Excretory System - AP Biology

Cells and large proteins are too large to cross the capillary membranes of the glomerulus, and cannot be filtered into Bowman's capsule. After this initial step of filtration, urine in the nephron is only affected by secretion and reabsorbtion of water, ions, and other small molecules. Finally, each kidney is composed of about one million nephrons.

The nephrons are the smallest functional unit in the kidney. They are responsible for the filtration and concentration of urine.

Neurons are the smallest functional unit of the nervous system. The renal medulla is the region on the interior of the kidney, as opposed to the renal cortex. Renal corpuscles are used to filter blood into the nephron, and consist of the glomerulus and Bowman's capsule.

The kidney-nephron system is one of the few portal systems in the human body. Portal systems link two arteriole or capillary systems together, in this case the afferent and efferent arterioles. The efferent arterioles leave the glomerulus after it has been filtered into Bowman's capsule, and follow along the nephron to pick up any reabsorbed material from the nephrons via the vasa recta capillary bed. Both the glomerulus and vasa recta are groups of capillaries, which qualifies the renal circulation as a portal system.

The proximal convoluted tubule is incredible for its ability to reabsorb glucose at levels of nearly 100%. This function is due to specialized proteins that help transport the glucose out of the filtrate. Damage to the proximal convoluted tubule can lead to glucose in the urine. Another possible cause would be extremely high glucose levels in the filtrate, such that the proximal convoluted tubule is incapable of properly removing all of the solute. This condition is a trademark of diabetes.

The loop of Henle, the distal convoluted tubule, and, under special circumstances, the collection duct are responsible for the reabsorption of other nutrients (such as water and various ions).

The glomerulus, proximal tubule, distal tubule are all located in the cortex (outer portion) of the kidney, where the osmolarity of the interstitial fluid is relatively low. Both limbs of the loop of Henle and the collecting duct are located in the medulla (central portion) of the kidney, where the osmolarity of the interstitial fluid is much greater.

Filtrate movement through the nephron into different surrounding osmolarities is what allows water and sodium to be retained, if necessary, while other waste products are concentrated in the urine. The evolution of the loop of Henle is specifically designed to create the countercurrent multiplier system that allows for urine concentration and water retention in land animals. Water-based animals generally have shorter loops of Henle that may not intersect with the renal medulla, since water retention is less important.

The function of the kidney is to filter wastes out of the blood and concentrate them into a filtrate that can be excreted from the body. Nephrons are the functional unit of the excretory system, meaning that each nephron is capable of concentrating wastes into filtrate. Each nephron is made of a single long tubule, with different regions modified to transport different ions and wastes into or out of the filtrate. The proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule are the principle regions of the nephron.

The glomerulus and Bowman's capsule form the renal corpuscle, the site of blood filtration. While the nephron serves to concentrate filtrate, the renal corpuscle separates the filtrate from the blood. The glomerulus is a system of capillaries, and carries blood rather than filtrate. It is kept separate from the nephron by the barriers in the glomerulus walls and Bowman's capsule.

Together, the glomerulus and Bowman's capsule form the structure known as the renal corpuscle. Blood in the capillaries of the glomerulus is forced against the walls of the vessels, where specialized epithelium and cell junctions allows fluids and small solutes to diffuse across the walls of the glomerulus and into Bowman's capsule. This process is highly dependent on pressure differentials; higher hydrostatic pressure in the glomerulus and greater solute concentration in Bowman's capsule will work to remove water and fluids from the capillary. Blood cells and large proteins are unable to pass through filtration and remain in circulation. Ions, small sugars, amino acids, and nitrogenous wastes pass through filtration, and are either reabsorbed back into the blood or excreted in the filtrate.

The proximal and distal tubules are regions of the nephron that are located in the renal cortex and specialize in ion reabsorption. The renal pelvis is located in the renal medulla and serves as the final collecting point for filtrate from multiple collecting ducts before transferring it to the ureter.

Filtration of blood occurs in the renal corpuscle, which is composed of the glomerulus and Bowman's capsule. The glomerulus is a complex net of capillaries that carry blood adjacent to Bowman's capsule. Bowman's capsule collects fluid filtrate from the blood and transfers to the rest of the nephron. The glomerulus is the only portion of the nephron to contain blood rather than filtrate.

After entering Bowman's capsule, filtrate is passed to the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and finally the collecting duct.

The kidneys are responsible for the excretion of urine, while the skin secretes sweat from the body. The pancreas secretes digestive enzymes into glands and ducts. Exocrine function differs from endocrine function, in that exocrine excretions involve ducts that generally lead to the body's exterior. Endocrine functions, in contrast, will enter the blood stream and be distributed throughout the body.

The overall organ systems that perform excretory functions are highly variable between animal groups. All animals, however, share some common features. Water and ion balances must be tightly regulated by reabsorption mechanisms, in order to maintain organismal homeostasis and functions. To assist in this process, the vessels for filtrate transport must have a large surface area. Proteins, pumps, and channels along this surface serve to moderate water and ion transport.

Blood is filtered from the renal artery into the kidneys, where it is processed into urine. Each of the two kidneys is connected to the bladder via a ureter. The bladder has only one exit, the urethra, which carries the urine out of the body.

In summary, there are two kidneys, two ureters, one bladder, and one urethra.

The filtrate is processed as it moves throughout the nephron prior to entering the ureters to be expelled. The glomerulus is a capillary bed that deposits filtrate from the blood into the nephron. The proximal tubule does not alter the osmolarity of the primary filtrate. The loop of Henle then concentrates the urine, only to have the concentration lowered again in the distal convoluted tubule.

The kidneys are able to help regulate blood pressure by releasing the hormone renin. Renin initiates a cascade that ends with the adrenal cortex releasing aldosterone. Aldosterone increases the reabsorption of sodium ions in the distal convoluted tubule and collecting duct. The increase in surrounding ion concentration resulting from the reabsorption pulls water out of the nephron via osmosis, concentrating the urine and promoting water retention.

A sphincter is a circular band of mucle that, when constricted, closes an orifice. For example, the pyloric sphincter prevents stomach contents from entering the small intestine before it has been properly digested.

A varix is a twisted, dilated vein, as in varicose vein. Plica refers to one of the folds of the intestinal mucosa. The cecum is a blind pouch at the end of the ascending colon. At its end is the appendix. A vilus is the singular term for "villi," and refers to the minute projections on the intestinal mucosa that function in the absorption of nutrients.

Jellyfish possess a gastrovascular cavity, which serves as both mouth and anus. Food is captured and enters the gastrovascular cavity, where enzymes break it down and cells absorb the nutrients. The undigested remains leave through the same opening by which they entered.

A food vacule is a temporary stomach found in sponges. A crop is a storage organ that earthworms use to collect food. The gizzard receives food from the crop and, through the process of contraction, grinds the food into small particles. The crop and gizzard are parts of what is termed a "tubular digestive tract," which consists of a series of regions that process the food.

Glomerulus, Bowman's capsule, loop of Henle, ureter is the correct order. Blood is filtered from the glomerulus and becomes the filtrate as it enters the Bowman's capsule. The filtrate then passes through the loop of Henle of the nephron and exits the kidney, entering the ureter as urine where it is collected in the urinary bladder.

The liver is found on the right side of the body in the abdomen. Remember the liver is on the right side of the abdominal cavity. The liver cells (hepatocytes) function to detoxify products such as alcohols and other drugs.

The renal corpuscle is comprised of the glomerulus and Bowman's capsule, and is responsible for creating the primary filtrate that will enter the tubules of the nephron. This is accomplished via filtration. The proximal convoluted tubule is primarily responsible for reabsorption and secretion, and the loop of Henle is the site where filtrate is concentrated.

ATP is needed to help power the excretion of urine through the nephron. The thick ascending limb, for example, is thicker because it contains more mitochondria than other portions of the loop of Henle. The changing environment of the nephron is a product of the continual reabsorbtion of water and ions throughout the excretion process.

The renal medulla contains the loops of Henle and collecting ducts. As filtrate travels toward the interior of the kidney, the increased ion concentration (hypertonic environment) aids in the reabsorption of water. This ion gradient is established by ion flow from the concentrated filtrate in the descending limb toward less concentrated blood and interstitial fluids.

One of the key adaptations of the mammalian kidney is the ability to conserve water through reabsorption and excretion of concentrated urine. This is accomplished by maintenance of an osmolarity gradient, suitable for extracting water from the filtrate. The two primary solutes are sodium, which is deposited in the renal medulla by the loop of Henle, and urea, which crosses the epithelium of the collecting duct in the inner medulla. The increased osmolarity of the interstitial fluid enables water to be extracted and conserved through aquaporin proteins in the collecting duct.