Blood

Description Of Blood

Blood is the life sustaining "river of life" that flows within our bodies. It transports everything that must be carried from one place to another within the body nutrients, and body heat through blood vessels. Man recognized that blood was vital, and its loss was always considered to be a possible cause of death.

BLOOD COMPOSITION AND FUNCTION :

Blood is a thick, or viscous, substance composed of :

A. A fluid portion (plasma) in which

B. Living cells (formed elements) are suspended.

PLASMA :

Plasma (plaz'mah), which is approximately 90% water, is the liquid part of the blood. Over 100 different substances are dissolved in the plasma.

Composition of plasma varies continuously as cells remove or add substances to the blood.However, assuming a normal healthy diet, the composition of plasma is kept relatively constant by various homeostatic mechanisms of the body.

When the blood starts to become too acid (acidosis) or too basic (alkalosis), both the respiratory system and kidneys are called into action to restore it to its normal slightly alkaline pH range of 7.35 - 7.45.

In addition to transporting various substances around the body, plasma has another important role  - it helps distribute body heat evenly through the body tissues.

FORMED ELEMENTS :

Three types of formed elements, or blood corpuscles are present in the blood.

1.The most numerous are erythrocytes ( RBCs ) which are oxygen-carriers.

2.Leukocytes ( WBCs) are part of the body's immune system.

3.Thrombocytes or platelets help repair leaks in the blood vessels.

Formed elements make up 40%-45% of whole blood whereas plasma accounts for the remaining 55%-60%.

Only WBCs are complete cells

RBCs have no nuclei or organelles, and platelets are just cell fragments

Most formed elements survive in the bloodstream for only a few days

Most blood cells do not divide but are renewed by cells in bone marrow

ERYTIlROCYTES :

Erythrocytes, or red blood cells (RBCs), are small cells with a distinct biconcave disk shape. RBCs are anucleate. are unable to divide and have a limited lift span of 100-120days.

When mature and circulating in the blood, RBCs are literally sacs of hemoglobin molecules.

Hemoglobin (Hb) transports the bulk of the oxygen that is carried in the blood.

After that time, they begin to fragment, or fall apart, and their remains are eliminated by the spleen, liver, and other body tissues.

Lost cells are replaced more-or-Iess continuously by the division of cells called hemocytoblasts, located in the red bone marrow.

The rate of RBC production,hemopoiesis is controlled by a hormone, erythropoietin.

Which released by the kidneys in response to decreasing oxygen levels in the blood.

RBCs outnumber white blood cells by about 1000 to 1 and are the major factor contributing to blood viscosity.

Production of Erythrocytes :

Hematopoiesis –blood cell formation

Hematopoiesis occurs in thered bone marrow of the:

Axial skeleton and girdles

Epiphyses of the humerus and femur

Fate and Destruction of Erythrocytes :

The life span of an erythrocyte is 100–120 days

Old erythrocytes become rigid and fragile, and their hemoglobin begins to degenerate

Dying erythrocytes are engulfed by macrophages

Heme and globin are separated and the iron is reused

Heme is degraded to a yellow pigment called bilirubin

The liver secretes bilirubin into the intestines as bile

The intestines metabolize it into uro bilinogen

This degraded pigment leaves the body in feces, in a pigment called stercobilin

Globin is metabolized into amino acids and is released into the circulation

Hb released into the blood is captured by haptoglobin and phgocytized

Erythrocytes (RBCs) :

Although the numbers of RBCs in the circulation do vary, there are normally 4.5-5.5million cells per cubic millimeter of blood.

(Acubic millimeter [mm3] is a very tiny drop of blood, almost not enough to be seen.)

When thenumber of RBC/mm increases, blood viscosity increases.

Similarily; as the number of RBCs decreases, blood thins and flows more rapidly.

  

Erythrocyte Disorders :

1. Anemia: Is a decrease in the oxygen-carrying ability of the blood.

May be result of:

A. A lower than normal number of RBCs or,

B. A lower than normal hemoglobin content in the RBCs.

·It is a symptom rather than a disease itself

·Blood oxygen levels cannot support normal metabolism

·Signs/symptoms include fatigue, paleness, shortness of breath, and chills

Some types of anemia :

> Hemorrhagic anemia : result of acute or chronic loss of blood

>Hemolytic anemia : prematurely ruptured erythrocytes

>Aplastic anemia : destruction or inhibition of red bone marrow

Iron deficiency anemia results from :

A secondary result of hemorrhagic anemia

Inadequate intake of iron containing foods

Impaired iron absorption

Pernicious anemia results from :

Deficiency of vitamin B12

Lack of intrinsic factor needed for absorption of B12

Treatment is intramuscular injection of B12

Thalassemias - absent or faulty globin chain in hemoglobin

Erythrocytes are thin, delicate, and deficient in hemoglobin

Sickle cell anemia results from : defective gene coding for an abnormal hemoglobin called hemoglobinS (HbS)

HbS has a single amino acid substitution in the beta chain 

2. Polycythemia: Is an abnormal increase in number of erythrocytes ( RBCs).

May be result from:

A. Bone marrow cancer or,

B. Living at a high altitudes where less oxygen is available (secondary polycythemia).

The problem that results from excessive number of RBCs is increased viscosity of blood, which causes it to flow sluggishly in the body.

Leukocytes (WBCs) :

Basically WBCs are the blood's protective "movabie army." which helps protect the body from damage by bacteria, viruses, and tumor cells.

Leukocytes, the only blood components that are complete cells:

Are less numerous than RBCs and average from 5000 - 10,000 cells mm3

WBCs are more like most body cells than RBCs are that is, they contain nuclei

Make up 1% of the total blood volume

Can leave capillaries via diapedesis

Move through tissue spaces

Leukocyte Disorders :

1.Leukocytosis : WBC count over 10,000 per cubic millimeter

Normal response to bacterial or viral invasion

2.Leukemia : (literally, white blood) is a situation in which the

bone marrow becomes cancerous, and huge numbers of WBCs are turned out rapidly.

Classification of Leukocytes :

The WBCs are classified into two major groups, depending on whether or not they contain

Visible granules in their cytoplasm:

A. Granulocytes \–neutrophils, eosinophils, and basophils :

They are formed in red bone marrow as RBCs

Contain cytoplasmic granules that stain specifically (acidic,basic, or both)

Are larger and usually shorter-lived than RBCs

Have lobed nuclei

Are all phagocytic cells

1. A. Neutrophils

Neutrophils have two types of granules that:

Take up both acidic and basic dyes

Give the cytoplasm a lilac color

Contain peroxidases, hydrolytic enzymes, and defensins (antibiotic-like proteins)

Active phagocytes; their number increases rapidly during short-term or acute infections

2. A. Eosinophils

Eosinophils account for 1–3% of WBCs

Have red-staining, bilobed nuclei connected via a broad band of nuclear material

Have red to crimson (acidophilic) large, coarse, lysosome-like granules

Lead the body’s counterattack against parasitic worms

3. A. Basophils

Basophils account for 0.5% of WBCs and:

Cytoplasm has a few large granules, which stain deep purple with Wright's stain, large U or S shaped nucleus with constrictions, which stains dark blue

Granules believed to contain histamine (a vasodilator chemical ) and heparin (an anticoagulant), which are discharged on exposure to foreign substances (bacteria, viruses, and toxins)

B. Agranulocytes –lymphocytes and monocytes:

Lack visible cytoplasmic granules

They also arise from hemocytoblasts but migrate to lympnatic tissues.

Are similar structurally, but arefunctionally distinct and unrelated cell types

Have spherical (lymphocytes) or kidney-shaped (monocytes)nuclei.

        1. B. Lymphocytes :

Account for 20% -35% of WBCs and:

Have large, dark purple, circular nuclei with a thin rim of blue cytoplasm

2.B.Monocytes :

account for 3–8% of leukocytes

They are the largest leukocytes

  

Platelets :

Platelets are fragments of megakaryocytes with a blue-staining outer region and a purple granular center.

The normal platelet count in blood is 300,000 mm3.

Platelets are needed for the clotting process that occurs inplasma when blood vessels are ruptured or broken.

Genesis of Platelets :

The stem cell for platelets is the hemocytoblast.

The sequential developmental pathway is hemocytoblast, megakaryoblast, promegakaryocyte, megakaryocyte, and platelets.

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