CorpusHuman Anatomy
CardiovascularTissue

Blood

Sanguis

Also known as: whole blood, haematology, hematology

Blood is a fluid connective tissue made of plasma carrying erythrocytes, leukocytes and platelets. It transports gases, nutrients, wastes, hormones and heat, buffers pH, defends against infection and seals injured vessels.

Overview

Volume≈70 mL/kg (≈5 L)
Plasma : cells≈55 : 45
Arterial pH7.35–7.45
Osmolality275–295 mOsm/kg
Viscosity≈3–4 × water

Volume. An adult has about 5 L of blood (4–6 L), about 70 mL/kg in men and 65 mL/kg in women. This is about 7–8% of body weight.

Composition. When blood is spun in a centrifuge it separates into three layers. Plasma forms about 55% of the volume. Erythrocytes form about 45%; this fraction is the haematocrit, about 40–50% in men and 36–46% in women. The thin buffy coat between them (less than 1%) contains the leukocytes and platelets.

Physical properties. Blood is 3–4 times as viscous as water, and its viscosity rises with the haematocrit. Arterial pH is 7.35–7.45, plasma osmolality is 275–295 mOsm/kg and core blood temperature is about 38 °C. Oxygenated blood is bright red and deoxygenated blood is dark red, never blue.

Formation. All blood cells come from haematopoietic stem cells in the red bone marrow (haematopoiesis). The marrow releases about 2–3 million red cells every second. Production is driven by growth factors: erythropoietin for red cells, thrombopoietin for platelets and colony-stimulating factors for white cells.

Parts & subdivisions

  1. Plasma
    About 55% of blood volume. It is water containing proteins, electrolytes, nutrients, hormones and gases.
  2. Erythrocytes
    About 45% of blood volume (the haematocrit). They carry O₂ bound to haemoglobin.
  3. Leukocytes
    About 4–11 × 10⁹/L. Five types defend against infection.
  4. Platelets
    About 150–400 × 10⁹/L. They are needed for primary haemostasis.

Function

  • Transports O₂, CO₂, nutrients, wastes, hormones and heat.
  • Regulates pH (buffers), body temperature and osmotic and oncotic pressure.
  • Protects against blood loss (haemostasis) and infection (leukocytes, antibodies, complement).

Clinical significance

Haemorrhage and hypovolaemic shock

The ATLS classification grades blood loss in four classes. In class I (less than 15%, about 750 mL) signs are minimal. In class II (15–30%) the heart rate rises and the pulse pressure narrows. In class III (30–40%) hypotension and confusion appear. Class IV (more than 40%) is immediately life-threatening.

The haematocrit drops only after fluid has shifted into the blood vessels from the tissues, so it can be normal straight after an acute bleed.

Polycythaemia

In polycythaemia the haematocrit is raised. Primary polycythaemia (polycythaemia vera) is a bone marrow disorder driven by a JAK2 mutation. Secondary polycythaemia is driven by erythropoietin, in response to chronic hypoxia or an EPO-secreting tumour. Relative polycythaemia is caused by a low plasma volume. A high haematocrit makes blood more viscous and increases the risk of thrombosis.

Test yourself

Left ventricular myocardium receives most of its coronary blood flow during:
  1. Atrial systole
  2. Isovolumetric contraction
  3. Rapid ejection
  4. Diastole

Answer: Diastole

During systole the contracting myocardium squeezes the intramural vessels shut, especially in the subendocardium. Most left ventricular perfusion therefore happens in diastole, driven by the recoiling aorta. Tachycardia shortens diastole and so reduces coronary perfusion time.

In an emergency when the patient’s blood group is unknown, which red cells are given?
  1. AB RhD-positive
  2. O RhD-negative
  3. A RhD-negative
  4. B RhD-positive

Answer: O RhD-negative

Group O red cells carry no A or B antigens, so the recipient's anti-A and anti-B antibodies have nothing to attack. RhD-negative cells avoid sensitising RhD-negative recipients, which matters especially in women of child-bearing age. AB is the universal recipient for red cells and the universal donor for plasma.

A patient’s blood pressure is 120/80 mmHg. What is their approximate mean arterial pressure?
  1. 80 mmHg
  2. 93 mmHg
  3. 100 mmHg
  4. 107 mmHg

Answer: 93 mmHg

MAP ≈ diastolic + ⅓ pulse pressure = 80 + 40/3 ≈ 93 mmHg. MAP sits closer to diastolic than systolic pressure because diastole takes up about two-thirds of the cardiac cycle at rest.

More cardiovascular system questions

Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.