Disturbances Of Oxygen Transport In Blood.....
(1) Haemoglobin. Since most of the oxygen in the blood is carried in chemical combination with haemoglobin, a diminished concentration of haemoglobin in the blood, anaemia, reduces the availability of oxygen to the tissues. The normal blood haemoglobin concentration is 14 to 15 gper 100 ml, but in severe chronic anaemia the fire may fall to less than 5 g per 100 ml. All the tissues and organs of the body are chronically starved of oxygen and they are therefore peculiarly sensitive to any sudden insult: in these circumstances a blood transfusion meant to correct the anaemia may overload the damaged heart and provoke cardiac failure and death.
The haemoglobin may also be present in normal concentration, but not available for carrying oxygen because it has combined with carbon monoxide gas. Carbon monoxide has an even greater chemical affinity for haemoglobin than has oxygen. This property accounts for the dangers of poor ventilation in buildings such as garages where an accumulation of carbon monoxide (from the exhaust gases of a car's engine) may be expected.
(2) Circulation. Interference with the circulation may be complete, when it is due to the heart stopping, cardiac arrest. It is important to remember that only a few seconds after the heart stops, the part of the brain that controls the respiratory system, the respiratory centre, is damaged and ventilation ceases. Complete stoppage of the circulation is better called cardio-respiratory arrest to emphasize this point. Incomplete interference with the circulation, usually called circulatory failure, results from a variety of causes.
(i) Failure of the pump. This may be called central circulatory failure. When the heart is damaged by disease or by poison so that it cannot cope with pumping onwards the whole of the blood being brought back from the tissues by the veins, the venous return, the chambers of the heart and the great veins which are responsible for the venous return dilate. The pressure rises in the great veins, the superior and inferior venae cavae, which enter the right side of the heart within the thoracic cavity: this raised central venous pressure is diagnostic of a failing heart.
(ii) Failure of the venous return. The heart, no matter how strong, cannot pump out to the tissues any more blood than it receives. An inadequate cardiac output may therefore be due to an inadequate venous return. Since the cells receive an inadequate supply of blood despite the efforts of a normal heart, this situation is sometimes called peripheral (as distinct from central) circulatory failure. It is important to realize, however, that central circulatory failure reduces the cardiac output and hence the arterial supply to the periphery and to this extent the two terms are confusing. Failure of the venous return has two possible causes: a reduction in the volume of the circulating blood, hypovolaemia, and a dilatation of the small vessels, arterioles, capillaries, which bring the blood closest to the tissue cells.
Of these two types, hypovolaemia is the more important in surgery because it is frequently due to a loss of blood, haemorrhage . It is obvious that haemorrhage results in a reduction in the venous return, and hence in a reduction in the cardiac output. But a dilatation of the arteriolar-capillary bed means that at any given moment there is more blood in these small vessels than usual, so that if the cardiac output into the arteries is normal in volume there must be less blood in the veins.
The classical example ofa diminished cardiac output due to vasodilatation is the simple fainting attack, in which the small blood vessels of the muscles dilateusually as a result of an emotional disturbance; another important example is the vasodilatation produced by toxic substances derived from bacteria growing in the bloodstream, a condition known as septicaemia.
The important distinction between failure of the venous return and failure of the pump is that in the former the central venous pressure is low instead of high.
(1) Haemoglobin. Since most of the oxygen in the blood is carried in chemical combination with haemoglobin, a diminished concentration of haemoglobin in the blood, anaemia, reduces the availability of oxygen to the tissues. The normal blood haemoglobin concentration is 14 to 15 gper 100 ml, but in severe chronic anaemia the fire may fall to less than 5 g per 100 ml. All the tissues and organs of the body are chronically starved of oxygen and they are therefore peculiarly sensitive to any sudden insult: in these circumstances a blood transfusion meant to correct the anaemia may overload the damaged heart and provoke cardiac failure and death.
The haemoglobin may also be present in normal concentration, but not available for carrying oxygen because it has combined with carbon monoxide gas. Carbon monoxide has an even greater chemical affinity for haemoglobin than has oxygen. This property accounts for the dangers of poor ventilation in buildings such as garages where an accumulation of carbon monoxide (from the exhaust gases of a car's engine) may be expected.
(2) Circulation. Interference with the circulation may be complete, when it is due to the heart stopping, cardiac arrest. It is important to remember that only a few seconds after the heart stops, the part of the brain that controls the respiratory system, the respiratory centre, is damaged and ventilation ceases. Complete stoppage of the circulation is better called cardio-respiratory arrest to emphasize this point. Incomplete interference with the circulation, usually called circulatory failure, results from a variety of causes.
(i) Failure of the pump. This may be called central circulatory failure. When the heart is damaged by disease or by poison so that it cannot cope with pumping onwards the whole of the blood being brought back from the tissues by the veins, the venous return, the chambers of the heart and the great veins which are responsible for the venous return dilate. The pressure rises in the great veins, the superior and inferior venae cavae, which enter the right side of the heart within the thoracic cavity: this raised central venous pressure is diagnostic of a failing heart.
(ii) Failure of the venous return. The heart, no matter how strong, cannot pump out to the tissues any more blood than it receives. An inadequate cardiac output may therefore be due to an inadequate venous return. Since the cells receive an inadequate supply of blood despite the efforts of a normal heart, this situation is sometimes called peripheral (as distinct from central) circulatory failure. It is important to realize, however, that central circulatory failure reduces the cardiac output and hence the arterial supply to the periphery and to this extent the two terms are confusing. Failure of the venous return has two possible causes: a reduction in the volume of the circulating blood, hypovolaemia, and a dilatation of the small vessels, arterioles, capillaries, which bring the blood closest to the tissue cells.
Of these two types, hypovolaemia is the more important in surgery because it is frequently due to a loss of blood, haemorrhage . It is obvious that haemorrhage results in a reduction in the venous return, and hence in a reduction in the cardiac output. But a dilatation of the arteriolar-capillary bed means that at any given moment there is more blood in these small vessels than usual, so that if the cardiac output into the arteries is normal in volume there must be less blood in the veins.
The classical example ofa diminished cardiac output due to vasodilatation is the simple fainting attack, in which the small blood vessels of the muscles dilateusually as a result of an emotional disturbance; another important example is the vasodilatation produced by toxic substances derived from bacteria growing in the bloodstream, a condition known as septicaemia.
The important distinction between failure of the venous return and failure of the pump is that in the former the central venous pressure is low instead of high.
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