Why regional blood flow fails

A normal mean arterial pressure does not mean a normally perfused organ. Four circulations that fail while the monitor looks acceptable, and the mechanism behind each one.

Reviewed and updated September 2026 · Written by Anastasia Jackson, DNAP, CRNA

Every clinical claim on this page is cited to its source below. How we source clinical content — and the corrections we have made, with what each one was and why it changed.

Each of these produces the same clinical picture — an acceptable number and an underperfused organ — by a different route.

Why is splanchnic perfusion the first circulation the body sacrifices?

The splanchnic bed carries roughly a quarter of cardiac output to the gut, liver, pancreas and spleen through the celiac, superior mesenteric and inferior mesenteric arteries, with the liver additionally receiving portal inflow that has already crossed the gut.

It is a reservoir before it is a conduit

Veins hold about 70% of total blood volume and are roughly thirty times more compliant than arteries. The circulation behaves as two compartments — a compliant splanchnic one and a much less compliant non-splanchnic one — and the splanchnic compartment holds the recruitable unstressed volume that sympathetic venoconstriction mobilizes.^1,2

That architecture is what makes this bed the designated donor. Sympathetic activation from hypovolemia, pain or surgical stress constricts splanchnic arterioles and squeezes the venous reservoir, defending central filling pressure at the cost of regional flow.^2,3 The sacrifice is not a failure of the system; it is the system working.

What worsens it in the operating room

Why does the kidney lose autoregulation before the brain does?

Renal blood flow is autoregulated across a range of perfusion pressures by changes in afferent arteriolar resistance, and autoregulation is the major mechanism protecting the kidney from ischemia during hypotension.⁴ The problem is where the lower limit sits.

The medulla is hypoxic by design

The renal cortex and medulla exhibit an inherent mismatch between oxygen supply and consumption, with the outer medulla characterized by physiological relative hypoxia because it combines low blood flow with high oxygen consumption — the thick ascending limb does energy-expensive active transport on a marginal supply.^5,6 It is the first tissue to cross into pathological hypoxia when the cortical–medullary oxygen gradient is disrupted.⁵

What blunts the autoregulation itself

The protection is not fixed. Renal autoregulation is impaired in chronic hypertension, chronic kidney disease, diabetes, atherosclerotic renal artery stenosis and aging, and during acute kidney injury from ischemia–reperfusion. Intraoperatively it is further blunted by hemodilution, hypothermia and renal tissue hypoxia, and some — though not all — anesthetics blunt it in addition to reducing renal blood flow directly. Vasopressor agents can also alter it.⁴

Cardiopulmonary bypass reduces renal blood flow and renal oxygen delivery further, leading to medullary hypoxia.⁴

The compensation that makes it worse

Systemic hypotension unloads arterial baroreceptors, producing a reflex increase in sympathetic activity and activation of the renin–angiotensin–aldosterone system. That causes vasoconstriction including the renal bed, which may restore systemic blood pressure and glomerular filtration rate — at the risk of further reducing renal blood flow.⁷ The number recovers while the organ does not.

This is the origin of “covert hypoxia”: local tissue hypoxia can persist even when systemic hemodynamic parameters have normalized.^5,6 Low urinary oxygen tension during and after bypass, a proxy for medullary tissue oxygen tension, is associated with postoperative acute kidney injury.⁴

Why does hepatic blood flow fall under anesthesia, and how far does the liver compensate?

The liver has a dual supply — the portal vein and the hepatic artery — and it cannot control the portal side at all. Portal inflow is whatever the gut and spleen deliver. What the liver can do is adjust the arterial side to buffer changes in the portal side, a mechanism called the hepatic arterial buffer response.^8,9

The adenosine washout mechanism

Adenosine is secreted at a constant rate into the space of Mall, a small isolated fluid compartment through which the portal triad passes — the terminal branches of the hepatic artery, portal vein and bile ductule, all in intimate contact. The concentration of adenosine, a potent vasodilator, is regulated by the rate at which it is washed out into the blood vessels.¹⁰

When portal flow falls, less adenosine is washed away. The local concentration rises and the hepatic artery dilates, partly offsetting the loss.^8,9,10 The mechanism is not neural — it persists in denervated livers — and it is not myogenic, since portal pressure changes only slightly even when portal flow changes markedly.¹¹

Two limits that matter clinically

The liver also acts as a compliance reservoir alongside the buffer: a reduction in portal flow lowers intrahepatic distending pressure, and the highly compliant hepatic vasculature passively expels up to 50% of its blood volume, adding to venous return.¹¹

Why does aortocaval compression cause hypotension only once the block is in?

Aortocaval compression is mechanical compression of the inferior vena cava and, less so, the aorta by the gravid uterus in the supine position. When it produces a fall in systolic pressure of 15–30 mmHg or a rise in heart rate of 20 beats per minute, with or without symptoms, it is called supine hypotensive syndrome.¹⁴

The compression is there the whole time

The inferior vena cava is thin-walled and low-pressure, lying directly anterior to the vertebral column and posterior to the uterus. Supine, the uterus compresses it against the spine. Venous return falls, preload falls, and cardiac output falls with it.

What keeps most parturients asymptomatic is compensation: sympathetic vasoconstriction, tachycardia, and collateral return through the intraosseous, vertebral, azygos, paravertebral and epidural veins, which raise right atrial pressure and maintain cardiac output.^14,15 Only 10–15% of parturients experience the syndrome despite significant compression in the third trimester — the compression is measurable in women who feel entirely well.¹⁴ Aortocaval compression reduces cardiac output by about 30% and uteroplacental blood flow by about 20%.¹⁴

Why the cuff can lie

Aortic compression contributes separately by lowering distal perfusion pressure. It matters because the uterine arteries arise below the level of compression, so uteroplacental flow can fall while maternal brachial pressure still looks acceptable. A normal maternal blood pressure does not guarantee normal uteroplacental perfusion, and the fetal heart rate can deteriorate before the mother appears compromised.

Compression begins around 18–20 weeks and is most pronounced in the third trimester.¹⁴ It worsens with multiple gestation, polyhydramnios and macrosomia. Left uterine displacement, wedging or left lateral tilt relieves it, which is why displacement is required regardless of anesthetic technique.

Frequently asked questions

Is a normal blood pressure enough to guarantee organ perfusion?

No, and the reasons differ by organ. The lower limit of renal autoregulation is higher than the cerebral one, so a pressure protecting the brain may not protect the kidney.⁴ Splanchnic flow is actively sacrificed by sympathetic activation to defend central pressure, so the number can be maintained by the very mechanism reducing gut flow.^2,3 In pregnancy the uterine arteries arise below the level of aortic compression, so uteroplacental flow can fall while the brachial cuff reads normally.

Why does perioperative acute kidney injury happen at pressures that seem acceptable?

Three converging reasons. The renal autoregulatory lower limit is higher than the cerebral one.⁴ The outer medulla is relatively hypoxic by design, combining low blood flow with high oxygen consumption, so it has little reserve.^5,6 And autoregulation itself is blunted by chronic hypertension, chronic kidney disease, diabetes, aging, hemodilution, hypothermia, some anesthetics and vasopressors.⁴ Local tissue hypoxia can persist even after systemic hemodynamic parameters normalize.⁵

Does the liver protect itself when splanchnic flow falls?

Partially. The hepatic arterial buffer response dilates the hepatic artery when portal flow falls, through reduced washout of adenosine from the space of Mall.¹⁰ But the compensation covered only about 25% of the portal flow reduction in the classical experiments — that is the active response measured with arterial pressure held steady; with pressure uncontrolled, active and passive effects together reached 44%^12,13 — the portal vein cannot return the favor, since it does not control its own flow,⁹ and at low portal flows the artery is already nearly maximally dilated with no reserve left.¹³

Why does my parturient become hypotensive after the spinal when she was fine before it?

The aortocaval compression was present before the block and was being masked by sympathetic vasoconstriction, tachycardia and collateral venous return. Neuraxial blockade abolishes that compensation, so the same mechanical compression now produces hypotension.¹⁴ Two mechanisms are additive — sympathectomy and caval compression — and only the first responds to a vasopressor. This is why left uterine displacement is required regardless of technique.

References

1. Gelman S. Venous function and central venous pressure: a physiologic story. Anesthesiology. 2008;108(4):735–748. PMID 18362606
2. Greenway CV, Lister GE. Capacitance effects and blood reservoir function in the splanchnic vascular bed during non-hypotensive haemorrhage and blood volume expansion in anaesthetized cats. J Physiol. 1974;237(2):279–294. PMID 4825450
3. Greenway CV. Role of splanchnic venous system in overall cardiovascular homeostasis. Fed Proc. 1983;42(6):1678–1684. PMID 6832386
4. Lankadeva YR, et al. Role of perioperative hypotension in postoperative acute kidney injury: a narrative review. Br J Anaesth. 2022;128(6):931–948. PMID 35465952. Source of the renal versus cerebral autoregulatory limit, the factors blunting renal autoregulation, the bypass effect, and urinary oxygen tension as a medullary proxy.
5. Li S, et al. Pathological triad of perioperative acute kidney injury: renal microcirculatory hypoxia, mitochondrial damage, and immuno-metabolic reprogramming. Front Immunol. 2026;17:1843391. PMID 42338604
6. Brezis M, Rosen S, Epstein F. The pathophysiologic implications of medullary hypoxia. Am J Kidney Dis. 1989;13:253–258
7. Franzén S, et al. Anesthesia and the renal sympathetic nervous system in perioperative acute kidney injury. Semin Nephrol. 2022. doi:10.1016/j.semnephrol.2022.10.009. Source of the baroreceptor and renin–angiotensin–aldosterone compensation point.
8. Lautt WW. Mechanism and role of intrinsic regulation of hepatic arterial blood flow: hepatic arterial buffer response. Am J Physiol. 1985;249(5):G549
9. Eipel C, Abshagen K, Vollmar B. Regulation of hepatic blood flow: the hepatic arterial buffer response revisited. World J Gastroenterol. 2010;16(48):6046–6057. PMID 21182219
10. Lautt WW. Hepatic Circulation: Physiology and Pathophysiology. NCBI Bookshelf NBK53066. Source of the space of Mall description and the washout mechanism.
11. Lautt WW. Regulatory processes interacting to maintain hepatic blood flow constancy: vascular compliance, hepatic arterial buffer response, hepatorenal reflex, liver regeneration, escape from vasoconstriction. Hepatol Res. 2007;37(11):891–903. doi:10.1111/j.1872-034X.2007.00148.x. PMID 17854463. Source of the passive volume expulsion and the 50% passive volume expulsion.
12. Lautt WW, Legare DJ, d’Almeida MS. Adenosine as putative regulator of hepatic arterial flow (the buffer response). Am J Physiol. 1985;248(3 Pt 2):H331–H338. PMID 2579585. Buffering of 25.5 ± 2.7% of the decrease in portal flow.
13. Lautt WW, Legare DJ, Ezzat WR. Quantitation of the hepatic arterial buffer response to graded changes in portal blood flow. Gastroenterology. 1990;98(4):1024–1028. PMID 2311859. Hepatic arterial flow changes inversely with portal flow in both directions: nearly full dilation at low portal flows, shown by inability to dilate further to adenosine, and nearly full constriction at high portal flows, shown by lack of further constriction to norepinephrine. The active buffer, with arterial pressure held steady, is the 25% figure; with pressure uncontrolled, active and passive effects combined reach 44% ± 4%.
14. Ahmed MF, Kamel I. Supine hypotensive syndrome. In: Abd-Elsayed A, ed. Advanced Anesthesia Review. Oxford University Press; 2023. Chapter 311, pp. 787–788. ISBN 9780197584521. doi:10.1093/med/9780197584521.003.0310. Source of the 15–30 mmHg and 20 beats/min definition, the 18–20 week onset, the 10–15% incidence, the 30% and 20% flow reductions, and the collateral routes.
15. Kacmar RM, Gaiser R. Physiologic changes of pregnancy. In: Chestnut DH, ed. Chestnut’s Obstetric Anesthesia: Principles and Practice. 6th ed. Elsevier; 2020:13–37. The source the chapter above rests its own definition and onset figures on.