Function of human kidneys

Function of human kidneys

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Why are the kidneys called metanephric? What does the term metanephric actually mean? I tried to search that on Google but couldn't find the exact meaning.

There are metanephric diseases, which refer to the location of pathologic tissue relative to the main kidney mass, but unless someone with more expertise wants to weigh in, I don't think that kidneys themselves are called metanephric.

There is also a developmental stage of the kidneys, in which they are referred to as metanephros. Perhaps you're thinking of that?


The kidneys of vertebrates have the vital function of removing metabolic wastes from the blood and otherwise maintaining its normal composition. The two kidneys of a normal human adult produce 1 to 2 liters (about 30 to 70 fluid ounces) of urine each day that contain wastes, excess water, and other unneeded molecules. Production of less than 0.4 liter (13.5 fluid ounces) of urine per day is insufficient to eliminate wastes and regulate the composition of blood. Such a condition is always fatal within a few weeks unless the underlying cause is corrected, a new kidney is transplanted, or the blood is artificially cleared by dialysis .

The human kidney belongs to one of three kinds of kidneys that occur among different vertebrates at various developmental stages. The first type, called the pronephros, lies toward the front of some fishes and the embryos of many vertebrates. The mesonephros lies more posteriorly and occurs in most adult fishes and amphibians and in the embryo of humans and other mammals. The metanephros occurs still farther posteriorly and is the type of kidney in adult reptiles, birds, and mammals, including humans.

Each human kidney is about the size of a fist, shaped like a kidney bean, and located on one side of the lower abdomen toward the back. At any given

From this "plumbing diagram" one can get an overview of renal function: blood enters the kidney, wastes and excess molecules are removed with the urine, and the blood is returned to the circulatory system. To appreciate how the kidneys function, however, one must take a microscopic view of one of the million or so structures called nephrons within each kidney. Each nephron begins its work by producing a filtrate of blood. Filtration occurs in a tuft of capillaries called the glomerulus. The lining of the glomerulus is leaky enough to allow blood pressure to force water, ions , and small molecules out while retaining cells and very large molecules in the blood. The filtrate, which is very much like the fluid portion of blood (plasma), enters Bowman's capsule, which encloses the glomerulus like a helmet. Bowman's capsule conducts the filtrate into the first part of the nephron tubule, called the proximal convoluted tubule. In humans approximately 180 liters of filtrate (almost enough to fill a 50-gallon drum) make it this far each day. Fortunately, not all of it goes into urine. In the proximal tubule, many of the inorganic ions and almost all of the glucose and amino acids get pumped out of the filtrate and go back into the blood. Most of the water in the filtrate is also drawn back into the blood.

The tubular fluid next passes through a hairpin turn called the loop of Henle, which helps the nephron return more water to the bloodstream rather than allowing it to be lost in the urine. How this works will be explained later. Tubular fluid then enters the distal convoluted tubule of the nephron. Here further transport of particular ions may occur, depending on whether the concentration of that ion in the blood is too high or too low. For example, if the pH of the blood is too low, hydrogen ions (H + ) are transported out of the blood and into the tubular fluid. If the pH is too high, H + ions are transported from the fluid into the blood.

By the time the fluid has completed its journey through the distal convoluted tubule, it is essentially dilute urine, called preurine. Preurine from several nephrons enters a tube called the collecting duct. As preurine passes through the collecting duct, more water can be removed and returned to the blood.

Water is drawn out of the collecting duct by osmosis due to an increasing concentration of ions surrounding the collecting duct. The loops of Henle produce this concentration gradient by a combination of transport and diffusion of ions and urea. Urea is a molecule that temporarily stores the nitrogen produced by the metabolism of proteins . After helping to create the concentration gradient, urea is eventually eliminated with the urine.

Parts of the kidney and their functions

Each kidney is composed of three sections — the outer cortex, the medulla, and the hollow inner pelvis where urine accumulates before it travels down the ureters.

Within the cortex and medulla of each kidney are about one million tiny filters called nephrons.

Each nephron consists of five parts:

  1. the Bowman’s capsule,
  2. the proximal tubule,
  3. the loop of Henle,
  4. the distal tubule, .

The upper portions of the nephron are found in the renal cortex, while the loop of Henle is located in the renal medulla.

The tubes of the nephron are surrounded by cells, and a network of blood vessels spreads throughout the tissue. Any material that leaves the nephron enters the surrounding cells and eventually returns to the bloodstream through the network of blood vessels.

The Bowman’s capsule

Blood enters the cavity of the ball-shaped Bowman’s capsule through a tiny artery that branches to form a network of porous, thin-walled capillaries called the glomerulus.

Under the influence of blood pressure, some blood plasma and small particles are forced out of the capillaries and into the surrounding capsule.

Larger blood components, such as blood cells and proteins, remain in the capillaries.

The fluid in the Bowman’s capsule is called nephric filtrate, and it is pushed out of the capsule into the proximal tubule.

About 20 percent of the blood plasma that enters the kidney becomes nephric filtrate.

The proximal tubule

When the nephric filtrate enters the proximal tubule, reabsorption begins.

Osmosis, diffusion, and active transport draw water, glucose, amino acids, and ions from the filtrate into the surrounding cells. From here the materials return to the bloodstream. This process is aided by active transport of glucose and amino acids out of the filtrate.

When the filtrate reaches the end of the proximal tubule, the fluid is isotonic with the surrounding cells, and the glucose and amino acids have been removed from the filtrate.

A fluid is isotonic when it has the same concentration of water and solutes as that in the cells surrounding it.

The loop of Henle

From the proximal tubule, the filtrate moves to the loop of Henle.

The primary function of the loop of Henle, which first descends into the inner renal medulla and then turns to ascend back towards the cortex, is reabsorption of water from the filtrate by the process of osmosis.

The cells of the medulla have an increased concentration of sodium ions (Na + ). These ions increase in a gradient starting from the area closest to the cortex and moving toward the inner pelvis of the kidney. This increasing gradient acts to draw water from the filtrate in the loop of Henle. This process continues down the length of the descending loop due to the increasing level of Na + in the surrounding tissue. The high levels of Na + in the surrounding medulla tissue are the result of active transport of Na + out of the ascending loop of Henle.

The amount of water removed from the filtrate by the time it reaches the bottom of the loop of Henle results in an increased concentration of all of the materials dissolved in the remaining filtrate, including Na + .

Thus, as the filtrate moves up the ascending loop of Henle, Na + is actively pulled from the filtrate into the surrounding tissue. At the same time, the water that left the descending loop cannot re-enter the ascending loop because this loop is impermeable to water.

Chloride ions tend to follow the sodium ions because of the electrical attraction between the negative chloride ions and the positive sodium ions. In addition, as the water concentration in the filtrate decreases, the chloride ion concentration in the filtrate increases, resulting in still more chloride diffusion out of the ascending loop.

The distal tubule

The distal tubule is responsible for a process called tubular secretion.

Tubular secretion involves active transport to pull substances such as hydrogen ions, creatinine, and drugs such as penicillin out of the blood and into the filtrate.

The collecting duct

The fluid from a number of nephrons moves from the distal tubules into a common collecting duct, which carries what can now be called urine into the renal pelvis.

At that point, 99 percent of the water that entered the proximal tubule as nephric filtrate has been returned to the body. In addition, nutrients such as glucose and amino acids have been reclaimed.

What do the kidneys do?

The kidneys are a pair of bean-shaped organs present in all vertebrates. They remove waste products from the body, maintain balanced electrolyte levels, and regulate blood pressure.

The kidneys are some of the most important organs. The Ancient Egyptians left only the brain and kidneys in position before embalming a body, inferring that the held a higher value.

In this article, we will look at the structure and function of the kidneys, diseases that affect them, and how to keep the kidneys healthy.

Share on Pinterest The kidneys play a role in maintaining the balance of body fluids and regulating blood pressure, among other functions.

The kidneys are at the back of the abdominal cavity, with one sitting on each side of the spine.

The right kidney is generally slightly smaller and lower than the left, to make space for the liver.

Each kidney weighs 125–170 grams (g) in males and 115–155 g in females.

A tough, fibrous renal capsule surrounds each kidney. Beyond that, two layers of fat serve as protection. The adrenal glands lay on top of the kidneys.

Inside the kidneys are a number of pyramid-shaped lobes. Each consists of an outer renal cortex and an inner renal medulla. Nephrons flow between these sections. These are the urine-producing structures of the kidneys.

Blood enters the kidneys through the renal arteries and leaves through the renal veins. The kidneys are relatively small organs but receive 20–25 percent of the heart’s output.

Each kidney excretes urine through a tube called the ureter that leads to the bladder.

The main role of the kidneys is maintaining homeostasis. This means they manage fluid levels, electrolyte balance, and other factors that keep the internal environment of the body consistent and comfortable.

They serve a wide range of functions.

Waste excretion

The kidneys remove a number of waste products and get rid of them in the urine. Two major compounds that the kidneys remove are:

  • urea, which results from the breakdown of proteins
  • uric acid from the breakdown of nucleic acids

Reabsorption of nutrients

The kidneys reabsorb nutrients from the blood and transport them to where they would best support health.

They also reabsorb other products to help maintain homeostasis.

Reabsorbed products include:

  • glucose
  • amino acids
  • bicarbonate
  • sodium
  • water
  • phosphate
  • chloride, sodium, magnesium, and potassium ions

Maintaining pH

In humans, the acceptable pH level is between 7.38 and 7.42. Below this boundary, the body enters a state of acidemia, and above it, alkalemia.

Outside this range, proteins and enzymes break down and can no longer function. In extreme cases, this can be fatal.

The kidneys and lungs help keep a stable pH within the human body. The lungs achieve this by moderating the concentration of carbon dioxide.

The kidneys manage the pH through two processes:

  • Reabsorbing and regenerating bicarbonate from urine: Bicarbonate helps neutralize acids. The kidneys can either retain it if the pH is tolerable or release it if acid levels rise.
  • Excreting hydrogen ions and fixed acids: Fixed or nonvolatile acids are any acids that do not occur as a result of carbon dioxide. They result from the incomplete metabolism of carbohydrates, fats, and proteins. They include lactic acid, sulfuric acid, and phosphoric acid.

Osmolality regulation

Osmolality is a measure of the body’s electrolyte-water balance, or the ratio between fluid and minerals in the body. Dehydration is a primary cause of electrolyte imbalance.

If osmolality rises in the blood plasma, the hypothalamus in the brain responds by passing a message to the pituitary gland. This, in turn, releases antidiuretic hormone (ADH).

In response to ADH, the kidney makes a number of changes, including:

  • increasing urine concentration
  • increasing water reabsorption
  • reopening portions of the collecting duct that water cannot normally enter, allowing water back into the body
  • retaining urea in the medulla of the kidney rather than excreting it, as it draws in water

Regulating blood pressure

The kidneys regulate blood pressure when necessary, but they are responsible for slower adjustments.

They adjust long-term pressure in the arteries by causing changes in the fluid outside of cells. The medical term for this fluid is extracellular fluid.

These fluid changes occur after the release of a vasoconstrictor called angiotensin II. Vasoconstrictors are hormones that cause blood vessels to narrow.

They work with other functions to increase the kidneys’ absorption of sodium chloride, or salt. This effectively increases the size of the extracellular fluid compartment and raises blood pressure.

Anything that alters blood pressure can damage the kidneys over time, including excessive alcohol consumption, smoking, and obesity.

9 Major Functions of Kidney

The major role of kidney is as follows.

Functions of Kidney
1. Removal of metabolic wastes: Both nitrogenous and non nitrogenous metabolic wastes are filtered out of blood by kidneys for removal from the body.

2. Regulation of water balance: Kidneys secrete excess hypotonic urine if the body has excess water and hypertonic urine if the body has deficiency of water.

3. Regulation of pH: Kidneys regulate pH of the body fluids by removing the excess acid or base. The ions which take part in this regulation are H+ and HCO3-.

4. Regulation of Salt balance: A proper Na+ -K+ ion balance is a must for functioning of nerves, muscles and other cells. Kidney help in maintaining the same through their retention or excretion.

5. Fluid Homeostasis: Blood volume of the body is maintained through controlling the amount of fluid loss in urine.

6. Elimination of Extra materials: Extra vitamins, drugs, pigments, salts and toxic chemicals are flushed out of the body by the kidneys.

7. Regulation of Blood Pressure: Blood pressure is regulated through secretion or non secretion of renin.

8. Conservation of Water: Kidneys have a very high osmotic concentration in interstitial fluid of medulla, about 1200mosm/1l. This removes water from urine and conserves the same.

9. Erythropoietin is a hormone produced by juxtaglomerular cells in response to decreased RBC count. Erythropoietin stimulates bone marrow to increase the rate of erythropoiesis or formation of red blood corpuscles (RBC).

Kidney function

The kidney carries out very varied functions. In the following lines, we are going to tell you the most important ones.

1. Excretion

The most popularly known purpose is to expel disposable substances through the urine. For this, the kidney is in charge of creating urine where it puts all those substances the body does not need, and that could be harmful, such as ammonia and urea.

2. Homeostasis

Homeostasis consists of a state of stability and self-regulation. The kidney has to achieve a balance of the body (animal or human). To accomplish this, it uses a lot of mechanisms that are highly complex, among which we can find:

  • Regulation of the ionic composition in the blood.
  • Regulation of blood osmolarity.
  • Plasma volume regulation.
  • Blood pressure regulation.
  • Regulation of the pH of the blood.

3. Hormone secretion

Another function of the kidney is the secretion of different hormones, for different purposes. Among them, we have erythropoietin, renin, vitamin D and kallikrein.

4. Metabolic function

Through a metabolic process called gluconeogenesis, the kidney generates glucose thanks to the amino acids present in our organism. This is done in emergencies when the body has been without food for long periods. However, the liver is mainly responsible for this function.

What is the Excretory System?

First of all, let us talk about What is the Excretory System? And how does it work.

The excretory system is a biological system that removes excess, unnecessary materials from the body fluids of an organism. retaining the proper amounts of water, salts, and nutrients.

Whatever an organism eats in the form of food, some or some waste material is left in our food which serves as a waste to our body and its gathering can have a bad effect on our body. Therefore it becomes necessary that this toxic substance comes out from our side and the process of getting the toxic substance out is called excretion.

There are many useless substances that come out of the body after metabolism, such as carbon dioxide and vapor are formed by digestion of carbohydrates and fats. Which is in the form of our waste material.

Protein digestion creates amino acids and when the body uses proteins as amino acids, our body produces three types of nitrogenous wastes, they are –

Here ammonia is the most toxic substance, because ammonia and uric acid are very toxic, so our body cannot remove it directly. To get out of the body, you have to change to urea, which is done by the liver, it is through the liver that urea is converted into uric acid.

The excretory system Waste substances like sodium chloride are removed from the body by the skin Any toxic waste product that is excreted from the body by some organ is called excretory organs.

The heart is a part of the cardiovascular system responsible for bringing blood to the various tissue in the body. The blood carries oxygen and white blood cells, which is a part of the immune system. The heart receives deoxygenated blood from veins and pumps it to the lungs where red blood cells pick up more oxygen for delivery. The blood is returned to the heart where it pumps oxygenated blood to all organs in the body.

The lungs are the major organ that provides oxygen exchange. The lungs contain tiny bronchiol alveoli, which is the site for absorption of oxygen and elimination of carbon dioxide. The oxygenated blood is then sent back to the heart to provide tissue with the necessary oxygen. The lungs also contain tiny cilia that push foreign objects out of the lungs. This leads to coughing to keep the lungs clear from bacteria, dirt, and smoke. Smoking causes these cells to die, making it difficult for lungs to clear.

The kidneys

Our bodies need to get rid of waste products. Three waste products our bodies must excrete are CO2, urea and sweat. This is known as homeostasis (controlling conditions inside the body).

Urea (a waste product from the breakdown of amino acids) is produced in the liver. Urea is toxic in high concentrations, although the liver releases it into the blood stream to be filtered out by the kidneys.

We take in water from food and drink, and water is a waste product of respiration. We lose water in sweat, faeces, urine and breathing out.

For our cells to work properly their water content must be maintained at the correct level. Our kidneys help us to maintain that balance.

Stages of blood filtration in the kidneys:

Stage 1: Ultrafiltration. Blood is brought to the kidneys to be filtered – blood passes through tiny tubules and water, salt, glucose and urea are squeezed out.

Stage 2: Selective reabsorption. The kidneys send all of the glucose and as much water and salt as the body needs into the blood. Sugar and dissolved ions may be actively absorbed against a concentration gradient.

Stage 3: Waste. Water, salt and urea are left – this is urine. Urine is sent to the through the ureter to the bladder where it is stored before being excreted.

Kidney failure:

Sometimes the kidneys can fail due to infections, toxic substances or genetic reasons. A patient with kidney failure will soon die unless there is a way to rid the body of the urea and excess salt.

A kidney dialysis machine provides an artificial kidney for the sufferers of kidney failure. The patient must use a dialysis machine for 3-4 hours three times a week.

The patients’ blood flows alongside a partially permeable membrane, surrounded by dialysis fluid which contains the same concentration of dissolved ions and glucose as the blood (this ensures that glucose and useful mineral ions are not lost)

Ions and waste can pass through, but big molecules like blood cells and proteins can’t pass through (like in the kidneys).

Dialysis removes urea and maintains blood sodium and glucose levels.

Instead of dialysis a kidney could be transplanted into the patient. This option is cheaper than dialysis but it requires a donor (a normal person can still function with one kidney). The new kidney might be rejected by the body’s immune system.

To prevent rejection of the transplanted kidney a donor kidney with a ‘tissue-type’ similar to the recipient is used and the patient can take immunosuppressant drugs.

Transplanted kidneys only work for around 9 years, then the patient has to return to dialysis.

Mycoprotein is a low-fat, protein-rich food suitable for vegetarians. It

is made from the fungus Fusarium.The fungus grows and reproduces rapidly on a cheap energy supply


1. The tubule begins with a hollow enlargement called Bowman's capsule, which is where water and solutes initially enter the tubule from the bloodstream. This process is known as filtration. The structure comprised of Bowman's capsule and associated capillaries is called the renal corpuscle.

2. From Bowman's capsule the tubular fluid flows towards the proximal tubule, which remains in the outer layer (cortex) of the kidney. The proximal tubule is the major site of reabsorption of water and solutes in equal proportions from the filtered tubular fluid.

3. Then the tubule dips into the hairpin loop of Henle, which descends toward the center of the kidney (medulla) and then rises back to the cortex. The loop of Henle is also a major site of reabsorption, but unlike the proximal tubule, proportionately more solute than water is reabsorbed, so the tubular fluid is dilute relative to plasma by the end of this segment.

4. The next segment is the distal tubule, which like the proximal tubule remains in the cortex. Both reabsorption and secretion take place in this segment, which is where sodium and potassium concentrations (and other electrolytes) and the pH of the tubular fluid are adjusted to ensure homeostasis.

5. The final segment of the nephron is the collecting duct, where multiple tubules join and descend toward the center of the kidney, where the ureter collects the remaining tubular fluid as urine. The collecting duct is a major site of regulation of water balance, where additional water may be reabsorbed from the tubular fluid depending on the body's hydration status.

Surrounding each tubule is a complex system of blood vessels that exchange water and solutes with the tubule. This system is special in that blood must pass through two capillary beds.

1. An afferent arteriole takes blood to the renal corpuscle, where the blood passes through the first capillary bed, a ball-shape tuft known as the glomerulus.

2. An efferent arteriole takes blood away from the glomerulus.

3. From there the blood passes into a set of peritubular capillaries, which follow the remainder of the tubule and are the site of further exchange of water and solutes between plasma and tubular fluid.