Lecture 9: blood, RBCs, platelets, hemostasis (66 cards)

Blood is classified as

A fluid connective tissue.

List the 5 functions of blood.

1) Transport (gases, nutrients, waste, hormones). 2) Regulation of pH and ion concentration in interstitial fluid. 3) Restriction of fluid loss in injury (clotting). 4) Defense against toxins and pathogens. 5) Stabilization of body temperature.

Basic blood facts: temperature, viscosity, pH

About 38 C (slightly higher than average body temp); about 5x more viscous than water; slightly alkaline, pH 7.35-7.45.

Blood volume in males vs females

Males 5-6 L; females 4-5 L.

Whole blood = ?

Plasma (about 55%) + formed elements (about 45%).

What is plasma?

The noncellular fraction of blood, made of fluid and solutes (ions and proteins). About 92% water, protein rich, ion rich.

What are the formed elements?

The cellular fraction of blood: red blood cells, white blood cells, and platelets.

Plasma vs interstitial fluid: similarities and differences

Similar: about 92% water and similar ion concentrations. Different: O2 and CO2 levels, and protein concentration and types of proteins.

Albumins: percentage and functions

About 60% of plasma proteins. Maintain osmotic pressure and transport fatty acids, thyroid hormones, and steroid hormones.

Globulins: percentage and functions

About 35% of plasma proteins. Include immunoglobulins (antibodies) and transport globulins (bind insoluble molecules: metalloproteins, hormones, lipoproteins).

Fibrinogen: percentage and function

About 4% of plasma proteins. Precursor to the clotting protein fibrin.

What is serum?

Plasma with the fibrinogen removed.

Where are plasma proteins made?

About 90% are secreted by the liver (albumins, fibrinogen, globulins). Immunoglobulins are secreted by white blood cells; peptide hormones by glands and organs.

Blood after centrifugation WITH anticoagulant

Three layers: plasma on top, buffy coat (platelets and leukocytes) in the middle, erythrocytes on the bottom.

Blood after centrifugation WITHOUT anticoagulant

Blood clots: serum on top and the clot (cells plus fibrin) on the bottom.

What is the buffy coat?

The thin layer between plasma and RBCs after centrifugation, made of platelets and leukocytes.

Erythrocytes: function, number, and hematocrit

Red blood cells that transport O2 and CO2. Most numerous cell in the body (about 30 trillion; 5-6 million per uL). Hematocrit is the medical measure of the amount of RBCs.

Normal counts: leukocytes and platelets per uL

Leukocytes 5,000-10,000 per uL; platelets 250,000-400,000 per uL.

What are platelets?

Cell fragments that function in blood clotting.

Which stains are used for blood smears?

Romanowsky-family stains: contain eosin like H&E, but use multiple blue-purple dyes instead of hematoxylin, which allows differentiation of leukocyte subtypes.

What is hematopoiesis and where does it occur?

Formation of blood cells in red bone marrow (immature nucleated RBCs and WBCs, megakaryocytes, and sinusoidal capillaries are visible).

What are megakaryocytes?

Very large bone marrow cells with a multilobed nucleus. Each produces several thousand platelets by budding off and shedding most of its plasma membrane and cytoplasm.

Why are bone marrow sinusoidal capillaries important?

Their discontinuous endothelium allows newly matured cells to enter circulation (movement across is still regulated).

Which stem cell line gives rise to most blood cells and platelets?

The myeloid line. The lymphoid line forms a small minority of leukocytes but is critical for adaptive immunity.

What hormone drives erythrocyte differentiation and where is it made?

Erythropoietin (EPO), secreted by the kidney when blood oxygen, blood pressure, or blood volume is low. Androgens, growth hormone, and thyroid hormone also indirectly drive RBC formation.

Order of RBC maturation in bone marrow

Proerythroblast (day 1) -> basophilic erythroblast (day 2) -> polychromatophilic erythroblast (day 3) -> normoblast (day 4) -> nucleus ejected -> reticulocyte (days 5-7, enters circulation) -> mature RBC.

When does hemoglobin synthesis begin and how far along is it by the normoblast stage?

Begins at the basophilic erythroblast stage (day 2); about 80% done by the normoblast stage (day 4).

What percent of a mature RBC's macromolecular content is hemoglobin?

About 95%. Hb can be used as a marker of maturation level.

RBC shape and size

Biconcave disc, about 8 um in diameter.

Why do mature RBCs lack a nucleus?

It lets them bend and squeeze through tiny capillaries (as small as 4 um in diameter).

Why is the biconcave shape useful?

Large surface-to-volume ratio, which is good for easy gas exchange.

What is rouleaux?

RBCs stacking on each other like coins (RBCs are stackable).

What do mature RBCs lack, and what are the consequences?

No nucleus (no gene transcription changes), no ribosomes (no translation changes), and no mitochondria (they would steal O2 from hemoglobin). RBCs cannot divide or repair themselves.

RBC lifespan and turnover

About 120 days; about 1% of RBCs are replaced per day. At the end of life they must be disposed of by the body.

Hemoglobin structure

Heterotetramer: 2 alpha and 2 beta subunits. Each subunit carries a heme molecule that chelates an iron atom (Fe2+).

How many oxygen molecules does one RBC carry?

About 1 billion.

How much oxygen is dissolved in plasma vs bound to hemoglobin?

Only about 1.5% is dissolved in plasma; the vast majority is bound to hemoglobin.

What is the Bohr effect?

Hemoglobin's affinity for O2 decreases as acidity increases (pH falls). CO2 from tissues forms carbonic acid, lowering pH and promoting O2 release from Hb in tissues.

Write the CO2 / carbonic acid reaction (with the enzyme)

CO2 + H2O

Which direction does the oxygen-Hb curve shift as pH drops?

To the right: lower O2 saturation at a given PO2 (lower affinity), so more O2 is released.

Hemoglobin subunit changes during development

Embryonic hemoglobin uses epsilon chains; fetal hemoglobin uses gamma chains; adult hemoglobin uses beta chains (plus a small amount of HbA2 with delta chains).

Why does fetal hemoglobin (gamma chain) have higher O2 affinity, and why does that matter?

Higher affinity lets the fetus pull O2 from maternal blood across the placenta (the fetal curve is shifted left of the adult).

What causes sickle cell disease?

Mutations in the beta hemoglobin gene. RBCs gradually assume a rigid sickle shape that hinders circulation and dramatically shortens cell lifespan.

Why does anemia occur in sickle cell disease?

RBC production cannot keep up with RBC death.

Treatments that can cure sickle cell disease

Bone marrow transplants and gene therapy (2023).

What happens to old RBCs?

About 90% are phagocytosed and recycled by macrophages (concentrated in the spleen); about 10% lyse (hemolysis). Hb from lysed cells is excreted in urine and lost.

What does elevated hemoglobin in urine indicate?

A clinical sign of damage to RBCs.

Heme breakdown pathway

Heme -> biliverdin (after losing Fe2+) -> bilirubin. These pigments give bruises their color and are excreted by the liver (bile), in urine, or in feces.

What is recycled and what is eliminated from hemoglobin?

Amino acids and iron (Fe2+) are reused; heme metabolites (bilirubin etc.) are eliminated.

Iron needs for RBC production

About 26 mg/day is needed, but efficient recycling means only 1-2 mg/day must be consumed. Iron is carried in blood by transferrin.

What causes jaundice?

Blockage of elimination of heme metabolites (bilirubin).

What is hemostasis?

The response that stops blood loss after vessel damage. It begins almost immediately and has 3 phases.

What are the 3 phases of hemostasis and their timing?

Vascular phase (almost immediate), platelet phase (starts in under 15 seconds, overlaps with vascular), coagulation phase (slower, starts after more than 30 seconds).

Vascular phase: what happens and what mediates it?

Vascular spasm (smooth muscle contraction near the injury) triggered by local signals, especially endothelins (peptide hormones). Endothelial cells also contract (exposing the extracellular matrix) and become sticky with cell adhesion proteins for platelets.

Why do endothelins also act as growth factors?

They stimulate nearby endothelial, smooth muscle, and fibroblast cells to divide so the vessel wall can be repaired.

Platelet phase: what happens?

Thrombocytes (platelets) adhere to endothelium/exposed matrix and aggregate with each other, forming a platelet plug in the gap in the vessel wall.

What do platelets release during the platelet phase?

Thromboxanes (continue vessel contraction), clotting factors (start coagulation), growth factors (promote cell division for repair); also ADP and Ca2+.

Coagulation phase: what happens?

Fibrinogen is converted to insoluble fibrin by thrombin. The fibrin network combines with the platelet plug to complete the clot.

Name the 3 coagulation pathways

Extrinsic pathway, intrinsic pathway, and common pathway.

Extrinsic pathway begins with

Clotting factors from outside the blood, in damaged tissue (tissue factor, factor III, plus factor VII and Ca2+).

Intrinsic pathway begins with

Clotting factors that circulate in the blood itself (activated proenzymes, usually factor XII; factors VIII, IX; platelet factor PF-3; Ca2+).

Common pathway

Point of convergence of extrinsic and intrinsic pathways. Begins with activation of factor X, which leads to prothrombinase, then prothrombin -> thrombin, then fibrinogen -> fibrin.

Which enzyme ends the common pathway?

Thrombin, which converts soluble fibrinogen into insoluble fibrin.

Which clotting factors are critical inputs?

Calcium ions (factor IV) and vitamin K (needed to synthesize factors II, VII, IX, and X).

How are positive feedback loops in clotting terminated?

By anticoagulants such as heparin, released by basophils when they arrive at the injury.

What is thrombocytopenia?

An abnormally low platelet count. Causes include reduced production (nutritional deficiencies, sepsis, genetic causes) or increased usage/destruction (autoimmune disorders, viral infections). Bruising is a typical sign.