Formed Elements
Formed Elements
Includes all cellular
parts of blood
 Composes approx.
45% of total blood
volume
 Originate from stem
cells

Red Blood Cells
A.K.A. erythrocytes
 Most abundant formed element – 95% of
elements and 40% of total blood volume
 Small, flexible cells shaped like discs; thin
in center, thicker on the edges—biconcave
 No nuclei or other organelles
 Shape provides more surface area; lack of
organelles means more room for
hemoglobin

Hemoglobin
RBCs continued
Function is to bond to oxygen (O2) to
carry it from lungs to body tissues
 a small amount of CO2 can also be carried
by RBCs, but most of that is dissolved in
plasma
 Short life cycle – last 120 days on average
 Any shortage in number of RBC’s or their
capacity to carry O2 is referred to as
anemia

White Blood Cells
Also known as leukocytes
 Represent less than 1% of total blood
volume, increase or decrease in numbers
indicate a diseased condition
 All contain a nucleus and can wander
outside of the blood to fight infections in
body tissues by either phagocytosis or
secretion of antibodies
 See p. 351 for types of WBCs

Platelets
Also known as thrombocytes
 Fragments of complete cells
 Consist of cytoplasm wrapped with a
membrane; contain most types of
organelles
 Play a role in hemostasis

Hemostasis


Loss of blood can threaten homeostasis;
hemostasis is the process by which the
body prevents blood loss
It is a process that involves three steps
1. Blood vessel spasm
2. Platelet plug formation
3. Coagulation
Blood Vessel Spasm
Walls of blood vessels contain smooth
muscle tissue
 When vessel is broken these muscles
contract, causing the ends of the vessel to
“pinch in,” decreasing the size of the
opening
 Sometimes this completely stops bleeding,
sometimes not
 Spasm can last for up to 30 minutes

Platelet Plug Formation
Platelets passing through area of damage
change in characteristics; increase in size,
become irregular shapes, and become
very sticky
 Sticky platelets adhere to rough edges of
injury, usually collagen fibers
 This forms a plug which can stop the
bleeding

Coagulation
Most effective and complex of clotting
mechanisms
 Involves substances that are normally present in
plasma (fibrinogen) and substances secreted by
platelets at the site of injury
 Begins when platelets release thromboplastin,
which interacts with Ca ions to convert
prothrombin (present in small quantities in
plasma) into thrombin

Coagulation cont’d
Thrombin works as an enzyme to link
molecule of fibrinogen into fibrin
 Fibrin molecules very long, sticky, and
insoluble in water
 They form a net that traps formed
elements; this makes a blood clot
 Process takes 2 – 8 minutes, depending
on size of injury to blood vessel

Coagulation cont’d
Once the clot is formed, repair of the injury can
begin through the action of fibroblasts from
neighboring connective tissues
 Blood clots sometimes form where they are not
needed, especially if the blood vessel is not
smooth
 Undesirable clot which is stationary can is called
a thrombus, free-floating ones are called an
embolus
 Both can block blood vessels and be dangerous

Animation of Hemostasis

Animation
Blood Groups
First discovered when blood transfusions
became medically possible
 In blood types that are incompatible cells
will clump together (agglutination)
 Blood grouping relies on proteins located
on the surface of RBCs called antigens and
their interaction with proteins found in
plasma called antibodies

Blood Groups cont’d
Red Blood Cell
Plasma
Type A
Antigen A
Type B
Antigen B
Type AB
Antigens A and B
Antibody B
(anti-B)
Antibody A
(anti-A)
No antibodies
Type O
No antigens
Anitibodies A and B
(anti-A and anti-B)
The Rh System
First discovered in the rhesus monkey
 People with Rh antigens on cells are referred to
as Rh positive, those lacking them are called Rh
negative
 Unlike anti-A and anti-B, anti-Rh is not present
from birth
 In order for the plasma to contain anti-Rh
antibodies, the individual must first be sensitized
through exposure to Rh positive blood
