Blood: What Is Actually Moving Around the Circulation?

You take blood from a patient and send several tubes to the laboratory. A little later the results begin appearing: haemoglobin, platelets, white cells, sodium, potassium, albumin, creatinine.

They look like separate tests for separate things. But they all started in the same circulating tissue.

Blood is often introduced as a list: plasma, red blood cells, white blood cells and platelets. That list is correct, but it misses the useful part. Blood makes more sense when you see those components working together. Plasma provides the fluid environment through which cells and dissolved substances travel. Red blood cells carry respiratory gases. White blood cells circulate as part of immune defence and can move into tissues. Platelets and plasma coagulation proteins help limit blood loss when a vessel is damaged.

Because these components share the same circulating volume, changes in plasma volume, cell number, production, destruction or loss can alter what appears on a blood test. Understanding the sample therefore means understanding both what is in the blood and what has happened to the patient.

If you separated a tube of blood, what would you actually see?

Take anticoagulated blood and centrifuge it and the components separate largely according to density. Red blood cells settle at the bottom. Above them is a very thin buffy coat containing most of the white blood cells and platelets. The upper layer is plasma.

The proportion of blood volume occupied by red blood cells is the haematocrit. It is a useful visual starting point because it immediately shows that blood is not simply a collection of cells. A substantial part of circulating blood volume is fluid.

Plasma is predominantly water, but calling it only ‘the liquid part’ undersells it. Electrolytes, nutrients, hormones, metabolic products, medicines and proteins are dissolved in or transported through it. Albumin is the most abundant plasma protein and contributes importantly to plasma colloid osmotic pressure. Immunoglobulins are involved in adaptive immunity, while coagulation proteins circulate in plasma ready to participate in haemostasis when appropriately activated.

Red blood cells, or erythrocytes, dominate the cellular volume. They contain haemoglobin, which provides the principal means of transporting oxygen in blood. White blood cells, or leucocytes, are far fewer but have specialised roles in immune defence. Platelets are small cell fragments derived from megakaryocytes in the bone marrow and become particularly important when vascular integrity is disrupted.

The important point is that these proportions are not fixed. Fluid intake and loss, haemorrhage, pregnancy, intravenous fluids, renal function, inflammation and disease can all alter the relationship between the cellular and liquid compartments.

A blood result is often a concentration, not a count of everything in the body

This distinction explains something students often meet in practice: a laboratory value can change even when the total amount of that substance or cell population has not changed by the same proportion.

Many laboratory results are expressed as concentrations. Haemoglobin, for example, is reported as the amount of haemoglobin within a given volume of blood. That measurement is influenced by the red-cell compartment, but also by the volume of plasma in which those cells are circulating. Research comparing haemoglobin concentration with total haemoglobin mass has demonstrated why plasma-volume changes matter when anaemia is interpreted.

Imagine David arrives after two days of profuse diarrhoea and poor oral intake. He is thirsty, tachycardic and clinically dehydrated. Loss of body water can reduce plasma volume and concentrate circulating constituents. A relatively high haemoglobin concentration therefore does not automatically mean David has suddenly produced more red blood cells. Dehydration itself can contribute to a raised measured red-cell concentration.

Now consider the opposite situation. If plasma volume expands, for example following substantial intravenous fluid administration, measured concentrations can fall through haemodilution. The analyser has not necessarily made an error and the patient's red cells have not necessarily disappeared. The relationship between the circulating cells and plasma volume has changed.

This is why a number on a screen should not be separated from fluid balance, bleeding, intravenous therapy and the patient's wider clinical picture.

The components of blood depend on one another

Consider oxygen transport. Oxygen entering the pulmonary capillaries diffuses into red blood cells and binds predominantly to haemoglobin. Those cells then travel through the left side of the heart and systemic circulation before reaching tissue capillaries, where oxygen can unload according to local conditions.

Haemoglobin is essential to that process, but red cells still need blood flow to carry them to the tissues. Oxygen delivery therefore depends on more than a haemoglobin result alone. Circulation, cardiac output, oxygenation and tissue demand all form part of the physiological picture.

Now consider infection. Neutrophils circulate in blood and can be recruited into infected or inflamed tissue. Lymphocytes move between blood, lymphoid tissues and other sites. Antibodies and complement proteins circulate in plasma. Blood is therefore not only transporting immune cells; it is one of the routes through which cellular and soluble parts of the immune response reach the tissues where they are needed.

Haemostasis shows this cooperation particularly clearly. When a blood vessel is injured, the vessel wall changes, platelets adhere and activate at the damaged site, and coagulation reactions involving plasma proteins lead to fibrin formation. Platelet activity and the coagulation system are related but not interchangeable. Effective haemostasis depends on their interaction with the injured vessel.

Blood also links organs that can seem separate when anatomy and physiology are learned system by system. The kidneys produce erythropoietin, an important regulator of red-cell production. Bone marrow produces circulating blood cells. The spleen filters blood and removes or sequesters cells. The liver synthesises albumin and many proteins involved in haemostasis. Endocrine organs release hormones into the circulation so that a signal produced in one part of the body can act elsewhere.

That is why an abnormal blood result often points you towards a process occurring outside the blood itself.

One abnormal result can have several physiological explanations

A low haemoglobin concentration is a good example. It may reflect blood loss, reduced red-cell or haemoglobin production, increased red-cell destruction, or dilution caused by an expanded plasma volume. The same laboratory finding can therefore arise through very different mechanisms.

The same reasoning applies elsewhere. A low platelet count may result from reduced production, increased destruction or consumption, or sequestration. Albumin concentration can be influenced by synthesis, losses, distribution, inflammation and hydration. A raised white-cell count may accompany infection or inflammation, but the total white-cell count alone does not tell you which leucocyte population has changed or why.

The useful question is therefore not simply, ‘Is this result high or low?’ It is, ‘What physiological process could produce this result in this patient?’

That shift in thinking matters for student nurses because it connects anatomy and physiology to clinical reasoning without asking you to diagnose independently. You are learning to recognise patterns, relate results to the patient in front of you, and discuss unexpected or concerning findings with your supervising registered professional and the wider clinical team.

When the results come back, put them back into the patient

Suppose David's results return and several values sit outside the laboratory reference intervals. Start with what you know about him. He has had significant gastrointestinal fluid loss, poor intake and clinical signs of dehydration. Those details change how you think about the concentrations in his sample.

If haemoglobin is low, think about oxygen-carrying capacity and the possible mechanisms behind the fall. If white cells are abnormal, look at the differential rather than treating all leucocytes as interchangeable. If platelets are low, relate the result to the patient's condition, medicines, bleeding risk and the clinical context. If albumin is low, avoid reducing the explanation to dietary protein alone. Albumin is affected by several physiological and pathological processes, including inflammation, synthesis, losses and fluid distribution.

Trends can be more informative than isolated measurements. A haemoglobin concentration falling over serial samples tells a different story from a stable longstanding result. A platelet count dropping rapidly during acute illness raises different questions from mild, stable thrombocytopenia already known to the clinical team. The meaning comes from the relationship between the result, the patient and what has changed over time.

Reference intervals need context too. They are produced for particular laboratory methods and populations and can vary between laboratories. A value within the stated interval can still represent an important change for an individual patient, while an isolated result just outside it does not automatically establish disease.

Before interpreting the physiology, ask whether the sample is reliable

There is another reason not to treat every laboratory number as a perfect representation of the patient's physiology: what happens before the sample reaches the analyser matters.

Haemolysis during collection or handling can interfere with laboratory measurements and can falsely alter some analytes. Prolonged venous stasis, delays in processing and contamination between collection tubes can also affect results. NHS pathology guidance specifically warns about haemolysis, drip contamination and prolonged venous constriction as sources of poor-quality or misleading samples.

Blood collected from an arm receiving an intravenous infusion can also be diluted or contaminated by the infused fluid. The exact procedure for obtaining blood around intravenous devices is governed by local policy, so students should follow local sampling guidance and supervision rather than applying a single technique everywhere.

This gives you another useful clinical question when a result looks completely inconsistent with the patient: could there be a pre-analytical problem? That does not mean dismissing an unexpected result. It means recognising it, checking the clinical picture and escalating it appropriately so that the team can decide whether further assessment or repeat sampling is required.

From one tube back to one patient

At the beginning, the blood sample seemed to become a collection of unrelated numbers: haemoglobin, platelets, white cells, electrolytes, albumin and creatinine.

The laboratory separates them because measuring individual components is clinically useful. At the bedside, your learning is to begin putting those measurements back into the context of the patient.

A haemoglobin result is connected to red cells, haemoglobin production, blood loss and plasma volume. Platelets connect bone marrow, haemostasis and disease processes. Albumin connects plasma with the liver, inflammation, losses and fluid distribution. Even the reliability of the numbers connects back to how the sample was collected and handled.

Once you understand blood as a circulating tissue rather than a list of components, those results stop looking quite so separate.

Blood is the connection between them.

References

NHS (2023) Blood groups. Available at: NHS.

NHS (2022) Red blood cell count. Available at: NHS.

Oxford University Hospitals NHS Foundation Trust (n.d.) Tips for blood collection. Available at: Oxford University Hospitals.

University Hospitals of North Midlands NHS Trust (n.d.) Common problems affecting blood test results. Available at: University Hospitals of North Midlands.

Otto, J.M. et al. (2017) ‘Hemoglobin concentration, total hemoglobin mass and plasma volume in patients: implications for anemia’, Haematologica, 102(9), pp. 1477–1485. Available at: Haematologica.

Joles, J.A. et al. (2022) ‘Albumin is an interface between blood plasma and cell membrane, and not just a sponge’, Clinical Kidney Journal, 15(4), pp. 624–634. Available at: Clinical Kidney Journal.

Pries, A.R. and Secomb, T.W. (2016) ‘Regulation of blood flow and volume exchange across the microcirculation’, Critical Care, 20, 319. Available at: Critical Care.