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In closed circulatory system
A. The cells and tissues are directly bathed in the blood pumped out by heart
B. Arteries and veins are lacking
C. The capillaries are largest blood vessels and closed at their ends
D. Blood circulates through a series of vessels of varying diameters

Last updated date: 20th Jun 2024
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Hint: Closed circulatory systems (evolved in echinoderms and vertebrates) have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities. Blood flow is not sluggish.

Complete Answer:
- The circulatory system is effectively a network of cylindrical vessels (arteries, veins and capillaries) that emanate from a pump (the heart). The circulatory system can either be open or closed, depending on whether the blood flows freely in a cavity or is contained in vessels.
- Open circulatory systems (evolved in insects, mollusks and other invertebrates) pump blood into a hemocoel with the blood diffusing back to the circulatory system between cells. Blood is pumped by a heart into the body cavities, where tissues are surrounded by the blood. The resulting blood flow is sluggish.
- In all vertebrate organisms, as well as some invertebrate organisms, there is a closed-loop system in which the blood is not moving freely in a cavity. In a closed circulatory system, blood is contained inside blood vessels, circulating unidirectionally from the heart around the systemic circulatory route, then returning to the heart again, blood circulates through a series of vessels of varying diameters.

Therefore the correct option is D.

Additional information: In contrast to a closed system, arthropods (including insects, crustaceans and mollusks) have an open circulatory system. In an open circulatory system, the blood is not enclosed in the blood vessels, but is pumped into a cavity called hemocoel.
- The blood is called hemolymph because it mixes with the interstitial fluid. As the heart beats, and the animal moves, the hemolymph circulates around the organs with the body cavity, re-entering the heart through openings called ostia. This movement allows for gas and nutrient exchange.
- An open circulatory system does not use as much energy to operate and maintain as a closed system; however, there is a trade-off with the amount of blood that can be moved to metabolically-active organs and tissues that require high levels of oxygen.
- In fact, one reason that insects with wing spans of up to two feet wide (70 cm) are not around today is probably because they were outmatched by the arrival of birds 150 million years ago. Birds, having a closed circulatory system, are thought to have moved more agilely, allowing them to obtain food faster and possibly to prey on the insects.

Note: In closed circulatory systems, the heart pumps blood through vessels that are separate from the interstitial fluid of the body. Most vertebrates and some invertebrates such as annelid earthworm have a closed circulatory system.