EXCRETORY SYSTEM

The excretory system excretes wastes. It is responsible for the elimination of the nitrogeneous waste products of metabolism as well as other non-useful nitrogeneous materials. Lungs are two organs localized on each side of the thorax. They are constituted by pulmonary alveoli. They are responsible for converting oxygen into carbon dioxide, but to maintain the organism's cells can take the oxygen through passive diffusion from the bloodstream and use it in its own metabolism, thus Every organism, from the smallest protist to the largest mammal, must rid itself of the potentially harmful by-products of its own vital activities. This process in living things is called elimination, which may be considered to encompass all of the various mechanisms and processes by which life forms dispose of or throw off waste products, toxic substances, and dead portions of the organism. The nature of the process and of the specialized structures developed for waste complexity of the organism.
Four terms are commonly associated with waste-disposal processes and are often used interchangeably, though not always correctly: excretion, secretion, egestion, and elimination.

The separation, elaboration, and elimination of certain products arising from cellular functions in multicellular organisms is called secretion. Though these substances may be a waste product of the cell producing them, they are frequently useful to other cells of the organism. Examples of secretions are the digestive enzymes produced by intestinal and pancreatic tissue cells of vertebrate animals, the hormones synthesized by specialized glandular cells of plants and animals, and sweat secreted by glandular cells in the skins of some mammals. Secretion implies that the chemical compounds being secreted were synthesized by specialized cells and that they are of functional value to the organism. The disposal of common waste products should not, therefore, be considered to be of a secretory nature.
Egestion is the act of excreting unusable or undigested material from a cell, as in the case of single-celled organisms, or from the digestive tract of multicellular animals.
As defined above, elimination broadly defines the mechanisms of waste disposal by living systems at all levels of complexity. The term may be used interchangeably with excretion. The waste then travels to anus and is released.
Skin
The skin is another part of the excretory system, containing sweat that help regulate the concentration in one’s body while also keeping him or her cool. The salt helps evaporate the water, cooling off the skin. Sweat is excreted through sweat glands. There are two types of sweat glands: eccrine sweat glands and apocrine sweat glands. The basic purpose of skin is to provide a waterproof, protective, covering for the body's complex internal environment. The skin also plays a key role in helping to maintain the circulatory and nervous system
Liver
The liver is an accessory of the digestive system.It weighs more than 1.5 kg in typical maturity. It also helps in excreting wastes from the body in a variety of processes. Laboratory analysis reveals a high concentration of a small organelle called a peroxisome, responsible for breakdown of several toxic substances.
The liver absorbs drugs and other poisonous substances in the blood. It changes the chemical structure of these substances and are then excreted in the bile. The bile is secretion of the liver. It makes digestion of fats easier and also carries away waste production.
Kidney
The key organs in the excretory system of vertebrates are the kidneys (for a more detailed explanation, follow this link). Please see protonephridia/flame-bulb system for Platyhelminthes, metanephridia for Annelida, or the Malpighian tubes for Insects and terrestrial arthropods. The kidneys are placed on either side of the spinal column near the lower back. They are responsible for removing nitrogenous wastes from the blood and they also regulate blood pressure in a process called Osmoregulation and also assist with the production of red blood cells. The kidneys filter the blood and remove any wastes.
The Kidney filters via its three layers of transport epithelia called the Cortex, the Medulla and the Pelvis. In the Cortex and Medulla there are Nephrons. These Nephrons comprise of a Glomerulus (bundle of capilaries), a Bowman's Capsule, a Proximal Convoluted Tubuale, the descending and ascending Loop of Henle, the Distal Convoluted Tubual and Collecting Ducts. The collecting ducts come together in the Pelvis. When your body gets ready to pass waste products, it goes through the kidneys and mixes with water and urine. Then, the aqueous waste travels into the bladder. The tubes responsible for transport are known as Ureters. The urea is stored in the bladder until excretion is either imperative or convenient. This occurs in a well-known process known as urination. When this happens, a tube called the urethra takes the urine to the outside of the body. It also produces nubile.

Human Excretory System
(From www.cliffsnotes.com)
The human excretory system functions to remove waste from the human body. This system consists of specialized structures and capillary networks that assist in the excretory process. The human excretory system includes the kidney and its functional unit, the nephron. The excretory activity of the kidney is modulated by specialized hormones that regulate the amount of absorption within the nephron.
Kidneys
The human kidneys are the major organs of bodily excretion (see Figure 1 ). They are bean-shaped organs located on either side of the backbone at about the level of the stomach and liver. Blood enters the kidneys through renal arteries and leaves through renal veins. Tubes called ureters carry waste products from the kidneys to the urinary bladder for storage or for release.






Figure 1 Details of the human excretory system. Position and allied structures of the kidneys (top). A cross section of the kidney showing the two major portions (left). Details of the nephron, the functional unit of the kidney (right).


The product of the kidneys is urine, a watery solution of waste products, salts, organic compounds, and two important nitrogen compounds: uric acid and urea. Uric acid results from nucleic acid decomposition, and urea results from amino acid breakdown in the liver. Both of these nitrogen products can be poisonous to the body and must be removed in the urine.
Nephron
The functional and structural unit of the kidney is the nephron. The nephron produces urine and is the primary unit of homeostasis in the body. It is essentially a long tubule with a series of associated blood vessels. The upper end of the tubule is an enlarged cuplike structure called the Bowman's capsule. Below the Bowman's capsule, the tubule coils to form the proximal tubule, and then it follows a hairpin turn called the loop of Henle. After the loop of Henle, the tubule coils once more as the distal tubule. It then enters a collecting duct, which also receives urine from other distal tubules.
Within the Bowman's capsule is a coiled ball of capillaries known as a glomerulus. Blood from the renal artery enters the glomerulus. The force of the blood pressure induces plasma to pass through the walls of the glomerulus, pass through the walls of the Bowman's capsule, and flow into the proximal tubule. Red blood cells and large proteins remain in the blood.
After plasma enters the proximal tubule, it passes through the coils, where usable materials and water are reclaimed. Salts, glucose, amino acids, and other useful compounds flow back through tubular cells into the blood by active transport. Osmosis and the activity of hormones assist the movement. The blood fluid then flows through the loop of Henle into the distal tubule. Once more, salts, water, and other useful materials flow back into the bloodstream. Homeostasis is achieved by this process: A selected amount of hydrogen, ammonium, sodium, chloride, and other ions maintain the delicate salt balance in the body.
The fluid moving from the distal tubules into the collecting duct contains materials not needed by the body. This fluid is referred to as urine. Urea, uric acid, salts, and other metabolic waste products are the main components of urine. The urine flows through the ureters toward the urinary bladder. When the bladder is full, the urine flows through the urethra to the exterior.
Control of kidney function
The activity of the nephron in the kidney is controlled by a person's choices and environment as well as hormones. For example, if a person consumes large amounts of protein, much urea will be in the blood from the digestion of the protein. Also, on a hot day, a body will retain water for sweating and cooling, so the amount of urine is reduced.
Humans produce a hormone called antidiuretic hormone (ADH), also known as vasopressin, which is secreted by the posterior lobe of the pituitary gland. It regulates the amount of urine by controlling the rate of water absorption in the nephron tubules.
Some individuals suffer from a condition in which they secrete very low levels of ADH. The result is excessive urination and a disease called diabetes insipidus. Another unrelated form of diabetes, diabetes mellitus, is more widespread. Persons with this disease produce insufficient levels of insulin. Insulin normally transports glucose molecules into the cells. But when insulin is not available, the glucose remains in the bloodstream. The glucose is removed from the bloodstream in the nephron; to dilute the glucose, the nephron removes large amounts of water from the blood. Thus, the urine tends to be plentiful.
Hormones from the cortex of the adrenal glands also control the content of urine. These hormones promote reabsorption of sodium and chloride ions in the tubules. Thus, they affect the water balance in the body, because water flows in the direction of high sodium and chloride content.