The life processes that occur inside the body a living being produce some substances which may become toxic to the organism if allowed to accumulate within the body. These wastes need to be expelled (thrown out) from the body. The process of removal of wastes from the body is called excretion.
The Excretory Organs
The wastes formed as a result of the various metabolic activities (biochemical processes that occur to maintain life) inside the body are removed by different organs such as the kidneys, lungs, skin and liver. Excess water, salts, carbon dioxide and other substances are excreted as wastes from the body. As you already know, undigested food, which passes down the large intestine, is expelled through the anus as faeces. The large intestine also expels wastes produced from the breaking down of haemoglobin. This waste gives faeces its colour. However, we will not discuss the large intestine in this chapter. We will learn about the roles of the other excretory organs of the body,
The kidneys
The kidneys are the body's main excretory organs and are a part of the renal system (renes means 'kidneys' in Latin), or urinary system. We will discuss the urinary system in greater detail later in this chapter.
The lungs
As you already know, carbon dioxide formed
during internal respiration passes from the blood stream into the lungs. From the lungs, it goes up the bronchi and windpipe, to be exhaled through the nose.
The skin
The body excretes salts and urea through the skin in the form of sweat. Urea is a waste produced as a result of the breaking down of amino acids. You have learnt that amino acids are the end products of the digestion of proteins. The body uses some of these amino acids to make new proteins. What it does not use is broken down into urea.
Sweat is formed in sweat glands, which lie under the top layer (epidermis) of the skin. Waste products and water from the blood flowing through the capillaries in the skin pass through the walls of the capillaries into
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