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Subject: General Science

Topic: The Circulatory System

INTRODUCTION

Living things must be capable of transporting nutrients, wastes and gases to and from cells. Single-celled organisms use their cell surface as a point of exchange with the outside environment. Multicellular organisms have developed transport and circulatory systems to deliver oxygen and food to cells and remove carbon dioxide and metabolic wastes.

CIRCULATORY SYSTEMS IN SINGLE CELLED ORGANISMS

Single-celled organisms use their cell surface as a point of exchange with the outside environment. Sponges are the simplest animals, yet even they have a transport system. Seawater is the medium of transport and is propelled in and out of the sponge by ciliary action. Simple animals, such as the hydra and planaria lack specialized organs such as hearts and blood vessels, instead using their skin as an exchange point for materials. This, however, limits the size an animal can attain. To become larger, they need specialized organs and organ systems.

CIRCULATORY SYSTEMS IN MULTICELLULAR ORGANISMS

Multicellular animals do not have most of their cells in contact with the external environment and so have developed circulatory systems to transport nutrients, oxygen, carbon dioxide andmetabolicwastes.Components of the circulatory system include

i. Blood: a connective tissue of liquid plasma and cells
ii. Heart: a muscular pump to move the blood
iii. Blood vessels: arteries, capillaries and veins that deliver blood to all tissues

TYPES OF CIRCULATORY SYSTEMS

The open circulatory system, examples molluscs and arthropods. 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.

Vertebrates, and a few invertebrates, have a closed circulatory system. 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. Hemoglobin causes vertebrate blood to turn red in the presence of oxygen; but more importantly hemoglobin molecules in blood cells transport oxygen. The human closed circulatory system is sometimes called the cardiovascular system. The lymphatic circulation, which is also secondary circulatory system collects fluid and cells and returns them to the cardiovascular system.