Most Things Biology

20.3 Hydrostatic vs Oncotic Pressure 

Case home  ·  Start at the beginning  ·  The Reveal

Time To Read

4–6 minutes

Date Last Modified

MODULE 20:
FLUIDS, ELECTROLYTES, and pH
PART 3 OF 7

Stina’s low blood albumin is the missing pull: with less protein in the plasma, the pressure that should draw fluid back into her capillaries has weakened, and fluid stays in the tissue as edema.

Slide 1 Transcript

PATIENT CHART

STINA

THIS PART

Age 44

PRESENTING WITH

Pitting ankle, periorbital edema

DIAGNOSIS

Partially compensated renal aciosis

CHART CLUES
COLLECTED AS YOU GO

1

Edema + Low Albumin + ‘Low’ Calcium 

2

Edema Localized to a Plasma → Interstitium Shift

3

Hypoalbuminemia → Low Oncotic Pressure

4

A ‘Low’ Calcium That Is Really Lost Albumin

5

A Low-Bicarbonate Drift = Metabolic Acidosis

6

Renal Metabolic Acidosis, Only Partly Compensated

7

Amyloid-damaged glomerulus

Put your money down: Two pressures fight over the fluid at every capillary. Which one did Stina’s low albumin weaken?

Bet before the mechanism. Hydrostatic pressure pushes fluid out of the vessel; oncotic pressure, built by plasma proteins, pulls it back in. Commit now: when Stina’s albumin leaks away, which pressure falls — and does the fluid stay out or come back?

Pick the answer you believe now. We’ll come back to it later!

Two Pressures at Every Capillary

At every capillary, two pressures fight over the fluid. Hydrostatic pressure is the push of the blood itself against the vessel wall, driving fluid out into the interstitial space, strongest at the arterial end. Opposing it is oncotic (colloid osmotic) pressure — the inward pull created by large plasma proteins, above all albumin, that are too big to leak out and so hold water in the vessel by osmosis.


Two Pressures

One pushes fluid out; one pulls it back in.

Slide 2 Transcript

Cash out the clip

Drag the Words: Label the opposing capillary pressures on a diagram.

The Normal Balance

Filter out, reabsorb back — the healthy capillary.

Slide 3 Transcript

Cash out the clip

Summary: Pick the true statement about net filtration and reabsorption at the capillary.

The Normal Balance — and the Anchor

In a healthy capillary these pressures roughly balance: fluid filters out at the arterial end and most is drawn back in at the venular end — the very system this course met as capillary exchange in the vascular module. The plasma proteins are the anchor that keeps the inward pull strong enough to recover the fluid. Albumin is the main plasma protein doing that job; it is the anchor holding water where it belongs.

Remove the Anchor

Lose albumin, lose the inward pull.

Slide 4 Transcript

Cash out the clip

Multiple Choice: Choose the correct step: how does low albumin cause edema?

Placeholder

Placeholder

Slide 5 Transcript

Cash out the clip

Placeholder

Remove the Anchor

Now remove the anchor. Stina’s amyloid-damaged glomeruli are leaking albumin into her urine, and as blood albumin falls, plasma oncotic pressure falls with it. Hydrostatic pressure keeps pushing fluid out, but the weakened oncotic pull can no longer draw enough back — so fluid accumulates as pitting ankle edema and periorbital puffiness. This is the heart of her swelling: not too much salt, but too little protein holding water in the vessels. And the same low albumin is about to explain her ‘low’ calcium too.

Placeholder

Placeholder

Slide 6 Transcript

Cash out the clip

Placeholder

Placeholder

Placeholder

Slide 7 Transcript

Cash out the clip

Placeholder

Placeholder

Placeholder

Slide 8 Transcript

Cash out the clip

Placeholder

FINDING!

Hypoalbuminemia → Low Oncotic Pressure → Edema

Stina’s proteinuria drops her plasma albumin, which lowers oncotic pressure so hydrostatic push wins and fluid stays in the tissue. Chart entry: her edema is a low-protein (oncotic) edema from the leaking glomerulus — mechanistically like liver-failure swelling — not a dietary-salt problem.

Confirm or refute your bet: Two pressures fight over the fluid at every capillary. Which one did Stina’s low albumin weaken?

Now that you’ve seen the two pressures, return to the bet. Stina’s lost albumin weakened the inward oncotic pull, so hydrostatic pressure wins and the fluid stays out as edema. Chart entry: the swelling is a protein problem, not a salt problem — restore the filter, not the salt restriction.

Slide 7 Transcript

NEXT PART

Albumin holds water in the vessel — but it also carries cargo. Half of the blood’s calcium rides on albumin, which is about to reframe what ‘low calcium’ even means for Stina.

PREVIOUS

The Body's Fluid Compartments & Water Movement

NEXT

The Major Electrolytes & the Albumin-Corrected Calcium

QUICK QUESTIONS

QUICK QUESTIONS

Hop to:

← Familial Mediterranean Fever — case home

All pages for Module 20 — Fluids, Electrolytes & Acid–Base Balance

20.0 Module Overview
20.1 The Swollen Ankles That Point Back to the Kidney
20.2 The Body’s Fluid Compartments & Water Movement
20.3 Hydrostatic vs Oncotic Pressure
20.4 The Major Electrolytes & the Albumin-Corrected Calcium
20.5 Acid-Base Balance & the Bicarbonate Buffer System
20.6 Metabolic Acidosis from a Failing Kidney
20.7 One Filter, Not Three Problems
20.8 Conclusion and Assessment
20.9 Evidence Behind the Case

All Modules
  1. Anatomical Language, Membranes & Homeostasis
  2. Just Enough Chemistry
  3. The Cell & Its Transport
  4. Making Cells & Proteins
  5. The Integumentary System
  6. The Skeletal System
  7. The Muscular System
  8. Nervous Tissue & the Senses
  9. The Spinal Cord
  10. The Brain & the Blood–Brain Barrier
  11. The Autonomic Nervous System
  12. Special Senses (in development)
  13. The Endocrine System
  14. Blood
  15. The Heart
  16. Blood Vessels
  17. The Digestive System
  18. The Respiratory System
  19. The Urinary System
  20. Fluids, Electrolytes & Acid–Base Balance
  21. The Reproductive System
  22. The Immune System

List of terms