Blood Physiology

Function of Blood (overview)

• Transportation

• Regulation

• Protection

Transportation of blood

• Deliver O2 and nutrients to body cells

• Transport metabolic waste to lungs and kidney for elimination 

• Transport hormones from endocrine organs to target orders 

Regulation of blood

• Maintaining appropriate body temperature

  • absorbing and distributing heat

• Maintaining normal pH in body tissues

  • Blood proteins/solutes act as buffers to prevent significant changes to pH

  • Acts as a reservoir for the body’s “alkaline reserve: of bicarbonate ions

• Maintaining adequate fluid volume in circulatory system

  • Blood proteins (albumin) prevent fluid loss from blood vessels

Protection of blood

• Preventing blood loss 

  • Plasma proteins and platelets in blood initiate clot formation if a vessel is damaged

• Preventing infection

  • Agents of immunity are carried in blood, defending body against bacteria, virus, etc

    • Antibodies

    • Complement proteins

    • leukocytes/WBC 

Composition of blood

• Blood is the only fluid tissue in body

• Appears as thick homogeneous liquid

• Microscopically it contains both cellular and liquid components

• Specialized type of connective tissue

  • Living blood cells, formed elements, suspended in nonliving fluid matrix, plasma

    • Formed elements:

    • Erythrocytes (RBCs)

    • Leukocytes (WBCs)

    • Platelets (PLT)

• Lacks collagen and elastic fibers, typical to CT, but dissolved fibrous proteins form during blood clotting

Three layers of blood

• Erythrocytes on bottom

  • Most dense component, 45% of whole blood

  •  Hematocrit: percent of blood volume that is RBCs (45%)

• WBCs and platelets in Buffy coat, middle layer

  • < 1% whole blood

  • Thin, whitish layer between RBCs and plasma layers

• Plasma on top

  •  55% of whole blood, Least dense component

Physical characteristics and volume of blood

• Blood is a sticky, opaque fluid, with characteristic metallic taste/smell

• Color varies with O2 content

  •  High O2 levels show a scarlet red

  • Low O2 levels show a dark red/purplish

• Slightly alkaline: normal range for blood pH 7.35–7.45

• More dense than water and 5x more viscous (due to formed elements)

  • Women have slightly lower RBC counts= thinner blood compared to men

• Makes up ~8% of body weight

• Average blood volume: 5L

  • Males: 5–6 L (1.5 gallons)

  • Females: 4–5 L

Blood Plasma

• Blood plasma is straw-colored sticky fluid

• About 90% water

  • Dissolving and suspending medium for solutes

  • Absorbs heat

• Over 100 dissolved solutes (10%)

  • Plasma Proteins

  • Inorganic ions/electrolytes

  • Waste/ Nutrients

  • Respiratory Gases

  • Hormones

Plasma Proteins

• Most abundant plasma solutes by weight, 8% of plasma

• Remain in blood; not taken up by cells

• All contribute to osmotic/oncotic pressure to maintain water balance

  • Albumin: 60%

    • Main contributor to oncotic/osmotic pressure

  • Globulins: 36%

    • Alpha, beta: transport proteins

    • Gamma: antibodies made by plasma cells (B lymphocytes) in lymphoid tissue

  • Fibrinogen: 4%

    • Forms fibrin threads for clotting

  • Prothrombin: 1%

    • Involved in coagulation/ clot formation

• Most are produced in the liver, except for gamma globulins

Electrolytes

• Cations

  • Sodium

  • Potassium

  • Calcium

  • Magnesium

• Anions

  • Chloride

  • Phosphate 

  • Sulfate

  • Bicarbonate 

• Maintain plasma osmotic pressure and blood pH 

Nutrients and waste for cellular function

• Nutrients

  • Material absorbed from GI tract

  •  Glucose, simple carbohydrates, amino acids, fatty acids, triglycerides, cholesterol, vitamins

•  Waste

  •  By-products of cellular metabolism

  • Urea, uric acid, creatinine, ammonium salts

Respiratory gases and hormones

• Respiratory Gases

  • Oxygen

    • Mostly bound to hemoglobin inside RBCs

    • Some dissolved in plasma

  • Carbon dioxide

    • Transported dissolved as bicarbonate

    • Dissolved in plasma

    • Bound to hemoglobin in the RBCs

• Hormones 

  • Thyroid and steroid hormones are bound to plasma proteins for transportation

• Electrolytes, nutrients/waste, gases, and hormones = 1-2% plasma volume

Formed Elements (overview)

• Erythrocytes

  • Not true cells

  • They have no nuclei or organelles

• Leukocytes (WBCs) 

  • Complete cells

  • Many types 

• Platelets (PLT) 

  • Cell fragments

• Formed elements survive in the bloodstream for varying amounts of time

• Blood cells do not divide once mature

• Stem cells divide in bone marrow constantly, to replace them

Structural characteristics of erythrocytes

• Erythrocytes are small-diameter (7.5 μm) cells that contribute to gas transport

• Biconcave disc shape, flattened disc with depressed center

• Bound by plasma membrane, anucleate (cannot regenerate), has no organelles

  • ”Bags” of hemoglobin (Hb), a protein used for gastransport

• Contains plasma membrane protein, spectrin

  • Spectrin provides flexibility to change shape, bend fold twist as needed as they pass through capillaries that are smaller in diameter than themselves

Three features make for efficient gas transport of erythrocytes

• Small size and biconcave shape offers huge surface area relative to volume for gas exchange

  • Everything is near a surface

• Hemoglobin makes up 97% of cell volume

• RBCs have no mitochondria

  • ATP production is anaerobic

  • They do not consume O2 they transport

Function of erythrocytes

• RBCs are dedicated to respiratory gas transport

• Hemoglobin binds easily and reversibly with oxygen

• Normal hemoglobin levels in the blood are:

  • Males: 13-18 g/dL of blood

  • Females: 12-16 g/dL

Hemoglobin

• Hemoglobin consists of red heme pigments bound to the protein globin

  • Globin is composed of four polypeptide chains

  • Two alpha and two beta chains

  • Each binding a red, ringlike, heme group

• Each heme group contains and iron ion (Fe2+) at it’s center

  • The iron ion can reversibly bind to one oxygen molecule

• Each hemoglobin can carry 4 oxygen molecules

  • Single RBC contains about 250 million hemoglobin molecules

  • So single RBC can take up 1 billion oxygen molecules!

RBC cell count

• RBC do not leave the bloodstream

•  Red blood cell count

  • Male: 4.3-5.9 million/microliter (uL)

  • Female: 3.5- 5.5 million/microliter

Respiratory reminder

• O2 loading in lungs

  • Produces oxyhemoglobin

  • Changes shape and becomes ruby red

• O2 unloading in tissues

  • Produces deoxyhemoglobin

  • Changes shape, reduced hemoglobin appears dark red

• CO2 loading in tissues

  • 20% of CO2 in blood binds to globin chains in Hb, not the heme group

  • producing carbaminohemoglobin

  • Increasing CO2 levels decrease hemoglobin’s affinity for O2, bumping O2 off to diffuse into tissues

Hematopoiesis

• Hematopoiesis: formation of blood cells

  • Occurs in red bone marrow;

    • Soft network of reticular connective tissue

    • Containing wide capillaries: blood sinusoids

    • Network also contains:

      • Immature blood cells, macrophages, fat cells, reticular cells

  • In adult, red marrow found in axial skeleton, girdles, and proximal epiphyses of humerus and femur

Hematopoietic stem cells

• Hematopoietic stem cells

  • Undifferentiated precursor cells reside in red marrow

  • Stem cell that gives rise to all formed elements

    • Maturation pathways vary between formed elements

  • Once a cell is committed to a specific cell pathway it cannot change

  • Specific hormones and growth factors determine which pathway a cell will take

• New blood cells enter circulation via blood sinusoids

  • Marrow makes one ounce of new blood per day

    •  100 billion new cells

Erythropoiesis

• Erythropoiesis: process of formation of RBCs

• 8 stages 

  • 1. Hematopoietic stem cell produces a myeloid stem cell

  • 2. Myeloid stem cell transforms into proerythroblast

  • 3. Proerythroblast: divide many times, giving rise to basophilic erythroblasts

  • 4. Basophilic erythroblasts: synthesize many ribosomes, which stain blue, transforms into: Polychromatic erythroblasts

  • 5. Polychromatic erythroblasts:

    • Synthesize large amounts of red-hued hemoglobin; accumulates iron

    • Cell now shows both pink and blue areas

    • Transforms into Orthochromatic erythroblasts

  • 6. Orthochromatic erythroblasts:

    • Contain mostly accumulated hemoglobin

    • Appear just pink, ribosomes are masked by hemoglobin

    • Once full of hemoglobin, eject organelles and nucleus, ribosomes start to degrade collapses inward, causing concave shape, resulting in a: reticulocyte

  • 7. Reticulocyte: young RBC

    • Takes 15 days to form reticulocytes

    • Still contain small amount of ribosomes

    • Packed full of hemoglobin

    • Enter blood stream via sinusoids

    • Account for 1-2% of erythrocytes in blood

      • Reticulocyte count indicates approximate rate of RBC formation

  • 8. Erythrocyte (RBC)

    • Mature within two days of release into circulation

    • Once all ribosomes are degraded by intercellular enzymes

Regulation and Requirements for erythropoiesis

• Number of circulating erythrocytes in a given individual is constant

• Reflects a balance between: RBC production and destruction.

• Important:

  •  Too few erythrocytes leads to hypoxia

  • Too many makes blood more viscous

• 2 million per second are produced in a healthy individual

• Maintaining normal level of RBC depends on

  • Hormonal controls

  • Dietary Supplies of iron, amino acids, B vitamins

Hormonal controls of erythropoiesis

• Erythropoietin (EPO):

• Glycoprotein hormone, stimulates formation of RBCs

• Stimulates red marrow cells that are already committed to mature more rapidly

• Always small amount of EPO circulating in blood to maintain normal RBC count

• Kidneys produce and release EPO (liver produce small amt)

  • EPO released in response to hypoxia (low O2) by renal cells

    • Oxygen-sensitive enzymes can no longer degrade the intracellular signaling molecule: hypoxia –inducible factor (HIF)

    • HIF accumulates, triggering synthesis and release of EPO

• Causes of hypoxia:

  • Decreased RBCs due to increased loss (hemorrhage) or increased destruction

  • Insufficient hemoglobin per RBC (iron deficiency)

  • Reduced availability of O2 (high altitudes or lung problems that decrease ventilation)

• 2-3 days after a rise in EPO in the blood; the rate of reticulocyte release and the reticulocyte count rise markedly

• Inversely: Too many erythrocytes or high oxygen levels in blood inhibit EPO production

• Testosterone enhances EPO production which may be the reason males have higher RBC counts and hemoglobin compared to females

Dietary requirements

• Raw materials required for erythropoiesis:

  • Nutrients for the synthesis of cells

  • Amino acids

  • Lipids

  • Carbohydrates

• Two B-complex vitamins for normal DNA synthesis

  • Slight deficits jeopardize rapidly dividing cell populations (low can cause anemia)

    • Vitamin B12

    • Folic acid

Iron for blood

• Available from diet

• Intestinal cells precisely control absorption in response to changes in amount of body’s iron stores

• Iron stores in the body:

  • 65% of iron is found in hemoglobin

  • 35% stored in liver/ spleen >> bone marrow

• Free iron ions (Fe2+ , Fe3+) are toxic

  • Iron is stored inside the cells as protein-iron complexes

  • Ferritin – primary storage form of iron

  • Hemosiderin

  • Both complexes stores large amount of iron

  • Found in the spleen, liver, bone marrow, skeletal muscle

• Transported in blood bound to protein

  • Transferrin

• Iron lost daily to feces, urine, perspiration, and menstrual flow (women)

Fate and Destruction of Erythrocytes

• Life span - 100-120 days 

• RBCS are anucleate so cannot synthesize new proteins or grow or divide 

• Old RBCs become fragile, and Hb begins to degenerate

• Get trapped and fragmented in smaller circulatory channels, particularly in spleen

  • Spleen is the RBC graveyard

• Macrophages in spleen engulf and destroy dying RBCs

• Hemes are split off from globin

  • Iron binds to ferritin, hemosiderin (via transferrin) and is stored for reuse 

Heme vs globin

• Heme 

  • Heme is degraded into unconjugated bilirubin, released into blood, binds to albumin

  • Transported to the liver, where it is conjugated and added to bile, secreted into the small intestine

  • Small intestine metabolizes bilirubin into urobilinogen

  • In the large intestine most urobilinogen is broken down into stercobilin by bacteria in the lumen

  • Stercobilin is a brown pigment excreted in feces

• Globin

  • Metabolized into amino acids

  • Released into circulation to be reused

Leukocytes (overview)

• Only formed element in blood that is a complete cell with nuclei and organelles

• Make up <1% of total blood volume

  • 4,800 - 10,800 WBCs per microliter of blood (normal range of WBC count)

• Types:

  • Granulocytes

    • Neutrophils

    • Eosinophils

    • Basophils

  • Agranulocytes

    • Lymphocytes

    • Monocytes

Function of leukocytes

• Defense against disease/infections

  • Form mobile army to defend against damage by: virus, fungus, bacteria, parasite, toxins, tumor cells

  • Can leave capillaries via diapedesis (para/transcellular)

  • Use bloodstream as mode of transportation to area of body to mount inflammatory or immune response(s)

• Once in extracellular space leukocytes move by amoeboid motion (cytoplasmic extensions that move them) following chemical trail left by damaged cells, process referred to as positive chemotaxis

  • Gather in large numbers to destroy invader and/or damaged cells

  • Phagocytosis of foreign material and dead cells

Percentage of leukocytes

• Granulocytes - contain visible cytoplasmic granules

  • Neutrophils (50-70%

  • Eosinophils (2-4%)

  • Basophils (0.5-1%

• Agranulocytes: do not contain visible cytoplasmic granules

  • Lymphocytes (25-45%)

  • Monocytes (3-8%)

• Most abundant to least abundant in the blood:

  • Never Let Monkeys Eat Bananas

  • Neutrophils

  • Lymphocytes

  • Monocytes

  • Eosinophils

  • Basophils

Granulocytes

• Roughly spherical in shape

• Larger but much shorter lived than erythrocytes

• They have lobed nuclei (rounded nuclear masses connected by thinner strands of nuclear material)

• Membrane bound cytoplasmic granules stain with Wright’s

• Stain

  • Wright’s stain hematologic stain that facilitates the differentiation of blood cell types.

Neutrophils

• Most numerous WBCs

  • Account for 50–70% of WBCs

• Live 6 hours- few days

• About twice the size of RBCs

• Also called polymorphonuclear leukocytes (PMNs or polys) because many possible shapes of nucleus

  • Nucleus has anywhere from three to six lobes

• Neutrophil cytoplasm contains very fine granules, difficult to see

  • Granules stain with both acid (red) and basic (blue) dyes (neutral) looks purple

• Granules contain either

  • Lysosomes- hydrolytic enzymes

  • Defensins - antimicrobial proteins

• Are the body’s bacteria slayers/numbers increase explosively during acute bacterial infections

• Chemically attracted to sites of inflammation

• Are active phagocytes

  • Can ingest bacteria and some types of fungi into a vesicle called a phagosome

• Respiratory/oxidative burst: one way neutrophils kill bacteria

  • Triggered when a microbe is engulfed

  • During phagocytosis, the neutrophil quickly synthesizes potent oxidizing substances (bleach or hydrogen peroxide) that are released into the phagosome

  • Defensin granules also merge with microbe containing phagosome releasing its antimicrobial proteins

  • Defensins form “spears” that pierce holes in membrane of ingested microbe

Eosinophils

• Account for 2-4% of all leukocytes

  • Same size as neutrophils

• Lifespan ~ 5 days

• Bilobed nucleus (purple) connected by a broad band of nuclear material (resembles ear muffs)

  • Granules- red

• Cytoplasm packed with large course granules, stain brick red, contain digestive enzymes

• Attack parasitic worms, too large to be phagocytized

  • Release enzymes to digest their surface

  • Flatworms – tapeworms/flukes

  • Round worms – pinworms/hookworms

• Worms enter body via ingestion or through skin, burrow to respiratory or intestinal mucosa, where many eosinophils reside

• Do not kill bacteria

Also play role in allergies and asthma, as well as immune response modulators

Basophils

• Rarest WBCs, accounting for only 0.5–1% of leukocytes

• Lifespan few hours- few days

• Bilobed nucleus, deep purple generally U or S shaped

• Large, purplish black (basophilic) granules contain histamine

  • Histamine: inflammatory response/allergic reaction chemical that acts as vasodilator and attracts WBCs to inflamed sites

• Are functionally similar to mast cells, arise by different cell lines (Mast cells found in connective tissue)

  •  Bind to particular antibodies, immunoglobulin E, and cause the release of histamine

Agranulocytes

• Lack visible cytoplasmic granules

• Nuclei are typically spherical or kidney shaped

• Include:

  • Lymphocytes

  • Monocytes

• Structurally similar to each other but functionally distinct, and unrelated (from different cell linage lines)

Lymphocytes 

• Second most abundant WBC, accounts for about 25%

• Lifespan = hours - years

• Large, dark purple, circular nuclei with thin rim of blue cytoplasm

• Classified by size/diameter: small, medium, large

• Large number exist in body, but very few in bloodstream

• Mostly found in lymphoid tissue (lymph nodes, spleen, etc.)

• Play a crucial role in immunity

• Two types 

  • T lymphocytes (T cells) act directly against virus-infected cells and tumor cells

  • B lymphocytes (B cells) give rise to plasma cells

    • Plasma cells produce antibodies (immunoglobulins)

    • Released into the blood

Monocytes

• Largest of all leukocytes

• 3-8% WBCs

• Lifespan = months

• Abundant pale blue cytoplasm

• Dark purple-staining nucleus, U- or kidney-shaped

• When they leave circulation and enter tissues, they differentiate into

• highly mobile macrophages:

• Macrophages are actively phagocytic cells; crucial against:

  • Viruses

  • intracellular bacterial parasites

  • chronic infections

• Can activate lymphocytes to mount an immune response

Leukopoiesis

• Leukopoiesis: production of WBCs

  • Stimulated by two types of chemical messengers

  • Released from supporting cells of red bone marrow and mature WBCs

• Hematopoietic factors are the chemical messengers

  •  Interleukins are numbered (e.g., IL-3, IL-5)

  • Colony-stimulating factors (CSFs) are named for WBC type they stimulate (e.g., granulocyte-CSF stimulates granulocytes)

• Signal WBC precursors to divide and mature

• Enhance potency of mature leukocytes

Leukocyte origin

• All leukocytes originate from hematopoietic stem cell that branches into two pathways:

  • lymphoid stem cells produces

    • B and T lymphocytes

  • Myeloid stem cells produce all other elements

    • All Granulocytes

    • Monocytes

Granulocyte production

 Hematopoietic stem cell gives rise to Myeloid stem cell which differentiates into:

2. Myeloblasts: the committed cell line, accumulate lysosomes

3. Become Promyelocytes:

4. Myelocytes: accumulate distinctive granules (of particular granulocyte type)

At this stage cell division stops

5. Band cells: nuclei become U shaped

6. Mature granulocyte: nuclei become segmented just before being released in blood

10× more mature granulocytes are stored in bone marrow than in blood

3× more granulocytes are formed than RBCs

Because they have a shorter life, most die fighting microbes

Agranulocyte production

• Monocytes: derived from myeloid line 

  • Hematopoietic stem cell → Myeloid stem cell → Monoblast → promonocyte → monocyte → (macrophage)

  • Share common precursor with neutrophils/granulocytes

  • Can live for several months

• Lymphocytes: derived from lymphoid line

  • Hematopoietic stem cell lymphoid stem cell

  • B or T Lymphocyte precursor cells

  • T lymphocyte precursors give rise to Immature T lymphocytes that leave marrow and go to thymus to further differentiate/ mature

  • B lymphocyte precursors give rise to mmature B lymphocytes mature within bone marrow

Platelets (overview)

• Fragments of megakaryocyte

  • Megakaryocyte are extremely large cells found only in red bone marrow

• Lighter staining outer region; inner area with purple staining granules, and large nucleus

• Granules contain several chemicals involved in clotting process

  • Serotonin, calcium, enzymes, ADP, platelet-derived growth factor (PDGF)

Function of platelets

• Function: essential for the clotting process to stop blood loss when a vessel is damaged

  • Form temporary platelet plug that helps seal break(s) in blood vessel

  • Anucleate so they age and die quickly, about 10 days, if they are not involved in clotting

  • Circulating platelets are kept mobile but inactive by: nitric oxide (NO) and prostacyclin

  • Both secreted from endothelial cells lining blood vessels

Platelet formation

• Formation is regulated by the hormone thrombopoietin

• Hematopoietic stem cell  myeloid stem cell Megakaryoblast (stage I megakaryocyte) stages ⅔

• Stage IV megakaryocyte (mature form)

  • Bizarre shaped cell with huge multilobed nucleus, and large cytoplasmic mass

• Mature megakaryocyte press against sinusoid capillary

  • Sends cytoplasmic projections into lumen of capillary

  • Projections break off into disc shaped platelet fragments

• Platelets age quickly and degenerate in about 10 days

• Normal = 150,000– 400,000 platelets/ml of blood

Blood typing

• Determined genetically

• Surface proteins are referred to as antigens or agglutinogens

• A persons immune system does not respond to its own RBC surface antigens

• RBC surface markers become true antigens if exposed to another persons immune system that doesn’t “match”

  • RBC antigen detection and the immune systems response can cause a transfusion reaction: agglutination (clumping together) so they are also referred to as agglutinogens

• Situations where one would encounter another’s blood antigens

  • Transfusion

  • Pregnancy/delivery

• There are at least 30 groups of naturally occurring RBC antigens

• The presence or absence of these antigens allow us to classify a persons blood type into a specific group

• The antigen groups that are highly associated with serious transfusion reactions are the ABO and Rh blood groupings

• Therefore these blood typings are always done prior to blood transfusion

• Further typing for other antigens is possible if multiple transfusions are anticipated/needed

ABO Blood Groups

• Based on presence or absence of agglutinogens A and/or B

  • Type A

  • Type B

  • Type AB (both)

  • Type O (neither)

• Unique to the ABO groups is the presence of preformed antibodies or agglutinins circulating in the plasma

• These act against RBCs carrying ABO antigens that are not present on a persons own RBCs

• Circulating in the blood, do not require an immune response

Blood Types

• Type A blood contains

  • A antigen (agglutinogen)- RBC

  • Anti-B antibody (agglutinins)- plasma

• Type B blood contains

  • B antigen

  • Anti-A antibody

• Type AB blood contains

  • BOTH the A and B antigen

  • NO antibody

  • Universal recipient, no antibodies

  • Difficult to donate due to both antigens present

  • Least prevalent in North America

• Type O blood contains

  • NO antigen

  • BOTH Anti-A AND Anti-B antibody

  • Universal donor

  • Difficult to receive as it has both antibodies

  • Most prevalent in North America

Rh Blood Groups

• Rh antigen (agglutinogen D)

  • Positive- carry the antigen D

    • 85% of Americans

  • Negative- have no antigen D

• Unlike ABO Rh antibodies do not spontaneously form in the blood

  • But if Rh negative person receives RH positive blood the immune system will produce the antibodies

  • First exposure may not result in hemolysis but second exposure and there after would

• Rh Typing 

  • Rh negative

    • Donate to Rh+ and Rh-

    • Receive only Rh-

  • Rh positive

    • Donate only to Rh+

    • Receive from Rh+ and Rh-