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Human Physiology · Blood and immunity

Blood and Plasma Proteins

Blood is a fluid tissue: about five litres that carry oxygen, nutrients, heat, hormones and defence cells. This page describes what blood is made of, what the plasma proteins do, how blood volume adapts to training, and how to read common blood markers such as albumin and CRP.

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10Questions

Part 6 · Blood and Immunity · Chapter 25 of 47

Learning objectives

By the end of this page you should be able to:

  • Describe the composition and functions of blood.
  • List the plasma proteins and their roles.
  • Explain the causes and effects of low albumin.
  • Describe acute-phase proteins, including CRP and ESR.
  • Explain how blood volume adapts to training and to bed rest.
  • Relate haematocrit to viscosity and oxygen delivery.

Why it matters

Blood results arrive with almost every hospital referral. A physiotherapist who can read albumin, CRP and haemoglobin understands why a patient is swollen, tired, slow to heal or at risk of complications.

Blood volume also changes with training and inactivity, which explains why trained athletes have low heart rates and why patients after bed rest feel faint on standing.

Composition of blood

Figure 1 · What blood is made of

Blood separated in a tube and the composition of plasma A spun tube of blood: pale yellow plasma about 55 per cent on top, a thin buffy coat of white cells and platelets under 1 per cent, and red cells about 45 per cent below. Plasma is about 92 per cent water, 7 per cent protein and 1 per cent salts, nutrients, gases, hormones and waste. Plasma about 55%Buffy coat under 1%white cells and plateletsRed cells about 45%= haematocritWater 92%Proteins 7%Other solutes about 1%Plasma
The haematocrit is the red cell fraction. Typical values: about 40–50% in men and 36–46% in women.

An adult has about 70 mL of blood per kilogram, roughly 5 litres. Blood is a suspension of cells in plasma. When clotted blood is spun, the fluid left is serum: plasma without its clotting proteins.

FunctionHow blood does it
TransportOxygen, carbon dioxide, nutrients, hormones, waste
RegulationHeat distribution, pH buffering, fluid balance
ProtectionWhite cells, antibodies, clotting

The plasma proteins

Figure 2 · The plasma proteins

Proportions and functions of the main plasma protein groups Three bars. Albumin about 60 per cent, made by the liver, holds water in the blood and carries drugs and hormones. Globulins about 36 per cent include antibodies and transport proteins. Fibrinogen about 4 per cent is needed for clotting. Albumin 60%made by liveroncotic pressure; carries drugs, calcium, hormonesGlobulins 36%liver and immune cellsantibodies, transport proteins, acute-phase proteinsFibrinogen 4%made by liverclotting; rises in inflammationTotal plasma protein about 60–80 g/L; albumin about 35–50 g/L
Low albumin means oedema and a sick liver, kidney or diet. Approximate proportions.
ProteinMain rolesLow inHigh in
AlbuminOncotic pressure; transport of drugs, calcium, bilirubin, hormonesLiver disease, kidney protein loss, malnutrition, inflammationDehydration
ImmunoglobulinsAntibodiesImmune deficiencyChronic infection, myeloma
Transport globulinsCarry iron, copper, hormonesVariedVaried
FibrinogenClot formationLiver failure, consumption in bleedingInflammation, pregnancy

Albumin is made in the liver and holds water in the circulation by its oncotic pressure (Chapters 4 and 14). Low albumin contributes to oedema and is a marker of poor nutrition and severe illness, so it is also linked with slower recovery.

Many drugs travel bound to albumin. When albumin is low, more drug is free and active, so side effects such as drowsiness may appear at normal doses.

Acute-phase proteins

Injury, infection and inflammation release cytokines that make the liver change its output. Some proteins rise, such as CRP and fibrinogen; others fall, such as albumin.

Pepys and Hirschfield describe CRP as the classic acute-phase protein. It is made by the liver, rises sharply within hours, and has a half-life of about 19 hours, so its level tracks how active the process is [2].

Figure 3 · C-reactive protein after an insult

Time course of C-reactive protein after injury or surgery CRP stays low for the first few hours, rises from about six hours, peaks at about 48 hours and then halves about every 19 hours once the stimulus stops. injury or surgerypeak about 48 hrises within about 6 hfalls by half about every 19 honce the stimulus stops0day 1day 2day 3day 4day 5day 6day 7CRP (relative)
A fast, sensitive marker of inflammation or infection. A CRP that keeps rising after the second day after surgery suggests a complication [2]. Schematic.
MarkerWhat it reflectsSpeedPractical point
CRPAcute inflammation or infection [2]Rises in hours, falls in daysUseful for monitoring post-operative course
ESRRed cells clumping due to fibrinogen and globulinsRises and falls slowlyRaised in chronic inflammation, anaemia, age
AlbuminNutrition and illness severityFalls over daysLow values mean higher risk

Red flags in musculoskeletal practice

Back or joint pain with fever, night pain and a high CRP may indicate infection, such as septic arthritis or discitis. Very high ESR with new headache and jaw pain in an older adult suggests giant cell arteritis. These need urgent medical review.

Blood volume and training

Endurance training expands blood volume. Convertino showed that plasma volume expansion accounts for nearly all of the increase in the first 2–4 weeks, after which red cell volume also rises [1].

Figure 4 · Training expands blood volume

Plasma and red cell volume during the first weeks of endurance training Plasma volume rises quickly in the first days and weeks of endurance training. Red cell volume begins to rise later, after two to four weeks. 02468Weeks of endurance trainingIncrease in volume (%)Plasma volumerises within daysRed cell volumefollows after2–4 weeks
More volume, bigger stroke volume. Early expansion is almost all plasma, which can lower haemoglobin concentration slightly: dilution, not true anaemia [1]. Schematic.

The kidneys retain more sodium and water, and urine output falls [1]. The extra volume raises cardiac filling and stroke volume, lowers heart rate during exercise, and gives more fluid for sweating and heat loss [1].

Because plasma expands first, haemoglobin concentration can fall slightly in newly training athletes. This ‘sports anaemia’ is usually dilution, not a lack of red cells.

The reverse happens with bed rest and inactivity. Plasma volume falls within days, which contributes to orthostatic intolerance and reduced exercise capacity after illness (Chapter 15).

Haematocrit and viscosity

Figure 5 · More red cells is not always better

Viscosity, oxygen carrying capacity and oxygen delivery against haematocrit Oxygen carried per litre rises steadily with haematocrit, but viscosity rises steeply. Oxygen delivery, the product of flow and content, peaks near the normal haematocrit of about 40 to 50 per cent and falls at higher values. normal rangeViscosityOxygen per litreOxygen delivered20%30%40%50%60%70%Haematocrit
Thick blood flows slowly. Very high haematocrit, from dehydration, disease or blood doping, raises the risk of clots and strains the heart. Schematic.

Red cells carry oxygen, but they also make blood thicker. As haematocrit rises, viscosity increases steeply, slowing flow. Oxygen delivery, which depends on both content and flow, is greatest around the normal haematocrit.

Very high haematocrit, from dehydration, chronic hypoxia, some blood disorders or illegal blood doping, increases clot risk and cardiac work.

Clinical relevance for the physiotherapist

  • Oedema. Low albumin contributes; elevation and movement help, but nutrition and medical care matter.
  • Post-operative monitoring. CRP normally peaks around day two; a further rise suggests a complication [2].
  • Training. Blood volume expansion underpins early aerobic gains [1].
  • Bed rest. Lost plasma volume causes dizziness on standing; mobilise early and gradually [1].
  • Drugs. Low albumin increases free drug levels; watch for sedation and falls.

Assessment and application

FindingInterpretationAction
Albumin 24 g/L, swollen legs, slow wound healingMalnutrition or severe illnessInvolve dietitian; protect skin; graded activity
CRP rising on day 4 after knee replacement, warm swollen kneePossible infection [2]Report urgently; do not push range
Haemoglobin slightly low in a runner early in a training blockLikely dilution from plasma expansion [1]Check iron if symptoms; monitor
Dizziness on first standing after a week in bedReduced plasma volume, deconditioning [1]Graded mobilisation; postural BP

Common mistakes

  • Assuming CRP stays high for weeks after a single insult [2].
  • Calling every low haemoglobin in an athlete iron deficiency [1].
  • Ignoring albumin when explaining oedema.
  • Confusing plasma with serum.
  • Thinking more red cells always means more oxygen delivery.

Exam preparation

Long answer

List the plasma proteins and describe their functions.

  • Composition of blood (Figure 1).
  • Albumin, globulins, fibrinogen (Figure 2 and table).
  • Oncotic pressure, transport, clotting, immunity, buffering.
  • Acute-phase response and CRP [2] (Figure 3).
  • Clinical: hypoalbuminaemia and oedema.

Short notes

  • Haematocrit.
  • ESR.
  • C-reactive protein [2].
  • Blood volume and training [1].

Viva and practical

  • “Which protein holds fluid in the blood?” — albumin.
  • “When does CRP peak after surgery?” — about 48 hours [2].
  • “What expands first with training?” — plasma volume [1].
  • OSCE: interpret a blood report before exercise.

Current evidence

Exam and traditional teaching

Blood is described by its composition, and volume is treated as fixed.

What the evidence adds

Blood volume is plastic: it expands with endurance training and shrinks with inactivity [1].

CRP is a fast, dynamic marker that tracks inflammatory activity [2].

What the student should understand

Read blood results as a story over time, not single numbers, and remember that activity changes blood itself.

Check yourself

10 questions on this page. Tap one to see the answer and the reasoning.

Q1. In the first 2–4 weeks of endurance training, the rise in blood volume is mainly due to:
  1. (A) Red cell volume
  2. (B) Plasma volume
  3. (C) White cell volume
  4. (D) Platelets

Answer: (B) Plasma volume expansion accounts for nearly all early hypervolaemia [1].

Q2. Training-induced plasma volume expansion is associated with:
  1. (A) Increased urine output
  2. (B) Increased renal sodium reabsorption and reduced urine output
  3. (C) Loss of plasma protein
  4. (D) Dehydration

Answer: (B) Reduced urine output via increased tubular sodium reabsorption contributes [1].

Q3. A slightly low haemoglobin concentration in a newly training endurance athlete most likely reflects:
  1. (A) Iron deficiency in all cases
  2. (B) Dilution from expanded plasma volume
  3. (C) Leukaemia
  4. (D) Bone marrow failure

Answer: (B) Early expansion is mainly plasma, which dilutes red cells [1].

Q4. A benefit of training-induced hypervolaemia is:
  1. (A) Lower stroke volume
  2. (B) Greater stroke volume and better heat dissipation
  3. (C) Higher resting heart rate
  4. (D) Less sweating

Answer: (B) Larger volume raises filling pressure and stroke volume and aids thermoregulation [1].

Q5. C-reactive protein is best described as:
  1. (A) A clotting factor
  2. (B) An acute-phase protein that rises in inflammation and infection
  3. (C) A red cell enzyme
  4. (D) A hormone from the kidney

Answer: (B) CRP is a classic acute-phase protein [2].

Q6. CRP is made mainly by the:
  1. (A) Liver
  2. (B) Kidney
  3. (C) Spleen
  4. (D) Bone marrow

Answer: (A) Hepatocytes produce CRP under the influence of cytokines [2].

Q7. After a single inflammatory stimulus, CRP typically peaks at about:
  1. (A) 1 hour
  2. (B) 48 hours
  3. (C) 2 weeks
  4. (D) 6 months

Answer: (B) CRP rises within hours and peaks around 48 hours [2].

Q8. Once the stimulus stops, CRP falls quickly because its plasma half-life is about:
  1. (A) 19 hours
  2. (B) 19 days
  3. (C) 19 minutes
  4. (D) 19 weeks

Answer: (A) Its half-life is about 19 hours, so levels track the activity of the process [2].

Q9. The main plasma protein responsible for oncotic pressure is:
  1. (A) Fibrinogen
  2. (B) Albumin
  3. (C) Immunoglobulin
  4. (D) Haemoglobin

Answer: (B) Albumin is the most abundant plasma protein; CRP, by contrast, is a minor acute-phase protein [2].

Q10. Bed rest and detraining tend to:
  1. (A) Expand plasma volume
  2. (B) Reduce plasma volume, contributing to dizziness on standing
  3. (C) Have no effect on blood volume
  4. (D) Raise haematocrit to dangerous levels

Answer: (B) Blood volume adapts to activity; it expands with training and contracts without it [1].

Quick review

Key takeaways

  • Blood about 5 L; plasma 55%, red cells 45%.
  • Albumin: oncotic pressure and transport; low in liver disease, kidney loss, malnutrition.
  • CRP rises in hours, peaks at about 48 h, half-life about 19 h [2].
  • Training expands plasma volume first, red cells later [1].
  • Early training can dilute haemoglobin: not true anaemia [1].
  • Oxygen delivery is best near normal haematocrit.

Further reading

SourceWhy it earns its place
Convertino VA — Med Sci Sports Exerc, 1991Blood volume and training [1]
Pepys MB, Hirschfield GM — J Clin Invest, 2003The biology of CRP [2]
Hoffbrand AV — Essential HaematologyStandard haematology text

References

  1. Convertino VA. Blood volume: its adaptation to endurance training. Medicine and Science in Sports and Exercise. 1991;23(12):1338–1348. PMID 1798375 Review
  2. Pepys MB, Hirschfield GM. C-reactive protein: a critical update. Journal of Clinical Investigation. 2003;111(12):1805–1812. doi:10.1172/JCI18921 PMID 12813013 Review

Related topics

Reviewed by the Physiotherapist India Team. · Human Physiology contents