THE BLOOD

Blood is classified as a connective tissue, since nearly half of it is made up of cells. However, it differs from other connective tissues in that its cells are not fixed in position, instead they move freely in the liquid portion of the blood, the plasma.

Blood is a viscous (thick) fluid that varies in colour from bright to dark red, depending on how much oxygen it is carrying. Its quantity differs with the size of the person; the average adult male, weighing 70 kg has about 5-6 litres of blood.

This volume accounts for about 8% of the total body weight. It is carried through a closed system of vessels pumped by the heart. The circulating blood is of fundamental importance in maintaining the internal environment in a constant state (homeaostasis).

Characteristics of Blood

• Colour: bright red is well oxygenated eg that from the artery. Dark red when de-oxygenated eg that from the vein.
• Quantity. Approximately 7% of body weight.
•  Erythrocyte Sedimentation rate (ESR); Separation  of blood from plasma Rages from 4-10m/hr
• Reaction (PH) slightly alkaline 7.35-7.

Functions of the Blood

 Transportation

• Oxygen- from inhaled air diffuses into the blood through the thin lung membranes and is carried to all the tissue of the body. Carbon dioxide, a waste product of cell metabolism, is carried from the tissues to the lungs, where it is breathed out.
• The blood transports foods and other needed substances such as minerals and vitamins, to the cells. These materials may inter the blood from the digestive system or may be released into the blood from body stores.
• The blood transports waste products from the cells to the sites from which they are released. The kidney removes excess water, minerals, and urea from protein metabolism and maintains the acid-base balance of the blood. The liver removes bile pigments and drugs.
• The blood carries hormones from their sites of origin to the organs they affect.

 Regulation

• Buffers in the blood help keep the PH of body fluids at about 7.4
• The blood serves to regulate the amount of fluid in the tissues by means of substances (mainly proteins) that maintain the proper osmotic pressure
• The blood transports heat that is generated in the muscles to other parts of the body thus aiding in the regulation of body temperature by the blood, thus aiding in the regulation of body temperature.

 Protection

• The blood carries the cells that are among the body’s defenders against pathogens. It also contains substances (antibodies) that are concerned with immunity to disease.
• The blood contains factors that protect against blood loss.

Composition of Blood

The blood is composed of two prime elements: the liquid element is called plasma; the cells and fragments of cells are called formed elements or corpuscles

Plasma making 55% of blood, Cells suspended in plasma consists of 45% of blood.

 The formed elements are classified as follows:

• Erythrocytes, from erythro, meaning “red,” are the red blood cells, which transport oxygen.
• Leukocytes, from leuko, meaning “white,” are the several types of white blood cells, which protect against infection.
• Platelets, also called thrombocytes, are cell fragments that participate in blood clotting

BLOOD PLASMA

Over half of the total volume of blood is plasma. The plasma itself is 90% water. Many different substances dissolved or suspended in the water, make up the other 10%. The plasma content varies somewhat, since the substances carried by the blood to and from the organs get used and added to. However, the body tends to maintain a fairly constant level of these substances. For example, the level of glucose, a simple sugar, is maintained at a remarkably constant level of about a 10nth of a 1% solution.

After water, the next largest percentage of material in the plasma is protein. Proteins are the principal constituents of cytoplasm and are essential to the growth and the rebuilding of body tissues.

  Plasma proteins

These make up about 7% of plasma, are normally retained within blood, because they are too big to escape through the capillary pores into the tissues.

Functions

• They are largely responsible for creating the osmotic pressure of blood, which keeps plasma fluid within circulation.
• If plasma protein levels fall, because of either reduced production or loss from the blood vessels, osmotic pressure is also reduced, and fluid moves into the tissues (oedema) and body cavities.
• Plasma visconsity (thickens) is due to plasma proteins, mainly albumin and fibrinogen.
• Alblumins: These are the most abundant plasma proteins (about 60% of total). Their main function is to maintain normal plasma osmotic pressure
• They also act as carrier molecules for free falty acids, some drugs and stevoid hormones.
• Albumin, the most abundant protein in plasma, is important for maintaining the osmotic pressure of the blood. This protein is manufactured in the liver.
• The antibodies combat infection.
• The blood clotting factors are also manufactured in the liver.
• A system of enzymes made of several proteins, collectively known as complement, helps antibodies in their fight against pathogens.

Nutrients  also found in the plasma:

  carbohydrates. The principal form of carbohydrate found in the plasma is glucose, which is absorbed by the capillaries of the intestine following digestion. Glucose is stored mainly in the liver as glycogen and released as needed to supply energy.

Amino acids, the products of protein digestion, are also found in the plasma. These are also absorbed into the blood through the intestinal capillaries.

Lipids constitute a small percentage of blood plasma. Lipids include fats. They may be stored as fat for reserve energy or carried to the cells as a source of energy.

The mineral salts in the plasma appear primarily as chloride, carbonate, or phosphate salts of sodium, potassium, and magnesium. These salts have a variety of functions, including the formation of bone (calcium and phosphorus), the production of hormones by certain glands (iodine for the production of thyroid hormone), the transportation of the gases oxygen and carbon dioxide (iron), and the maintenance of the acid base balance (sodium and potassium carbonates and phosphates).

Small amounts of other elements also help maintain homeostasis: Many other materials, such as waste products and hormones, are also transported in the plasma.

THE FORMED ELEMENTS (cellular part of blood)

Erythrocytes

Erythrocytes, the red cells, are tiny, disk-shaped bodies with a central area that is thinner than the edges. They are different from other cells in that the mature form found in the circulating blood does not have a nucleus. These cells, like almost all the blood cells, live a much shorter time (120 days) than most other cells in the body, some of which last a lifetime. One purpose of the red cells is to carry oxygen from the lungs to the tissues. The oxygen is bound in the red cells to haemoglobin, a protein that contains iron.

Haemoglobin combined with oxygen gives the blood its characteristic red color. The more oxygen carried by the haemoglobin, the brighter is the red color of the blood.

Therefore, the blood that goes from the lungs to the tissues is a bright red because it carries a great supply of oxygen; in contrast, the blood that returns to the lings is a much darker red, since it has given up much of its oxygen to the tissues.

Haemoglobin that has given up its oxygen is able to carry hydrogen ions; in this way, haemoglobin acts as a buffer and plays an important role in acid-base balance. The red cells also carry a small amount of carbon dioxide from the tissues to the tlings for elimination in exhalation.

Carbon monoxide is a harmful gas that combines with haemoglobin to form a stable compound. It displaces the oxygen that is normally carried by the haemoglobin and reduces the oxygen-carrying ability of the blood. Carbon monoxide may be produced by the incomplete burning of various fuels, such as gasoline, coal, wood, and other carbon containing materials. It also occurs in automobile exhaust fumes and in cigarette smoke.

The erythrocytes are by far the most numerous of the corpuscles, averaging from 4.5 to 5 million per cubic millimeter of blood.

Leukocytes

The leukocytes, or white blood cells, are very different from the erythrocytes in appearance, quantity, and function. They contain nuclei of varying shapes and sizes; the cells themselves are round. Leukocytes are outnumbered by red cells by 700 to 1, numbering 5,000 to 10,000 per cubic millimetre of blood.

Whereas the red cells have a definite color, the leukocytes tend to be colorless.

The different types of white blood cells are identified by their size, the shape of the nucleus, and the appearance of granules in the cytoplasm when the cells are stained, usually with Wright’s blood stain.

 Granulocytes include neutrophils, which show lavender granules; eosinophils, which have beadlike, bright pink granules; and basophils, which have large, dark blue granules that often obscure the nucleus.

The neutrophils are the most numerous of the white cells, constituting up to 60% of all leukocytes.

Because the nuclei of the nuclei of the neutrophils are of various shapes, they are also called polymorphs (meaning “many forms”)

The agranulocytes, so named because they lack easily visible granules, are the lymphocytes and monocytes.

The most important function of the leukocytes is to destroy pathogens. Whenever pathogens enter the tissues, as through a wound, certain white blood cells (neutrophils and monocytes) are attracted to that area. They leave the blood vessels and proceed by ameboid or ameba-like motion to the area of infection.

There they engulf the invaders by a process called phagocytosis. If the pathogens are extremely strong or numerous, they may destroy the leukocytes.

 A collection of dead and living leukocytes, forms pus. A collection of pus localized in one area is known as abscess.

The Lymphocytes destroy foreign invaders by attacking the cells directly or by producing antibodies that circulate in the blood and help destroy the cells.

Platelets

Of all the formed elements, the blood platelets (thrombocytes) are the smallest (Figure 9-1). These tiny structures are not cells in themselves, but fragments of cells. The number of platelets in the circulating blood has been estimated at 200, 000 to 400,000 per cubic millimeter. Platelets are essential to blood coagulation (clotting). When, as a result of injury, blood comes in contact with any tissue other than the lining of the blood vessels, the platelets stick together and form a plug that seals the wound. They then release chemicals that take part in a series of reactions that eventually results in the formation of a clot. The last step in these reactions is the conversion of a plasma protein called fibrinogen into solid threads of fibrin, which form the clot.

Blood Clotting

Blood clotting, or coagulation, is a protective device that prevents blood loss when a blood vessel is ruptured by an injury. The many substances necessary for clotting are normally inactive in the blood stream. A balance is maintained between compounds that promote clotting, known as procoagulants, and those that prevent clotting known as anticoagulants. In addition, there are also chemicals in the circulation that act to dissolve clots. Under normal conditions the substances that prevent clotting prevail.

However, when an injury occurs, the procoagulants are activated and a clot is formed.

Basically, the clotting process consists of the following essential steps:

1. The injured tissues release thromboplastin, a substance that triggers the clotting mechanism.
2. Thromboplastin reacts with certain protein factors and calcium ions to form prothrombin activator, which in turn reacts with calcium ions to convert the prothrombin to thrombin.
3. Thrombin, in turn, converts soluble fibrinogen into insoluble fibrin. Fibrin forms a network of threads that entraps red blood cells and platelets to form clot.

 Formation of a clot.

Thromboplastin

Ca++

Prothrombin – Thrombin- Fibrinogen – Fibrin threads

+ Blood cells and plasma

Clot

ABO BLOOD GROUP SYSTEM

• If for some reason the amount of blood in the body is severely reduced, through haemorrhage or disease, the body cells suffer from lack of oxygen and food. The obvious measure to take in such an emergency is to inject blood from another person into the veins of the patient, a procedure called transfusion.
• The patient’s plasma may contain substances called antibodies that can cause the red cells of the donor’s blood to become clumped, a process called agglutination. Alternatively, donor’s red blood cells may rupture and release their haemoglobin; such cells are said to be haemolysed, and the resulting condition can be very dangerous.
• These reactions are determined largely by certain proteins, called antigens, on the surface membrane of the red blood cells.
• There are many types of these proteins but only two groups are particularly likely to cause a transfusion reaction, the so-called A and B antigens and the Rh factor.
• Four blood types involving the A and B antigens have been recognized: A, B, AB, and O. these letters indicate the types of antigen present on the red cells, with O indicating that neither A nor B antigen is present. It is these antigens to the donor’s red fells that react with the antibodiesin the patient’s plasma and cause a transfusion reaction.
• Blood serum containing antibodies that can agglutinate and destroy red cells that have A antigens on the surface is called anti-A serum; blood serum containing antibodies that can destroy red cells with B antigen on the surface is called anti-B serum. These sera are used to test for blood type.
• Persons with type O blood are said to be universal donors because they lack the AB red cell antigens and in an emergency their blood can be given to anyone. Type AB individuals are called universal recipients, since their blood contains no antibodies to agglutinate red cells and they can therefore receive blood from most donors.
• Usually a person can safely give blood to any person with the same blood type. However, because of other factors that may be present in the blood, determination of blood type must be accompanied by additional tests (cross matching) for compatibility before a transfusion is given.

The Rh factor

Rh factor is another red cell antigen that determines the blood group. Those individuals who possess this antigen in their red cell surface are said to be Rh positive. Those who lack this antigen are said to be Rh negative. If Rh positive blood is given to an Rh negative person, he or she may become sensitized to the protein in the Rh positive blood. The sensitized person’s blood cells may then produce antibodies to the “foreign” Rh antigens and destroy the transfused red cells.

A pregnant woman who is Rh negative may become sensitized by proteins from her Rh positive fetus (this factor having been inherited from the father) if these proteins enter the mother’s circulation before or during childbirth. During a subsequent pregnancy with an Rh positive fetus, some of the anti Rh antibodies may pass from the mother’s blood in to the blood of her fetus and cause destruction of the fetus’s red cells. This condition is called erythroblastosis fetalis, or hemolytic disease of the newborn.

Erythroblastosis fetalis may be prevented by administration of immune globulin Rho (D), ofr RhoGAM, to the mother shortly after delivery. This destroys the Rh-positive fetal cells in the mother’s blood and prevents her sensitization.

CARDIOVASCULAR SYSTEM

The cardiovascular system is the transport system of the body by which food, oxygen, water and all other essentials are carried to the tissue cells and their waste products are carried away. It consists of three parts:

• The blood, which is the fluid in which materials are carried to and from the tissue
• The heart, which is the driving force which propels the blood
• The blood vessels, the routes by which the blood travels to and through the tissues and back to the heart.

The Heart

The heart is a muscular pump that drives the blood through the blood vessels. Slightly bigger than a fist, this organ is located between the lungs in the center (mediastinum) and a bit to the left on the midline of the body.