organisms thrive side by side peacefully after some struggle for their existence. During the process of infection, immunity and allergy, many host-parasite relationship and adaptations are established.

Infection is a condition where the microorganisms enter the human body tissues, grow in or on them, may damage them and the host may react to this damage by manifesting symptoms like fever, chills, nausea and headache. Infection may be fatal or serious. A pimple is a simple reaction to the infection of the skin; whereas Typhoid is serious if untreated by antibiotics.

Factors of Infection:

(a) Dose of infecting microorganisms;

(b) Virulence of microorganisms:

(i) Aggressiveness, and

(ii) Toxigenicity.

(a) Dose of infecting microorganisms:

There is possibility of infection if the number of microorganism entering the body is very large; a million organisms may set up the infection, if the tissue resistance is lowered by fatigue, disease or alcoholism, or after entry into the body, by overcoming the host defence mechanism, whereas a single bacterium (coccus or bacillus) may not be able to establish in the healthy body.

(b) Virulence of organisms is the ability of organisms to enter, establish, infect, damage the tissues and produce the disease.

The virulence depends upon two factors:

(i) Aggressiveness and

(ii) Toxigenicity.

(i) Aggressiveness: Skin inhabiting bacteria are not very aggressive as they are unable to penetrate the skin barrier of the body. If they get entry into the skin, they will become aggressive in the favourable environment for their growth. Heavily capsulated organisms (Pneumococci, Anthrax bacilli, Klebsiella) are highly aggressive and dangerous as they cannot be engulfed by leukocytes.

(ii) Toxigenicity is the ability of organisms to elaborate the toxin. Bacteria producing toxin may not be aggressive (i.e. invasive), but they damage the body tissues by their toxin; for example, toxigenic strains of diphtheria bacilli lodging on the tonsils or in the throat will produce the toxin which is absorbed in the blood (toxaemia), carried to various parts of the body, and, ultimately, can cause death.

Tetanus bacilli are also toxigenic as diphtheria bacilli.

Toxins:

Toxigenic strains of organisms produce two types of toxins which are proteins in nature. Toxin may damage the tissues of the body after combining with nerves, heart muscle, liver.

1. Endotoxins (Greek, endon — inside):

They are found in the cytoplasm enclosed by the cytoplasmic membrane. These intracellular toxins, known as endotoxins, are released after the death and lysis of bacterial cells (e.g. Gram-negative bacteria, gonococci; typhoid bacilli) produce endotoxin.

2. Exotoxin (Greek, exo — outside):

Toxin producing bacteria liberate toxins through their cytoplasmic membrane. These are known as exotoxins. Corynebacterium diphtheriae, Clostridium tetani produce exotoxins. Few bacteria produce both endotoxin and exotoxin.

Specificity of toxin:

Toxins of organisms are specified in their action. Tetanus toxin has an affinity for nerve cells, causing spasm of jaw muscles, i.e., lock jaw. Diphtheria toxin has affinity for heart muscles, death in the severe infections is usually due to acute myocarditis and heart failure. This toxin may lead to such sequelae as post-diphtheric paralysis.

In staphylococcal food poisoning, enterotoxin of Staphylococcus aureus produces vomiting and diarrhoea. This enterotoxin is also known as preformed toxin, as this toxin is preformed in the food contaminated by Staphylococcus aureus and consumed later.

Application to Nursing:

Professionals should know that plasma and serum contain antibodies (Gamma globulins) which play a major role by protecting from diseases. She should also be aware of the fact that the patient with low platelet count may be affected by surgical hazards, as his blood may not clot immediately.

A knowledge of blood clotting mechanisms is necessary, the soluble fibrinogen is changed into insoluble firbrin which causes clotting. Antibiotics and drugs injected or swallowed are carried by the blood to combat the infection. A sound knowledge of toxin is essential, as the toxins are responsible for many diseases.