Overview of blood tissue

Introduction

Blood connects all the various tissues of the body

• Blood is part of every organ and every system
• Is a “connective” tissue in a functional sense
• Will relate directly to remaining topics of this course: cardiovascular, respiration, etc.

Structure

Connective tissue with liquid matrix (plasma)

Cells and cell fragments are also called formed elements

Flows within cardiovascular system (heart and blood vessels)

Blood volume

• 71 ml/kg (70 kg adult male would have about 5,000 ml total blood volume)
• About 55% is fluid and about 45% formed elements

Function

Primary functions

• Transportation of molecules within the internal fluid environment
• Exchange between tissues and between internal/external environment

Secondary functions

• Immunity — defense
• Thermoregulation — acts as “radiator fluid”
• Fluid volume homeostasis
• pH homeostasis

Blood and cardiovascular problems are a major health concern

Formed elements – overview

Hematopoiesis

Hematopoiesis — formation of new blood cells

• All are descended from stem cells in red marrow (myeloid tissue)
• Divide into “lines” of daughter cells, each line producing different groups of blood cells (differentiation)

Main types

• Platelets — clotting cells
• Red blood cells — carry blood gases
• White blood cells — immune cells

Blood cell types
Red cell (left), platelet (center), white cell (T lymphocyte; right). SEM

Red blood cells

Name

Also called RBCs or erythrocytes [lit. “red cells”]

Number

4-6 million per cubic mm of blood (RBC count)

45% of total blood volume (packed cell volume [PCV] or hematocrit)

Hematocrit (PCV “packed cell volume”)

Shape

Biconcave disk

• 7 micron (µm) diameter

Deformable

• PGE increases deformability
• PGE₂ decreases deformability

Crenation — shrinkage from water loss (as in a hypertonic solution)

Schizocyte — broken cell (schizo = “split”)

RBC inside blood capillary. The RBC must deform to flow through this tiny blood vessel.

Cytoplasm

Organelles few in number (no nucleus)

• Lost during development

Contain about 300 million molecules of hemoglobin (Hb)

• Quaternary protein made of four polypeptides, each with an iron (Fe) containing heme group in the center (of each folded polypeptide)
• Oxyhemoglobin (HbO₂) forms when O₂ combines with heme groups
• Carbaminohemoglobin (HbCO₂) forms when CO₂ combines with amino acids in polypeptide chains
• Anemia — less normal hemoglobin than usual (< 10g/100ml)
  • Normal male: 14-16 g/dl (g/dl = g/100ml)
  • Normal female: 12-14 g/dl
  • Examples
    • Sickle cell anemia
    • Pernicious anemia
    • Fe-deficiency anemia (target cells/codocytes)

Life cycle

RBC hematopoiesis

• Erythropoiesis
  • Stem cells (hemocytoblasts) in red bone marrow (myeloid tissue)
• Stimulated by hormone erythropoietin (EPO), which is formed in kidney in response to low O₂ delivery to tissue cells (tissue hypoxia)
• Example: hematocrit (RBC%) goes UP when you stay at higher elevations, where O₂ concentration in air is lower
• Requires: vitamin B₁₂ (and intrinsic factor), folic acid (folate or vitamin B₉), iron (Fe)
• Blood doping
  • Add blood to increase hematocrit, and thus O₂ capacity
  • Add EPO to increase RBC production, increasing O₂ capacity

RBC destruction

• Last about 4 months (can’t repair themselves because they lack nuclei)
• Run through “obstacle courses” in spleen and liver and aged cells break apart
• Macrophages eat them
• Globin (polypeptide) part is reduced to amino acids and recycled
• Heme is stripped of its iron, which is recycled
  • Remain part of heme is converted to bilirubin (yellowish pigment that gives plasma its characteristic “straw” color)

Formation of blood cells
The RBCs in this diagram look strange. What is odd about them?

Blood types

Blood typing is the earliest form of “tissue typing”

• Karl Landsteiner discovered ABO system (1902), Rh system (1904) and later other blood types, for which he won a Nobel Prize

Based on glycoprotein markers (antigens) on surface of RBCs

ABO system looks for A or B markers (antigen)

• Type A — antigen A only
• Type B — antigen B only
• Type AB — both A and B antigens
• Type 0 — neither A nor B antigen present

Rh (D antigen) system (named for Rhesus monkeys)

• Rh positive — Rh (D) antigen present
• Rh negative — Rh antigen absent

Other systems of blood typing exist —looking for different antigens (markers)

Read over textbook material on practical applications of this theory

White blood cells

Name

White blood cells are also called WBCs or leukocytes [lit. “white cells”]

Number

Less than 1% of whole blood

• 5,000 – 10,000 per cubic mm of blood (WBC count)
• Leukopenia — abnormally low WBC count
• Leukocytosis — abnormally high WBC count
  • Often observed in leukemia (tumor of WBCs)

Life cycle

Leukopoiesis is development of new WBCs

• Produced in bone marrow, but some migrate to lymphoid tissue for most of development (lymphoid tissue is found in lymph nodes, thymus gland, spleen, liver, elsewhere)

Lifespan is uncertain, perhaps 3 days to a year, depending on type

Location

Not confined to blood stream

Chemotaxis — move toward chemical attractants (chemotactic factors)released at site of injury/infection

Diapedesis — move between, or even through, other cells to get to their target

Diapedesis of WBC across wall of blood vessel.

Functions

Immune functions of various sorts, for example

• Phagocytosis
• Release of cell toxins
• Release of immune function regulators
• More on immunity later (see Chapter 21)

Types

Types distinguished by how they stain with Wright’s stain —a mix of different cell stains

Differential WBC count — distinguished how many of each type of WBC in sample

Granulocytes (look granular when stained)

• Basophils (stain with the basic stain in Wright’s mixture)
• Eosinophils (stain with the acid stain called eosin)
• Neutrophils (stain with a neutral stain)

Agranulocytes (do not look granular when stained)

• Lymphocytes (small—8 microns (µm); develop in lymphoid tissue)
• Monocytes (large—15-20 microns (µm); develop into macrophages)

click image to enlarge