Animal Transport Systems: Circulation, Fluids, and Blood
Animal Transport Systems
In animals, the transport system serves a dual purpose: it carries nutrients (gaseous and non-gaseous) and cells, and it eliminates waste products. Single-celled organisms do not require a transport system because they rely on diffusion. However, diffusion is a slow process effective only over short distances, making it insufficient for multicellular organisms.
The animal body consists largely of fluids, approximately 56% of its mass. These fluids are found inside cells (intracellular fluid) and in the spaces between cells (extracellular fluid). The extracellular fluid, the internal environment, contains the nutrients needed for cellular activity and is in continuous exchange with the cells. For cells to function properly, the internal environment must maintain a constant composition and volume. The ability of organisms to regulate these values is called homeostasis. Three systems are primarily involved in maintaining homeostasis: the circulatory system, which governs the physical properties of the transported fluids, and the respiratory and excretory systems, which control the chemical properties.
Specialized Transport Systems: Circulatory Systems
Complex animals have specialized transport systems called circulatory systems. These systems consist of a circulating fluid, a pipe system (which can be open or closed), and a propulsive organ called the heart.
Circulating Fluids
Circulating fluids are located inside the tubes and renew extracellular components. They are composed of water, salts, and cells in suspension. The main circulating fluids are:
- Hemolymph: Transports nutrients and excretion products. Contains cells called phagocytes. Typical of echinoderms.
- Hemolymph: Transports nutrients and excretion products. It is colorless and contains hemocyanin (a respiratory pigment). Consists of phagocytes and hemocytes.
- Blood: Circulates through closed ducts and contains a respiratory pigment. In annelids, this can be chlorocruorin, hemoerythrin, or hemoglobin; in vertebrates, it is hemoglobin. Blood is formed by plasma, a fluid composed of water, salts, and cells (erythrocytes, leukocytes, and platelets).
- Lymph: A yellowish fluid that circulates through lymphatic vessels. It is composed of cells (lymphocytes), water, and blood plasma, containing less protein than blood.
Composition of Blood
Plasma:
- Fibrinogen: A protein dissolved in plasma involved in blood clotting.
- Blood serum: Water, protein, glucose, lipids, salts.
Blood Cells:
- Red blood cells (erythrocytes): Carries gases. In mammals, they are circular, biconcave, and lack a nucleus. In other vertebrates, they are elliptical, biconvex, and nucleated.
- Platelets: Cell fragments from megakaryocytes. Their function is to initiate the process of blood clotting.
- Leukocytes (white blood cells): Possess a nucleus and are involved in defending the body against infections.
Types of Leukocytes:
- Granulocytes: Basophils, neutrophils, and eosinophils.
- Agranulocytes: Monocytes (phagocytic function) and lymphocytes (synthesize antibodies against antigens).
Circulatory Systems
Circulatory systems consist of a propulsive organ (the heart) and a pipe system (conducting vessels). The heart is made of a special muscle called the myocardium, which contracts and relaxes to propel the circulating fluid. There are two types of hearts: septated (mollusks and vertebrates) and tubular (arthropods). Accessory hearts can also accelerate circulation in a specific area.