[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Gastrointestinal](https://durgical.com/en/physiology/areas/gi/) › Hepatic lymph, the liver as a blood reservoir, and how ascites forms

# Hepatic lymph, the liver as a blood reservoir, and how ascites forms

- Mechanism
- Exam priority: medium

The liver's leaky sinusoids make lots of protein-rich lymph; when their pressure rises and the kidneys hold salt, fluid spills into the belly.

Updated Nov 13, 2026

Also called: Hepatic lymph flow, SAAG, Sinusoidal pressure

**See it in 3D:** [Liver](https://durgical.com/en/anatomy/liver/) · [Portal vein](https://durgical.com/en/anatomy/hepatic-portal-vein/) · [Hepatic veins](https://durgical.com/en/anatomy/hepatic-veins/) · [Peritoneum](https://durgical.com/en/anatomy/peritoneum/) · [Cisterna chyli](https://durgical.com/en/anatomy/cisterna-chyli/)

## Key points

- The liver produces roughly half of the body's resting lymph, with a protein content close to plasma (≈ 6 g/dL).
- Ascites needs sinusoidal (or post-sinusoidal) hypertension: pre-hepatic portal vein block alone causes varices but little ascites.
- Cirrhosis: splanchnic vasodilation (NO) → low effective arterial volume → RAAS, sympathetic and ADH activation → Na⁺ and water retention.
- SAAG ≥ 1.1 g/dL means portal hypertension (cirrhosis, heart failure, Budd–Chiari);

On this page

- The basics Key points
- What it is
- Key facts
- How it works, step by step
- Equations
- Reference values (adult)
- Reading ascitic fluid: SAAG and total protein
- Memory aid

- Clinical Clinical notes

- Study Test yourself

- Connections Connections

## What it is

Liver sinusoids have large fenestrae and no basement membrane, so plasma and its proteins pass freely into the space of Disse and drain as lymph. The liver is also a soft, expandable blood store. Raised sinusoidal pressure makes more lymph than the lymphatics can remove, fluid weeps from the liver surface, and splanchnic vasodilation drives renal Na⁺ retention that keeps refilling the cavity.

## Key facts

- Liver blood flow is ≈ 1.3–1.5 L/min (about a quarter of cardiac output), roughly three quarters from the portal vein and one quarter from the hepatic artery.
- Portal pressure is ≈ 9 mmHg and hepatic vein pressure near 0, so normal sinusoidal resistance is very low.
- Because sinusoids leak protein, the oncotic gradient across them is small: sinusoidal fluid balance is set mostly by hydrostatic pressure.
- Hepatic lymph drains to hilar nodes and then through the cisterna chyli into the thoracic duct; in portal hypertension thoracic duct flow rises many-fold.
- Low serum albumin (poor synthesis in cirrhosis) lowers plasma oncotic pressure in the splanchnic and peritoneal capillaries, adding to the leak.
- Sympathetic venoconstriction mobilizes hepatic blood in hemorrhage; back-pressure from the right atrium distends the liver (congestive hepatomegaly).

## How it works, step by step

- Fibrosis and regenerative nodules (or outflow block) raise resistance in or after the sinusoids, so sinusoidal pressure rises.
- More plasma filters into the space of Disse; hepatic lymph flow rises until the lymphatics cannot keep up.
- Excess fluid weeps from the liver capsule and from congested splanchnic capillaries into the peritoneal cavity.
- Portal hypertension releases vasodilators (mainly NO) in the splanchnic bed; arterial pressure and effective arterial volume fall.
- Baroreceptors and the kidney sense underfilling: renin–angiotensin–aldosterone, sympathetic tone and ADH rise.
- The kidneys retain Na⁺ (urine Na⁺ falls) and free water; the extra volume is drawn into the leaky splanchnic bed and ascites grows.
- Late on, intense renal vasoconstriction lowers GFR (hepatorenal physiology) and free-water retention causes dilutional hyponatremia.

### Starling forces along a capillary

**Arteriolar NFP** +22 mmHg **Venular NFP** +5 mmHg **Filtration** 13.5 **Reabsorption** 0.0 **Net filtration** 13.5 a.u. (lymph capacity 7.5)

Hypoalbuminemia (cirrhosis, nephrotic syndrome, malnutrition): πc falls and the force that holds water in the vessel is lost: generalized edema and ascites.

Edema: net filtration (13.5 a.u.) is more than the lymph can return (7.5 a.u.), so fluid collects in the interstitium.

Tap a force (Pc, πc, Pif, πif), the lymph or the net pressure value.

- Filtration (fluid out)
- Absorption (fluid in)
- Net filtration pressure
- Normal net pressure
- Lymphatic (returns the excess)
- Edema

Illustrative model of a systemic capillary with low albumin. Liver sinusoids leak protein, so there pressure, not oncotic force, decides the leak.

## Equations

**Serum–ascites albumin gradient**SAAG = albuminserum − albuminascites

**albumin**
g/dL, samples taken the same day

SAAG ≥ 1.1 g/dL tracks raised sinusoidal pressure with ≈ 97% accuracy; it does not depend on how much protein leaks.

**Hepatic venous pressure gradient**HVPG = Pwedged hepatic vein − Pfree hepatic vein

**P wedged**
estimate of sinusoidal pressure

Normal 1–5 mmHg; > 5 mmHg is portal hypertension; ≥ 10 mmHg is clinically significant (varices, ascites).

## Reference values (adult)

| Parameter | Value | Note |
| --- | --- | --- |
| Liver blood volume | ≈ 450 mL | ≈ 10% of blood volume |
| Liver blood flow | ≈ 1.3–1.5 L/min | ≈ 25% of cardiac output |
| Portal vein pressure | ≈ 7–10 mmHg | |
| Hepatic lymph protein | ≈ 6 g/dL | close to plasma |
| HVPG | 1–5 mmHg | ≥ 10 mmHg clinically significant |
| SAAG cut-off | 1.1 g/dL | |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Reading ascitic fluid: SAAG and total protein

| SAAG | Ascitic protein | Physiology | Examples |
| --- | --- | --- | --- |
| [≥ 1.1](https://durgical.com/en/conditions/cirrhosis/) |
- Low-protein ascites (

## Memory aid

Leaky sieve, high water, thirsty kidney: leaky sinusoids, high sinusoidal pressure, and kidneys that think the body is dry.

## Test yourself

Think of the answer, then tap the question to see it.

## Why does portal vein thrombosis without liver disease cause varices but usually little ascites?

The block is before the sinusoids, so sinusoidal pressure and hepatic lymph production stay near normal; ascites needs sinusoidal or post-sinusoidal hypertension.

## A patient has ascites with SAAG 1.8 g/dL and ascitic protein 3.2 g/dL. What is the most likely physiology?

High sinusoidal pressure with still-leaky sinusoids, as in right heart failure or Budd–Chiari: high SAAG from portal pressure, high protein from intact fenestrae.

## Why is urine sodium very low in a patient with cirrhotic ascites?

Splanchnic vasodilation lowers effective arterial volume, activating renin–angiotensin–aldosterone and sympathetic nerves, so the kidneys reabsorb nearly all filtered Na⁺.

## Why is hepatic lymph much richer in protein than lymph from skeletal muscle?

Sinusoidal endothelium has large fenestrae and no basement membrane, so plasma proteins enter the space of Disse almost freely.

## How does the liver act as a blood reservoir after acute hemorrhage?

Sympathetic venoconstriction squeezes blood out of the distensible hepatic veins and sinusoids, returning part of the ≈ 450 mL they hold to the circulation.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/physiology/hepatic-lymph-the-liver-as-a-blood-reservoir-and-how-ascites-forms/)

[More Physiology questions →](https://durgical.com/en/practice/?f=physiology)

## Connections

## See it in 3D 5

The structures of Durgical's 3D anatomy where it happens.

- [Liver](https://durgical.com/en/anatomy/liver/)
- [Portal vein](https://durgical.com/en/anatomy/hepatic-portal-vein/)
- [Hepatic veins](https://durgical.com/en/anatomy/hepatic-veins/)
- [Peritoneum](https://durgical.com/en/anatomy/peritoneum/)
- [Cisterna chyli](https://durgical.com/en/anatomy/cisterna-chyli/)

## Related topics 5

- [Splanchnic and portal circulation](https://durgical.com/en/physiology/splanchnic-and-portal-circulation/)
- [Functions of the liver](https://durgical.com/en/physiology/functions-of-the-liver/)
- [Starling forces at the capillary](https://durgical.com/en/physiology/starling-forces-at-the-capillary/)
- [How edema forms](https://durgical.com/en/physiology/how-edema-forms/)
- [The renin–angiotensin–aldosterone system](https://durgical.com/en/physiology/the-renin-angiotensin-aldosterone-system/)

## Related conditions 4

- [Cirrhosis](https://durgical.com/en/conditions/cirrhosis/)
- [Portal hypertension](https://durgical.com/en/conditions/portal-hypertension/)
- [Ascites and spontaneous bacterial peritonitis](https://durgical.com/en/conditions/ascites-and-spontaneous-bacterial-peritonitis/)
- [Budd-Chiari syndrome](https://durgical.com/en/conditions/budd-chiari-syndrome/)

## In Biochemistry 1

- [Albumin](https://durgical.com/en/biochemistry/enzymes/albumin/)

## In Pharmacology 3

- [Spironolactone](https://durgical.com/en/pharmacology/spironolactone/)
- [Furosemide](https://durgical.com/en/pharmacology/furosemide/)
- [Human albumin](https://durgical.com/en/pharmacology/human-albumin/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [Hepatic Cirrhosis](https://www.ncbi.nlm.nih.gov/books/NBK482419/) (StatPearls (NCBI Bookshelf), 2025); [Portal Hypertension](https://www.ncbi.nlm.nih.gov/books/NBK507718/) (StatPearls (NCBI Bookshelf), 2025); [Ascites: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000286.htm) (MedlinePlus, US National Library of Medicine, 2025).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/physiology/hepatic-lymph-the-liver-as-a-blood-reservoir-and-how-ascites-forms/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/hepatic-lymph-the-liver-as-a-blood-reservoir-and-how-ascites-forms/