How does mammalian kidney function as an excretory

Since the surrounding fluid has been made more concentrated, water leaves the tubule by osmosis. As a result, the fluid in the descending limb becomes more concentrated as it flows toward the bottom of the medulla. As the collecting duct descends from the cortex to the tip of the renal pyramid, the concentration gradient established by the loop of Henle increases.

This increasing solute concentration causes more and more water to be absorbed from the fluid, thus concentrating the urine in the collecting duct. Most of this volume, along with valuable molecules such as glucose and amino acids, is reabsorbed from the proximal convoluted tubules.

As the filtrate flows up through the thick segment of the ascending limb, NaCl is transported out of the filtrate and into the extracellular spaces of the medulla. The resulting increase in the osmotic concentration of the extracellular fluids draws water out of the filtrate in the thin descending limb making it more concentrated. As this more concentrated filtrate flows up through the thick ascending limb, more NaCl is transported out and the extracellular fluid becomes even more concentrated.

Thus, the opposing—or countercurrent—directions of flow through the two limbs of the loop of Henle, along with differential permeability and transport capacities of these limbs results in "multiplying" the concentration difference between the filtrate and the extracellular fluid of the medulla. In response to this gradient, as the urine flows down through the collecting duct from the cortex to the tip of the renal pyramid, the urine can lose water osmotically and become more concentrated.

This countercurrent multiplier mechanism makes it possible for mammals to produce a urine that is hypertonic to their blood plasma. Mammals inhabit an enormous range of environments on Earth, including some of the most arid. The major adaptation that allows mammals to maintain homeostasis in the face of a wide range of osmotic stress is the variable ability of their kidneys to concentrate urine. The branch that exits the glomerulus is called the efferent arteriole. Within the glomerulus, the network of capillaries is called the glomerular capillary bed.

Once the efferent arteriole exits the glomerulus, it forms the peritubular capillary network , which surrounds and interacts with parts of the renal tubule.

In juxtamedullary nephrons, the peritubular capillary network forms a network around the loop of Henle and is called the vasa recta. Kidneys filter blood in a three-step process. First, the nephrons filter blood that runs through the capillary network in the glomerulus. Almost all solutes, except for proteins, are filtered out into the glomerulus by a process called glomerular filtration. Second, the filtrate is collected in the renal tubules. Most of the solutes get reabsorbed in the PCT by a process called tubular reabsorption.

In the loop of Henle, the filtrate continues to exchange solutes and water with the renal medulla and the peritubular capillary network.

Water is also reabsorbed during this step. Then, additional solutes and wastes are secreted into the kidney tubules during tubular secretion , which is, in essence, the opposite process to tubular reabsorption. The collecting ducts collect filtrate coming from the nephrons and fuse in the medullary papillae.

From here, the papillae deliver the filtrate, now called urine, into the minor calyces that eventually connect to the ureters through the renal pelvis. Each part of the nephron performs a different function in filtering waste and maintaining homeostatic balance.

The more acidic the filtrate, the more ammonia is secreted. In the thick part, these same ions are actively transported into the interstitial fluid. Because salt but not water is lost, the filtrate becomes more dilute as it travels up the limb. Glomerular Filtration Glomerular filtration filters out most of the solutes due to high blood pressure and specialized membranes in the afferent arteriole.

The blood pressure in the glomerulus is maintained independent of factors that affect systemic blood pressure. All solutes in the glomerular capillaries, except for macromolecules like proteins, pass through by passive diffusion. There is no energy requirement at this stage of the filtration process. Glomerular filtration rate GFR is the volume of glomerular filtrate formed per minute by the kidneys.

GFR is regulated by multiple mechanisms and is an important indicator of kidney function. Tubular reabsorption occurs in the PCT part of the renal tubule. Almost all nutrients are reabsorbed, and this occurs either by passive or active transport. Reabsorption of water and some key electrolytes are regulated and can be influenced by hormones. Water is also independently reabsorbed into the peritubular capillaries due to the presence of aquaporins, or water channels, in the PCT.

This occurs due to the low blood pressure and high osmotic pressure in the peritubular capillaries. However, every solute has a transport maximum and the excess is not reabsorbed. In the loop of Henle, the permeability of the membrane changes. The descending limb is permeable to water, not solutes; the opposite is true for the ascending limb. Additionally, the loop of Henle invades the renal medulla, which is naturally high in salt concentration and tends to absorb water from the renal tubule and concentrate the filtrate.

The osmotic gradient increases as it moves deeper into the medulla. Because two sides of the loop of Henle perform opposing functions, it acts as a countercurrent multiplier. The vasa recta around it acts as the countercurrent exchanger.

The loop of Henle acts as a countercurrent multiplier that uses energy to create concentration gradients. The descending limb is water permeable. Water flows from the filtrate to the interstitial fluid, so osmolality inside the limb increases as it descends into the renal medulla. At the bottom, the osmolality is higher inside the loop than in the interstitial fluid.

By the time the filtrate reaches the DCT, most of the urine and solutes have been reabsorbed. If the body requires additional water, all of it can be reabsorbed at this point. Further reabsorption is controlled by hormones, which will be discussed in a later section. Excretion of wastes occurs due to lack of reabsorption combined with tubular secretion. Undesirable products like metabolic wastes, urea, uric acid, and certain drugs, are excreted by tubular secretion.

Most of the tubular secretion happens in the DCT, but some occurs in the early part of the collecting duct. Although parts of the renal tubules are named proximal and distal, in a cross-section of the kidney, the tubules are placed close together and in contact with each other and the glomerulus.

The nephron is the functional unit of the mammalian kidney. Which of the following structures is not considered part of the nephron? 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. 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 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 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. If you've found an issue with this question, please let us know. With the help of the community we can continue to improve our educational resources. If Varsity Tutors takes action in response to an Infringement Notice, it will make a good faith attempt to contact the party that made such content available by means of the most recent email address, if any, provided by such party to Varsity Tutors.

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Subject optional. Email address: Your name:. Example Question 1 : Excretory Anatomy. Which of the following statements about kidney structure and function is true? Possible Answers: Cells and large proteins are filtered into urine from the glomerulus.

Each kidney is composed of approximately one thousand nephrons. No filtration occurs after Bowman's capsule, only reabsorbtion and secretion.