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Iron Deficiency Anemia

Subject: Pathology Topic group: Microcytic Anemias

Iron deficiency anemia is a microcytic anemia caused by decreased iron availability for hemoglobin synthesis. The core exam pattern is low MCV, low ferritin, low serum iron, high TIBC, and low transferrin saturation.

Interactive Lab Simulator

Microcytic Anemia Lab Simulator

This simulator is for microcytic anemia patterns. Use the smear + iron studies to narrow the diagnosis.

Visual smear pattern

Small pale cells with more size variation suggest depleted iron stores.

Smear view

Real peripheral blood smear showing thalassemia target cells

Hover over the smear to make the cells react.

Diagnosis snapshot

Iron Deficiency Anemia

Classic microcytic pattern

Iron deficiency shows depleted stores and a compensatory TIBC increase, with more variable RBC size.

Ferritin Low

Serum Iron Low

TIBC High

RDW High

MCV Low

Key clue

Empty stores → body increases transferrin/TIBC.

Common trap

Do not confuse iron deficiency with anemia of chronic disease just because both can have low serum iron.

✦ HEY — pay attention

AUC + NBME favorite

Professors at AUC love iron deficiency anemia, and it is also a major Step 1 concept. The first move is recognizing the patient context: a young woman with fatigue, pallor, and heavy menstrual bleeding should immediately make you include iron deficiency anemia in the differential.

Core Lab Pattern

Ferritin Low

Serum iron Low

TIBC High

RDW High

MCV Low

NBME rule: Low ferritin = iron deficiency anemia until proven otherwise.

What Is Iron Doing in This Case?

Follow the iron story

Do not memorize isolated facts. Follow the sequence: empty stores → poor hemoglobin synthesis → extra RBC divisions → small pale cells.

🧰 Iron stores become depleted

There is not enough stored iron available to support normal hemoglobin production.

🩸 Hemoglobin synthesis decreases

With less usable iron, the marrow cannot build hemoglobin at the normal rate.

➗ Bone marrow forces extra RBC divisions

The cell keeps dividing while trying to reach an adequate hemoglobin concentration.

🔬 Final RBC becomes microcytic and hypochromic

Small cell, pale center.

Why this matters: Extra divisions create smaller cells. Less hemoglobin creates a larger pale center.

How Does a Patient Develop Iron Deficiency Anemia?

Recognize the patient context

Think in three buckets: losing iron, not absorbing iron, or needing more iron.

#1 NBME Favorite

Chronic Blood Loss

Heavy menstrual bleeding → classic young woman clue
GI bleeding in older adults → especially older adults
Parasites: lower yield, but possible depending on context

Absorption problem

Decreased Iron Intake or Absorption

Poor diet
Celiac disease/sprue
Gastric surgery
Low gastric acid → decreased iron absorption

Need more iron

Increased Demand

Pregnancy
Growth

Classic Board Triad

Plummer-Vinson Syndrome

If you see a question about a middle-aged woman with iron deficiency anemia who suddenly can’t swallow her food, stop and look for this triad.

🩸

Severe IDA Microcytic anemia

🕸️

Esophageal Webs Upper esophagus

🍽️

Dysphagia Solids, not liquids

Physical Exam Buzzwords: Look for koilonychia (spoon-shaped nails), pica, and a beefy red tongue from glossitis.

The Hepcidin Switch

The Hepcidin Switch: IDA vs Anemia of Chronic Disease

Understanding hepcidin is essential because both iron deficiency anemia and anemia of chronic disease can have low serum iron. The difference is where the iron is. In IDA, the body is truly out of iron. In anemia of chronic disease, iron is present but trapped.

✦ HEY — pay attention

AUC/NBME Favorite

Low ferritin = empty iron stores. Normal/high ferritin + low iron + low TIBC = iron is trapped by hepcidin.

Iron exits through ferroportin

Enterocyte / Macrophage

Stored iron

Hepcidin

Ferroportin

Low hepcidin → gate open

Transporter status OPEN

Bloodstream

Iron stored in enterocytes/macrophages.

Ferroportin open.

Iron enters blood for hemoglobin synthesis.

Diagnosis

Iron Deficiency Anemia

Iron can move into blood.

Why it behaves this way

The body lowers hepcidin to absorb and mobilize iron. The problem is that total iron stores are depleted.

Low ferritin • Low serum iron • High TIBC

NBME Takeaway

Pattern first

If ferritin is low, think iron deficiency first. If serum iron is low but ferritin is normal/high and TIBC is low, think anemia of chronic disease. The hepcidin switch explains the difference.

High-Yield / Test Favorites

NBME Clinical Pearls

↔

Low iron? Check TIBC.

High TIBC → iron deficiency. Low TIBC → anemia of chronic disease.

★

Young woman + fatigue = think IDA

Especially with heavy menstrual bleeding. This is a classic AUC/NBME setup.

◌

High RDW helps confirm IDA

Variation in RBC size (anisocytosis) is common in iron deficiency.

If ferritin is low, think iron deficiency first. If iron is low but ferritin is normal/high, think hepcidin and anemia of chronic disease.

Topic Practice

Lock It In

Make sure you can recognize the pattern before moving on.

Question 1 of 5

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✦ HEY — pay attention

AUC + NBME favorite

Do not confuse thalassemia with iron deficiency anemia. Both are microcytic and hypochromic, but thalassemia usually keeps serum iron and ferritin normal or high. If the vignette mentions target cells and a normal RDW, think thalassemia trait first.

Genetics

Deletions vs mutations

Alpha chain logic

Alpha-Thalassemia

Gene deletions on chromosome 16

Chromosome 16

α α

Parent 1 Parent 2

4 total α genes → severity depends on how many are deleted.

1 deletion: Silent carrier 2 deletions: Trait 3 deletions: HbH disease 4 deletions: Hydrops fetalis

Beta chain logic

Beta-Thalassemia

Gene mutations on chromosome 11

Chromosome 11

β β

Maternal β gene Paternal β gene

2 total β genes → severity depends on β+ versus β0 mutations.

β⁺ → decreased β-chain production (HbA ↓) β⁰ → absent β-chain production (HbA absent)

Interactive Hemoglobin Electrophoresis Lab

See the bands change

Alkaline electrophoresis: bands migrate from cathode (-) toward anode (+).

Selected Patient: Beta-Thalassemia Minor Pattern

Compared with normal: HbA slightly decreased, HbA2 increased, HbF mildly increased.

Normal Control

Cathode (-) at the top · Anode (+) at the bottom

Band thickness represents relative abundance, not exact concentration.

Compare to Normal

Compared with normal: HbA slightly decreased, HbA2 increased, HbF mildly increased.

Genetic Defect

β-globin point mutation causing decreased β-chain production.

Key Electrophoresis Finding

Increased HbA2 with mild increase in HbF.

Clinical Clue

Usually mild microcytic anemia with normal or elevated RBC count and normal iron studies.

★

NBME Trap

Do not confuse with iron deficiency anemia. Thalassemia usually has normal ferritin and a normal RDW.

Management & Therapies

Treatment depends on disease severity. Minor disease usually needs counseling only, while major disease creates a transfusion–iron overload–chelation pathway.

Minor / Trait

Beta-Thalassemia Minor requires no specific treatment. Do not give iron supplements unless there is concomitant iron deficiency anemia. Primary management is genetic counseling.

Beta-Thalassemia Major Pathway

Major disease creates a treatment cascade: severe anemia → chronic transfusions → secondary iron overload → iron chelation.

The Fix

Chronic Transfusions

Correct severe anemia and suppress extramedullary hematopoiesis.

The Consequence

Secondary Iron Overload

The body cannot excrete excess iron from repeated transfusions, so iron deposits in the heart and liver.

Heart: arrhythmias/cardiomyopathy • Liver: fibrosis/cirrhosis

The Rescue

Iron Chelation Therapy

Deferoxamine or deferasirox binds excess iron and allows excretion in urine or feces. Chelation is mandatory for long-term survival.

Definitive Cure

The only definitive cure for Beta-Thalassemia Major is allogeneic hematopoietic stem cell transplant / bone marrow transplant.

Clinical Correlate

Beta-Thalassemia Major

Because beta chains are not needed in utero, this disease presents 6 to 9 months after birth when the infant tries to switch from fetal hemoglobin to adult hemoglobin A.

Hover Pearl

Always expect hepatosplenomegaly when extramedullary hematopoiesis is occurring because the liver and spleen become active sites of blood cell production.

Bone changes

Extramedullary Hematopoiesis

The marrow expands massively to keep up with RBC demand, producing the classic crew-cut skull pattern and chipmunk facies.

Hover Pearl

Secondary hemochromatosis means iron overload caused by repeated transfusions, not a primary genetic iron absorption disorder.

Transfusion burden

Secondary Hemochromatosis

These patients often need lifelong transfusions, which can cause fatal iron overload in the heart and liver if iron is not chelated.

Topic Practice

Lock It In

Make sure you can recognize the thalassemia patterns before moving on.

Question 1 of 5

Core

⚠️ The AUC / NBME Trap: Soluble Transferrin Receptor (sTfR)

Highest Yield

Both ACD and IDA have low serum iron. Ferritin can sometimes be tricky if there’s a mixed picture. The ultimate tie-breaker is the Soluble Transferrin Receptor (sTfR). In true Iron Deficiency, sTfR is highly elevated. In ACD, sTfR is NORMAL or DECREASED.

Starvation in the midst of plenty!

The body has plenty of iron in storage (High Ferritin), but the inflammation locks it away in the bone marrow macrophages and enterocytes, starving the erythroid precursors.

Epidemiology

3 Causes to Note!

Microbial Infections

TB
Osteomyelitis
Bacterial Endocarditis
Lung Abscess

Immune Disorders

Rheumatoid Arthritis
SLE
Chronic Renal Failure (CKD)
Regional Enteritis
Obesity

Malignancy

Hodgkin Disease
Lung Cancer
Breast Cancer

Mechanism

Molecular Domino Effect

The Trigger

Inflammation → Macrophages release sustained IL-6

The Liver

IL-6 stimulates hepatocytes to synthesize hepcidin

The Blockade

Hepcidin binds ferroportin, causing its internalization and degradation. Iron is trapped!

The Hepcidin Switch

The Hepcidin Switch: IDA vs Anemia of Chronic Disease

✦ HEY — pay attention

AUC/NBME Favorite

Low ferritin = empty iron stores. Normal/high ferritin + low iron + low TIBC = iron is trapped by hepcidin.

Iron exits through ferroportin

Enterocyte / Macrophage

Stored iron

Hepcidin

Ferroportin

Low hepcidin → gate open

Transporter status OPEN

Bloodstream

Iron stored in enterocytes/macrophages.

Ferroportin open.

Iron enters blood for hemoglobin synthesis.

Diagnosis

Iron Deficiency Anemia

Iron can move into blood.

Why it behaves this way

The body lowers hepcidin to absorb and mobilize iron. The problem is that total iron stores are depleted.

Low ferritin • Low serum iron • High TIBC

Shared iron study clue Both ACD and IDA can give you low serum iron.

Storage clue ACD keeps ferritin high because the iron is stored but inaccessible.

Tie-breaker sTfR is elevated in true IDA but normal/decreased in ACD.

Core hemodynamics concept

Vessel storage logic

Compliance is the ability of a blood vessel to accept fluid or blood. You can describe this as distensibility: how easily the vessel stretches when volume enters.

Compliance Equation

Volume accepted for a pressure change

C = ΔV ΔP

C = mL/mmHg ΔV = mL ΔP = mmHg

Higher compliance means the vessel accepts more blood volume with less pressure rise.

Interactive Equation

Rearrange the equation

Choose the target, then adjust the known values.

Compliance 4.0 mL/mmHg

100 mL divided by 25 mmHg gives the vessel's compliance.

Artery vs Vein

How easily can each vessel accept blood?

Low compliance

Arteries

Accept less blood volume before pressure rises.
They stretch with each stroke volume, then recoil strongly.
That pressure rise helps keep blood moving forward.

High compliance

Veins

Accept much more blood volume with little pressure rise.
They are very distensible, so they stretch easily.
They have less recoil than arteries.

Compliance and elasticity move in opposite directions.

More elastic recoil means lower compliance: the vessel snaps back harder and accepts less blood volume before pressure rises.

Interactive Simulator

Slow-motion hemodynamics simulator

Fluid Dynamics

Active recoil vs passive pooling

In an artery, elastic walls push back against incoming blood, so pressure rises quickly and the vessel accepts only a smaller volume.

Stroke Volume Surge LowHigh

Pressure monitor Time

C = ΔV / ΔP ΔV 80 mL / ΔP 40 mmHg = C 2.0 mL/mmHg

Clinical equation mindset

Flow is not just math

Read every flow equation as a clinical story. NBME questions usually ask what the equation means at the bedside: where velocity slows, why murmurs happen, and which variable quietly dominates the answer.

Velocity Trap

Capillaries are slow because the network is huge

Velocity equation

v = Q A

A = πr². Velocity falls when total cross-sectional area gets large.

Interactive Equation

Rearrange velocity

Choose v, Q, or A, then use the arrows to change the known values.

Velocity 4.0 cm/s

Q divided by A gives velocity.

NBME trap

Capillaries have the slowest blood velocity.

Not because each capillary is wide. Each one is tiny. The key is that all capillaries in parallel create a massive total cross-sectional area, so blood moves slowly enough for gas and nutrient exchange.

One capillary Small radius (r) and small area (A).

All capillaries Huge total A because many tiny vessels run in parallel.

Clinical result Velocity (v) slows so exchange can happen.

Diffusion note Exchange depends on contact time: slower v means more time at the capillary wall, so diffusion increases.

Reynolds Number Trap

Murmurs are turbulence problems

Reynolds number

N_r = ρdv η

ρ density d diameter v velocity η viscosity

Interactive Equation

Rearrange turbulence

Choose N_r, v, or η, then adjust the known values.

Reynolds number 2333

Higher N_r means turbulence becomes more likely.

Reynolds context

Any increase in Reynolds number can shift flow from laminar to turbulent.

Murmur trap

Anemia can cause a systolic flow murmur.

Anemia lowers blood viscosity (η). Since viscosity is in the denominator, lower viscosity increases Reynolds number and makes turbulent flow more likely.

Anemia Lower viscosity (η) Higher N_r Systolic flow murmur

Stenosis Concept

Key Concept: Why Stenosis Causes Murmurs

Stenosis narrows the opening, so the same amount of blood must pass through a smaller space. That makes velocity rise, and high velocity creates turbulence — which is heard as a murmur.

Narrow opening Faster blood flow Turbulence/murmur

Why velocity matters

Velocity = Flow / Area

Area depends on radius squared, so small decreases in radius cause large decreases in area. If area falls, velocity rises.

Vessel opening

Smooth laminar flow

NBME Trap You might expect a smaller diameter to lower Reynolds number. But narrowing massively increases velocity (v ∝ 1 / r²). Velocity outweighs diameter → Reynolds number increases → turbulence.

Velocity is the reason stenosis murmurs.

Vascular Circuitry

Vascular Circuitry: Series vs. Parallel Resistance

Series

Series Circuit (The Local Pathway)

R_total = R_artery + R_arteriole + R_capillary...

Blood vessels within a single organ act in series. Resistance is additive. The arterioles provide the greatest resistance in this linear pathway.

Parallel

Parallel Circuit (The Systemic Network)

1R_total = 1R_renal + 1R_GI + 1R_skin...

The major organs receive blood from the aorta in parallel. This design ensures that every organ receives fully oxygenated blood at the same mean arterial pressure, while keeping the overall systemic resistance incredibly low.

High-yield trap

The Nephrectomy Paradox Highly Counterintuitive

If you remove an organ like a kidney, you are not just removing blood flow to that organ. You are removing one of the body's parallel pathways for blood to move through. With fewer pathways open, blood has fewer places to go, so the overall resistance to flow goes up. That means after nephrectomy, Total Peripheral Resistance (TPR) increases.

4 Organs (R=10 each) 1/10 + 1/10 + 1/10 + 1/10 = 4/10 TPR = 2.5

3 Organs (Post-Nephrectomy) 1/10 + 1/10 + 1/10 = 3/10 TPR = 3.33

The Organ Perfusion Engine

Series and parallel resistance in motion

Change resistance in a local organ pathway, then remove systemic parallel pathways to see why total resistance behaves differently.

Artery Resistance Arteriole Resistance Capillary Resistance

R Total = 6 + 14 + 8 28

1/R Total = 1/10 + 1/10 + 1/10 + 1/10 = 4/10 2.5

Pressure & perfusion mindset

From a vessel to the whole circulation

Arterial pressure represents the pulsatile output of the heart against systemic resistance. For NBME questions, anchor everything to MAP as the perfusion pressure, and remember that flow obeys Q = ΔP/R at every scale — from a single vessel to the entire circulation.

Clinical anchor

MAP is the perfusion pressure that matters.

Systolic and diastolic pressures are the upper and lower bounds of a pulsation, but neither one is what actually drives blood into the kidneys, brain, or coronaries. MAP is the average pressure across the entire cardiac cycle and the only pressure that organs feel as a steady perfusing force.

Heart pumps SP/DP fluctuate MAP averages out Organs see steady pressure

Core definitions

The Four Pressures You Need to Know

Lock these vocabulary terms in before the interactive curve. Every NBME stem about hemodynamics builds on them.

Systolic Pressure

Peak arterial pressure during ventricular contraction.

Normal range 90 – 120 mmHg

Reflects how forcefully the left ventricle ejects blood.

Diastolic Pressure

Lowest arterial pressure during ventricular relaxation.

Normal range 60 – 80 mmHg

Reflects systemic vascular resistance and elastic recoil.

Pulse Pressure

The difference between SP and DP — the size of the pulsation.

Formula PP = SP − DP

Widens with stiff arteries (arteriosclerosis) or aortic regurgitation. Narrows with aortic stenosis or low stroke volume.

MAP

Mean Arterial Pressure

The time-weighted average pressure across one cardiac cycle.

Formula MAP = DP + ⅓PP

The actual pressure organs feel — the perfusion pressure that matters.

Interactive equations

Calculate the Pressures

Two rearrangers using the same pattern as Physics of Flow's velocity and Reynolds widgets. Pick a variable to solve for, then nudge the others with the arrows.

Pulse pressure

PP = SP − DP

PP pulse pressure SP systolic DP diastolic

Interactive Equation

Rearrange pulse pressure

Choose PP, SP, or DP, then adjust the other values with the arrows.

Pulse pressure 40 mmHg

Pulse pressure widens with stiff arteries or large stroke volume.

Mean arterial pressure

MAP = DP + ⅓ × PP

MAP mean arterial DP diastolic PP pulse pressure

Interactive Equation

Rearrange MAP

Choose MAP, DP, or PP, then adjust the other values with the arrows.

Mean arterial pressure 93 mmHg

MAP is the perfusion pressure organs feel.

Other ways to calculate MAP

⅓SP + ⅔DP

Algebraically identical to DP + ⅓ × PP. Some textbooks teach this form — pick whichever you remember faster.

CO × TPR

The hemodynamic identity: cardiac output multiplied by total peripheral resistance. The form you'll use when reasoning about pressors and shock.

True time-weighted integral

What arterial lines actually compute — average pressure across the entire cycle. Bedside formulas approximate this; modern monitors don't need to.

Centerpiece interactive

The Arterial Pressure Waveform

Drag the SP and DP handles to reshape one cardiac cycle. Adjust heart rate to see how diastole's share of the cycle changes — and how that pulls the simple (SP+DP)/2 average away from the real MAP.

0 ms One cardiac cycle · 800 ms 800 ms

Systolic 120 mmHg

Diastolic 80 mmHg

Pulse pressure 40 mmHg

Heart rate 75 bpm

Diastole is 67% of the cycle

Mean arterial pressure DP + ⅓PP

93 mmHg

80 + 40÷3 ≈ 93

Wrong shortcut · (SP+DP)/2 100

misses the true MAP by 7 mmHg

Why this formula?

The heart spends roughly two-thirds of each cycle in diastole and one-third in systole. Pressure sits closer to DP for most of the cycle, so the time-weighted average lands near DP — not halfway between SP and DP.

(SP + DP) / 2 — assumes a 50/50 time split. Wrong because the simple average ignores diastole length.
⅓SP + ⅔DP — the same idea written by weighting DP twice. Equivalent algebra.
DP + ⅓PP — the bedside-friendly form: start at the floor, add a third of the bounce. The version you'll actually use.

Drag the heart rate down to 50 bpm. Diastole stretches further, and the gap between (SP+DP)/2 and the true MAP grows.

NBME trap

Why isn't MAP just the average of SP and DP?

Because the heart spends about two-thirds of each cycle in diastole and only one-third in systole. The simple average overweights the brief systolic peak. The weighted formula reflects the time spent at each pressure.

⅓ time in systole ⅔ time in diastole DP weighted twice MAP ≈ DP + ⅓PP

Pathology

How Disease Reshapes the Curve

Pick a pathology to see its waveform side-by-side with normal. The deep dive below the curves walks through the mechanism step by step.

Normal

120 / 80 mmHg

Arteriosclerosis

160 / 80 mmHg

Healthy aortic compliance absorbs systolic stroke volume, then recoils during diastole. The dicrotic notch marks aortic valve closure.

SP— DP— PP— MAP—

NBME hook Reference shape. Use this as your baseline for spotting the others.

All disease deep dives

NBME trap

Wide pulse pressure: notch or no notch?

Both PDA and aortic regurgitation produce a wide pulse pressure with a strong, "bounding" pulse. The discriminator is the dicrotic notch — it marks aortic valve closure, so it disappears only when the valve never fully closes.

Patent Ductus Arteriosus

Valve still closes → notch persists. Diastolic runoff happens through the open ductus shunt after closure.

Aortic Regurgitation

Valve never closes → no notch. Diastolic blood regurgitates straight back into the LV through the leaky valve.

Flow & resistance

Flow obeys ΔP / R at every scale

Single vessel

Q = ΔP R

Flow through a vessel equals the pressure difference across it divided by its resistance.

Whole circulation

CO = MAP TPR

Cardiac output equals mean arterial pressure divided by total peripheral resistance.

Same equation. Different names. NBME loves the rebrand.

Poiseuille

What Actually Changes Resistance

Resistance

R = 8ηL πr⁴

η viscosity L length r radius

Interactive equation

Move the variables

Adjust viscosity and radius to see how resistance changes. Length is grayed out — vessel length is essentially fixed in the body.

η Viscosity (relative) 3.0 L Length (relative) 1.0 Vessel length is fixed in the body — not a testable variable. r Radius (relative) 1.00

R = 8(3.0)(1) / π(1)⁴ = 7.64

Q = ΔP / R → Q is 100% of normal flow at the same ΔP

Baseline radius: resistance is 1.00×.

High-yield trap

Radius dominates everything.

Length is fixed. Viscosity changes a little (anemia, polycythemia). But radius gets squared — then squared again. A 50% reduction in vessel radius doesn't double resistance: it multiplies it 16-fold. This is why arterioles, which can dilate and constrict their radius significantly, control nearly all of TPR.

r = 1.0 1×

r = 0.75 3.16×

r = 0.5 16×

r = 0.25 256×

Clinical correlates

What changes each variable in real life

Viscosity

Anemia ↓ η
Polycythemia ↑ η
Dehydration ↑ η

Directly proportional to resistance.

Length

Essentially fixed
Obesity slight ↑ L
Not testable

You can ignore L for almost every NBME question.

Radius

Vasoconstriction ↓ r
Vasodilation ↑ r
Sympathetic tone, local metabolites, drugs

The dominant variable. r⁴.

Topic Practice

Lock It In

Make sure you can recognize the pattern before moving on.

Question 1 of 5

Loading questions…

Exchange mindset

Where the circulation does its job

Microcirculation is where the cardiovascular system actually does its job. Capillaries are the only vessels thin enough for exchange — and the rules governing what crosses, in which direction, and how fast are dictated by structure (capillary type), forces (Starling), and pathology (edema).

Clinical anchor

Capillaries are where exchange happens.

Arterioles control flow. Venules collect it. But the capillary — a single layer of endothelial cells — is the only place in the circulation where fluid, gases, and solutes actually move between blood and tissue. Everything in this module is about what crosses, why, and what happens when the system breaks.

Capillary bed: a network of vessels connecting an arteriole on the left to a venule on the right — Capillaries run in parallel networks — many tiny vessels connecting one arteriole to one venule.

Structure

Three capillary types, three permeabilities

Capillary permeability is set by the structure of the endothelial wall. The same blood arrives at every capillary — what differs is what the wall lets out.

Continuous capillary cross-section: intact endothelial wall with narrow clefts between cells

Continuous capillaries

Locations

Skeletal muscle, brain, skin, gonads, fingers

What crosses

Water and ions (through clefts only)

Tightest. Brain capillaries are the basis of the blood-brain barrier.

Fenestrated capillary cross-section: endothelial wall with round pores punched through the cells

Fenestrated capillaries

Locations

Endocrine glands, pancreas, intestine, kidney glomeruli

What crosses

Water, ions, small proteins

Found wherever rapid solute exchange is needed — kidney filtration, hormone secretion, nutrient absorption.

Sinusoidal capillary cross-section: large irregular gaps in both endothelium and basement membrane, with an RBC and WBC crossing the wall

Sinusoidal capillaries

Locations

Liver, bone marrow, spleen, lymph nodes, adrenal glands

What crosses

Water, ions, proteins, AND cells (RBCs, WBCs)

Leaky on purpose. Bone marrow releases new cells; liver processes large proteins.

Permeability Continuous Fenestrated Sinusoidal

Transport

How molecules cross the capillary wall

Three routes, picked by molecule size and lipid solubility.

Simple Diffusion

Lipid-soluble: gases (O₂, CO₂) — dissolve through endothelial membrane.
Water-soluble: ions, glucose, amino acids — pass through intercellular clefts.

The default route for small molecules.

Vesicular Transport (Transcytosis)

For molecules too large for clefts or pores.
Endothelial cells engulf molecule on one side, release on the other.

Important at the blood-brain barrier.

Osmosis

Pure water movement, low solute → high solute.
Through endothelial cell junctions.

Driven by oncotic gradients — sets up the Starling balance.

Foundation

Both hydrostatic AND oncotic pressure act at every point in the capillary.

Forget “Pc lives at the arterial end and πc lives at the venous end.” Both forces are present everywhere along the capillary. What changes is their magnitudes — and which one wins at each location.

Hydrostatic pressure = water pressing outward on the vessel walls.

Imagine air pressure inside a balloon. Blood is constantly pushing against the inside of the capillary wall, trying to force fluid out. This force exists at every point along the capillary. But it gets weaker from the arterial end (30 mmHg) to the venous end (10 mmHg) because the vessel has resistance — energy is lost as blood travels through. The pressure doesn’t disappear; it just drops.

πc

Oncotic pressure = osmotic pull from proteins trapped inside the vessel.

Plasma proteins (mainly albumin) can’t cross the capillary wall. They sit inside the vessel and exert an osmotic pull on water, like a magnet drawing water back in. This force also exists at every point along the capillary (~28 mmHg). But it becomes relatively more powerful toward the venous end — because water has been filtering out along the way, concentrating the proteins. Same proteins, less water → stronger pull.

At the arterial end: Pc (30) > πc (28) → fluid leaves the vessel. At the venous end: Pc (10) < πc (28) → fluid returns. Same two forces. Different magnitudes. Different winners.

30 Pc

28 πc

Net OUTWARD 2 mmHg → fluid LEAVES the vessel.

Mental model

Hydrostatic pressure is like air inside a balloon — it pushes outward on the walls everywhere. In the capillary, blood pressure pushes fluid out through the wall. It’s strongest at the arterial end where blood pressure is highest. It drops along the length as energy is lost to vessel resistance.

Mental model

Oncotic pressure is osmosis — water moves from low solute to high solute. Plasma proteins are stuck inside the capillary (too big to cross). They make the inside protein-rich, pulling water in. As water filters out at the arterial end, the remaining plasma becomes even more concentrated, making the osmotic pull stronger at the venous end.

Starling forces

The four pressures across every capillary

Each Starling force acts across the capillary wall — pushing fluid OUT (filtration) or pulling it IN (absorption). Lock the vocabulary before the equation.

→ OUT

Capillary Hydrostatic

Blood pressure pushing fluid OUT.

Typical ~30 art · ~10 ven mmHg

→ OUT

Interstitial Hydrostatic

Sub-atmospheric pressure pulling fluid OUT.

Typical ~ −3 mmHg (negative)

πc

← IN

Plasma Colloid Osmotic

Albumin pulling fluid IN.

Typical ~28 mmHg

πi

→ OUT

Interstitial Colloid Osmotic

Tissue proteins pulling fluid OUT.

Typical ~8 mmHg

Visualize the forces

See Starling forces in motion

Each force isolated. Each force animated. Then layered together. Click a button to see how that force alone moves fluid.

Filtration at the arterial end. Absorption at the venous end. Lymphatics clear the surplus.

Starling forces base diagram: capillary lumen flanked by interstitial spaces with a lymphatic vessel below

Net filtration at the arterial end (Pc dominates). Net absorption at the venous end (πc dominates). The small leftover fluid is cleared by lymphatics.

High-yield trap

Why is interstitial pressure NEGATIVE?

Most students assume interstitial pressure is positive — fluid is in the tissue, so it must push back, right? It's actually slightly negative (~ −3 mmHg) because the lymphatic system actively pumps interstitial fluid back into circulation. This negative pressure is what helps draw fluid in across the venous end of the capillary.

Lymphatics pump Pi drops below zero Helps absorption at venous end

NBME hook. Block the lymphatics (filariasis, post-mastectomy, radiation), and Pi flips from −3 to positive. Filtration outpaces absorption. Edema.

Interactive equation

Net Fluid Movement (Jv)

Adjust the four pressures to see how net fluid flow (Jv) changes — or click a clinical scenario below to load realistic values.

NBME high yield

The Starling equation is heavily tested. Expect questions that give you four pressures and ask the direction of net fluid movement, OR that describe a clinical scenario (heart failure, nephrotic syndrome, burns) and ask which Starling variable changed. Practice with the presets below.

Clinical Presets

Click a scenario to load realistic values and see how Jv changes.

Jv = Kf × [(Pc − Pi) − σ(πc − πi)]

Jv = net flow · Kf = filtration constant · σ = reflection coefficient · Pc/Pi = hydrostatic pressures · πc/πi = oncotic pressures

Pc — Capillary hydrostatic

mmHg

Pi — Interstitial hydrostatic

−3

mmHg

πc — Plasma oncotic

mmHg

πi — Interstitial oncotic

mmHg

σ = 1.0 Reflection coefficient — assume 1 unless capillaries leak protein.

Kf = 1.0 Filtration constant — assume 1 unless capillaries leak.

Sign convention: + Jv = filtration (OUT)

Net fluid movement (Jv) +13 mmHg

Net filtration — fluid moves OUT of capillary into tissue.

Show forces summed at each capillary end

	Arterial end	Venous end
Outward forces (filtration)
Pc (capillary hydrostatic)	30	10
Pi (interstitial hydrostatic)	3	3
πi (interstitial oncotic)	8	8
Total outward	41	21
Inward forces (absorption)
πc (plasma oncotic)	28	28
Total inward	28	28
Net force	+13 mmHg filtration	−7 mmHg absorption

Pc dominates at the arterial end → net filtration. As blood flows through the capillary, Pc drops and πc becomes the dominant force → net absorption at the venous end. Pi and πi are smaller outward contributors that add to the filtration side. (Pi is sub-atmospheric, so its magnitude is shown in the outward column.)

Visual

Filtration and absorption across a capillary

Pc dominates at the arterial end (filtration). As Pc drops along the length, πc becomes dominant at the venous end (absorption). Lymphatics catch the small fraction not reabsorbed.

OUTWARD force (Pc dominates filtration)

Pc (capillary hydrostatic) 30 mmHg

INWARD force (πc dominates absorption)

πc (plasma oncotic) 28 mmHg

Net filtration along 57% of capillary length.

Flow regulation

Arterioles set the capillary pressure

Capillary pressure (Pc) is downstream of arteriolar tone. Open the arteriole and Pc rises; constrict it and Pc falls.

Arteriolar dilation

↑ Pc ↑ filtration

Exercising muscle (active hyperemia).

Arteriolar constriction

↓ Pc ↓ filtration

Fluid stays in vessel.

NBME trap

Think of arterioles as a pressure valve sitting upstream of the capillary — they clamp down to shield the capillary from the high arterial pressure pushing on them from behind. On the downstream side, between the capillary and the vein, there’s no equivalent valve. So when venous pressure backs up (heart failure can’t empty blood forward, DVT blocks the return flow), that pressure travels backward straight into the capillary and forces Pc up. The vein is where the problem starts — but the capillary is where edema actually forms.

Pathology

Edema is broken Starling balance

Edema is abnormal fluid accumulation in the interstitial space. It happens when any Starling force breaks: ↑Pc, ↓πc, ↑Kf, or blocked lymphatics. The four mechanisms below cover every cause you’ll encounter. Click through to see each in clinical depth in the pathology module.

Engorged capillary with elevated hydrostatic pressure pushing fluid into the interstitium

↑ Pc

Capillary hydrostatic pressure rises

Venous backpressure raises Pc. Filtration outpaces absorption → fluid leaks into tissue.

Heart failure Venous obstruction DVT

See Pc-mediated edema in pathology

↓ πc

Plasma oncotic pressure drops

Less albumin → weaker inward pull. Filtration wins by default → fluid escapes the vessel.

Nephrotic syndrome Liver failure Malnutrition

See hypoalbuminemic edema in pathology

↑ Kf

Capillary permeability rises

Damaged walls leak protein AND fluid into tissue → severe third-spacing edema.

Burns Sepsis Anaphylaxis

See inflammatory edema in pathology

Blocked lymphatics

Lymphatic drainage fails

No clearance → fluid stays in tissue. Pi flips positive → progressive lymphedema.

Filariasis Post-mastectomy Tumor compression

See lymphedema in pathology

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Pathology

Iron Deficiency Anemia

Microcytic Anemia Lab Simulator

Visual smear pattern

Iron Deficiency Anemia

Core Lab Pattern

Follow the iron story

Recognize the patient context

Chronic Blood Loss

Decreased Iron Intake or Absorption

Increased Demand

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The Hepcidin Switch: IDA vs Anemia of Chronic Disease

Iron Deficiency Anemia

Pattern first

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Deletions vs mutations

Alpha-Thalassemia

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Management & Therapies

Chronic Transfusions

Secondary Iron Overload