Assimilating the course of the veins to that of the arteries, follow them from trunk to branch. The general veins of the body commence by minute nodes from the ultimate termination of the aortic system. The mode of origin, which will be considered when the capillary vessels are described, exhibits several varieties in the different organs and tissues, relating to the size of the primitive roots, to the kind of connection existing between them and the terminal arteries, etc. The veins of the lungs begin similarly from the last ramifications of the pulmonary arteries.
The capacity of the venous system exceeds that of the arterial due to the number and size of the veins being greater than those of the arteries. Thus we may remark that the two vena cavae and the vena coronaria correspond with the single trunk of the aorta; and that there are four pulmonary veins, and only one corresponding artery. In most parts of the body each artery is accompanied by two veins; in some organs, however, as in the stomach, intestines, spleen, kidneys, testicles, etc., the two species of vessels are in equal number; and a similar arrangement is observed in some of the great trunks, as in the iliac and subclavian arteries and veins. In a few instances the veins are less numerous than the arteries they accompany; this disposition is exemplified in the penis, clitoris, umbilical cord, etc.; but in these rare cases the capacity of the veins makes up for their want of number.
In addition to the deep-seated veins, there are others placed on the external surface of the body, which are in some parts very large and numerous, and do not possess any corresponding arteries. The veins, taken collectively, are then much more capacious than the arteries, and consequently they contain a larger quantity of blood; but, as Richet has shown, it is impossible to calculate with precision the amount of this excess. According to Haller the size of the veins is at least double that of the arteries, and frequently it is triple or even more.
The disposition of the venous system bears a general resemblance to that of the arterial; but it is distinguished by the veins being arranged in a superficial and a deep set. The former are placed immediately under the skin, and are therefore called subcutaneous veins; they freely anastomose and form a network, which in some parts, as in the forearm, is very evident through the skin. The deep-seated veins generally accompany the arteries to which they are often intimately joined by a common sheath of condensed cellular membrane; there are, however, exceptions to this arrangement, as in the liver, brain, and spinal cord, in which organs the two orders of vessels run apart from each other.
In those internal parts of the body where the veins cross the arteries, they are usually placed more externally than the latter; thus the subclavian veins lie before their arteries, the left renal vein before the aorta, the inferior vena cava before the right renal artery, etc.; but the relative situation is sometimes reversed, as in the descent of the iliac veins crossing beneath their arteries. The branches of the veins are in the aggregate larger than the trunks, from which it results that the blood, moving in a space which successively decreases, must circulate with greater velocity in proportion as it approaches the heart.
<Callout type="important" title="Venous Capacity">The venous system is significantly more capacious compared to the arterial system due to its larger number and size of veins.</Callout>
Having in the former part of this chapter described rather minutely the anastomosis of the vessels, I shall only add in this place that the veins are characterized by the freedom of their communications, which are established not only by small branches, as in the arterial system, but very frequently by large trunks. They are also distinguished in some parts of the body by forming intricate plexuses; these are met with on the surface of the body and in a few of the internal organs, such as in the spermatic cord, broad ligaments of the uterus, around the neck of the bladder, etc.
The free anastomosis which exists between the veins is a wise provision of nature to guard against any interruption in the return of the blood to the heart, which might otherwise have happened due to these flexible tubes being very liable to compression. Before considering the structure of the veins, I shall briefly allude to the peculiar system formed by the vessels of the abdominal viscera.
The vena portae, consisting of a central trunk and two sets of branches, is distinct both from the arterial and venous systems, being placed between the terminations of the stomachic, intestinal, and splenic arteries, from which it arises, and the radicles of the hepatic veins in which it ends. This vessel is equally peculiar with respect to its functions; for on one hand, it acts as a vein by returning blood from the viscera; whilst on the other, it performs the office of an artery by secreting bile.
The vena portae is, however, more allied to the veins than to the arteries in consequence of its structure and the kind of blood that it contains.
Key Takeaways
- The venous system is larger and more capacious compared to the arterial system.
- Veins are arranged in superficial and deep sets, with many forming intricate plexuses.
- Free anastomosis between veins ensures continuous blood flow despite potential compression.
Practical Tips
- Understanding the venous system's capacity can help in assessing blood loss during emergencies.
- Recognizing the arrangement of veins can aid in proper vein puncture or cannulation techniques.
Warnings & Risks
- Veins are more prone to compression than arteries, which could disrupt blood flow if not managed properly.
- The vena portae system's unique functions and structure require special consideration during medical procedures involving the abdominal viscera.
Modern Application
While this chapter focuses on historical anatomical descriptions of the venous system, its insights into the capacity and arrangement of veins are still relevant today. Understanding these principles can enhance modern emergency response techniques such as blood loss assessment and vein puncture methods.
Frequently Asked Questions
Q: What is the relationship between the number and size of arteries and veins in the human body?
The venous system has a larger number and size of veins compared to the arterial system, making it more capacious overall.
Q: How does the vena portae differ from other vessels in the body?
The vena portae is distinct both structurally and functionally; it acts as a vein by returning blood from the viscera but also performs arterial functions like secreting bile.
Q: Why are veins arranged in superficial and deep sets?
This arrangement allows for efficient blood return to the heart, with superficial veins forming networks under the skin while deep-seated veins accompany arteries.