Mechanism of Concentration of Filtrate: How much water do our bodies need each day? The daily intake of water recommended for the human body is approximately eight to ten glasses. A significant percentage of the human body is water which involves both intracellular as well as extracellular fluid. Waste material gets excreted out of the body, the process known as excretion.  

The kidney produces urine, and during the process of excretion, urine gets out of the body in concentrated form so that the maximum amount of water gets absorbed in the body. Mammals have the ability to produce concentrated urine, and Henle’s loop and vasa recta play an important role in the process. This process of forming concentrated urine is known as the mechanism of concentration of the filtrate. Read this article to know more about the excretion process, the structure of a nephron, countercurrent mechanism, or the mechanism of concentration of the filtrate.

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Excretion

Excretion is described as the process by which unwanted substances and metabolic wastes are eliminated from the body. During the metabolic process, large amounts of waste materials and carbon dioxide are produced in the tissues. Besides this, the residue of undigested food, heavy metals, drugs, toxic substances, and pathogenic bacteria are also present in the body. All these substances needed to be removed from the body to keep the body in a healthy condition.

The renal system helps in the process of excretion, which includes:

1. A pair of kidneys
2. Ureters
3. Urinary bladder
4. Urethra.

Kidneys produce urine. Ureters transport the urine to the urinary bladder. The urinary bladder stores the urine until it is emptied. Urine is voided from the bladder through the urethra.

Functions of Kidney

Kidneys perform several important functions besides the formation of urine. By excreting urine, kidneys play the main role in homeostasis. Thus, the functions of the kidney are:

1. Excretion of Waste Material
   Kidneys excrete the unwanted waste products formed during metabolic activities. These are in the form of:
   a. Urea (end product of amino acid metabolism)
   b. Uric acid (end product of metabolism of nucleic acid)
   c. Creatinine (end product of metabolism in muscles)
   d. Bilirubin (end product of hemoglobin degradation)
2. Maintaining Water Balance
   Kidneys maintain the water balance in the body by conserving water when it is decreased in the body and excreting water when it is excess in the body.
3. Maintaining Electrolyte Balance
   Maintenance of electrolyte balance, mainly sodium, is in relation to water balance. Kidneys retain sodium if the osmolarity of water decreases and eliminate sodium when osmolarity increases.
4. Maintaining Acid-Base Balance
   The \({\rm{pH}}\) of the blood and body fluids should be maintained within a specific range in the body. This is maintained by the kidneys.

Structure of Nephron

The nephron is defined as the basic structural and functional unit of the kidney. In each kidney, \(1\) to \(1.3\) million nephrons are present. The number of nephrons starts decreasing as the age increases. Each nephron consists of two parts:

1. A blind end is known as renal corpuscle or Malpighian corpuscle
2. A tubular portion called a renal tubule.

Fig: Structure of Nephron

The renal corpuscle consists of two parts:

1. Glomerulus
2. Bowman capsule.

Glomerulus:  It is a tuft of capillaries that is enclosed by Bowman’s capsule. It consists of glomerular capillaries, which contain afferent arteriole on one end and efferent arteriole on the other end. Glomerular capillaries arise from the afferent arteriole.

Bowman Capsule: It is a capsular structure, which encloses the glomerulus. It is formed by two layers:

i. Inner visceral layer
ii. Outer parietal layer

The visceral layer covers the glomerular capillaries. It is continued as the parietal layer at the visceral end.

Tubular Portion of the Nephron

The tubular portion of the nephron is continued from the Bowman capsule. It consists of three parts:

1. Proximal convoluted tubule
2. Loop of Henle
3. Distal convoluted tubule.

1. Proximal convoluted tubule: It is the coiled portion that arises from the Bowman capsule. It is situated in the cortex. It is formed by a single layer of cuboidal epithelial cells. Characteristic features of these cells are that there is the presence of hair-like projections, which are directed towards the lumen of the tubule. Because of the presence of these projections, the epithelial cells are called brush-border cells.

2. Loop of Henle: It consists of:
i. Descending limb
ii. Hairpin bend
iii. Ascending limb.

i. Descending Limb
It is made up of two segments:
a. Thick descending segment: It is a continuation of the proximal convoluted tubule. It descends down into the medulla. Brush-bordered cuboidal epithelial cells are present here.
b. Thin descending segment: It is continued as a thin descending segment. It is formed by flattened epithelial cells without brush border epithelium and continues as the hairpin bend of the loop.

ii. Hairpin Bend
Hairpin bend is formed by flattened epithelial cells without brush border epithelium and is continued as the ascending limb of the loop of Henle.

 iii. Ascending Limb
It has two parts:
a. Thin ascending segment
b. Thick ascending segment.

3. Distal convoluted tubule: It is the continued portion of a thick ascending segment and occupies the cortex of the kidney. It is continued as a collecting duct. Single layers of cuboidal epithelial cells without brush border epithelium are present here.
Collecting duct: Distal convoluted tubule continues as the arched collecting duct, which lies in the cortex. The lower part of the collecting duct lies within the medulla. \(7 – 10\) initial collecting ducts unite to form the straight collecting duct.
Two types of epithelial cells are present in the collecting duct:
1. Principal or \({\rm{P}}\) cells
2. Intercalated or \({\rm{I}}\) cells.

Passage of Urine

In the inner zone of the medulla, the medullary pyramid contains straight collecting ducts which unite to form papillary ducts or ducts of Bellini. They open into a \(‘{\rm{V}}’\) shaped area called a papilla. Urine is collected in the papilla, which drains into a minor calyx. Three or four minor calyces unite to form one major calyx. In each kidney, \(8\) minor calyces and \(2\) to \(3\) major calyces are there.
From minor calyces, urine passes through major calyces. This then opens into the pelvis of the ureter. From the renal pelvis, urine passes through the remaining portion of the ureter and reaches the urinary bladder.

Everyday \(180{\rm{ L}}\) of glomerular filtrate is created with a large amount of water. If this amount of water gets excreted out in the form of urine, the body will face serious threats. So the concentration of urine is essential.

The osmolarity of glomerular filtrate is equal to that of plasma that is \(300\,{\rm{mOsm}}/{\rm{L}}\). But, the osmolarity of urine is four times more than that of plasma, that is \(1,200\,{\rm{mOsm}}/{\rm{L}}\).

The osmolarity of urine depends upon two factors:

1. Water content in the body
2. Antidiuretic hormone (ADH).

The mechanism of urine formation is the same for dilute urine and concentrated urine until the fluid reaches the distal convoluted tubule.

Concentration of Urine

When the water content is more in the body, the kidney excretes dilute urine. This is obtained by inhibition of secretion of ADH from the posterior pituitary gland. Hence, water reabsorption does not take place from renal tubules. This makes the urine dilute.
When the water content decreases in the body, the kidney retains water and excretes concentrated urine. It involves two processes:
1. Medullary gradient by countercurrent system
2. Secretion of ADH.

Urine Formation

Urine formation involves the following processes:

1. Glomerular filtration or ultrafiltration of the blood plasma by the glomeruli
2. Tubular reabsorption or selective reabsorption
3. Tubular secretion

1. Glomerular filtration: Glomerular filtration occurs with the help of glomerular capillaries, which causes filtration of blood through three layers, namely, endothelium of glomerular blood vessels epithelium of Bowman’s capsule, and a basement membrane between these two layers.
Epithelial cells called podocytes are present in Bowman’s capsule. Blood is filtered through these membranes, and all the constituents of plasma except protein pass through it.
GFR of a healthy individual is \(125\,{\rm{ml}}/{\rm{min}}\) or \(180\,{\rm{l}}/{\rm{day}}\).

2. Tubular reabsorption: As a result of tubular reabsorption, most of the filtrate passes out of tubules and returns to the blood through peritubular capillaries. Epithelial cells in PCT have numerous microvilli, which increases the surface area for reabsorption.
Depending upon the type of molecules being reabsorbed, movement occurs by active or passive transport. For example, water and urea are reabsorbed by passive transport, that is, from a high concentration area to a low concentration area. Glucose and amino acids are reabsorbed by active transport, while sodium ions are reabsorbed by both active and passive means.

3. Tubular secretion: Certain chemicals that are not removed by filtration from the glomerular capillaries are removed by this process. These chemicals are removed by both active as well as passive transport. These chemicals are mixed with glomerular filtrate and are eliminated from the body with urine. These include ions such as potassium, hydrogen, and ammonium ions; medicines such as penicillin, harmful drugs such as cocaine, marijuana, etc.

Difference between Tubular Secretion and Tubular Reabsorption

Tubular reabsorption	Tubular secretion
It is the absorption of selected materials from the nephric filtrate into the blood of peritubular blood capillaries.	It is the removal of selected materials from the blood of peritubular blood capillaries into the nephric filtrate.
Reabsorption of glucose, amino acids, water, inorganic ions like sodium, potassium, etc., takes place.	Removal of ammonia, urea, uric acid, creatine, creatinine, etc. takes place.
It takes place by diffusion and active transport.	It takes place by active transport only.

Medullary Gradient

Medullary Hyperosmolarity
The osmolarity of medullary interstitial fluid is similar to that of the cortex, which is \(300\,{\rm{mOsm}}/{\rm{L}}\).
As the urine moves from the outer part towards the inner part of the medulla, the osmolarity increases slowly and reaches a maximum at the innermost part of the medulla. Here, the osmolarity is \(1,200\,{\rm{mOsm}}/{\rm{L}}\).

Development and Maintenance of Medullary Gradient
Kidneys possess a unique mechanism called the countercurrent mechanism, which is responsible for the development and maintenance of medullary gradient and hyperosmolarity of interstitial fluid in the inner medulla.

Countercurrent Mechanism

A countercurrent system is a system of \({\rm{‘U}}\) Shaped tubular system in which the fluid flows in the opposite direction in two limbs of the \({\rm{‘U}}\) Shaped tubules.

Divisions of Countercurrent System
The countercurrent system has two divisions:

1. Countercurrent multiplier which is formed by a loop of Henle.
2. Countercurrent exchanger is formed by the vasa recta.

Countercurrent Multiplier

Loop of Henle
Loop of Henle functions as a countercurrent multiplier. Active reabsorption of sodium chloride and other solutes from the ascending limb of the Henle loop into the medullary interstitium results in the development of hyperosmolarity. These solutes accumulate and increase the osmolarity.
Moreover, the continuous addition of sodium and chloride ions increases the osmolarity of fluid. Hence, it is called a countercurrent mechanism.

Fig: Countercurrent Multiplier

Countercurrent Exchanger

Vasa Recta
Vasa recta functions as a countercurrent exchanger. It acts as a countercurrent exchanger because of its position. It is a \({\rm{‘U}}\) Shaped tubule having a descending limb, hairpin bend, and an ascending limb.
The Vasa recta run parallel to the loop of Henle. Its descending limb is parallel to the ascending limb of the Henle loop, and its ascending limb is parallel to the descending limb of the Henle loop.

1. The sodium chloride reabsorbed from the ascending limb of the Henle loop enters the medullary interstitium.
2. Then it enters the descending limb of the vasa recta.
3. Simultaneously water diffuses from the descending limb of the vasa recta into the medullary interstitium.
4. The blood flows very slowly through the vasa recta. Till the time the blood reaches the ascending limb of the vasa recta, the concentration of sodium chloride increases.
5. This causes diffusion of sodium chloride into the medullary interstitium.
6. Simultaneously, water from the medullary interstitium enters the ascending limb of the vasa recta. And the cycle is repeated.

Thus, the vasa recta maintain the hyperosmolarity by retaining sodium chloride in the medullary interstitium and removing water from it.
Hence this system is called a countercurrent exchanger.

Fig: Countercurrent Exchanger

Role of Antidiuretic Hormone
The final concentration of urine is obtained by the action of ADH. The presence of ADH makes DCT and collecting duct permeable to water, resulting in water reabsorption.

Summary

Excretion is the process of eliminating waste material from the body. The excretory system produces, stores, and eliminates the urine after it produces and modifies a urinary filtrate consisting of a large volume of hypotonic blood filtrate. Each nephron consists of a filtering component called renal corpuscle and a tubule called renal tubule for reabsorption and secretion. The formation of urine is the result of glomerular filtration, tubular reabsorption, and tubular secretion. In the presence of ADH, the distal convoluted tubule and collecting duct becomes permeable to water, resulting in water reabsorption and the final concentration of urine.

Frequently Asked Questions(FAQs) on Mechanism of Concentration of Filtrate

Q.1. Define the excretion process.
Ans: Excretion is the process of removal of unwanted waste material from the body.

Q.2. What does the renal corpuscle consist of?
Ans: The renal corpuscle consists of the glomerulus and Bowman’s capsule.

Q.3. What is the GFR (glomerular filtration rate) of a healthy individual?
Ans: GFR of a healthy individual is \(125\,{\rm{ml}}/{\rm{min}}\) or \(180\,{\rm{l}}/{\rm{day}}\).

Q.4. What is the role of ADH in urine concentration?
Ans: The presence of ADH (antidiuretic hormone) makes the distal convoluted tubule and collecting duct permeable to water, which results in water reabsorption.

Q.5. Define medullary hyperosmolarity.
Ans: As the urine moves from the outer part towards the inner part of the medulla, the osmolarity increases slowly and reaches. Here, the osmolarity is \(1,200\,{\rm{mOsm}}/{\rm{L}}\). This is known as medullary hyperosmolarity.

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