The lungs themselves are not abnormally large and all pinnipedes exhale before or on diving so that there is literally little or no oxygen in the lungs to provide for tissue respiration during the activities of swimming and catching prey below the surface. The thorax, which also contains the heart and great vessels besides the lungs, is more elongate than normal in mammals and the diaphragm, which separates its cavity from that of the abdomen, is set much more obliquely, its upper attachment to the body wall being set farther back than normal and the sternal support of the lower margin is shorter than usual (Fig. 4). This means that the cavity can be more completely compressed and a greater proportion of the air in the lungs exhaled than in normal land carnivores and other mammals. The small residium is driven into the nonrespiratory trachea and bronchi. When the seal returns to the surface breathing recommences and a series of deep inhalations and expirations takes place. From my own observations on a southern elephant seal the number of such breaths is roughly proportional to the length of time that the nostrils have been closed. Even when on land and hauled-out seals will continue to remain with closed nostrils for considerable periods separated by series of breathings.
By way of contrast Cetaceans dive with full lungs and all the modifications are towards the prevention of collapse and the transmission of the external pressure to the air contained in the lungs; another ‘anti-bends’ device.
In the pinnipedes we are still left with the puzzle of how they obtain and maintain sufficient oxygen for their activities below the surface, and we must turn to the blood system for further information.
First it must be clear that if there is little or no air in the lungs there is no profit in circulating the blood from the lungs for there is no oxygen to pass on to the active tissues. It is therefore not altogether surprising to find that seals exhibit a phenomenon known as ‘bradycardia’. This is a reduction in the heart beat both in the number per minute and in the strength of the beat. In fact it is reduced to little more than an occasional flutter by which some blood is circulated along the carotid artery to the brain. This has been shown to arise almost immediately the seal has dived, and in this it differs from the bradycardia of Cetaceans in which the rate and strength of the heart beat is gradually reduced to a low level. This difference must be associated with the difference of lung contents, the gradual bradycardia of Cetaceans keeping pace with the gradual exhaustion of the oxygen in the lungs (Fig. 5).
To prevent the ‘used’ blood from the tissues of the body being circulated even to a very minor degree in the pinnipedes, they have evolved a powerful sphincter muscle which closes the huge venous blood vessel leading to the heart and which draws blood from the hinder part of the body, the viscera and liver. This large blood vessel (posterior vena cava) is disproportionately large (usually double and enlarged) and so can act as a reservoir for the non-circulating blood. In addition there is a large vein lying below and up the sides of the spinal cord (extradural vein) which is enormously enlarged in pinnipedes. From it only a little blood can find its way back to the heart in the front region. Elsewhere this vein is connected by special large veins both directly to the posterior vena cava (at the hinder end where it is double) and indirectly round the kidneys in huge blood sinuses (Fig. 6). All these peculiarities increase the storage capacity of the venous system when bradycardia is in action. Some idea of the size of the veins will be conveyed by saying that in the grey seal the posterior vena cava in its posterior part is ‘nearly as thick as your wrist’ and King (1964) refers to their size in the walrus ‘by the often quoted reference that they can be “pulled on like a pair of trousers”’.
But all these modifications tend to show that the blood is not the continuing source of oxygen during active diving. This is confirmed by two other facts; firstly the red blood cell count (the number of red blood cells per unit volume) is nothing out of the way, about 5–6 million (cf. human 4–5 million), and secondly that the amount of haemoglobin in a unit volume is not very high either, about 1.2 compared to the standard in man of 1.0. If this is all true, then how do seals manage to respire in their tissues during diving?
Part of the answer lies in another pigment known as myoglobin because it is present in muscle. Here we find an enormous difference from the normal and we hardly need figures to show it. Myoglobin is also coloured though not so deeply as haemoglobin and it gives the normal pink colour to muscle meat. In the Pinnipedia the muscle is almost black in colour, certainly very deep red. Those who remember the whale meat which was available after the second world war will recognise that Cetaceans too have a very high myoglobin content. To both of these groups then part of the answer is the ability to store oxygen attached to the myoglobin on the site where it is required for the respiration of the active muscle cells.
There is also evidence that these animals can run into oxygen debt, particularly in respect of metabolising the waste products, which are normally toxic if allowed to accumulate without treatment. This adaptation is not so extraordinary as it sounds, for it is known that, in humans, when slow starvation is prolonged and the organism begins to live on its muscle protein (autolysis of the muscle) a level of waste nitrogen products can build up to many times the normally lethal concentration. In these diving mammals this ability has become normal rather than a pathological occurrence. On return to the surface the repeated inhalations and exhalations rapidly restore the oxygen balance.