Department of Physiology: Blood Physiology-Review & Illustrations

Functions of the blood include:

1. Transport of O₂, nutrients, hormones, substances, and carries substances, CO₂ to the lungs and other products of metabolism to the kidneys to be excreted.

3. Help in the regulation of body temperature.

4. Helps to maintain the pH and electrolyte concentrations of interstitial fluid.

5. Blood also serves essential body protective functions.

Plasma Proteins:

Albumin, 58% of plasma proteins, 4.5 g/dl .
Globulins, 38% of plasma proteins, 2.5 g/dl.
Fibrinogen, 4% of plasma proteins, 0.3 g/dl .

Liver forms all the albumin and fibrinogen of the plasma proteins, as well as, 50-80% of the globulins. The lymphoid tissues form the remainders of the globulins. They are mainly the gamma globulins that constitute the antibodies.

Functions of plasma proteins, in general, include:

1. Plasma proteins exert an osmotic pressure of about 25 mm Hg across the capillary wall.

2. Plasma proteins are responsible for 15% of the buffering capacity of blood.

3. Plasma proteins transport hormones and different substances in blood.

4. Plasma proteins in form of antibodies are providing the body with immunity.

5. Plasma proteins are concerned with blood clotting.

6. Plasma proteins can act as a source for rapid replacement of the tissue proteins.

Hemopoiesis:

After birth, blood cells are produced by the bone marrow of all bones.
With the advancing of age up to 20 years of age , the active red marrow of long bones has become inactive (yellow and fatty and produce no more blood cells) , except for the upper humerus and femur.
Beyond 20 years, blood cells are normally formed in the marrow of flat or membranous bones.

Red blood cells (erythrocytes):

Transport oxygen from the lungs to the various tissues of the body, and to transport carbon dioxide from the tissues to the lungs.
They are non-nucleated, biconcave discs.
Lack of mitochondria, therefore, RBCs are incapable of aerobic respiration.
Circulating erythrocytes live for about 120 days.
Hb A: It is the normal adult Hb. α₂ β₂. Hb A is the predominant type of Hb in adults (95-97% of total Hb).
Hb A2: In the normal adult. α₂δ₂(alpha 2, delta 2). About 2.5% of the total Hb is Hb A2.
Hb F (Fetal Hb): It is the main Hb in fetus and newborn. It is α₂γ₂ (alpha 2, gamma 2). Adult Hb replaces Hb F gradually soon after birth, usually at about 6 months to one year of age the normal adult Hb predominates. In normal adults Hb F may be found in a level of less than 2% of the total Hb.

Variant forms of normal Hb: Hb may be found in the blood in different forms;

Oxyhemoglobin: Hb combined with O₂.
Carbaminohemoglobin: Hb combined with CO₂.
Carboxyhemoglobin: Hb combined with CO. Carboxyhemoglobin molecule becomes useless for O₂ transport.
Sulfhemoglobin: Hb containing sulfur, and is incapable of transporting O₂. Certain oxidizing drugs form it.
Methemoglobin: Ferrous iron is converted into ferric iron forming methemoglobin. Methemoglobin is incapable of combining with O₂. The O₂binding site is occupied by water, in acidic solutions, and by an OH^- group, in alkaline solutions. Methemoglobin is normally found in the blood in a concentration of 1-2%.
Myoglobin (symbol Mb or MB): It is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals.
Neuroglobin: It is an intracellular hemoprotein expressed in the central and peripheral nervous system, cerebrospinal fluid, retina and endocrine tissues.

Destruction of red blood cells and catabolism of hemoglobin:

RBCs circulate in the blood for an average of 120 days, after that old RBCs are destroyed by macrophages in the mononuclear phagocyte system, in many parts of the body especially in the liver, spleen, and bone marrow.
Inside the mononuclear phagocyte system, the Hb is broken into its constituents (globin and heme). Globin is catabolized within mononuclear phagocyte system into its constituent amino acids and enters the circulating amino acid pool.
The heme is converted into biliverdin. Biliverdin is converted into bilirubin, which is released into the blood where it combines to albumin and later it is detached from albumin and enters the liver where it is conjugated and becomes water-soluble bilirubin and is secreted by the liver into the bile which empties into the intestine.
The iron from the heme is released into the blood to be carried to the bone marrow for production of new RBCs or to the liver and other tissues for storage.

Classification of anemia: Anemia can be classified, according to the cause, into anemia due to:

Inadequate production of normal RBCs by the bone marrow.
Excessive destruction of RBCs (hemolysis).
Blood loss (hemorrhage).

Anemia can also be classified, according to the blood indices: The red blood cells indices are used to define the size and hemoglobin content of the red blood cells. They consist of the mean corpuscular volume, mean corpuscular hemoglobin and mean corpuscular hemoglobin concentration (see figure).

If MCHC is normal, the RBC is called normochromic as in acute blood loss anemia.
If MCHC is below normal, the RBC is hypochromic as in iron deficiency anemia.
If MCV is is above normal, the RBC is macrocytic as in Megaloblastic anemia .
If MCV is below normal, RBC is microcytic as in iron deficiency anemia.

Polycythemia: It is an increased concentration of erythrocytes in the circulating blood that is above normal for sex and age. Polycythemia could be:

Relative polycythemia is due to reduction in plasma volume.
Absolute or true polycythemia, which could be;

Secondary polycythemia that is related to increased erythropoietin production. It is seen for example in those living at high altitudes.
Primary polycythemia (polycythemia vera) is caused by a gene aberration that occurs in the hemocytoblastic cell line that produces the blood cells. The blast cells continue producing red cells even when too many cells are already present. This causes excess production of red cells without erythropoietin stimulus, and usually there is excess production of white blood cells and platelets as well.

Consequences of anemia:

The tissues suffer hypoxia. and in severe anemic hypoxia can cause life-threatening necrosis of brain, heart, and kidney tissues.
Blood osmolarity is reduced. More fluid is thus transferred from the blood stream to the intercellular spaces, resulting in edema.
Blood viscosity is reduced and altered the hemodynamic blood flow..
The heart and lungs also must work harder to compensate for the blood’s low capacity to carry oxygen. Because the heart has to work harder to get blood and oxygen to the tissues, anemia, particularly severe anemia, can result in cardiac arrest.

Consequences of polycythemia:

Increased blood volume,
Increased blood pressure,
Increased blood viscosity.

The above three consequences put strain on cardiovascular system.

White blood cells (leukocytes): They provide the body with powerful defenses against tumors, and viral, fungal, bacterial, and parasitic infections. WBCs are nucleated cells. They are classified according to the presence or absence of specific-staining granules in the cytoplasm.

1. Granulocytes: Their nuclei are lobulated (also called polymorphonuclear leukocytes). These are neutrophils, eosinophils, and basophils.

2. Non granulocytes which are:

The lymphocytes. There are three main types: T-lymphocytes, B-lymphocytes, & natural killer (NK) lymphocytes.
The monocytes. The monocyte is the largest WBC.

Normal Values: The normal range of total WBC count in adults is 4000-11000 per microliter (mm³) of blood. The normal percentages of the different types of WBCs are:

Neutrophils = 50-70%, 2.0–7.0×10 ⁹/l
Lymphocytes = 20-40%, 1.0–3.0×10 ⁹/l
Monocytes = 2-8%, 0.2–1.0×10 ⁹/l
Eosinophils = 1-4%, 0.02–0.5×10 ⁹/l
Basophils = 0.4%, 0.02–0.1×10 ⁹/l

Immunity: Is the ability to resist damage from foreign substances, such as microorganisms; harmful chemicals; and internal threats, such as cancer cells. Immunity is categorized as innate immunity (also called nonspecific immunity) and adaptive immunity (also called specific immunity) (see figure), although the two systems are fully integrated in the body.

Innate immunity: The main components of innate immunity are:

Mechanical barriers.
Chemical barriers that act directly against microorganisms or that activate other mechanisms.
Cells involved in phagocytosis (mainly neutrophils and macrophages).
Natural killer (NK), “police”cells.
Fever.

Functions of leukocytes: The leukocytes use the blood to migrate from bone marrow or other production sites to the tissues where they function.

Neutrophils: = Phagocytosis; cellular ingestion of the offending agent.
Eosinophils: = Kill parasites, and detoxify some of the substances released during the allergic reactions.
Basophils contain heparin and histamine and play a role in allergic reactions.
Monocytes = Become tissue macrophages = Phagocytosis
Natural killer (NK) = Kill abnormal cells like cancer cells or cells infected with viruses or bacteria and cells of transplanted tissues.

The sequence of events of acute inflammation: (figure):

Activation of tissue macrophages and mast cells: Any irritation > Activates tissue macrophages and mast cells > release of chemical mediators (cytokines, as well as other substances).
The response to the chemical mediators are:

Greatly increase production of both granulocytes and monocytes by the bone marrow
Changes in vascular caliber (vasodilatation) and increased local blood flow
Increase the stickiness of endothelium
Increased vascular permeability
Chemotaxis and phagocytosis

Adaptive immunity (also called specific immunity): Adaptive immune system is a specific immune system acts against specific organisms or particles, and becomes effective only after prior exposure to the invading agent. Lymphocytes are the key constituents of the immune system. The body has two types of adaptive immune defense systems; humoral and cellular. Both react to antigens.

Humoral immunity (B-cell immunity) is immunity due to circulating antibodies (Abs) which are gamma globulins. It is a major defense against bacterial infections.

Cellular or cell–mediated immunity (T-cell immunity) is achieved by formation of large numbers of activated lymphocytes that are specially designed to destroy the foreign agent.

Development of the adaptive Immune System: > Review Figure.

Types of the involved cells and the interactions between humoral immunity and cell–mediated immunity > Review figure.

The T cells differentiate into four varieties:

Helper T cells (CD4 cell) > They serve as the major regulator of humoral and cellular immune functions, Most numerous

Suppressor T cell > Involved in regulation of humoral and cellular immunity by suppressing them.

Cytotoxic T cells (CD8) (killer cells) > Attacking and destroying invading antigen

Memory T & B cells > Cells that have been exposed to an Ag and are readily converted to effector cells by a later encounter with the same Ag. Unlike other lymphocytes, they persist in the body for months or even years.

Cellular & humoral immunity: (see figure)

Cellular immunity is mediated by T lymphocytes, humoral imunity is mediated by B lyphocytes.
Both Start when the antigen is presented by antigen-presenting cells (APC) (Macrophages, Dendritic cells and B lymphocytes) to helper T cells.
Activated helper T cells proliferate and the proliferated cells enter the circulation for activation of cytotoxic T cells, OR activation of B lymphocytes.
Activated Cytotoxic T cells circulate through blood, lymph and lymphatic tissues and destroy the invading organisms that have the Ag, which activated them by attacking them directly, and destroy cells. OR, the activated B cells are transformed into two types of cells: Plasma cells, and memory cells. The plasma cells secrete large quantities of Abs, which are specific to the Ag, into the general circulation.

Types of Abs:

The IgG → (75% of the Abs of the normal person), the only class of Ab that can pass the human placental barrier. It is the main Ab produced during secondary response.
The IgA → It is found in secretions of epithelium lining many organs such as of respiratory, gastrointestinal, and genitourinary tracts, where it can bind foreign Ags and impair their entry into the body, and has two or three basic units .
The IgM: → It is produced during the primary response and has 10 Ag binding sites.
The IgD → It acts as a receptor on the surface of B cells (for Ag recognition by B cells).
The IgE → is secreted in increased amounts in patients with allergic conditions and parasitic infections.

Antibodies protect the body from invading organisms in two ways:
1. by direct actions: Antibodies directly inactivate the invading organism by any one of the following methods:

Agglutination: In this, the foreign bodies like RBCs or bacteria with antigens on their surfaces are held together in a clump by the antibodies.
Precipitation: In this, the soluble antigens like tetanus toxin are converted into insoluble forms and then precipitated.
Neutralization: During this, the antibodies cover the toxic sites of antigenic products.
Lysis: It is done by the most potent antibodies. These antibodies rupture the cell membrane of the organisms and then destroy them.

2. by activation of complement system: Only IgG and IgM can activate this system. These enzymes or the byproducts formed during these events produce the following activities:

Opsonization: By which complement proteins, antibodies, and plasma protein called opsonin form a coat around pathogenic organisms so that neutrophil and macrophages identify them easily and phagocytize them.
Agglutination: Clumping of foreign bodies like RBCs or bacteria.
Neutralization: Covering the toxic sites of antigenic products.
Lysis: Destruction of bacteria by rupturing the cell membrane.
Chemotaxis: Attraction of leukocytes to the site of antigen-antibody reaction.
Activation of mast cells and basophils, which liberate histamine: Histamine dilates the blood vessels and increases capillary permeability. So, plasma proteins from blood enter the tissues and inactivate the antigenic products.

Blood types (blood groups): → Review Table.

The membranes of human RBCs contain a hundreds of Ags (integral membrane glycoproteins or glycolipids). Most of them are weak.
Two groups of Ags are the most important and are more likely to cause blood transfusion reactions; the ABO system of Ags and the Rh system of Ags. According to ABO system of Ags (also called agglutinogens), bloods are grouped into 4 major types: O, A, B, and AB, depending on the presence or absence of Ags A and B on the RBCs.
A and B Ags are found in many tissues in addition to blood, these include salivary glands, saliva, pancreas, kidney, liver, lungs, testes, semen, and amniotic fluid.
Immediately after birth, RBC has agglutinogens but the plasma has non agglutinins, the quantity of agglutinins in the plasma is almost zero. Two to eight months after birth, the baby begins to form agglutinins. A maximum level is reached usually at 8-10 years of age and this gradually declines throughout the remaining years of life.
The agglutinins are IgM and IgG immunoglobulin molecules like other Abs, and are produced by the same cells that produce Abs to any other Ags.
When recipient and donor bloods are mismatched, so that anti-A or anti-B agglutinins are mixed with RBCs containing A or B agglutinogens, respectively, transfusion reaction occurs → agglutinin can attach to two or more different red cells at the same time → causing the cells to adhere to each other → causing cells to clump together (agglutination).
Unlike the ABO Ags, the Rh system has not been detected in tissues other than RBCs. Again, Unlike the Abs of the ABO system, which develops spontaneously, anti-D Abs do not develop without exposure of a D-negative individual to D-positive red cells. This exposure occurs by:
1. Transfusion of Rh positive blood to Rh negative recipient.
2. Entrance of Rh positive fetal blood into the maternal circulation of an Rh negative mother.

If Rh positive blood is transfused into Rh-negative person for the first time → the anti-Rh agglutinins will develop slowly → there will be no immediate reaction → on subsequent transfusion of Rh-positive blood into the same person (after 2 weeks-4 months) → the transfusion reaction is occurred and can be severe.

when an Rh negative mother carries an Rh positive fetus → small amounts of fetal blood may enter the maternal circulation at the time of delivery or as result of fetal-maternal hemorrhage which may occurs during pregnancy → sensitization of the mother can occur and anti-Rh Abs are formed in the mother after delivery → during the next pregnancy → the mother’s anti-Rh Abs cross the placenta to the fetus → if the fetus is Rh positive → agglutination of fetal RBCs occur → the agglutinated RBCs are then hemolyzed releasing Hb → which will be converted to bilirubin and cause jaundice (erythroblastosis fetalis) → in severe cases death of the fetus or the newborn may occur.

Since sensitization of Rh-negative mother by carrying an Rh-positive fetus generally occurs at birth, the first child is usually normal if the mother has not been previously sensitized by transfusion of Rh-positive blood. However the incidence of erythroblastosis fetalis rises progressively with subsequent pregnancies because the mother who is already sensitized by the first Rh-positive baby will develop Abs rapidly and high concentrations of Abs will be present upon subsequent pregnancies with Rh positive fetuses.

It is possible to prevent sensitization from occurring the first time by administering a single dose of anti-D immunoglobulin within 72 hours of delivery of the Rh-positive baby. This does not harm the mother, and will destroy the baby’s cells that have leaked into the mother’s circulation and prevent Ab formation by the mother. Not all hemolytic disease of the newborn is due to Rh incompatibility, however. About 2% of cases result from incompatibility of ABO and other blood types.

Cross-matching test (see figure): In cross-matching test, the donor’s RBCs are mixed with recipient’s plasma on a slide and checked for agglutination. If agglutination occurs it means that the donor blood is incompatible with the recipient blood and blood transfusion cannot occur.

Persons with AB group have been called universal recipients (see figure) because they have no circulating agglutinins and can be given blood of any type without developing transfusion reaction due to ABO incompatibility. Persons with O group have been called universal donors, because they lack A and B Ags, and type O blood can be given to anyone without producing a transfusion reaction due to ABO incompatibility. However, this does not mean that blood should ever be transfused without being cross-matched, since the possibility of reactions or sensitization due to incompatibilities in systems (subgroups) other than ABO system always exists.

Platelets (thrombocytes):

Platelets are the smallest of the cellular elements of blood, 2-4 micrometers in diameter.
They are non-nucleated, granulated bodies. Their granules secrete:

ADP, causing platelets aggregatation,
ATP,
Serotonin, vasoconstrictor which cause vascular spasms in broken vessels.
Clotting factors, which promote blood clotting.
Thromboxane A_2, vasoconstrictor which cause vascular spasms in broken vessels, and also causing platelets aggregatation,
Platelet-derived growth factor (PDGF) which stimulates wound healing.

Their normal concentration in blood ranges from 150,000 to 400,000 per microliter.
Their life span is 7-10 days.
The membranes of platelets contain receptors for collagen, vessel wall von Willebrand factor and fibrinogen.
Their cytoplasm contains actin, myosin, glycogen, lysosomes.
The platelet membrane contains large amounts of phospholipids that play several activating roles at multiple points in the blood clotting process.

Hemostasis and blood coagulation: A spontaneous and natural process occurs to arrest bleeding when a small blood vessel is transected or injured. This process includes four events (see figure);

Contraction of the injured vessel,
Formation of platelet plug at the site of injury,
Activation of blood coagulation,
Clot retraction.

As a normal secondary response in hemostasis, activation of the fibrinolytic system, which gradually dissolves away the fibrin clot as tissue repair, is taking place.

1. Contraction of the vessel wall (vasoconstriction): This reduces the flow of blood from the vessel rupture due to release of vasoconstrictor substances (Endothelin) from the endothelial cells, and due to neural reflexes initiated by local pain receptors.

2. Formation of a platelet plug: When a vessel is broken, subendothelial collagen fibers are exposed to the blood → platelets receptors attached to collagen fibers and collagen von Willebrand factor → platelets become active (swell, become sticky, and start to put out long spiny pseudopods (see figure) that adhere to the vessel and to other platelets) → stickiness causes circulating platelets to adhere to the platelets already attached to the collagen and the pseudopods contract and draw the walls of the vessel together → the mass of platelets thus formed, called a platelet plug, may reduce or stop minor bleeding.

In addition, the activated platelets release [A] serotonin, [B] thromboxane A₂, and [C] ADP (figure).

Serotonin and thromboxane A₂ → enhance vasoconstriction.
Thromboxane A₂and ADP → activate other nearby platelets and increase their stickiness and this causes circulating platelets to adhere to the platelets already attached to the collagen, so platelets will aggregate (platelets stick to each other) and form platelet plug at the site of the injury.
ADP enhances the endothelium to release NO and thromboxane A₂,to prevent platelet aggregation in undamaged tissue and restrict platelets aggregation to the site of injury.

3. Blood coagulation: Platelet plug is converted into the definitive clot by formation of fibrin. > Review figure.
The intrinsic and extrinsic pathways usually work together and are interconnected in many ways, but there are significant differences between them (see table).

IMPORTANT NOTES TO REMEMBER

Both intrinsic pathway and extrinsic pathway occur within ruptured blood vessel. Under physiological conditions, however, the two pathways are not parallel but are actually activated sequentially, with thrombin serving as the link between them. There are also several points at which the two pathways interact.

Coagulation usually starts with the extrinsic pathway. As soon as you make a little Xa, the extrinsic pathway is turned off. The small amount of thrombin that has been generated during the action of the extrinsic system goes up and turns on the intrinsic pathway, which finishes out the job of making the rest of the fibrin.

4. Clot retraction: Within few minutes after a clot is formed, it begins to contract and usually expresses most of the fluid from the clot within 20-60 minutes. The fluid expressed is called serum. Serum differs from plasma in that it cannot clot because all its fibrinogen and most other clotting factors have been removed. In addition, serum has higher serotonin content because of breakdown of platelets during clotting. Platelets play essential role in clot retraction through the following mechanisms:

They bond different fibers together.
Platelets entrapped in the clot continue to release procoagulant substances (substances promoting coagulation), one of which is factor XIII (fibrin stabilizing factor) which cause more and more cross-linking bonds between the adjacent fibrin fibers.
Platelets themselves contribute directly to clot contraction by activating platelet contractile proteins (thrombosthenin, actin and myosin molecules) which cause strong contraction of platelet spicules attached to the fibrin. This also helps compress the fibrin meshwork into a smaller mass.

As the clot retracts, the edges of the broken blood vessel are pulled together. When the number of platelets in the circulating blood is low, there will be failure of clot retraction.

Platelets and endothelial cells secrete a mitotic stimulant named platelet-derived growth factor (PDGF). PDGF stimulates fibroblasts and smooth muscle cells to multiply and repair the damaged blood vessel. Fibroblasts also invade the clot and produce fibrous connective tissue, which helps to strengthen and seal the vessel while the repairs take place.

Fibrinolysis:

A process by which the formed clot by the coagulation process is disposed of. The main components of the fibrinolytic system are plasminogen and plasmin. Plasminogen activated by two ways . Review figure.

When a clot is formed, a large amount of plasminogen is bound to fibrin,
t-PA is released slowly from endothelial cells and is adsorbed on fibrin surface and activates the bind plasminogen
Plasmin is generated at the clot surface, and begins to dissolve the fibrin clot with the production of fibrin degradation products.
The fibrinolytic system removes clots from intravascular and extravascular sites.

An important function of the fibrinolytic system is to remove minute clots that form in tiny peripheral vessels which otherwise would become occluded.

The fibrinolysis is held in check by fibrinolysis inhibitors which otherwise the bleeding will resume once the clot is formed by dissoluting it. Inhibition of fibrinolysis is achieved by:

A free plasmin may escape to the circulation, is inactivated by a₂AP. The serum a₂-antiplasmin (a₂AP) which is the principal and most important physiologic inhibitor of plasmin (synthesized in the liver).

Plasminogen activator inhibitors, which inhibit t-PA and urokinase. They are synthesized by endothelial cells and by placenta. They are present in the plasma in low concentrations.

The anticlotting mechanisms:

Smooth and prostacyclin-coated endothelium.
The glycocalyx, on the surface of the endothelium.
The production of thrombomodulin (a thrombin-binding protein) on the surface of endothelial cells. Its anticlotting mechanisms are achieved through:

Inactivation of thrombin
Inactivates factors Va and VIIIa.
Inhibits plasminogen activator inhibitor (an inhibitor to t-PA) thereby, increasing the formation of plasmin.

The balanced interaction between the platelet-aggregating effect of thromboxane A2 and platelet-antiaggregating effect of prostacyclin (see figure).

Fibrin acts as antithrombin and the presence of antithrombin III.
The fibrinolytic system.