The primary function of blood is to deliver O₂ and energy sources to the tissues, and to remove CO₂ and waste products. It contains elements of the defence and immune systems, is important for regulation of temperature and transports hormones and other signalling molecules between tissues. In a 70-kg man blood volume is ∼5500 mL, or 8% of body weight. Blood consists of plasma and blood cells. If blood is centrifuged, the cells sediment as the packed cell volume (PCV, haematocrit), normally ∼45% of total volume (i.e. PCV = 0.45) in men, less in women (Figure 5).

Plasma

The plasma volume is ∼5% of body weight. It consists of ions in solution and a variety of plasma proteins. Normal ranges for key constituents are shown in Figure 5. After clotting, a straw-coloured fluid called serum remains, from which fibrinogen and other clotting factors have been removed. The relative osmotic pressures of plasma, interstitial and intracellular fluid are critical for maintenance of tissue cell volume, and are related to the amount of osmotically active particles (molecules) per litre, or osmolarity (mosmol/L); as plasma is not an ideal fluid (it contains slow diffusing proteins), the term osmolality (mosmol/kg H₂O) is often used instead. Plasma osmolality is ∼290 mosmol/kg H₂O, mostly due to dissolved ions and small diffusible molecules (e.g. glucose and urea). These diffuse easily across capillaries, and the crystalloid osmotic pressure they exert is therefore the same either side of the capillary wall. Proteins do not easily pass through capillary walls, and are responsible for the oncotic (or colloidal osmotic) pressure of the plasma. This is much smaller than crystalloid osmotic pressure, but is critical for fluid transfer across capillary walls because it differs between plasma and interstitial fluid (see Chapter 21). Oncotic pressure is expressed in terms of pressure, and in plasma is normally ∼25 mmHg. Maintenance of plasma osmolality is vital for regulation of blood volume (see Chapter 29).

Ionic Composition

Na⁺ is the most prevalent ion in plasma, and the main determinant of plasma osmolality. The figure shows concentrations of the major ions; others are present in smaller amounts. Changes in ionic concentration can have major consequences for excitable tissues (e.g. K⁺, Ca²⁺). Whereas Na⁺, K⁺ and Cl⁻ completely dissociate in plasma, Ca²⁺

Stay updated, free articles. Join our Telegram channel

Join