BIOLOGY Volume 2 - A Guide to General Biology - 2004

14. TRANSPORT IN ANIMALS

In simply organized animals, such as Cnidarians and Flatworms, specialized internal transport and distribution systems are absent. These animals are characterized by a high surface-area-to-volume ratio, and gas exchange across the outer body surface fully satisfies their needs, especially since the metabolic rate at this evolutionary stage is low. The distances that substances must travel within the body are also short, so they can easily be moved by diffusion or cytoplasmic streaming (see sections 5.9.8 and 5.10.2).

As the size and complexity of animals increase, the amounts of substances entering the body and requiring elimination also increase. The distances these substances must travel within the body likewise grow, creating a need for a more efficient mode of transport. This mode becomes transport via a fluid current, or bulk flow (see the beginning of Chapter 13, as well as Table 13.1). There are two circulatory systems that ensure the Transport of substances between different PARTS OF THE Organism, namely the vascular¹ (cardiovascular) and lymphatic systems. These systems are called vascular because the Blood or Lymph travels at least part of its journey inside specialized tubular structures—vessels.

14.1. General Features of the Circulatory System

The function of the Circulatory system is to maintain a rapid bulk flow of substances between body parts over distances too great for transport by diffusion. Upon reaching their destination, these substances must be able to cross vessel walls into the relevant Organs or Tissues. Similarly, substances produced by these organs or tissues must also enter the circulatory system. In other words, specialized exchange systems are coupled with this bulk-flow transport system.

Any circulatory system consists of three main components:

1) a circulating fluid (blood);

2) a contractile, pump-like organ that drives the fluid throughout the body, a role played either by specialized vessels or a Heart;

3) tubes, or vessels, through which the fluid moves.

Two Types of circulatory systems are known in animals: open (lacunar) and closed.

OPEN CIRCULATORY SYSTEM (found in most Arthropods, some cephalopod Mollusks, etc.). The Heart pumps blood into the aorta, which branches into several Arteries. These open into body spaces surrounding the Internal Organs, collectively called the hemocoel. Thus, the blood does not remain permanently confined within vessels, hence the name "open." The blood moves slowly through the hemocoel under low pressure, bathing the surrounding tissues, and gradually returns to the heart directly through ostia (apertures) in the heart itself or via open-ended Veins. The distribution of blood to different parts of the body is poorly regulated.

CLOSED CIRCULATORY SYSTEM (found in Echinoderms, most cephalopod mollusks, Annelids, vertebrates, including humans). This type of circulatory system is characterized by the following features.

1. Blood remains entirely within The Heart and Blood Vessels and does not come into direct contact with body tissues.

2. Blood flows rapidly and reaches all parts of the body and returns to the heart under relatively high pressure.

3. The distribution of blood to various organs is regulated according to their metabolic demands.

4. The exchange of substances into and out of the system occurs exclusively across vessel walls.

Blood vessels are named differently depending on their Structure and function. Vessels that carry blood away from the heart are called arteries. Arteries branch into smaller arterioles, which in turn branch repeatedly to form a dense network of microscopic capillaries permeating almost all body tissues. It is here that the exchange of substances between the blood and surrounding tissues takes place.

Converging within an organ or tissue, capillaries form venules, which mark the beginning of the blood's return journey to the heart; merging with one another, venules form progressively larger veins. Ultimately, all blood returns to the heart via the major veins. The structure of each of these vessel types is discussed in detail in section 14.5.

¹ The term "vascular" (or blood-related) is not entirely apt, since in some animal groups the circulating fluid does not strictly correspond to the definition of blood and is called hemolymph. — Transl. note.