Besides the kidney, the lungs, liver and skin also excrete waste products.

     The lungs excrete carbon dioxide and water , the waste products of cellular respiration.

      The liver cells make bile which is needed for the digestion of fats. The bile contains bilirubin, a greenish yellow pigment which is a break down product of haemoglobin. Bilirubin is harmful if allowed to accumulate in the body. When bile is secreted into the small intestine during digestion of food, the bilirubin is also got rid of . It undergoes changes in the intestine and leaves the body in the faeces, which owes its yellowish brown colour to this pigment. Thus, by excreting bilirubin via bile, the liver acts as an excretory organ.

       The skin of humans contains sweet glands. We control our body temperature through producing sweat. Sweat contains water with sodium chloride and traces of urea dissolved in it. The substances in sweat can be considered as excretory products. However, the only purpose of sweating is to cool the body. Excretion of substances in sweat is thus incidental.  

 An earthworm has a segmented body. Each segment has a pair of tubes called nephridia. These are the excretory organs of the earthworm. Each nephridium has a ciliated funnel, the nephrostome, at one end. This opens into the body cavity of the segment in front. The nephrostome leads into a long coiled tube made up of the following parts:

 • Narrow ciliated tube, •

• Middle ciliated tube,

• Wide, non ciliated tube, and

 • Muscular tube which opens to the exterior via an excretory pore.

     The nephridium thus opens at both ends, unlike the flame cells which are blind tubules opening only at one end to the exterior.

    Each nephridium is surrounded by s capillary network. The waste products, mainly urea are extracted from the blood in the capillaries surrounding the nephridia. Wastes are also removed from the fluid in the body cavity and wastes moves through the long tubes of the nephridia. Along the way, salts and other reabsorbed through the walls of the tubes. This composition and concentration of the body fluid constant.

     The unabsorbed substances, including water collect in the muscular tube as hypotonic urine. The sphincter (ring of muscle) guarding the excretory pore relaxes to allow the urine to escape to the exterior.

     Although the earthworm is terrestrial, it Lives in a moist environment since it needs a moist skin surface for gaseous exchange. Carbon dioxide is excreted by diffusion through the skin. The earthworm cannot survive in a dry environment. Within limits, it can tolerate dry conditions and conserve water by producing very little urine which is hypertonic.  

 The processes involved in the formation of urine are as follows:

 • ultra filtration,

 • selective reabsorption, and

 • tubular secretion.

     As blood circulates through the glomerulus, ultra filtration occurs. Small molecules such ad water, urea, mineral salts, sugar and plasma solutes pass through the one cell thick walls of the capillaries and the Bowman capsule into the capsular space. Bigger molecules like plasma proteins and the blood cells cannot pass through this barrier which thus act as a filter. a high pressure in the glomerulus is essential for the filtration process. This is brought about in the following ways:

 • The blood entering the kidney is already at high pressure because the renal artery branches off from the dorsal aorta at only a short distance from the heart.

 • The blood vessel leaving each glomerulus is narrower than the one entering it, thereby increasing the pressure of the blood in the glomerulus further.

    The fluid that filters into the Bowman's capsule is known as glomerular filtrate. It flows down of the tubule. As it passes through the proximal part of the tubule and the Henle loop, selective reabsorption takes place. In this process, water and useful substances like sugar, amino acids and salts are reabsorbed into the surrounding blood capillaries. The filtrate then moves into the distal part of the tubule. Here, large waste molecules like creatinine are secreted into the tubules. If necessary, ions (hydrogen, potassium and hydrogen carbonate) are secreted into the tubules to keep the osmotic concentration of the blood constant. The fluid that eventually remains in the tubule is concentrated and is known as urine.

    An average of 1.5 litters of urine is produced daily. The amount of urea excreted will depend on the protein content of the daily diet. The filtered blood leaving the kidney by the renal vein contains

 • less oxygen and glucose, and mor carbon dioxide, as a result of cellular respiration; and less nitrogenous wastes, salts and water as a result of excretion.

    The urine formed trickles down the ureter and collects in the bladder. When the bladder is full, it contracts discharging the urine out of the body through the urethra.

Note How the kidney helps to maintain a constant internal environment is going to be discussed in my next topics.  

 Insects are a very successful group of animals, with a remarkable ability to conserve water so that they can be found in the hottest and driest places on earth. Their ability to conserve water can be attributed to several factors:

 • their outer surface is waterproof because of a layer of wax;

• the spiracles prevent water loss from the gaseous exchange surface which is inside the body; and

 • they have an extremely efficient excretory system.

 The excretory organs are the malpighian tubules. They are found between the midgut and the rectum. One end of each tubules opens into the gut, while the other free end floats in the haemocoel.

     Nitrogenous waste products and water which are librated into the haemocoel are absorbed at the distal end of the malpighian tubule. The nitrogenous waste is converted to uric acid as it passes along the malpighian tubule towards the gut. A lot of water Is also reabsorbed so that by the time the uric acid reaches the proximal end of the malpighian tubule it is changed to solid crystals. In the hindgut, more water is reabsorbed by the rectal gland. Thus, the urine which eventually leaves the body is very concentrated, almost a dry solid.  

 The urinary tubule is the functional unit of the kidney. Each urinary tubule begins in the cortex as a cup like structure called the Bowman's capsule. The capsule opens into a short coiled tube, the proximal convoluted tubule. Then it straightens out as it passes into the medulla, where it makes a U-shaped loop, the Henle loop, before reentering the cortex. In the cortex. The tubule becomes coiled again to form the distal convoluted tubule. The tubule bends once more and completes its course in the medulla.

     The tubule widens as it approaches the pelvis. Together with many other tubules, it pours its contents into wider main collecting ducts which eventually join up and open into the pelvis st the apices of pyramids.

       All along its course, the tubule is closely associated with several networks of blood capillaries. The renal artery branches in the kidney. Each branch breaks into a made of blood capillaries in the Bowman's capsule. This knot of capillaries is called the glomerulus. The capsule and glomerulus form the malpighian capsule. The capillaries in the glomerulus rejoin to form a blood vessel leading out of the capsule. This vessel then branches into a capillary network around the urinary tubule before rejoining to form a branch of the renal vein.  

 The paired kidneys are the excretory organs of humans. They remove unwanted nitrogenous substances like urea and other ammonium compounds from the blood. they also maintain the osmotic pressure of the blood by controlling the excretion of water and salts.

       The kidney is supplied with blood vessels. The renal artery enters the kidney. It arises directly from the dorsal aorta and brings oxygenerated blood containing excretory products. The renal vein drains filtered deoxygenated blood from the kidney to the posterior vena cavae. A narrow tube, the ureter, connects the kidney to the urinary bladder. Urine is stored in the urinary bladder. The urinary bladder leads to the urethra which opens to the exterior.


       The kidney has two distinct regions, an outer cortex and an inner medulla. More than a million fine narrow tubules, the urinary tubules pass through both these regions. They open at the tips of triangular shaped masses of tissues called pyramids. The pyramids open into a funnel shaped cavity called the pelvis. The pelvis is continuous with the ureter. The kidney has many tiny capillaries which are branches of the renal artery and vein.  

 The following mechanism are involved in the formation of urine:

 • Active transport : this is the mechanism that is mainly responsible for selective reabsorption of substances that the body needs and excretion of urea. The energy needed comes from cellular respiration.

 • Varying permeability of tubule : various regions of the tubule are selectively permeable to water, ions and urea. For example, the descending limb of the loop of Henle is Kore permeable to water than the ascending limb.

 • Passive diffusion and osmosis ions and water move in and put of the fluid in the tubule by diffusion and Osmosis respectively. These processes are controlled by concentration gradients and selective permeability of the regions of the tubule concerned.

 • Hormonal control : Hormones act on various regions of the tubule to control reabsorption of ions and water.