Excretion is a natural process that helps animals, humans and other organisms to expel out the waste products (urea and uric acid) from the body. This waste product is generated inside the body during metabolism. Cells need energy and nutrients to survive. To get the required energy, they perform certain biochemical reactions. These reactions take place and some byproducts are also produced. This byproduct can be very toxic or lethal to the cells if allowed to accumulate inside the body. This waste product is get rid of by the process of excretion. The main excretory organ in humans are as follows: Skin: Remove excess salt in the form of sweat from the body. Lungs: Remove carbon dioxide that is generated during respiration. Kidney: Remove nitrogenous waste from the body.

Describe in brief how urine is produced in the human body.

The production of urine occurs in three major steps; they are as follows: Ultrafiltration: The blood flows through the glomerulus under great pressure which is much greater than in the capillaries elsewhere. High pressure causes the liquid part of the blood to filter out from the glomerulus into the renal tubule. This filtration under extraordinary force is called ultrafiltration. During ultrafiltration almost all the liquid part of the blood comes out of the glomerulus and passes into the funnel-shaped cavity of the Bowman's capsule. The fluid entering the renal tubule is called the glomerular filtrate. The glomerular filtrate consists of water, urea, salts, glucose and other plasma solutes. Reabsorption: The glomerular filtrate entering the renal tubule is not urine. It is an extremely dilute solution containing a lot of useful materials including glucose and some salts such as those of sodium. As the filtrate passes down the tubule, much of the water is reabsorbed together with the useful substances. Tubular secretion: Certain substances like potassium (K+) in the normal course, are passed into the forming urine in the distal convoluted tubule. This passage involves the activity of the cells of the tubular wall, and hence it is called tubular secretion. The filtrate left after reabsorption and tubular secretion is called urine.

Name the organs that form the excretory system in human beings.

Excretory system in humans: The process of removal of waste mainly the nitrogenous waste from the body is called excretion. Organs that are concerned with the formation, storage and elimination of waste products constitute the excretory system. The organs that form the excretory system in human beings are as follows: A pair of Kidneys: These are the primary excretory organs eliminating nitrogenous wastes (chiefly urea) from the blood and throwing it out in the form of urine. A pair of Ureters: These narrow tubes carry urine from the kidneys to the bladder. Urinary bladder: The bladder's walls relax and expand to store urine, and contract and flatten to empty urine through the urethra. Urethra: It is the tube that allows urine to pass out of the body.

Why is there a difference in the rate of breathing between aquatic organisms and terrestrial organisms? Explain.

Breathing: The process of taking in oxygen from inhaled air and releasing carbon dioxide by exhalation is called breathing. Breathing is the physical process of exchanging gases whilst respiration is a chemical process which takes place at a cellular level and produces energy. Aquatic animals have gills for breathing, whereas terrestrial organism has lungs and skin for breathing. The dissolved oxygen present in water is very little compared to the atmosphere. Thus, the aquatic animals have to breathe faster to get more oxygen in comparison to the terrestrial animals.