Blood

 Blood transports nutrients, wastes and body heat within the body through blood vessels.

Composition and Function of Blood

Components

 Among all of the body’s tissues, blood is unique as it is the only fluid tissue.
 It is a thick homogeneous liquid.
 It is a complex connective tissue in which living blood cells, the formed elements are
suspended in a nonliving fluid matrix called plasma.
 If a sample of blood is spun in a centrifuge, heavier formed elements are packed down by
centrifugal force and the plasma rises to the top.
 The reddish mass consists of erythrocytes which function in oxygen transport.
 There is a barely visible thin whitish layer called the buffy coat at the junction between
the formed elements and the plasma. This layer contains leukocytes which protect the
body and platelets that function in blood clotting.
 Erythrocytes account for about 45% of the total volume of a blood sample, a percentage
known as the hematocrit.
 WBCs and platelets contribute less than 1% and plasma makes up most of the remaining
55% of the whole blood.

Physical Characteristics and Volume

 Blood is a sticky opaque fluid with a characteristic metallic taste. Blood has the taste of
saltiness.
 The colour of blood varies from scarlet (oxygen-rich) to a dull red (oxygen-poor).
 Blood is heavier than water and about 5 times thicker or more viscous because of its
formed elements.
 Blood is slightly alkaline with a pH between 7.35 and 7.45.
 Its temperature is always slightly higher than body temperature, which is 38°C.

Plasma
 Plasma approximately 90% water, is the liquid part of the blood.
 There are over 100 different substances dissolved in this straw-coloured fluid. For
example, nutrients, salts, respiratory gases, hormones, plasma proteins and various wastes
and products of cell metabolism.
 Plasma proteins are most abundant solutes in the plasma. Except for antibodies and
protein-based hormones, most plasma proteins are made by the liver.
  The plasma serves a variety of functions. The albumin contributes to the osmotic pressure
of blood, which acts to keep water in the bloodstream; clotting proteins help stem blood
loss when a blood vessel is injured; antibodies help protect the body from pathogens.
 Plasma proteins are not taken up by cells to be used as food fuels or metabolic nutrients.
 The composition of plasma varies continuously as cells remove or add substances to the
blood.
 When blood proteins drop to undesirable levels, liver is stimulated to make more
proteins; when the blood starts to become too acid (acidosis) or too basic (alkalosis), the
respiratory system and the kidney are called into action to restore it to its normal.
 Besides transporting various substances around the body, plasma helps to distribute body
heat evenly throughout the body.

Erythrocytes
 Erythrocytes or red blood cells (RBCs) function primarily to ferry oxygen in blood to all
cells of the body.
 RBCs differ from other blood cells because they are anucleate, which they lack a nucleus,
and contain very few organelles.
 Mature RBCs circulating in the blood are literally sacs of hemoglobin molecules.
 Hemoglobin (Hb), an iron bering-protein, transports the bulk of the oxygen that is carried
in the blood. (It also binds with a small amount of carbon dioxide).
 Erythrocytes which are lack of mitochondria make ATP by anaerobic mechanisms. They
do not use up any of the oxygen they are transporting.
 Erythrocytes are biconcave discs – flattened discs with depressed centers on both sides.
Their small size and peculiar shape provide a large surface area relative to their volume,
making them ideally suited for gas exchange.
 RBCs outnumber WBCs by about 1000 to 1 and is the major factor contributing to blood
viscosity.
 When the number of RBC/mm3 increases, blood viscosity increases.
 The more hemoglobin molecules the RBCs contain, the more oxygen they will be able to
carry.
 A single RBCs contains about 250 million hemoglobin molecules, each capable of
binging 4 molecules of oxygen.
 Normal blood contains 12-18 grams of hemoglobin per 100 ml of blood.

Leukocytes
 Leukocytes or white blood cells are far less numerous than red blood cells that only
account for less than 1% of total blood volume.
 They are crucial to body defense against disease.
 WBCs are the only completes cells in blood which contain nuclei and the usual
organelles.
 Leukocytes form a protective, movable army that helps defend the body against damage
by bacteria, viruses, parasites and tumor cells.
 WBCs are able to slip into and out of the blood vessels, a process called diapedesis.
 The circulatory system is simply their means of transportation to areas of the body where
their services are needed for inflammatory or immune responses.
 WBCs can locate areas of tissue damage and infection in the body by responding to
certain chemicals that diffuse from the damaged cells. This is called positive chemotaxis.
 WBCs move through the tissue spaces by ameboid motion (they form flowing
cytoplasmic extensions that help move them along). By following the diffusion gradient,
they pinpoint areas of tissue damage and rally round in large numbers to destroy
microorganisms or dead cells.
 Whenever WBCs mobilize for action, the body speeds up their production.
 A total WBC count above 11000 cells/mm3 is referred to as leukocytosis which indicates
that a bacterial or viral infection is stewing in the body.
 The opposite condition , leucopenia is an abnormally low WBC count is caused by
certain drugs, such as corticosteroids and anticancer agents.
 The two major groups of WBCs depend on whether or not they contain visible granules
in their cytoplasm.
 Granulocytes are granules containing WBCs. They have lobed nuclei, which typically
consist by thin strands of nuclear material. The granules in their cytoplasm stain
specifically with Wright’s stain. The granulocytes include neutrophils, eosinophils and
basophils.
 Neutrophils have multiblobed nucleus and very fine granules that respond to both acid
and basic stains. Neutrophils are avid phagocytes at sites of acute infection.
 Eosinophils have a blue-red nucleus and sport large brick-red cytoplasmic granules. Their
number increases rapidly during allergies and infections by parasitic worms.
 Basophils contain large histamine-containing granules that stain dark blue. Histamine is
an inflammatory chemical that makes blood vessels leaky and attracts other WBCs to the
inflammatory site.
 Agranulocytes lack of visible cytoplasmic granules. Their nuclei are spherical, oval, or
kidney-shaped. Lymphocytes and monocytes are examples of agranulocytes.
 Lymphocytes have a large dark purple nucleus that occupies most of the cell volume.
They tend to take up residence in lymphatic tissues where they play an important role in
the immune response.
 Monocytes are the largest of the WBCs. They are the most abundant cytoplasm and
indented nucleus. When they migrate into tissues, they change into macrophages with
huge appetites. They are very important in fighting chronic infections.
Platelets
 Platelets are not cells but fragments of bizarre multinucleate cells called megakaryocytes.
 Platelets appear as darkly staining, irregularly shaped bodies scattered among the other
blood cells.
 The normal platelet count in blood is about 300000/mm3.
 Platelets are needed for the clotting when blood vessels are ruptured or broken.

Hematopoiesis
 Blood cell formation or hematopoiesis occurs in red bone marrow or myeloid tissue.
 In adults, this tissue is found chiefly in the flat bones of the skull and pelvis, the ribs,
sternum, and proximal epiphyses of the humerus and femur. Each type of blood cells are
produced in different numbers. After they matured, they are discharged into the blood
vessels surrounding the area.
 All the formed elements arise from a common type of stem cell, the hemocytoblast which
residues in the red bone marrow. Their development differs and once a cell is committed
to a specific blood pathway it cannot change.
 Lymphoid stem cell produces lymphocytes and myeloid stem cells which can produce all
other classes of formed elements.
 As RBCs are anuclueate, they are unable to synthesize proteins, grow or divide. As they
age, RBCs become more rigid and begin to fragment in 100 to 120 days.
 Their remains are eliminated by phagocytes in the spleen, liver and other body tissues.
 Lost cells are replaced continuously by the division of hemocytoblasts in the red bone
marrow.
 The developing RBCs divide many times and then begin synthesizing huge amount of
hemoglobin. When enough hemoglobin is accumulated, the nucleus and most of the
organelles are ejected and the cell collapse inward.
 The young RBC is called a reticulocyte as it still contains some rough endoplasmic
reticulum (ER).
 The reticulocytes enter the bloodstream to begin their task of transporting oxygen. Within
2 days of release, the remaining ER is ejected.
 The entire developmental process from hemocytoblast to mature RBC takes 3 to 5 days.
 The rate of erythrocytes production is controlled by erythropoietin (hormone). It
circulates in the blood at all times. It is produce in the liver in a small amount and kidney
in a large amount.
 When the blood levels of oxygen begin to decline, the kidneys step up their release of
erythropoietin. Erythropoietin targets the bone marrow.
 Excessive amount of oxygen in the bloodstream, depresses erythropoietin release and
RBC production.
  The formation of leukocytes and platelets is stimulated by hormones. The colony
stimulating factors (CSFs) and interleukins marshal up an army of WBCs to ward off
attacks by enhancing the ability of mature leukocytes to protect the body.
 They are released in response to specific chemical signals in the environment such as
inflammatory chemicals and certain bacteria or their toxins.
 The hormone thrombopoietin accelerates the production of platelets.

Hemostasis

 If a blood vessel wall breaks, a series of reactions is set in motion to accomplish

hemostasis or stoppage of blood flow.
 Hemostasis involves three major phases: platelet plug formation, vascular spasms, and
coagulation or blood clotting.
 Blood loss is permanently prevented when fibrous tissue grows into the clot and seals the
hole in the blood vessel.
 Hemostasis occurs as follow:
1) Platelet plug forms. Platelets are repelled by an intact endothelium, but when it is
broken so that the underlying collagen fibers are exposed, the platelets become “sticky”
and cling to the damaged site. Anchored platelets release chemicals that attract more
platelets to the site, and as more and more platelets pile up, a small mass called a platelet
plug or white thrombus is formed.
2) Vascular spasms occur. The anchored platelets also release serotonin, which causes
that blood vessels to go into spasms. The spasms narrow the blood loss until clotting can
occur. (Other factors causing vessel spasms include direct injury to the smooth muscle
cells and stimulation of local pain receptors.)
3) Coagulation events occurs. At the same time, the injured tissues are releasing tissue
factor (TF), a substance that plays an important role in clotting. PF3, a phospholipid that
coats the surfaces of the platelets, interacts with TF, vitamin K and other blood protein
clotting factors, and calcium ions (Ca2+) to form an activator that triggers the clotting
cascade. The prothrombin activator converts prothrombin, present in the plasma, to
thrombin. Thrombin then joins soluble fibrinogen proteins into long hairlike molecules of
insoluble fibrin which forms a meshwork that traps the RBCs and forms the basis of the
clot. Within the hour, the clot begins to retract, squeezing serum from the mass and
pulling the ruptured edges of the blood vessels closer together.

Human Blood Groups

 People have different blood groups, and transfusing incompatible or mismatched blood
can be fatal.
 The plasma membrane of RBCs bear genetically determined proteins (antigens) which
identify each person as unique.
  An antigen is a substance the body recognized as foreign if transfused into another person
with different RBC antigens.
 The antibodies present in the plasma that attach to the RBCs bearing surface antigens
different from those on the patient’s RBCs.
 Binding of the antibodies causes the RBCs to clump, a phenomenon called agglutination
which leads to clogging of small blood vessels throughout the body.
 The foreign RBCs are lysed and their hemoglobin is released into the bloodstream.
 The most devastating consequence of severe transfusion reactions is that the freed
hemoglobin molecules may block the kidney tubules and cause kidney failure. It may
also cause fever, chills, nausea and vomiting.
 There are over 30 common RBC antigens in humans, allowing each person’s blood cells
to be classified into different blood groups.
 The antigens of the ABO and Rh blood groups cause the most vigorous transfusion
reactions.
 ABO blood groups are based on which of two antigens, type A or type B. Absence of
both antigens results in type O blood; presence of both antigens leads to type AB, and the
possession of either A or B antigen yields type A or B blood respectively.
 Most Americans are Rh+, meaning that their RBCs carry the Rh antigen.
 Anti-Rh antibodies are not automatically formed and present in the blood of Rh-
individuals.
 If an Rh- person receives mismatched blood, shortly after the transfusion his or her
immune system becomes sensitized and begins producing antibodies.
 Hemolysis does not occur with the first transfusion. However, the second time and every
time thereafter, a typical transfusion reaction occurs in which the patient’s antibodies
attack and rupture the donor’s Rh+ RBCs.
 Pregnant Rh- women who are carrying Rh+ babies will deliver a healthy baby. However,
the mother is sensitized by Rh+ antigens that have passed through the placenta into her
bloodstream, she will form anti-Rh+ antibodies unless treated with RhoGAM shortly after
giving birth.
 RhoGAM is an immune system that prevents this sensitization and her subsequent
immune response.
 If not treated, her antibodies will cross through the placenta and destroy the baby’s RBCs,
producing a condition known as hemolytic disease of the newborn.
 Brain damage and even death may result unless fetal transfusions are done before birth to
provide more RBCs for oxygen transport.

Development Aspects of Blood

 In the young embryo, development of the entire circulatory system occurs early. Before
birth, there are many sites of blood cell formation – fetal liver and spleen.
 After seven months, the red marrow has become the chief site of hematopoiesis.
 Fetal hemoglobin (HbF) differs from the hemoglobin formed after birth. It has a greater
ability to pick up oxygen.
 After birth, fetal blood cells are gradually replaced by RBCs that contain the more typical
hemoglobin A (HbA).
 In situation where fetal RBCs are destroyed at such rapid rate that the immature infant
liver cannot get rid the body of hemoglobin breakdown products in the bile fast enough,
the infant becomes jaundiced.
 Jaundiced with no major problems is called physiologic jaundice.