The only fluid tissue in the human body
Classified as a connective tissue

Components of blood
Living cells 
Formed elements
Non-living matrix 

If blood is centrifuged
Erythrocytes sink to the bottom (45 percent of blood, a percentage known as the hematocrit)
Buffy coat contains leukocytes and platelets (less than 1 percent of blood)
Buffy coat is a thin, whitish layer between the erythrocytes and plasma
Plasma rises to the top (55 percent of blood)

Physical Characteristics of Blood

Color range

Oxygen-rich blood is scarlet red
Oxygen-poor blood is dull red
pH must remain between 7.35–7.45
Blood temperature is slightly higher than body temperature at 100.4°F
In a healthy man, blood volume is about 5–6 liters or about 6 quarts 
Blood makes up 8 percent of body weight

Blood Plasma
Composed of approximately 90 percent water
Includes many dissolved substances
Salts (electrolytes)
Respiratory gases
Plasma proteins
Waste products

Plasma proteins
Most abundant solutes in plasma
Most plasma proteins are made by liver
Various plasma proteins include
Albumin—regulates osmotic pressure
Clotting proteins—help to stem blood loss when a blood vessel is injured
Antibodies—help protect the body from pathogens

Blood becomes too acidic
Blood becomes too basic
In each scenario, the respiratory system and kidneys help restore blood pH to normal

Formed Elements
Red blood cells (RBCs)
White blood cells (WBCs)
Cell fragments

Erythrocytes (red blood cells or RBCs)
Main function is to carry oxygen
Anatomy of circulating erythrocytes
Biconcave disks
Essentially bags of hemoglobin
Anucleate (no nucleus)
Contain very few organelles
5 million RBCs per cubic millimeter of blood

Iron-containing protein
Binds strongly, but reversibly, to oxygen
Each hemoglobin molecule has four oxygen binding sites
Each erythrocyte has 250 million hemoglobin molecules
Normal blood contains 12–18 g of hemoglobin per 100 mL blood

Homeostatic imbalance of RBCs
Anemia is a decrease in the oxygen-carrying ability of the blood
Sickle cell anemia (SCA) results from abnormally shaped hemoglobin
Polycythemia is an excessive or abnormal increase in the number of erythrocytes

Disorder resulting from excessive or abnormal increase of RBC
May be caused by bone marrow cancer (polycythemia vera)
May be a response to life at higher altitudes (secondary polycythemia)
Increased RBC slows blood flow and increases blood viscosity

Leukocytes (white blood cells or WBCs)
Crucial in the body’s defense against disease
These are complete cells, with a nucleus and organelles
Able to move into and out of blood vessels (diapedesis)
Can move by ameboid motion
Can respond to chemicals released by damaged tissues
4,800 to 10,800 WBC per cubic millimeter of blood

Abnormal numbers of leukocytes
WBC count above 11,000 leukocytes/mm3
Generally indicates an infection
Abnormally low leukocyte level
Commonly caused by certain drugs such as corticosteroids and anticancer agents
Bone marrow becomes cancerous, turns out excess WBC

Types of leukocytes
Granules in their cytoplasm can be stained
Possess lobed nuclei
Include neutrophils, eosinophils, and basophils
Lack visible cytoplasmic granules
Nuclei are spherical, oval, or kidney-shaped
Include lymphocytes and monocytes

List of the WBCs from most to least abundant

Types of granulocytes

Cytoplasm stains pale pink and contains fine granules
Deep purple nucleus contains three to seven lobes
Function as phagocytes at active sites of infection 
Numbers increase during infection
3,000–7,000 neutrophils in a cubic millimeter of blood (40–70% of WBCs)

Red, coarse cytoplasmic granules
Figure-8 or bilobed nucleus stains blue-red
Function to kill parasitic worms and play a role in allergy attacks
100–400 eosinophils in a cubic millimeter of blood (1–4% of WBCs)

Sparse but large blue-purple granules
U- or S-shaped nucleus stains dark blue
Release histamine (vasodilator) at sites of inflammation
Contain heparin (anticoagulant)
20–50 basophils in a cubic millimeter of blood (0–1% of WBCs)

Types of agranulocytes

Cytoplasm is pale blue
Dark purple-blue nucleus
Functions as part of the immune response
B lymphocytes produce antibodies
T lymphocytes are involved in graft rejection, fighting tumors and viruses
1,500–3,000 lymphocytes in a cubic millimeter of blood (20–45% of WBCs)

Largest of the white blood cells
Gray-blue cytoplasm 
Dark blue-purple nucleus is often kidney shaped
Function as macrophages
Important in fighting chronic infection
100–700 monocytes per cubic millimeter of blood (4–8% of WBCs)

Derived from ruptured multinucleate cells (megakaryocytes)
Needed for the clotting process
Platelet count ranges from 150,000 to 400,000 per cubic millimeter of blood
300,000 is considered a normal number of platelets per cubic millimeter of blood

Blood cell formation
Occurs in red bone marrow
All blood cells are derived from a common stem cell (hemocytoblast)
Hemocytoblast differentiation
Lymphoid stem cell produces lymphocytes
Myeloid stem cell produces all other formed elements

Formation of Erythrocytes
Unable to divide, grow, or synthesize proteins
Wear out in 100 to 120 days
When worn out, RBCs are eliminated by phagocytes in the spleen or liver
Lost cells are replaced by division of hemocytoblasts in the red bone marrow

Control of Erythrocyte Production
Rate is controlled by a hormone (erythropoietin)
Kidneys produce most erythropoietin as a response to reduced oxygen levels in the blood
Homeostasis is maintained by negative feedback from blood oxygen levels

Formation of White Blood Cells and Platelets
Controlled by hormones
Colony stimulating factors (CSFs) and interleukins prompt bone marrow to generate leukocytes
Thrombopoietin stimulates production of platelets

Stoppage of bleeding resulting from a break in a blood vessel
Hemostasis involves three phases
Vascular spasms
Platelet plug formation
Coagulation (blood clotting)

Vascular spasms
Vasoconstriction causes blood vessel to spasm
Spasms narrow the blood vessel, decreasing blood loss

Platelet plug formation
Collagen fibers are exposed by a break in a blood vessel
Platelets become “sticky” and cling to fibers
Anchored platelets release chemicals to attract more platelets
Platelets pile up to form a platelet plug

Injured tissues release tissue factor (TF)
PF3 (a phospholipid) interacts with TF, blood protein clotting factors, and calcium ions to trigger a clotting cascade
Prothrombin activator converts prothrombin to thrombin (an enzyme)
Thrombin joins fibrinogen proteins into hair-like molecules of insoluble fibrin
Fibrin forms a meshwork (the basis for a clot)

Blood usually clots within 3 to 6 minutes
The clot remains as endothelium regenerates
The clot is broken down after tissue repair

Undesirable Clotting
A clot in an unbroken blood vessel
Can be deadly in areas like the heart
A thrombus that breaks away and floats freely in the bloodstream
Can later clog vessels in critical areas such as the brain

Bleeding Disorders
Platelet deficiency
Even normal movements can cause bleeding from small blood vessels that require platelets for clotting

Hereditary bleeding disorder
Normal clotting factors are missing

Blood Groups and Transfusions
Large losses of blood have serious consequences
Loss of 15 to 30 percent causes weakness
Loss of over 30 percent causes shock, which can be fatal
Transfusions are the only way to replace blood quickly
Transfused blood must be of the same blood group

Human Blood Groups
Blood contains genetically determined proteins
Antigens (a substance the body recognizes as foreign) may be attacked by the immune system
Antibodies are the “recognizers”
Blood is “typed” by using antibodies that will cause blood with certain proteins to clump (agglutination)

Human Blood Groups
There are over 30 common red blood cell antigens
The most vigorous transfusion reactions are caused by ABO and Rh blood group antigens

ABO Blood Groups

Based on the presence or absence of two antigens
Type A
Type B
The lack of these antigens is called type O

The presence of both antigens A and B is called type AB
The presence of antigen A is called type A 
The presence of antigen B is called type B
The lack of both antigens A and B is called type O

Blood type AB can receive A, B, AB, and O blood
Universal recipient
Blood type B can receive B and O blood
Blood type A can receive A and O blood
Blood type O can receive O blood
Universal donor

Rh Blood Groups
Named because of the presence or absence of one of eight Rh antigens (agglutinogen D) that was originally defined in Rhesus monkeys
Most Americans are Rh+ (Rh positive)
Problems can occur in mixing Rh+ blood into a body with Rh– (Rh negative) blood
Rh Dangers During Pregnancy
Danger occurs only when the mother is Rh– and the father is Rh+, and the child inherits the Rh+ factor
RhoGAM shot can prevent buildup of anti-Rh+ antibodies in mother’s blood
The mismatch of an Rh– mother carrying an Rh+ baby can cause problems for the unborn child
The first pregnancy usually proceeds without problems
The immune system is sensitized after the first pregnancy
In a second pregnancy, the mother’s immune system produces antibodies to attack the Rh+ blood (hemolytic disease of the newborn)

Blood Typing
Blood samples are mixed with anti-A and anti-B serum
Coagulation or no coagulation leads to determining blood type
Typing for ABO and Rh factors is done in the same manner
Cross matching—testing for agglutination of donor RBCs by the recipient’s serum, and vice versa

Developmental Aspects of Blood

Sites of blood cell formation
The fetal liver and spleen are early sites of blood cell formation
Bone marrow takes over hematopoiesis by the seventh month
Fetal hemoglobin differs from hemoglobin produced after birth
Physiologic jaundice results in infants in which the liver cannot rid the body of hemoglobin breakdown products fast enough