Blood: Structure, Function, and Clinical Aspects

Blood

Overview

Blood is the only fluid tissue in the human body and is classified as a connective tissue. It plays a vital role in transport, regulation, and protection within the body.

• Components of Blood:

  • Living cells: Formed elements (erythrocytes, leukocytes, platelets)

  • Non-living matrix: Plasma

• Blood Centrifugation:

  • Erythrocytes sink to the bottom (45% of blood, known as the hematocrit)

  • Buffy coat (leukocytes and platelets) forms a thin layer (<5% of blood) between erythrocytes and plasma

  • Plasma rises to the top (55% of blood)

Physical Characteristics of Blood

Color, pH, and Volume

• Color: Oxygen-rich blood is scarlet red; oxygen-poor blood is dull red.

• pH: Must remain between 7.35 and 7.45.

• Temperature: Slightly higher than body temperature (about 100.4°F or 38°C).

• Volume: In healthy adults, about 5–6 liters (6 quarts), making up 8% of body weight.

Blood Plasma

Composition and Function

• Composed of approximately 90% water.

• Contains many dissolved substances:

  • Nutrients

  • Salts (electrolytes)

  • Respiratory gases

  • Hormones

  • Plasma proteins

  • Waste products

• Plasma Proteins: Most are made by the liver.

  • Albumin: Regulates osmotic pressure.

  • Clotting proteins: Help stem blood loss when a vessel is injured.

  • Antibodies: Protect the body from pathogens.

• Acidosis: Blood becomes too acidic.

• Alkalosis: Blood becomes too basic.

• Respiratory system and kidneys help restore blood pH to normal in both cases.

Formed Elements

Types and Characteristics

• Erythrocytes (Red Blood Cells, RBCs):

  • Main function: Carry oxygen.

  • Anatomy: Biconcave disks, essentially bags of hemoglobin, anucleate (no nucleus), contain few organelles.

  • Normal count: 5 million RBCs per cubic millimeter of blood.

  • Hemoglobin: Iron-containing protein that binds strongly, but reversibly, to oxygen. Each molecule has four oxygen-binding sites; each erythrocyte contains about 250 million hemoglobin molecules.

• Leukocytes (White Blood Cells, WBCs):

  • Crucial in the body's defense against disease.

  • Complete cells with nucleus and organelles.

  • Can move in and out of blood vessels (diapedesis) and by amoeboid motion.

  • Respond to chemicals released by damaged tissues.

  • Normal count: 4,000 to 11,000 WBCs per cubic millimeter of blood.

• Platelets: Cell fragments derived from ruptured multinucleate cells. Normal count: 300,000/mm3.

Homeostatic Imbalances of RBCs

• Anemia: Decrease in the oxygen-carrying ability of the blood.

• Sickle cell anemia (SCA): Results from abnormally shaped hemoglobin.

• Polycythemia: Excessive or abnormal increase in the number of erythrocytes.

Abnormal Numbers of Leukocytes

• Leukocytosis: WBC count above 11,000/mm3; generally indicates infection.

• Leukopenia: Abnormally low WBC level; commonly caused by certain drugs (e.g., corticosteroids, anticancer agents).

• Leukemia: Bone marrow becomes cancerous, producing excess WBCs.

Types of Leukocytes

• Granulocytes: Granules in cytoplasm can be stained; possess lobed nuclei.

  • Neutrophils: Multilobed nucleus, fine granules, act as phagocytes at infection sites.

  • Eosinophils: Large brick-red cytoplasmic granules, found in response to allergies and parasitic worms.

  • Basophils: Have histamine-containing granules, initiate inflammation.

• Agranulocytes: Lack visible cytoplasmic granules; nuclei are spherical, oval, or kidney-shaped.

  • Lymphocytes: Nucleus fills most of the cell, play an important role in immune response.

  • Monocytes: Largest WBCs, function as macrophages, important in fighting chronic infection.

Order of WBCs from most to least abundant: Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils (mnemonic: Never Let Monkeys Eat Bananas).

Hematopoiesis

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; eliminated by phagocytes in the spleen or liver.

• Lost cells are replaced by division of hemocytoblasts in red bone marrow.

• Control of Erythrocyte Production:

  • Rate controlled by hormone erythropoietin.

  • Kidneys produce most erythropoietin in response to reduced oxygen levels in blood.

  • Homeostasis 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.

Hemostasis

Stopping Blood Loss

Hemostasis is the process of stopping bleeding resulting from a break in a blood vessel. It involves three phases:

1. Vascular Spasms: Vasoconstriction causes blood vessel to spasm, narrowing the vessel and decreasing blood loss.

2. Platelet Plug Formation: Platelets become "sticky" and cling to fibers; anchored platelets release chemicals to attract more platelets, forming a platelet plug.

3. Coagulation (Blood Clotting):

   • 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 (basis for a clot).

• Blood usually clots within 3 to 6 minutes.

• The clot remains as endothelium regenerates and is broken down after tissue repair.

Undesirable Clotting

• Thrombus: A clot in an unbroken blood vessel; can be deadly in areas like the heart.

• Embolus: A thrombus that breaks away and floats freely in the bloodstream; can clog vessels in critical areas such as the brain.

Bleeding Disorders

• Thrombocytopenia: Platelet deficiency; even normal movements can cause bleeding from small blood vessels.

• Hemophilia: Hereditary bleeding disorder; normal clotting factors are missing.

Blood Groups and Transfusions

Blood Transfusions

• Transfusions are the only way to replace blood quickly.

• Transfused blood must be of the same blood group.

• Loss of over 30% of blood causes shock, which can be fatal.

Human Blood Groups

• Blood contains genetically determined proteins (antigens).

• Antibodies recognize and may attack foreign antigens, causing agglutination (clumping).

• Over 30 common red blood cell antigens; most vigorous transfusion reactions are caused by ABO and Rh blood group antigens.

ABO Blood Groups

Blood Type	Antigens Present	Antibodies Present	Can Receive Blood From	Can Donate Blood To
A	A	Anti-B	A, O	A, AB
B	B	Anti-A	B, O	B, AB
AB	A, B	None	A, B, AB, O	AB
O	None	Anti-A, Anti-B	O	A, B, AB, O

• Type AB: Universal recipient.

• Type O: Universal donor.

Blood Typing

• Blood samples are mixed with anti-A and anti-B serum.

• Coagulation (clumping) or lack thereof determines blood type.

• Typing for ABO and Rh factors is done in the same manner.

Rh Blood Groups

• Named for the presence or absence of one of eight Rh antigens (agglutinogen D) originally defined in Rhesus monkeys.

• Most Americans are Rh positive (Rh+).

• Problems can occur when 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 the mother's blood.

• The mismatch of an Rh- mother carrying an Rh+ baby can cause problems for the unborn child (hemolytic disease of the newborn):

  • First pregnancy usually proceeds without problems.

  • Mother's immune system is sensitized after the first pregnancy.

  • In subsequent pregnancies, the mother's immune system produces antibodies to attack the Rh+ blood.

Developmental Aspects of Blood

Sites of Blood Cell Formation

• Fetal liver and spleen are early sites of blood cell formation.

• Bone marrow takes over hematopoiesis by the seventh month.

Fetal Hemoglobin

• Fetal hemoglobin differs from hemoglobin produced after birth.

• Physiologic jaundice results in infants whose liver cannot rid the body of hemoglobin breakdown products fast enough.