EXCRETORY SYSTEM Identify the major parts of the kidney. Relate the structure of a nephron to its function. Explain how the processes of filtration, reabsorption, and secretion help maintain homeostasis. Summarize the path in which urine is eliminated from the body. List the functions of each of the major excretory organs.

Kidneys The kidneys closely monitor the removal of nitrogenous waste from the body. The kidneys are the main organs of the urinary system and also regulate the chemical composition of the blood.

Kidneys Structure The renal cortex is the outermost part of the kidney that filters blood brought by the renal artery. The renal medulla is the inner part of the kidney that carries urine to the renal pelvis. The renal pelvis is a structure at the center of the kidney that carries urine to the ureter.

Wastes The nitrogenous wastes are initially brought to the liver as ammonia, which cannot remain in the body for long without damaging cells. The liver converts ammonia to urea. Urea is the principle nitrogenous product of the metabolism of proteins. Urea is found in urine and other body fluids. Substances such as toxins, urea, salts, and water that are removed from the blood by the kidneys are collectively called urine.

Nephrons Urine is made within the kidneys by nephrons. Nephrons are the functional units of the kidneys.

Nephrons At one end of the nephron is the Bowman’s capsule. The Bowman’s capsule is a cup-shaped capsule that surrounds a tight ball of capillaries. This ball of capillaries filters wastes from the blood, retains useful molecules, and produces urine. This ball of capillaries is called a glomerulus.

Nephrons: 3 Major Functions The three major processes that take place in the nephron are filtration, reabsorption, and secretion.

Filtration 1) Filtration is the process of separating dissolved substances, such as impurities in the blood, by passing them through a porous material.

Reabsorption 2) The process of reabsorption allows the body to retain many of the substances that were removed from the blood by filtration.

Secretion 3) In the distal convoluted tubule, some substances pass from the blood into the filtrate through a process called secretion

Nephron Functions

Reabsorption and Secretion The Loop of Henle The loop of Henle is a long U-shaped part of a nephron that reabsorbs water and salts from the urine. The loop of Henle uses osmosis to further concentrate urine.

Hormone Control No ADH Present - Collecting Duct is NOT permeable to water and large volume of urine is produced If a person is dehydrated, the hormone ADH (antidiuretic hormone) is released.  This causes aquaporin channels to be inserted in the membrane of the collecting duct so that large quantities of water can be reabsorbed. ADH Present - Collecting Duct is permeable to water and a small volume of urine is produced

Aldosterone If the Na+ levels in the blood are low, the hormone aldosterone is released. which leads to reabsorption of Na+ and Cl- in the distal tubule.  Water will also flow into the tubule via osmosis. 

The Excretory Organs The kidneys, lungs, and skin are all excretory organs. The kidneys are the primary excretory organs and play a vital role in maintaining the homeostasis of body fluids. The lungs are the primary site of carbon dioxide excretion and carry out detoxification and excretion of volatile substances. The skin excretes nitrogen waste, water, and some salts through perspiration.