Vascular Spasm and Platelet Plug Formation
Spasms of smooth muscle in blood vessel walls and accumulation
of platelets (platelet plug) at the site of vessel injury stop or slow
down blood loss temporarily until coagulation occurs.
Coagulation of blood may be initiated by either the intrinsic
or the extrinsic pathway. Platelet phospholipid (PF
) is crucial
to both pathways. Tissue factor (factor III) exposed by tissue
injury allows the extrinsic pathway to bypass many steps of the
intrinsic pathway. A series of activated clotting factors oversees
the intermediate steps of each cascade. ±e pathways converge as
prothrombin is converted to thrombin.
Clot Retraction and Fibrinolysis
A²er a clot is formed, clot retraction occurs. Serum is squeezed
out and the ruptured vessel edges are drawn together. Smooth
muscle, connective tissue, and endothelial cell proliferation and
migration repair the injured blood vessel.
When healing is complete, clot digestion (ﬁbrinolysis) occurs.
Factors Limiting Clot Growth or Formation
Abnormal expansion of clots is prevented by removal of
coagulation factors in contact with rapidly ﬂowing blood and
by inhibition of activated blood factors. Prostacyclin (PGI
and nitric oxide secreted by the endothelial cells help prevent
undesirable (unnecessary) clotting.
Disorders of Hemostasis
±romboembolic disorders involve undesirable clot formation,
which can block vessels.
±rombocytopenia, a deﬁcit of platelets, causes spontaneous
bleeding from small blood vessels. Hemophilia is caused by a
genetic deﬁciency of certain coagulation factors. Liver disease can
also cause bleeding disorders because many coagulation proteins
are formed by the liver.
Disseminated intravascular coagulation (DIC) is a condition of
bodywide clotting in undamaged blood vessels and subsequent
Transfusion and Blood Replacement
Transfusing Red Blood Cells
Whole blood transfusions are given to replace severe and rapid blood
loss. Packed RBCs are given to replace lost O
Blood group is based on agglutinogens (antigens) present on red
blood cell membranes.
When mismatched blood is transfused, the recipient’s agglutinins
(plasma antibodies) clump the foreign RBCs. ±e clumped RBCs
may block blood vessels temporarily and then are lysed. Released
hemoglobin may cause renal shutdown.
Before whole blood can be transfused, it must be typed and cross
matched to prevent transfusion reactions. ±e most important blood
groups for which blood must be typed are the ABO and Rh groups.
Restoring Blood Volume
Plasma volume can be replaced with balanced electrolyte
solutions, and these are generally preferred over plasma
Diagnostic Blood Tests
Diagnostic blood tests can provide valuable information about
the current status of the blood and of the body as a whole.
Plasma proteins, most made by the liver, include albumin,
globulins, and ﬁbrinogen. Albumin is an important blood buﬀer
and contributes to the osmotic pressure of blood.
Formed elements, accounting for 45% of whole blood, are
erythrocytes, leukocytes, and platelets. All formed elements arise
from hematopoietic stem cells in red bone marrow.
Erythrocytes (Red Blood Cells)
Erythrocytes (red blood cells, RBCs) are small, biconcave cells
containing large amounts of hemoglobin. ±ey have no nucleus
and few organelles. Spectrin allows the cells to change shape as
they pass through tiny capillaries.
Oxygen transport is the major function of erythrocytes. In the
lungs, oxygen binds to iron atoms in hemoglobin molecules,
producing oxyhemoglobin. In body tissues, oxygen dissociates
from iron, producing deoxyhemoglobin.
Red blood cells begin as hematopoietic stem cells and, through
erythropoiesis, proceed from the proerythroblast (committed
cell) stage to the basophilic, polychromatic and orthochromatic
erythroblast, and reticulocyte stages. During this process, hemoglobin
accumulates and the organelles and nucleus are extruded.
Diﬀerentiation of reticulocytes is completed in the bloodstream.
Erythropoietin and testosterone enhance erythropoiesis.
Iron, vitamin B
, and folic acid are essential for production of
Red blood cells have a life span of approximately 120 days.
Macrophages of the spleen and liver remove old and damaged
erythrocytes from the circulation. Released iron from
hemoglobin is stored as ferritin or hemosiderin to be reused. ±e
balance of the heme group is degraded to bilirubin and secreted
in bile. Amino acids of globin are metabolized or recycled.
Respiratory System; Topic: Gas Transport, pp. 3–5, 11–17.
Erythrocyte disorders include anemia and polycythemia.
Leukocytes (White Blood Cells)
Leukocytes are white blood cells (WBCs). All are nucleated, and
all have crucial roles in defending against disease. Two main
categories exist: granulocytes and agranulocytes.
Granulocytes include neutrophils, eosinophils, and basophils.
Neutrophils are active phagocytes. Eosinophils attack parasitic
worms, and their numbers increase during allergic reactions.
Basophils contain histamine, which promotes vasodilation and
enhances migration of leukocytes to inﬂammatory sites.
Agranulocytes have crucial roles in immunity. ±ey include
lymphocytes—the “immune cells”—and monocytes which
diﬀerentiate into macrophages.
Leukopoiesis is directed by colony-stimulating factors and
interleukins released by supporting cells of the red bone marrow
and mature WBCs.
Leukocyte disorders include leukemias and infectious
Platelets are fragments of large megakaryocytes formed in red
marrow. When a blood vessel is damaged, platelets form a plug to
help prevent blood loss and play a central role in the clotting cascade.
Hemostasis is prevention of blood loss. ±e three major steps of
hemostasis are vascular spasm, platelet plug formation, and blood