Endothelium Explained for UK Student Nurses: The Hidden System That Controls Circulation

Meet the Unsung System You’ve Been Ignoring: The Endothelium

If you asked most student nurses what controls circulation, they would probably mention the heart, blood pressure, blood vessels or kidneys.

All correct.

But quietly lining the entire vascular system is a structure that rarely gets the attention it deserves:

The endothelium: the ultra-thin, active lining inside blood vessels.

You cannot see it on a bedside monitor.

You will not find an “endothelial function” box on the observation chart.

But this microscopic layer helps regulate blood flow, vascular tone, inflammation, clotting and the movement of fluid between the circulation and surrounding tissues.

When endothelial function becomes seriously disrupted, the effects can be felt throughout the body.

Let’s give this hidden hero its moment.

🩸 What is the endothelium?

The endothelium is a continuous layer of specialised cells lining the inside of:

• arteries;
• veins;
• capillaries; and
• lymphatic vessels.

The endothelial lining of the heart is part of the endocardium.

Although it is only one cell thick in many vessels, the endothelium is not simply biological wallpaper. It forms an active interface between circulating blood and the vessel wall.

Endothelial cells respond to:

• blood flow;
• pressure;
• oxygen levels;
• inflammatory signals;
• hormones;
• platelets; and
• circulating immune cells.

Their behaviour varies across different organs. The endothelium within the kidneys, lungs and brain is specialised for the needs of those tissues.

What does the endothelium actually do?

🩸 Helps keep blood flowing

A healthy endothelium provides a smooth, regulated surface for blood flow.

It releases substances that influence whether a vessel relaxes or constricts. One of the best-known is nitric oxide, which helps promote vasodilation and influences platelet activity.

This means the endothelium contributes to vascular tone and blood-flow distribution,but it does not control blood pressure by itself. The heart, nervous system, kidneys, circulating hormones and vascular smooth muscle are also involved.

🧱 Maintains the vascular barrier

The endothelium helps control what moves between the blood and surrounding tissues.

It allows necessary exchange while helping retain fluid and proteins within the circulation. The permeability of this barrier differs between organs: the tight endothelial barrier in the brain behaves very differently from the specialised capillaries in the kidneys.

When the endothelial barrier is disrupted, fluid and proteins may move more readily into the tissues, contributing to oedema and reduced circulating volume.

🔥 Coordinates inflammation

During infection or injury, endothelial cells can become activated.

They produce signals and surface molecules that help white blood cells:

• slow down;
• attach to the vessel wall;
• move through the endothelium; and
• reach affected tissue.

This is an essential part of the immune response. However, excessive or widespread endothelial activation can contribute to tissue injury.

⚖️ Balances clotting and anti-clotting activity

Under normal conditions, the endothelium helps discourage inappropriate clot formation.

When a vessel is injured or the endothelium becomes activated, the balance may shift towards clotting. This helps control bleeding at an injury site, but widespread disruption can contribute to abnormal microvascular thrombosis.

The endothelium therefore has an important role in haemostasis, the tightly regulated balance between bleeding and clotting.

🫁 Supports microcirculation

Microcirculation is the movement of blood through the smallest vessels, including arterioles, capillaries and venules.

The endothelium helps regulate:

• local blood flow;
• vascular tone;
• capillary permeability;
• immune-cell movement; and
• exchange between blood and tissues.

This supports the delivery of oxygen and nutrients and the removal of metabolic waste.

The extra-hidden hero: the endothelial glycocalyx

Covering the blood-facing surface of vascular endothelial cells is an even finer structure called the endothelial glycocalyx.

Think of it as a delicate, gel-like protective coating.

It contributes to:

• vascular barrier function;
• sensing blood flow;
• controlling interactions with blood cells;
• limiting unnecessary platelet adhesion; and
• regulating the movement of fluid and proteins.

In severe inflammation, particularly sepsis, the glycocalyx can become damaged or shed. This is associated with increased permeability, endothelial activation and disturbed microcirculatory function.

Tiny layer. Very big job.

What happens when endothelial function is disrupted?

“Endothelial dysfunction” is a broad term. It means the endothelium is no longer performing one or more of its normal regulatory roles effectively.

Depending on the condition, this may involve:

• impaired vasodilation;
• increased vascular permeability;
• increased inflammatory activity;
• altered coagulation;
• increased platelet adhesion; or
• disturbed microvascular blood flow.

Endothelial dysfunction contributes to several chronic conditions, including:

• atherosclerosis;
• hypertension;
• diabetes mellitus; and
• cardiovascular disease.

Acute endothelial activation or injury is also involved in conditions such as:

• sepsis;
• severe systemic inflammation;
• anaphylaxis;
• major burns;
• significant trauma; and
• some shock states.

However, the endothelium is only one part of these complex conditions. It should not be presented as the sole cause of every change in circulation or organ function.

Sepsis: when endothelial regulation becomes disrupted

Sepsis provides one of the clearest examples of why endothelial function matters.

During sepsis, inflammatory and coagulation pathways can become dysregulated. The endothelium and its glycocalyx may be affected, contributing to:

• altered vascular tone;
• increased capillary permeability;
• movement of fluid into tissues;
• activation of clotting pathways;
• impaired regulation of the microcirculation; and
• reduced effective blood flow to some tissues.

Microcirculatory flow may become uneven. Some small vessels remain perfused while others receive much less flow, even when the larger circulation appears more stable.

This helps explain why a single blood-pressure reading cannot describe everything happening at tissue level.

But remember: endothelial dysfunction is one part of sepsis pathophysiology, not the whole story.

What might happen when the vascular barrier becomes leaky?

When endothelial permeability increases, fluid and proteins can move from the circulation into surrounding tissues.

Depending on the cause and severity, this may contribute to:

• peripheral or tissue oedema;
• reduced effective circulating volume;
• hypotension;
• impaired gas exchange in the lungs; or
• reduced organ perfusion.

In anaphylaxis, inflammatory mediators can rapidly cause vasodilation and increased vascular permeability. In severe burns, widespread tissue injury and inflammation can also produce major fluid shifts.

These are complex systemic responses. The clinical deterioration cannot be attributed to the endothelium alone, but endothelial barrier dysfunction is an important part of the physiology.

🔍 What should student nurses notice?

You cannot diagnose endothelial dysfunction through routine observations.

What you can recognise are signs that a patient may be deteriorating or experiencing impaired perfusion.

1. Skin changes

Look for:

• mottling;
• pallor or an ashen appearance;
• cool or clammy skin;
• unusual warmth or flushing;
• cyanosis; and
• delayed capillary refill.

These findings can provide information about peripheral circulation, but they are not specific to endothelial dysfunction. Temperature, medication, vascular disease and the clinical environment can also affect them.

Always interpret skin findings alongside the rest of the assessment.

2. A rising respiratory rate

Respiratory rate is an important marker of deterioration.

It may increase because of:

• hypoxaemia;
• metabolic acidosis;
• fever;
• pain;
• anxiety;
• sepsis;
• respiratory disease; or
• increased physiological demand.

A rising respiratory rate is not a direct measurement of endothelial function or tissue oxygen delivery. However, an unexpected change should never be brushed off.

Count it properly. Do not guess.

3. Changes in cognition or behaviour

New confusion, reduced alertness, agitation or an unusual change in behaviour may indicate acute illness.

Possible causes include:

• delirium;
• hypoxaemia;
• infection;
• hypoglycaemia;
• medication effects;
• pain; or
• reduced cerebral perfusion.

It is not possible to look at confusion and conclude, “That’s the endothelium.” But it may be an important sign that the patient needs prompt reassessment.

4. Reduced urine output

The kidneys are sensitive to changes in:

• circulating volume;
• renal blood flow;
• blood pressure;
• inflammation; and
• nephrotoxic injury.

Reduced urine output may therefore be an important sign of deterioration, but it is not specific to endothelial dysfunction.

Check:

• whether output has been accurately measured;
• fluid intake and balance;
• catheter patency, if present;
• recent blood pressure trends;
• relevant medicines;
• renal function; and
• the wider clinical picture.

Escalate unexpected oliguria according to local policy.

5. Abnormal lactate

A raised lactate may occur during serious illness and can be associated with impaired perfusion, but it does not automatically prove that tissues are receiving no oxygen.

Lactate may rise because of several mechanisms, including:

• tissue hypoperfusion;
• increased adrenergic stimulation;
• increased glycolysis during severe stress;
• reduced hepatic clearance;
• seizures;
• strenuous muscle activity; or
• medicines such as beta₂-agonists.

The trend, clinical context and response to treatment matter more than one isolated number.

A rising or persistently raised lactate should prompt clinical review, but students should avoid translating it simply as “the tissues are starving of oxygen.”

Can these signs appear before hypotension?

A patient can be seriously unwell while their blood pressure remains within the expected range. Compensatory mechanisms may temporarily maintain blood pressure despite developing circulatory problems.

However, it is too absolute to say that endothelial changes always begin before blood pressure becomes abnormal.

The safer clinical message is:

A normal blood pressure does not rule out serious illness or impaired perfusion.

Look at the patient, not one number.

Consider trends in:

• respiratory rate;
• heart rate;
• blood pressure;
• oxygen saturation;
• temperature;
• consciousness;
• skin appearance;
• capillary refill;
• urine output; and
• blood results when available.

Use the appropriate deterioration or sepsis pathway and escalate concerns promptly.

🧠 Bleepbook memory line

“One cell thick, but involved in the whole circulation.”

Or, another way to express this is:

“When the vascular lining loses control, the circulation feels it.”

What to remember

The endothelium is one of those structures that looks almost too simple to be important.

One thin layer of cells.

Yet it helps regulate vascular tone, permeability, inflammation, coagulation and microcirculatory flow across the body.

For student nurses, the lesson is not to diagnose “endothelial dysfunction” at the bedside. It is to understand some of the physiology behind the changes you may observe in a deteriorating patient.

Mottling, delayed capillary refill, altered cognition, reduced urine output, rising respiratory rate and abnormal lactate are warning signs that require interpretation and escalation.

They are pieces of the clinical picture, not a secret endothelial observation chart.

References

Chelazzi, C., Villa, G., Mancinelli, P., De Gaudio, A.R. and Adembri, C. (2015) ‘Glycocalyx and sepsis-induced alterations in vascular permeability’, Critical Care, 19, 26. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC4308932/

Godo, S. and Shimokawa, H. (2017) ‘Endothelial functions’, Arteriosclerosis, Thrombosis, and Vascular Biology, 37(9), pp. e108–e114. Available at: https://www.ahajournals.org/doi/10.1161/ATVBAHA.117.309813

National Institute for Health and Care Excellence (2025) Suspected sepsis in people aged 16 or over: recognition, assessment and early management (NG253). Available at: https://www.nice.org.uk/guidance/ng253

National Institute for Health and Care Excellence (2025) Suspected sepsis in under 16s: recognition, diagnosis and early management (NG254). Available at: https://www.nice.org.uk/guidance/ng254

Wu, M. et al. (2024) ‘Effect of endothelial responses on sepsis-associated organ dysfunction’, Chinese Medical Journal. Available at: https://mednexus.org/doi/10.1097/CM9.0000000000003342