Endothelium Explained for UK Student Nurses: Blood Flow, Clotting and Sepsis

Originally published 19 November 2025 · Content reviewed 21 October 2026

When you measure somebody's blood pressure, you get a useful picture of the pressure inside their arteries. What you do not see is how blood is moving through individual capillaries, whether fluid is escaping into the tissues, or how the vessel lining is responding to inflammation.

That lining is the endothelium. It is only a single layer of cells in most vessels, but those cells are doing far more than separating the blood from the vessel wall. They help regulate local blood flow, fluid exchange, immune-cell movement and clot formation. Understanding those jobs can make it easier to follow what is happening when a patient becomes unwell.

We are not going to use the endothelium as an explanation for every abnormal observation. You cannot diagnose endothelial dysfunction from a bedside assessment. Instead, we will connect the underlying physiology to the clinical findings you can actually assess.

Start with where the endothelium sits

Endothelial cells form the blood-facing surface of arteries, veins and capillaries. They also line lymphatic vessels, and the heart's inner endothelial lining forms part of the endocardium. On one side is blood or lymph; on the other are surrounding tissues, smooth muscle or other specialised structures.

These cells do not behave identically everywhere. The tight endothelium of the blood–brain barrier restricts the passage of many substances, whereas specialised capillaries in the kidney are adapted for filtration. In the lungs, a very thin exchange surface is essential, but it must also help prevent excess fluid entering the air spaces. The same cell type has to fulfil quite different demands depending on its location (Godo and Shimokawa, 2017; Wu et al., 2024).

How the lining helps a blood vessel widen or narrow

Think about an arteriole supplying an active muscle. As local conditions change, the vessel needs to adjust its diameter so blood flow can match the tissue's needs. The endothelium helps to coordinate this response, although it is not the only system involved.

One important signal is nitric oxide (NO). Endothelial nitric oxide synthase, often shortened to eNOS, produces NO from the amino acid L-arginine. NO then diffuses into the smooth muscle surrounding the vessel and activates an enzyme called soluble guanylyl cyclase. That increases cyclic GMP (cGMP), which favours smooth-muscle relaxation. The vessel widens, reducing resistance to flow.

You do not need to memorise every enzyme in that pathway to understand the useful sequence: endothelial signal → smooth-muscle relaxation → local vasodilation. NO also helps limit inappropriate platelet activation and adhesion.

The endothelium can send different messages too. Prostacyclin (PGI₂) encourages vasodilation and inhibits platelet aggregation, while endothelin-1 is a powerful vasoconstrictor. Vascular tone depends on the balance of these signals alongside autonomic activity, circulating hormones, vascular smooth muscle and the wider cardiovascular system. This is why it would be wrong to say that the endothelium controls blood pressure on its own (Godo and Shimokawa, 2017).

A selective barrier with a protective surface

Fluid and substances have to move between the blood and surrounding tissues. The endothelial barrier helps regulate that movement. Inflammation, injury and changes in endothelial junctions can increase permeability, allowing more fluid and proteins to move out of the circulation. Depending on the organ involved, this may contribute to oedema, impaired oxygen exchange or reduced effective circulating volume.

There is another layer here that is easy to miss. On the blood-facing surface of many endothelial cells sits the glycocalyx, a fine network containing proteoglycans and other molecules. It helps sense flowing blood, contributes to the vascular barrier and influences interactions with platelets and white blood cells.

During severe inflammation, including sepsis, parts of this glycocalyx may be damaged or shed. That can accompany greater vascular permeability and disturbed microvascular function. These processes vary between patients and organs; damage to the glycocalyx is not a bedside diagnosis and cannot be assumed in every case of infection (Chelazzi et al., 2015).

How inflammation changes the endothelial surface

Imagine an infection in the tissues. White blood cells need a way to leave the bloodstream and reach the affected area. Inflammatory signals cause nearby endothelial cells to display adhesion molecules on their surface. Selectins help white blood cells slow down and roll along the vessel wall. Molecules including ICAM-1 and VCAM-1 help them attach more firmly, before they move through the vessel lining towards the tissue.

That response is useful and necessary. Problems arise when activation becomes excessive or widespread, because it can contribute to barrier injury, altered blood flow and inflammation beyond the original site (Wu et al., 2024).

The endothelium is also involved in haemostasis. Under ordinary conditions it helps discourage unnecessary clotting. When a vessel is damaged, von Willebrand factor (vWF), including vWF released from endothelial storage granules, supports platelet adhesion at sites of injury. In severe systemic inflammation, increased vWF activity, tissue-factor-driven coagulation and weakened natural anticoagulant pathways may favour the formation of small clots in the microcirculation.

In some patients with severe sepsis, these changes contribute to microvascular thrombosis and, in extreme cases, disseminated intravascular coagulation. It is an interaction between the immune system, platelets, coagulation proteins and the endothelium, not a process caused by the vessel lining alone (Wu et al., 2024).

Sepsis: when several problems occur together

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection. Endothelial activation is one part of that response. Blood vessels may dilate abnormally, fluid may move into the tissues, and tiny regions of the microcirculation may receive uneven blood flow. Meanwhile, immune and coagulation pathways can be overactive.

It is tempting to explain a deteriorating patient's observations as a simple chain: leaking vessels cause low blood pressure, which causes poor oxygen delivery. The real physiology is less tidy. Cardiac function, circulating volume, vessel tone, cellular metabolism, oxygen use and inflammation can all be changing at once. Some capillaries may be poorly perfused while others remain open. A normal blood pressure reading therefore does not prove that every tissue is adequately perfused.

Sepsis does not always produce the same endothelial changes or the same haemodynamic picture. Nor does a patient with peripheral oedema, delayed capillary refill or a low blood pressure necessarily have endothelial dysfunction. Those findings should prompt broader clinical assessment rather than a conclusion about one microscopic mechanism.

What can you actually notice during an assessment?

At the bedside, you are looking for deterioration and impaired organ function, not trying to measure the endothelial glycocalyx.

Skin and peripheral circulation: mottling, an ashen appearance, unusual warmth or coolness, clamminess and delayed capillary refill may be relevant. But skin appearance also varies with lighting, baseline pigmentation, environmental temperature, medicines and vascular disease. Assess the whole patient and their usual baseline.

Breathing: a rising respiratory rate deserves attention even if oxygen saturation looks acceptable. Fever, pain, hypoxaemia, metabolic acidosis, anxiety and severe infection are among the possible causes. Count the respiratory rate rather than estimating it.

Consciousness: new confusion, agitation or reduced alertness may reflect serious illness, hypoglycaemia, hypoxaemia, medicines or a number of other problems. Check and report a new change promptly.

Kidney function: unexpectedly low urine output can be associated with changes in circulating volume, renal blood flow, inflammation or kidney injury. Check the accuracy of fluid-balance records and, where relevant, catheter patency, while reviewing trends and escalating the change.

Lactate: a raised lactate can accompany hypoperfusion, but it is not a direct measurement of endothelial function or proof that tissues have run out of oxygen. Adrenergic stimulation, accelerated glycolysis, seizures, medicines such as beta₂-agonists and reduced hepatic clearance can also contribute. The level, trend and clinical picture need interpretation together. Persistent or rising lactate in an unwell patient requires review; it should not be translated automatically as “the tissues are starving of oxygen”.

The important point is that these observations can be abnormal before hypotension develops, but they do not always precede it. A normal blood pressure does not rule out serious illness.

Putting it into practice: a patient with possible sepsis

Consider an adult on a ward with suspected infection. Their blood pressure is 118/72 mmHg, so at first glance that one reading may seem reassuring. But their respiratory rate is 27, pulse 118, temperature 38.6°C and oxygen saturation 96% on room air. They are newly confused.

For this illustrative patient, using NEWS2 SpO₂ Scale 1, the respiratory rate scores 3, pulse 2, temperature 1, new confusion 3, blood pressure 0, oxygen saturation 0 and supplemental oxygen 0. The total NEWS2 is 9. Those findings demand urgent escalation, regardless of the apparently preserved blood pressure.

For relevant people aged 16 and over who are not pregnant or recently pregnant, NICE NG253 recommends NEWS2 as part of suspected-sepsis assessment in acute hospitals, acute mental health settings and ambulances. A score of 7 or more suggests high risk, but a lower score does not cancel clinical concern. New mottling, a concerning single observation, worsening observations or failure to improve can all change the urgency of review. In children, and in people who are pregnant or recently pregnant, follow the appropriate separate risk assessment guidance rather than applying this adult pathway automatically (NICE, 2025a, 2025b, 2025c).

As a student nurse, raise the concern immediately with your supervising registered nurse and activate the local escalation or emergency response pathway as indicated. Assist with an ABCDE assessment within your competence, document the complete observations and their trend, and communicate the new confusion and concerns about possible infection. Reassessment and escalation should not be held up while waiting for a score or a blood result (Resuscitation Council UK, 2024; NICE, 2025a).

You might begin an SBAR handover: “I am concerned this patient with suspected infection is deteriorating. Their NEWS2 is 9. Their respiratory rate is 27, pulse 118 and they are newly confused, although their blood pressure is currently 118/72. They need urgent assessment.” Add the background, other observations, trend and actions already taken.

Some wards use the Sepsis Six to organise the early response: assessment of oxygen needs, blood cultures, antibiotics, intravenous fluids, lactate measurement and urine-output monitoring. These are team-based actions directed by the patient's assessment, local policy and the current NICE pathway, not six treatments to give indiscriminately. For example, oxygen is given when indicated and antibiotics and fluids are decided according to clinical risk and assessment. NICE NG253 differentiates the urgency of antibiotic treatment by risk rather than prescribing the same timing for every patient with a possible infection. Students should recognise the bundle and assist with appropriately delegated care, not make independent prescribing or fluid-resuscitation decisions (NICE, 2025a; UK Sepsis Trust, 2025).

What to take into your next shift

The endothelium helps regulate what flows through a vessel, what passes out of it, which immune cells can cross it and how the vessel surface interacts with platelets. Nitric oxide, the glycocalyx, adhesion molecules and clotting proteins give us a way to understand those functions in more depth.

You will not identify endothelial dysfunction by looking at an observation chart. What you can do is recognise when a patient is changing, measure observations accurately, consider trends, use the appropriate UK deterioration or sepsis pathway and get timely help. That is where the physiology becomes useful in practice.

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, article 26. https://doi.org/10.1186/s13054-015-0741-z.

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

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

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

National Institute for Health and Care Excellence (NICE) (2025c) Suspected sepsis in pregnant or recently pregnant people: recognition, diagnosis and early management (NG255). https://www.nice.org.uk/guidance/ng255.

Resuscitation Council UK (2024) The ABCDE approach. https://www.resus.org.uk/library/abcde-approach.

UK Sepsis Trust (2025) Clinical tools for healthcare professionals. https://sepsistrust.org/healthcare-professionals/clinical-tools/.

Wu, M., Yan, Y., Xie, X., Bai, J., Ma, C. and Du, X. (2024) ‘Effect of endothelial responses on sepsis-associated organ dysfunction’, Chinese Medical Journal, 137(23), pp. 2782–2792. https://doi.org/10.1097/CM9.0000000000003342.