Urine Formation: Definition, Mechanism, Significance

Urine Formation: The liquid waste product of the human body is urine. It is made up of waste products from the body’s many metabolic activities, such as urea, uric acid, salts, water, and other substances. It develops in the kidneys, which are the main excretory organs. Unwanted compounds are removed from the circulation by the kidneys, which also create urine to do so. Urine is produced in three stages: glomerular filtration, reabsorption, and secretion.

These procedures make sure that the body is solely expelled of waste and extra water. The nephrons are the structural and operational unit of the kidneys. The process of producing urine involves millions of nephrons. Each nephron has a glomerulus, which is where blood is filtered. The glomerular capsule (also known as Bowman’s capsule) surrounds the capillary network that makes up the glomerulus. Read through the article to learn about urine formation diagram, process and more!

What is Urine Formation?

Define urine formation: The process of urine formation refers to the build-up of yellow concentrated fluid that consists of wastes and toxic materials which are to be excreted out from the body.

Urine Formation Diagram

Fig: Reabsorption and Secretion of Major Substances at Different Parts of the Nephron (Arrows Indicate the Direction of Movement of Materials)

Definition of Urine Formation

The formation of yellow fluid called urine through different processes in the nephrons of the kidneys is called urine formation. The nephron is the unit of the kidney. Before getting into the mechanism of urine formation, we need to know about the unit of the kidney called the nephron.

Structure of Nephron

The nephron consists of a malpighian body and renal tubules.

• Malpighian Body- It consists of a cup-shaped Bowman’s capsule and glomerulus. The glomerulus is a tuft of capillaries. Afferent arteriole enters the Bowman’s capsule and leaves it through the efferent arteriole.

Fig: Bowman’s Capsule

Bowman’s capsule is a cup-shaped structure that surrounds the glomerulus. Its wall is double-layered and consists of special types of cells called podocytes. These cells have projections, and gaps between these projections form pores. These pores allow the filtration of blood from the glomerulus to the Bowman’s capsule.

Fig: Podocyte

• Renal Tubules- A tubule continues from the Bowman’s capsule called the renal tubule, which has the following distinct regions:
  • Proximal convoluted tubule
  • Loop of Henle
  • Distal convoluted tubule

Types of Nephrons

The types of nephrons are as follows:

• Cortical Nephron– These are short nephrons and almost lie in the cortical region of the kidney. Thus, it is called a cortical nephron.
• Juxtamedullary Nephron– These have a long loop of Henle and digs deep into the medulla and are called Juxtamedullary Nephron.

Fig: Types of Nephrons

Process of Urine Formation

Let us see how urine is formed in our bodies. The process of urine formation includes different processes like glomerular filtration, selective reabsorption, and tubular secretion. These urine formation steps involve different parts of the nephron. Students can check the physiology of urine formation below:

Glomerular Filtration

This is the first step of urine formation which is carried out by the glomerulus. About \(1100 – 1200\, ml\) of blood is filtered from the kidneys per minute. The glomerular capillaries are narrower than the glomerular arterioles. This creates pressure in the glomerular capillaries. Due to high pressure in the capillaries, blood is filtered through pores created by podocytes. The filtered blood is poured into the lumen of Bowman’s capsule.

Through these pores, large cells like RBCs, WBCs, plasma proteins, etc., are unable to pass through these pores. This process of filtration, which occurs through glomerular capillaries in the Bowman’s capsule, is known as Ultrafiltration, and the filtrate formed is called glomerular filtrate.

This glomerular filtrate is the same as that of blood plasma. The only difference is that filtrate does not contain fat and proteins.

After passing through the glomerular capillaries, blood enters the efferent arteriole. The amount of filtrate formed per minute by both the kidneys is called the glomerular filtrate rate. It is about \(125\,ml\) per minute or \(180\,L\) per day.

This glomerular filtration is regulated by-

• Myogenic Mechanism- It refers to the increase in blood pressure which in turn increases the pressure in afferent arterioles. This increase in pressure in afferent arterioles results in increased blood flow to the glomerulus. Increased pressure reduces the diameter of the blood vessels.
• Juxtaglomerular Mechanism- Juxtaglomerular apparatus secretes an enzyme called renin which regulates the blood pressure and thereby glomerular filtration rate (GFR). 
• Neural Control- Nervous system also controls the GFR by signalling the renal artery to constrict

Selective Reabsorption

Under this category, we will describe the reabsorption of useful materials back into the blood. 

• Proximal Convoluted Tubule– From Bowman’s capsule, filtrate enters the proximal convoluted tubule (PCT). Here, reabsorption of almost \(65\% \) of the filtrate is done before reaching the top of the loop of Henle. Reabsorption of glucose, amino acids, vitamins, hormones, chlorides, sodium, potassium, and much water are reabsorbed here. These are reabsorbed by active and passive transport. Water reabsorption occurs by osmosis. Here the filtrate is isotonic to the blood plasma.
• Loop of Henle- This part of the loop of Henle consists of its ascending and descending limb. After passing through the PCT, filtrate enters the loop of Henle.
  • Descending limb of the loop of Henle: Here water is reabsorbed when filtrate moves down the loop of Henle and not the solutes like sodium. This makes the filtrate hypertonic to the blood plasma.
  • Ascending limb of the loop of Henle: Here solutes like sodium, potassium, magnesium, calcium, and chloride are reabsorbed. This makes the filtrate hypotonic to the blood plasma.
• Distal Convoluted Tubule (DCT)- In this part of the nephron, under the influence of the aldosterone hormone secreted by the cortex of the adrenal gland, sodium ions are reabsorbed. Under the influence of ADH, water reabsorption occurs here. Chloride ions are reabsorbed too. Again here, filtrate becomes isotonic to the blood plasma.
• Collecting Duct- Now filtrate enters the collecting duct from DCT. Here more water is reabsorbed under the influence of ADH, which makes the filtrate more concentrated and hypertonic in the blood plasma. Sodium ions are also reabsorbed. This concentrated filtrate is now called urine.

Tubular Secretion

In the renal tubule, with reabsorption, secretion also occurs. Secretion of harmful toxic materials like ammonia, creatinine, urea, hippuric acid, drugs, etc., is actively secreted from the DCT. Mostly, secretion occurs in DCT. Some secretions also occur in the loop of Henle and DCT. Urea is secreted in the loop of Henle, while in DCT, potassium, and ammonia is reabsorbed.

Fig: Urine Formation

Mechanism of Concentration of Filtrate

Kidneys not only excrete harmful substances out from the body but also maintain the amount of water and salt in the body. The filtrate from the Bowman’s capsule enters the PCT, which is almost isotonic to the blood plasma. But here in PCT, as \(65\% \) of the filtrate is reabsorbed, it gets hypertonic to the blood plasma in the descending loop of Henle. Then in the ascending loop of Henle, the filtrate gets hypotonic to the blood plasma.

Then again, in DCT, due to the action of aldosterone and ADH, the filtrate becomes isotonic as sodium ions and water are reabsorbed by them, respectively. In the collecting duct, further reabsorption of water takes place which makes filtrate more hypertonic. Now this filtrate is called urine.

Counter Current Mechanism

This unique mechanism of the nephron helps in making the urine concentrated. In the limbs of the loop of Henle, filtrate flows in the opposite direction, which forms the counter current. The flow of blood in capillaries surrounding the loop of Henle is also in the opposite direction and forms a counter current.

Its function is to increase the concentration of sodium ions in the interstitial fluid and concentrate the filtrate in the collecting duct by allowing more reabsorption of water from it. This concentrates urine. Urine formed in the kidneys comes down to the urinary bladder through ureters and then is expelled out of the body. This expulsion is called micturition. 

Significance of Urine Formation

The significance of urine formation are as follows:

• Urine formation helps in the removal of wastes like creatinine and urea.
• It helps in the removal of extra fluid from the body, thereby regulating extracellular fluid volume.
• Urine helps to remove extra acidic components of the blood plasma and thus regulates the pH.
• Osmolarity or electrolyte-water balance is maintained by the kidneys by concentrating or diluting the urine.

Urine Formation Per Day

Glomerular filtration rate (GFR) is termed as the volume of filtrate formed by both kidneys per minute. On average, 125 mL/min filtrate is produced in men and 105 mL/min filtrate is produced in women. However, 99% of produced filtrate is returned to circulation by the process of reabsorption. Therefore, only about 1–2 litres of urine are produced per day in a healthy human body. Let’s see the table below for more information on the quantity of urine formed per day for men and women.

In the table below, we have mentioned the quantity of urine that is formed daily in men and women.

What is Net Filtration Pressure?

Net Filtration Pressure (NFP) is used to determine the filtration rates through the kidney.

NFP = Glomerular blood hydrostatic pressure (GBHP) – [capsular hydrostatic pressure (CHP) + blood colloid osmotic pressure (BCOP)] = 10 mm Hg

That is:

NFP = GBHP – [CHP + BCOP] = 10 mm Hg
Or:
NFP = 55 – [15 + 30] = 10 mm Hg

Multiple Choice Questions (MCQs) on Urine Formation

Here are some of the commonly asked multiple-choice questions on the “urine formation” topic, which will be asked in various competitive exams like NEET, Pharmacist, GPAT and various nursing exams.

Q: Where does capillary hydrostatic pressure build-up?

a) glomerulus
b) PCT
c) DCT
d) collecting ducts

Q: Which of the following products is reabsorbed very limited?

a) urea
b) uric acid
c) creatinine
d) all of the above

Q: What factor maintains the normal blood pH?

a) tubular secretion of H ions
b) tubular secretion of Ca ions
c) tubular secretion of K ions
d) tubular secretion of Na ions

Q: On average, how much volume of blood is filtered by the kidneys per minute?

a) 500 mL
b) 100-150 mL
c) 1100-1200 mL
d) 5000 mL

Q: What is the full form of GFR?

a) Glomerulus filtering unit
b) Glomerulur filtration rate
c) Globulin fast rate
d) Globulin filtering rate

Answers:

Summary on Urine Formation

Urine formation involves a few processes like glomerular filtration, selective reabsorption, and tubular secretion. The nephron is the unit of the kidney. Each nephron undergoes all three processes of urine formation and forms concentrated urine which is transported to the urinary bladder through ureters and then is excreted out of the body.

FAQs on Urine Formation

Find some of the frequently asked questions on the process of urine formation below:

Q.1: Where and how is urine produced?
Ans: Urine is produced in the nephrons of the kidneys. Urine formation involves the following processes:- 
a. Glomerular Filtration
b. Selective reabsorption
c. Tubular Secretion

Q.2: Why do the kidneys form urine?
Ans: Urine is a yellow fluid formed by the nephrons of the kidneys. Urine is formed to excrete excess water, salt and nitrogenous wastes like urea. Other toxic substances are also excreted through urine.

Q.3: What are the \(3\) steps of urine formation?
Ans: Following are the steps of urine formation:
a. Glomerular Filtration
b. Selective reabsorption
c. Tubular Secretion

Q.4: What is the composition of urine, and write the steps of urine formation?
Ans: Urine is a yellow colour fluid consisting of excess water, salt, and nitrogenous wastes like urea. Urine is produced in the nephrons of the kidneys. Urine formation involves the following processes:- 
a. Glomerular Filtration
b. Selective reabsorption
c. Tubular Secretion

Q.5: What are the \(7\) functions of the kidneys?
Ans: The functions are as follows:
a. maintains acid-base balance.
b. Maintains water balance
c. Maintains electrolyte balance
d. removes wastes and toxins from the body
e. Controls blood pressure
f. Produces hormone
g. Activates Vitamin D

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