Cardiac Muscle in Animals - April 24, 2010 by admin

Although even in the adult animals the fundamental modifications in the basic rate of the heart beat are brought about by the brain, the heart can dispense with these com­mands and set its rhythm independently. Figuratively speaking, our heart works on its own initiative, a peculiarity which we somehow do not appreciate. If the fibres of an embryonic cardiac muscle are grown in a tissue culture on a special nutrient medium, they will contract rhythmically in a vial too, without waiting for any orders. They just cannot live without contracting.

Nonetheless, work cannot be well co-ordinated without a headquarters. If every muscle fibre contracted of its own accord, the common contraction could take place only by pure chance. This is what really happens at the earliest stages of embryonic life. In the rat’s embryo individual sections of the heart contract quite independently until the headquarters is set up and starts to operate. In birds and mammals it is located in a special region of the heart known as sino-auricular node.

The cardiac muscle has no nerves, and commands are conducted over the muscle fibres at the rate of one metre per second. This rate is quite adequate for the auricles to contract normally. The ventricles of the heart, which are larger than the auricles and which require commands to be communi­cated more rapidly, have a system, known as Purkinje fibres, over which excitation spreads five or six times more quickly.

In the heart of every self-respecting animal there is only one headquarters known as the pacemaker. More pace­makers would certainly cause a mess. Strange things, however, are not uncommon. The ascidians and some tunicates have two pacemakers, one at each end of the pulsating vessel. In such animals the blood flow periodically changes its direction.

Pulsating Vessels & Blood Amount - April 22, 2010 by admin

It’s not a secret that the smaller the aquarium, the more intensively it is used and the more rapid the currents in it have to be so that the same liquid can be used over and over again. It is small wonder that insects can afford the luxury of having very slow currents in their aquaria, taking 30-35 minutes to make one complete cycle. Man cannot afford this. The blood in our internal aquarium completes a cycle in as little as 23 seconds and performs over 3700 cycles per day. This is, however, not the maximum. In a dog a complete cycle takes 16 seconds, in a rabbit only 7.5 seconds, and in the smaller animals even less.

In vertebrates the matter is complicated since the aquarium itself is very large, but has little water in it. Not can it be filled up. The total length of all man’s blood vessels is about 100 thousand kilometres. Most of them are usually empty since 7-10 litres of blood are far from enough to till them and only the most hard-working organs are supplied intensively. For this reason heavy-duty functions cannot be performed by many systems simultaneously. After a good meal the digestive organs are the most energetic. They receive a considerable amount of blood, while the brain is not adequately supplied to function normally. Hence, we experience drowsiness.

To set the waters of the internal aquarium in motion, it was necessary to have devices very different from the cilia of sponges. Muscle pumps proved much more dependable. The earliest pumps were nothing more than a pulsating vessel, i. e. a very simple heart, which drove hemolymph into the smaller vessels and thence into the interstitial and intercel­lular spaces. Having watered them, the hemolymph returned to the pulsating vessel. Such an open system could not provide proper circulation, and this is why insects, the highest representatives of the invertebrates, developed pumps which not only force out, but also suck in. For this purpose their hearts are freely attached to special muscles, known as the pterygoid muscles, that stretch the heart, thus creating a negative pressure that sucks in the liquid passing through the tissues.

A pulsating vessel is a low-capacity unit, and lower animals usually have many pumping devices. In the earthworm the main pulsating vessel, that extends throughout its entire body, drives the blood from the rear to the front end. On its way, the blood flows into side vessels which themselves act as hearts pushing the blood into even finer arteries. All these numerous hearts function independently, co-ordinating, at best, their work with the partner in the segment. And this is the extent of the organization.