Download Properties of blood

Properties of blood
• The colour of blood is red due to haemoglobin.
Arterial blood is scarlet red (due to oxygen)
and venous blood is purple red (due to carbon
dioxide)
• The pH under normal condition is 7.4 (venous
blood 7.36)
• Blood is opaque due to its cellular elements,
particularly the red blood cells. After
haemolysis blood becomes transparent.
• Blood has specific gravity of 1060 (that of
cells is about 1090 while that of plasma is
1025-1030)
• Blood is five times more viscous than water
due to its cellular elements and some plasma
proteins. The viscosity of plasma is twice as
water.
• The osmotic pressure of the plasma is about
5500 mmHg. It is mostly due to crystalloids.
Functions of blood
• It is the major transport medium in the
body. This is essential for:
• Nutrition: from the intestine to all the tissues
• Waste products: From the cells to the
excretory organs, e.g. urea to the kidneys, CO2
to the lungs, and bilirubin to the liver.
• Hormones : from the endocrine glands to their
target cells.
• Regulation of:
• Water content of the tissues: by fluid
exchange between the plasma and the
interstitial fluid.
• pH of tissue fluids: by removing any acidic
metabolites from the tissue fluids and
buffering them by the buffering systems of the
blood, e.g. the carbonic/ bicarbonate buffering
system (H2CO3/NaHCO3).
• Hormonal
secretion:
by
feedback
mechanism; e.g. parathormone raises the
plasma calcium level, a rise in the plasma
calcium level inhibits parathormone secretion,
and a drop of plasma calcium level stimulates
parathormone secretion.
• Body temperature: by heat transfer from deep
organs to the skin.
• This is facilitated by the high heat conductivity
and the high specific heat of water. Blood also
informs the hypothalamic thermostat about the
body core temperature.
• The hypothalamic thermostat then reacts
accordingly to adjust body temperature at its
normal level.
• Defence against:
• Microorganisms: blood phagocytes, with the
help of antibodies, attack and kill any invading
microorganisms.
• Toxic substances: antitoxins, i.e. antibodies
against toxins, neutralize toxins and poisons.
• Blood loss: blood clotting factors seal wounds
and prevent blood loss.
Plasma
• Blood plasma is a mixture of proteins,
enzymes, nutrients, wastes, hormones and
gases. The specific composition of its
components are as follows:
Composition of Plasma
Plasma Proteins
• The total plasma protein concentration is 7
gm/dL (range 6-8). Of this, 4 gm is albumin,
2.7 gm is a mixture of globulins and 0.3 gm is
fibrinogen.
• Globulins are classified according to their
physicochemical properties into 4 different
types; α1, α2, β and γ- globulins.
Separation and measurement of plasma
proteins
• Several methods are used to separate and
measure the different component of plasma
proteins.
• One method is chemical precipitation by
ammonium sulphate.
• A 25% saturation with ammonium sulphate
precipitates fibrinogen, 50% saturation
precipitates globulins, and full saturation
precipitates albumin.
• Electrophoresis is another method. If a mixture
of plasma proteins is put in an electric field
between two electrodes, different types of
proteins migrate towards the anode at different
speeds.
• Thus proteins can be spread and separated on a
paper strip and their concentration determined.
• Other methods include; isoelectric focusing,
ultracentrifugation
and
immunoelectrophoresis.
The A/G ratio
• The A/G is the ratio of albumin/globulin in the
plasma. The normal value depends on the method of
determination. It is 1.5 by electrophoresis and 1.2 by
the chemical precipitation method.
• The A/G is a non specific test that could help
clinically in diagnosis of some diseases, e.g. it is
decreased in liver diseases (due to decrease in
albumin synthesis) and in infections as tuberculosis
and typhoid (due to increase in γ- globulin synthesis).
• It is increased in hypogamma-globulinemia and in
AIDS.
Formation of the Plasma Proteins.
• Essentially all the albumin and fibrinogen of
the plasma proteins, as well as 50 to 80 per
cent of the globulins, are formed in the liver.
• The remainder of the globulins are formed
almost entirely in the lymphoid tissues.
• They are mainly the gamma globulins that
constitute the antibodies used in the immune
system.
• The rate of plasma protein formation by
the liver can be extremely high, as much
as 30 g/day.
• Certain disease conditions cause rapid
loss of plasma proteins; severe burns that
denude large surface areas of the skin can
cause the loss of several liters of plasma
through the denuded areas each day.
• The rapid production of plasma proteins by the
liver is valuable in preventing death in such
states.
• Occasionally, a person with severe renal
disease loses as much as 20 grams of plasma
protein in the urine each day for months, and it
is continually replaced mainly by liver
production of the required proteins.
• In cirrhosis of the liver, large amounts of
fibrous tissue develop among the liver
parenchymal cells, causing a reduction in their
ability to synthesize plasma proteins.
• This leads to decreased plasma colloid osmotic
pressure, which causes generalized oedema.
Functions of plasma proteins
• Haemostasis:
fibrinogen, prothrombin and most of other
blood clotting factors are plasma proteins.
• Tissue fluid formation:
Fluid interchange between plasma and
interstitial fluids occurs by filtration and
absorption at the capillaries. The filtration
force is mainly the hydrostatic capillary
pressure. The absorption force is the oncotic
pressure of the plasma proteins.
• In hypoproteinemia, the oncotic pressure
decreases leading to more filtration of
interstitial fluid than reapsorption which
causes accumulation of excess amounts of
interstitial fluids, i.e. edema
• Peripheral vascular resistance:
The peripheral vascular resistance is determined
by two main factors; i.e. the diameter of the
arterioles and the viscosity of the blood.
• The viscosity of the blood is the function of two
factors; i.e. the RBCs and plasma proteins. The
globulins and fibrinogen are particularly
important for the viscosity of the plasma.
• The peripheral vascular resistance is important for
maintaining the arterial blood pressure.
• Buffering action:
At the normal pH of arterial blood (7.4),
plasma proteins act as weak acids (proteinic
acids) that combine with bases, mainly
sodium.
• The mixture of a weak acid and its salt with a
strong base (Na+) makes a blood buffering
system. 15% of the buffering capacity of the
blood is due to plasma proteins.
• Defence and protection:
Gamma globulins are the antibodies which
attack any invading bacteria or foreign bodies
and neutralize toxins.
• Regulating the activity of the biologically
active substances:
• plasma proteins combine loosely with many
chemical substances including hormones (e.g.
thyroxin and cortisol), minerals (e.g. iron and
copper) and numerous drugs.
• The bonds are firm enough to limit free
diffusion of the small molecular substances,
and thus reduce their biological activity.
• The bond form serves as a reservoir from
which the free hormone, metal, or drug is
slowly released.
• Effect on capillary permeability:
Proteins clot the pores in the capillary wall.
Hypoproteinemia leads to increase in capillary
permeability which favours the development
of edema.
• Carbon dioxide transport:
10% of the tidal CO2 is carried from the tissues
to the lungs combined with plasma proteins in
the form of carbaminocompounds. At the lung,
CO2 is released and proteins restore their
amino structure.
• Nutritional function during starvation:
Plasma proteins are used by the tissues during
prolonged starvation after the body protein
reserves have been depleted.