(IGP) GS Paper 1 - General Science - "The Excretory System"

Subject - General Science
Chapter : The Excretory System

Nitrogen Wastes

Nitrogen wastes are by product of protein metabolism. Amino groups are removed from amino acids prior to energy conversion. The NH2 (amino group) combines with a hydrogen ion (proton) to form ammonia (NH3).

Ammonia is very toxic and usually is excreted directly by marine animals. Terrestrial animals usually need to conserve water. Ammonia is converted to urea, a compound the body can tolerate at higher concentrations than ammonia. Birds and insects secrete uric acid that they make through large energy expenditure but little water loss.

Water and Salt Balance

• The excretory system is responsible for regulating water balance in various body fluids.

• Osmoregulation refers to the state aquatic animals are in: they are surrounded by freshwater and must constantly deal with the influx of water. Animals, such as crabs, have an internal salt concentration very similar to that of the surrounding ocean. Such animals are known as osmocon-formers, as there is little water transport between the inside of the animal and the isotonic outside environment. Marincompositione vertebrates, however, have internal concentrations of salt that are about one-third of the surrounding seawater. They are said to be osmoregulators. Osmoregu-lators face two problems: prevention of water loss from the body and prevention of salts diffusing into the body. Fish deal with this by passing water out of their tissues through their gills by osmosis and salt through their gills by active transport.

Excretory System Functions

• Collect water and filter body fluids.

• Remove and concentrate waste products from body fluids and return other substances to body fluids as necessary for homeostasis.

• Eliminate excretory products from the body.

Invertebrate Excretory Organs

Many invertebrates such as flatworms use a nephridium as their excretory organ. At the end of each blind tubule of the nephridium is a ciliated flame cell. As fluid passes down the tubule, solutes are reabsorbed and returned to the body fluids. Body fluids are drawn into the Malphigian tubules by osmosis due to large concentrations of potassium inside the tubule. Body fluids pass back into the body, nitrogenous wastes empty into the insect’s gut. Water is reabsorbed and waste is expelled from the insect.

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Vertebrates excretory Organs

ALL vertebrates have paired kidneys. Excretion is not the primary function of kidneys. Kidneys regulate body fluid levels as a primary duty, and remove wastes as a secondary one.

The Human Excretory System

The urinary system is made-up of the kidneys, ureters, bladder, and urethra. The nephron, an evolutionary modification of the nephridium, is the kidney’s functional unit. Waste is filtered from the blood and collected as urine in each kidney. Urine leaves the kidneys by ureters, and collects in the bladder. The bladder can distend to store urine that eventually leaves through the urethra.

The Nephron

The nephron consists of a cup-shaped capsule containing capillaries and the glomerulus, and a long renal tube. Blood flows into the kidney through the renal artery, which branches into capillaries associated with the glomerulus. Arterial pressure causes water and solutes from the blood to filter into the capsule. Fluid flows through the proximal tubule, which include the loop of Henle, and then into the distal tubule. The distal tubule empties into a collecting duct. Fluids and solutes are returned to the capillaries that surround the nephron tubule. The nephron has three functions:

• Glomerular filtration of water and solutes from the blood. 
• Tubular reabsorption of water and conserved molecules back into the blood.
• Tubular secretion of ions and other waste products from surrounding capillaries into the distal tubule.

Urine Production

• Filtration in the glomerulus and nephron capsule.
• Reabsorption in the proximal tubule
• Tubular secretion in the Loop of Henle.

Components of the Nephron

• Glomerulus: mechanically filters blood
• Bowman’s Capsule: mechanically filters blood
• Proximal Convoluted Tubule: Reabsorbs 75% of the water, salts, glucose, and amino acids
• Loop of Henle: Countercurrent exchange, which maintains the concentration gradient
• Distal Convoluted Tubule: Tubular secretion of H ions, potassium, and certain drugs.

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