I found it possible to observe at least the superficial capillaries of muscles both in the frog and in mammals through a binocular microscope, using strong reflected light as a source of illumination. Resting muscles observed in this way are usually quite pale, and the microscope reveals only a few capillaries at fairly regular intervals.
August Krogh
The most direct evidence of the wonderful plasticity and elasticity of red corpuscles is obtained when they are watched in a current, where they can be caught against a projecting edge and bent by the pressure of the current flowing past them.
The respiratory mechanisms of birds are definitely adapted to the function of flight, as evidenced by the fact that birds which do not fly (Apteryx, Penguins) show these adaptations in a greatly reduced form.
Since muscular contractions are usually more or less regularly alternated with relaxations, the system of valves makes of the veins of every muscle a very effective pump, capable of maintaining a low pressure in the muscle capillaries.
The tremendous honour of the Nobel Prize is of the strongest incentive to me in my work, while the amount of the Prize will greatly simplify my task and provide me with much valuable help in my work.
Muscles do not use oxygen at a constant rate.
For a large number of problems, there will be some animal of choice, or a few such animals, on which it can be most conveniently studied.
In the numerous observations made in my laboratory upon this object, we have only once seen a combination of vessels in which there might be a direct communication between a small artery and a vein, though the two observers could not come to a final conclusion on the point.
It is no doubt technically possible to study metabolism and respiration of fishes during swimming at a constant rate, and of certain insects and birds during flight, and to obtain information similar to that obtained on man during work on a bicycle ergometer or a treadmill.
The circulatory system of man and the vertebrate animals can be considered as made up of a small number of organs or subordinate systems, which are easy to recognize anatomically, and the functions of which are on the whole quite distinct.
In the giraffe with a total height of 5 m., the heart is at a height of about 2.5 m., and it would be extremely interesting to know just how the giraffe avoids the development of filtration oedema in its long legs.
In the tissues generally, and in such a cavity as the abdomen, the pressure is everywhere and practically always very nearly atmospheric, and must be so, because the integuments give way very easily to any excess of pressure, whether positive or negative.
In many cases of inflammation, the vascular changes develop slowly and long after the application of the stimulus which is responsible for the inflammatory reaction.
In man, the mechanical breathing is essential to life, and it is one of the old tests for death to see whether these movements have ceased completely.
In many organisms, including man, the mechanical respiration and the circulation of the blood are 'regulated' so as to correspond to the demand of the moment.
There can be little doubt that fishes swimming rapidly do not make respiratory movements at all, but obtain the necessary ventilation of the gills simply by opening the mouth.
The affinity of blood or pure haemoglobins for oxygen is a complex phenomenon, depending upon a number of conditions, the most important of which are temperature and hydrogen ion concentration.
