Conversations on Chemistry Part 107

After circulating through the lungs, the blood is collected into four large vessels, by which it is conveyed into the left ventricle of the heart, whence it is propelled to all the different parts of the body by a large artery, which gradually ramifies into millions of small arteries through the whole frame. From the extremities of these little ramifications the blood is transmitted to the veins, which bring it back to the heart and lungs, to go round again and again in the manner we have just described. You see, therefore, that the blood actually undergoes two circulations; the one, through the lungs, by which it is converted into pure arterial blood; the other, or general circulation, by which nourishment is conveyed to every part of the body; and these are both equally indispensable to the support of animal life.

EMILY.

But whence proceeds the carbon with which the blood is impregnated when it comes into the lungs?

MRS. B.

Carbon exists in a greater proportion in blood than in organised animal matter. The blood, therefore, after supplying its various secretions, becomes loaded with an excess of carbon, which is carried off by respiration; and the formation of new chyle from the food affords a constant supply of carbonaceous matter.

CAROLINE.

I wonder what quant.i.ty of carbon may be expelled from the blood by respiration in the course of 24 hours?

MRS. B.

It appears by the experiments of Messrs. Allen and Pepys that about 40,000 cubic inches of carbonic acid gas are emitted from the lungs of a healthy person, daily; which is equivalent to _eleven ounces_ of solid carbon every 24 hours.

EMILY.

What an immense quant.i.ty! And pray how much of carbonic acid gas do we expel from our lungs at each expiration?

MRS. B.

The quant.i.ty of air which we take into our lungs at each inspiration, is about 40 cubic inches, which contain a little less than 10 cubic inches of oxygen; and of those 10 inches, one-eighth is converted into carbonic acid gas on pa.s.sing once through the lungs*, a change which is sufficient to prevent air which has only been breathed once from suffering a taper to burn in it.

[Footnote *: The bulk of carbonic acid gas formed by respiration, is exactly the same as that of the oxygen gas which disappears.]

CAROLINE.

Pray, how does the air come in contact with the blood in the lungs?

MRS. B.

I cannot answer this question without entering into an explanation of the nature and structure of the lungs. You recollect that the venous blood, on being expelled from the right ventricle, enters the lungs to go through what we may call the lesser circulation; the large trunk or vessel that conveys it branches out, at its entrance into the lungs, into an infinite number of very fine ramifications. The windpipe, which conveys the air from the mouth into the lungs, likewise spreads out into a corresponding number of air vessels, which follow the same course as the blood vessels, forming millions of very minute air-cells. These two sets of vessels are so interwoven as to form a sort of net-work, connected into a kind of spongy ma.s.s, in which every particle of blood must necessarily come in contact with a particle of air.

CAROLINE.

But since the blood and the air are contained in different vessels, how can they come into contact?

MRS. B.

They act on each other through the membrane which forms the coats of these vessels; for although this membrane prevents the blood and the air from mixing together in the lungs, yet it is no impediment to their chemical action on each other.

EMILY.

Are the lungs composed entirely of blood vessels and air vessels?

MRS. B.

I believe they are, with the addition only of nerves and of a small quant.i.ty of the cellular substance before mentioned, which connects the whole into an uniform ma.s.s.

EMILY.

Pray, why are the lungs always spoken of in the plural number? Are there more than one?

MRS. B.

Yes; for though they form but one organ, they really consist of two compartments called lobes, which are enclosed in separate membranes or bags, each occupying one side of the chest, and being in close contact with each other, but without communicating together. This is a beautiful provision of nature, in consequence of which, if one of the lobes be wounded, the other performs the whole process of respiration till the first is healed.

The blood, thus completed, by the process of respiration, forms the most complex of all animal compounds, since it contains not only the numerous materials necessary to form the various secretions, as saliva, tears, &c. but likewise all those that are required to nourish the several parts of the body, as the muscles, bones, nerves, glands, &c.

EMILY.

There seems to be a singular a.n.a.logy between the blood of animals and the sap of vegetables; for each of these fluids contains the several materials destined for the nutrition of the numerous cla.s.s of bodies to which they respectively belong.

MRS. B.

Nor is the production of these fluids in the animal and vegetable systems entirely different; for the absorbent vessels, which pump up the chyle from the stomach and intestines, may be compared to the absorbents of the roots of plants, which suck up the nourishment from the soil. And the a.n.a.logy between the sap and the blood may be still further traced, if we follow the latter in the course of its circulation; for, in the living animal, we find every where organs which are possessed of a power to secrete from the blood and appropriate to themselves the ingredients requisite for their support.

CAROLINE.

But whence do these organs derive their respective powers?

MRS. B.

From a peculiar organisation, the secret of which no one has yet been able to unfold. But it must be ultimately by means of the vital principle that both their mechanical and chemical powers are brought into action.

I cannot dismiss the subject of circulation without mentioning _perspiration_, a secretion which is immediately connected with it, and acts a most important part in the animal economy.

CAROLINE.

Is not this secretion likewise made by appropriate glands?

MRS. B.

No; it is performed by the extremities of the arteries, which penetrate through the skin and terminate under the cuticle, through the pores of which the perspiration issues. When this fluid is not secreted in excess, it is _insensible_, because it is dissolved by the air as it exudes from the pores; but when it is secreted faster than it can be dissolved, it becomes _sensible_, as it a.s.sumes its liquid state.

EMILY.

This secretion bears a striking resemblance to the transpiration of the sap of plants. They both consist of the most fluid part, and both exude from the surface by the extremities of the vessels through which they circulate.

MRS. B.

And the a.n.a.logy does not stop there; for, since it has been ascertained that the sap returns into the roots of the plants, the resemblance between the animal and vegetable circulation is become still more obvious. The latter, however, is far from being complete, since, as we observed before, it consists only in a rising and descending of the sap, whilst in animals the blood actually _circulates_ through every part of the system.

We have now, I think, traced the process of nutrition, from the introduction of the food into the stomach to its finally becoming a const.i.tuent part of the animal frame. This will, therefore, be a fit period to conclude our present conversation. What further remarks we have to make on the animal economy shall be reserved for our next interview.