Anatomy 2 lecture class 2

Lipoproteins, Cholesterol Transport, and Clinical Connections

Lipids in membranes and transport particles: a layer composed of phospholipids, cholesterol, other lipids, and proteins
Apoproteins: specialized proteins that combine with receptors on target cells to help lipoproteins attach to cells
Primary delivery role: lipoproteins deliver lipid components to other cells; liver is a major target for LDLs because liver cells have receptors for the apoproteins, enabling cholesterol supply
Composition: lipoproteins consist of protein and lipid components; density is determined by lipid-protein ratio
Density relationship:
- If lipid content is higher than protein, density is lower (LDL: low density lipoprotein; high cholesterol content)
- If protein content is higher than lipid, density is higher (HDL: high density lipoprotein; more protein, less lipid)
Examples and roles:
- LDLs: low protein, high cholesterol; major cholesterol-carrying molecules
- HDLs: high protein content, relatively low lipids; formed in liver and small intestines
- Chylomicrons: transit remnants to the liver; liver disposes of waste in bile
- Very low density lipoproteins (VLDLs, sometimes referred to as BLDL in notes): another lipoprotein class
Receptor-mediated uptake: liver cells have surface receptors that bind apoproteins on LDLs; LDLs are removed by endocytosis to deliver cholesterol
Blood tests: common to look at LDL/HDL ratios to assess cholesterol-related risk
Genetic condition: familial hypercholesterolemia (documented in notes as amelior hypocholesterolemia)
- Mechanism: lack of functional LDL receptor
- Consequence: elevated blood cholesterol with deposition in abnormal places (blood vessels, joints); promotes atherosclerosis
- Clinical outcome: early heart attacks; cholesterol can be high even in individuals who appear healthy or athletic
- Real-world context: an anecdote about a fitness-minded individual needing a bypass at a young age despite healthy habits
Real-world schemes and cautions:
- Visual media story about cholesterol-related commercial to illustrate public perception
- Emphasizes that outward health does not guarantee normal lipid handling
Transitional note: after discussing plasma and lipoproteins, focus shifts to red blood cells and blood components

Blood Components: Plasma vs Formed Elements

Blood has living (formed elements) and nonliving components (plasma)
Tissue context: blood is connected tissue (epithelial, connective, muscle, nervous); the plasma is nonliving, while formed elements are living
Formed elements categories:
- Erythrocytes (red blood cells)
- Leukocytes (white blood cells)
- Platelets (cell fragments)
True cells vs non-nucleus cells:
- White blood cells are true cells (nucleus present with cellular organelles)
- Red blood cells lack a nucleus and most organelles; platelets are fragments, not full cells
Lifespans and turnover:
- Red blood cells: ~100–120 days
- Platelets: ~5–10 days
- White blood cells: variable lifespans (some hours to days, some months or years depending on type)
Hematopoietic origin:
- All formed elements originate from hematopoietic stem cells in the red bone marrow (hemocytoblasts)
Practical point: blood diseases may be treated with stem cell transplantation (donor stem cells repopulate hematopoietic activity)
Clear distinction between donating whole blood (temporary cells) vs stem cell transplant (replaces stem cell source)

Red Blood Cells (Erythrocytes): Structure and Function

Role: primary oxygen transporters
Shape and size:
- Biconcave discs, thinner in the center and thicker at the rim
- Diameter around $7.5 ext{ μm}$
- Shape maximizes surface area-to-volume for rapid oxygen diffusion and efficient loading/unloading of O2
Key components:
- Lacking mitochondria and nucleus in mature form; retains cytoskeletal proteins (spectrin) for membrane flexibility
- RBC interior mainly filled with hemoglobin
Why no mitochondria? To avoid consuming the oxygen they are transporting; ATP is produced anaerobically (glycolysis) to preserve oxygen for tissues
Population and impact:
- RBCs are the most abundant formed element; contribute to about $45\%$ of blood volume (hematocrit ~0.45)
- Normal RBC count per microliter (per cubic millimeter): roughly $4.3\text{–}5.8 \times 10^6 \, ext{cells}/\mu L$ (males often higher than females due to testosterone effects on erythropoietin)
Visual concept: RBCs form stacks (rouleaux) as they pass through small vessels
Age-related changes and clinical clues:
- Abnormal RBC size, shape, or texture (e.g., sickle cell, target cells) reflect disease

Hematopoiesis and Erythropoiesis

Hematopoiesis (hemopoiesis): blood cell production; occurs in red bone marrow
Daily production: about $1.0\times 10^{11}$ new cells per day
Stem cell lineages:
- Hemocytoblast (hematopoietic stem cell) gives rise to two main lines: lymphoid and myeloid
- Lymphoid stem cells yield lymphocytes (B, T, NK cells)
- Myeloid stem cells yield erythrocytes, platelets, and four of the five white blood cell types
Erythropoiesis (erythrocyte production) details:
- Triggered by erythropoietin (EPO), a hormone that stimulates erythrocyte production
- Most EPO is produced by the kidneys; testosterone enhances EPO production, contributing to higher male RBC counts
Key stages in the erythroblast lineage (commitment to erythrocyte):
- Hemocytoblast → myeloid stem cell → proerythroblast (commitment begins)
- Early erythroblast: massive ribosome production to synthesize hemoglobin
- Late erythroblast: intense hemoglobin synthesis; iron accumulation for heme production
- Nucleus and organelle degradation: nucleus deteriorates and is ejected; major organelles are degraded and removed
- Formation of the biconcave shape occurs after nucleus expulsion
- Reticulocyte: immature RBC released into bloodstream; will mature within 1–2 days
Reticulocyte count as a clinical index:
- Normal circulating reticulocyte percentage: $1\%\text{–}2\%$ of RBCs
- Indicates rate of RBC production; higher or lower percentages reflect compensatory responses to blood loss or anemia
Hematopoietic stem cell sources:
- Red bone marrow is the site of hematopoiesis; stem cells reside in the marrow of flat bones and the epiphyses of long bones
- Bone marrow transplant uses donor stem cells to repopulate the recipient’s hematopoietic system
Nutritional and hormonal requirements for erythropoiesis:
- Iron, B vitamins (folic acid and B12) are essential for DNA synthesis and Hb production
- Intrinsic factor (in stomach) is required for B12 absorption; pernicious anemia occurs with insufficient intrinsic factor, requiring injections of B12
- Iron supplementation for iron-deficiency anemia; not all anemia responds to iron (e.g., pernicious anemia)
Other hormones and factors:
- EPO as the key stimulant for RBC production
- Leuko-/thrombopoiesis require their own regulatory signals (cytokines, interleukins) from immune cells and other tissues
Practical lab considerations:
- Distinguishing reticulocytes from mature erythrocytes in blood work helps gauge production rate
- Endogenous RBC production balances destruction to maintain homeostasis

Hemoglobin: Structure and Oxygen Transport Mechanism

Hemoglobin (Hb) composition:
- Tetrameric protein: four polypeptide chains (two alpha, two beta)
- Each chain contains a heme group with an iron ion at its center
- Each heme can bind one O2 molecule; thus one Hb can carry up to $4$ O2 molecules
Globin and heme details:
- Globin portion comprises the four polypeptide chains
- Heme group provides the red pigment; iron centers bind oxygen
Oxygen transport concepts:
- In lungs: hemoglobin binds O2 to form oxyhemoglobin
- In tissues: hemoglobin releases O2 (deoxyhemoglobin form)
- The relationship between oxygen and hemoglobin is strong but not permanent; the affinity changes with environment (lungs vs tissues)
Carbon dioxide transport and carboxyhemoglobin:
- Hb can also transport CO2, typically as carbaminohemoglobin (CO2 attaches at sites different from the iron-oxygen site)
- About $20\%\text{–}23\%$ of CO2 is transported bound to hemoglobin (carbaminohemoglobin)
- Carbon monoxide (CO) poisoning risk: CO binds to the same iron site as O2 with very high affinity, displacing O2 and forming carbon monoxide-hemoglobin complex; detachment is slow, causing hypoxia
Carbon monoxide poisoning details:
- Symptoms can be mild and delayed; severe cases require hyperbaric oxygen therapy to displace CO from Hb
- Prevention: CO detectors, proper exhaust maintenance, and safety measures in enclosed spaces
RBC color and hemoglobin status terms:
- Oxyhemoglobin: hemoglobin bound to O2 (bright red)
- Deoxyhemoglobin: no O2 bound (darker red)
- Carbaminohemoglobin: hemoglobin bound to CO2
Quantitative perspective:
- Inside one RBC: about $2.5\times 10^8$ Hb molecules
- Each Hb molecule carries up to $4$ O2 molecules, so one RBC can transport up to $4 \times 2.5\times 10^8 = 1.0\times 10^9$ O2 molecules
- Typical RBC count per μL: roughly $5.0\times 10^6$ RBCs/μL
- Total O2 transported per μL: $5.0\times 10^6 \times 1.0\times 10^9 = 5.0\times 10^{15}$ O2 molecules
Why four chains and four heme groups matter:
- Each Hb tetramer has four heme groups, each with its own iron center; enables high O2-carrying capacity
Visualizing Hb structure (conceptual):
- Two alpha chains and two beta chains; each chain has a heme group with an outer iron core inside the heme disc
- Oxygen binds to iron in the center of each heme group

Oxygen Transport: Diffusion, Diffusion Limits, and RBC Design Rationale

Diffusion and surface area considerations:
- RBCs are designed with a large surface area-to-volume ratio to maximize oxygen pickup and delivery efficiency
- The thin central region and rim design help saturate Hb with O2 in the lungs and release it in tissues
Oxygen-hemoglobin relationship nuances:
- Oxygen loading in lungs is favored by high O2 partial pressure and pH changes; unloading occurs in tissues with higher CO2, lower pH, or higher temperature
Spectrin and membrane flexibility:
- Spectrin proteins form a network (spectrin net) that maintains membrane integrity and provides flexibility to deform as RBCs pass through narrow capillaries
- Aging RBCs lose membrane flexibility and are more likely to be cleared in the spleen (red blood cell graveyard)
Spleen and RBC turnover:
- The spleen serves as a major site of RBC destruction, contributing to RBC lifespan and recycling

Erythrocyte Lifecycle and Destruction: Pathways and Clinical Insights

Lifespan andReplacement:
- RBCs live about $100\text{–}120\text{ days}$ ; platelets $5\text{–}10\ days; white blood cells vary widely</li></ul></li><li>Hemolysis and bilirubin:<ul><li>Destruction of RBCs releases hemoglobin, which is broken down into bilirubin and other waste products (upcoming cliffhanger topic)</li></ul></li><li>Hematocrit and blood viscosity:<ul><li>The number of RBCs contributes to blood viscosity; too many RBCs thickens blood (high viscosity) and can impair flow; too few reduces oxygen-carrying capacity</li></ul></li><li>Clinical importance of counts and indices:<ul><li>Hematocrit, RBC count, and hemoglobin levels are used together to diagnose and monitor anemia and other hematologic conditions</li></ul></li></ul><h3 collapsed="false" seolevelmigrated="true">Hematopoietic Stem Cells and Lineage Commitment</h3><ul><li>Hemocytoblasts (hematopoietic stem cells) reside in red bone marrow and give rise to all formed elements</li><li>Lineage bifurcation:<ul><li>Lymphoid stem cell → lymphocytes</li><li>Myeloid stem cell → erythrocytes, platelets, and most leukocytes</li></ul></li><li>Commitment concept:<ul><li>Once a cell becomes a committed erythroblast, it cannot switch to a different lineage (e.g., cannot become a platelet once committed to erythroid path)</li></ul></li><li>Proerythroblast and onward:<ul><li>Proerythroblast marks the start of erythroid commitment; subsequent steps lead toward mature erythrocyte</li></ul></li><li>Practical takeaway:<ul><li>Bone marrow transplants rely on donor hematopoietic stem cells to reconstitute all blood cell lineages in the recipient</li></ul></li></ul><h3 collapsed="false" seolevelmigrated="true">Erythropoiesis: Key Molecules, Hormones, and Nutritional Needs</h3><ul><li>Erythropoietin (EPO):<ul><li>The hormone that stimulates erythropoiesis</li><li>Primary source: kidneys (most EPO); liver also contributes at times</li><li>Testosterone enhances EPO release, contributing to higher male RBC counts</li></ul></li><li>Nutritional requirements:<ul><li>Iron: essential for heme synthesis</li><li>B vitamins: folic acid and B12 necessary for DNA synthesis in erythroid precursors</li><li>Intrinsic factor is required for B12 absorption; pernicious anemia results from B12 deficiency due to lack of intrinsic factor; treatment often requires B12 injections</li></ul></li><li>Pathologies linked to nutrients:<ul><li>Iron deficiency anemia treated with iron supplementation; not all anemias respond to iron if the underlying problem is B12 deficiency or intrinsic factor issues</li></ul></li><li>Hormonal and regulatory details:<ul><li>Cytokines and interleukins regulate white blood cell production, not directly erythropoiesis; they’ll be covered when discussing immune system regulation</li></ul></li><li>EPO and athletic performance:<ul><li>Blood doping with exogenous EPO or similar strategies can enhance RBC production and oxygen-carrying capacity, but carry significant health risks</li></ul></li><li>Summary:<ul><li>Erythropoiesis integrates stem cell biology, hormonal control, and nutrient availability to maintain adequate oxygen transport capacity</li></ul></li></ul><h3 collapsed="false" seolevelmigrated="true">Practical Lab Concepts: Counts, Units, and Interpretation</h3><ul><li>Units and measurements:<ul><li>RBC counts are typically expressed per cubic millimeter or per microliter: approximately$ 4.3\text{–}5.8 \times 10^6\, \text{cells/μL} $(males often higher)</li><li>Hematocrit (Hct) is the fraction of blood volume occupied by RBCs; often around$ 0.40\text{–}0.54 $depending on sex and age</li><li>Reticulocyte percent: typically$ 1\%\text{–}2\%$$ of circulating RBCs; used as a crude index of RBC production rate
Sex differences:
- Higher RBC counts in males due to testosterone-driven EPO activity
Interpreting changes:
- An elevated reticulocyte