Micturition: Human Urine, Neural Regulation of Micturition, Ornithine Cycle, Analysis of Urine For Disorders, Practice Problems and FAQs

We all have experienced an urge to pee. Sometimes we take permission from our teachers and rush to the restroom in the middle of a class. But do you know what causes this feeling of urge to pee? No, you need an answer. 

                                   Fig: Micturition

We know that a pair of kidneys are the organs of excretion in the human body. They filter the blood and the filtrate gets stored in the urinary bladder. This filtrate is known as urine. 

                                                      GIF: Formation of urine

This is stored in the urinary bladder for sometime and when the bladder gets full, it makes an urge to pee. Let’s take a deep dive into the details of micturition in this article.

Table of contents

• Micturition
• Neural regulation of micturition
• Human urine
• Ornithine cycle
• Analysis of urine for disorders
• Practice Problems
• FAQs

Micturition

Micturition is defined as the process by which urine is expelled out from the body. The excretory system, which is found in both animals and humans, is responsible for removing waste products from the body. Therefore, we can say that the process of discharging urine from the urinary bladder is known as micturition. 

Human excretory system

A pair of kidneys and ureters, a urinary bladder, and a urethra make up the human excretory system. The kidneys perform an important function in the generation and excretion of urine. The urinary bladder stores the urine which is produced. 

                Fig: Human excretory system

Micturition process

The micturition process consists of two phases as follows:

• Storage phase
• Voiding phase

Storage phase

The urinary bladder is a hollow, muscular structure in the shape of a balloon that stores urine. In a healthy urinary system, the urinary bladder can easily store up to 16 ounces (473.2ml) of urine for 2 to 5 hours. The sphincter muscles prevent urine from leaking out. It allows the urine formed to pass outside the body.

                                  Fig: Storage of urine in urinary bladder

Voiding phase

The nerves in the urinary bladder are stimulated when it is filled with urine, and the urge to urinate arises at that time. Then the brain sends a signal to the urinary bladder to contract. Now the urinary bladder receptors give a signal to the central nervous system through the sensory neurons. This causes the nervous system to produce a signal which reaches the urinary bladder through motor neurons. Now the urinary bladder contracts and micturition occurs. Micturition is the process of eliminating urine through the urinary opening at the urethra. In the micturition reflex the neurological mechanism is involved. 

Micturition reflex

The neural mechanism starting from sensory stretch receptors of the distended urinary bladder to the central nervous system and back to the urinary bladder for its emptying is called the micturition reflex.

                                        Fig: Micturition reflex

Neural regulation of micturition

When the urinary bladder is filled with urine, it stimulates stretch receptors to send signals to the central nervous system or spinal cord.

                                          Fig: Neural regulation of micturition

In response, CNS sends a voluntary signal to the smooth muscles via motor neurons. This results in the contraction of smooth muscles of the urinary bladder and relaxation of the urethral sphincter, which causes the release of urine.

                                      Fig: Relaxation of urethral sphincter

Human urine

An adult human excretes 1 to 1.5 litres of urine every day on average. Urine is a light yellow coloured watery liquid with a distinct odour. Due to vigorous hydrogen ion secretion, it is mildly acidic (pH - 6.0). The presence of a pigment called urochrome gives it a pale yellow tint. Every day, 25 - 30 grams of urea is excreted through urine. The properties of urine can be affected by multiple factors. 

The human urine is formed and stored in the urinary bladder. The filtrate is transported to the urinary bladder after the kidney filters the blood. The bladder gradually fills up, causing it to stretch. The stretching occurs because the stretch receptors are present in the urinary bladder. 

                   Fig: Urine

Urea formation

The ammonia is converted into urea by the liver cells. Nearly 80% of the nitrogen expelled by humans and other mammals is in the form of urea, which is created by a series of processes taking place in the cytosol and mitochondrial matrix of liver cells. The Krebs-Henseleit cycle, ornithine cycle or the urea cycle is the collective name for these reactions.

The toxic byproduct of nitrogen metabolism is ammonia which should be eliminated from the body. In the mitochondria of the liver cells, the urea cycle or ornithine cycle turns a surplus amount of ammonia into urea. After its formation, urea enters the bloodstream which then passes through the kidneys for filtration and is ultimately eliminated in the form of urine.

The overall reaction of urea formation from ammonia is given below:

2 Ammonia + CO2+ 3 ATP → Urea + Water + 3 ADP

Ornithine cycle

The five steps that make up the urea cycle are carried out by various important enzymes. The cycle's first two phases happen in the mitochondrial matrix, and the remaining steps happen in the cytosol. As a result, the urea cycle occurs in two distinct cellular regions of the liver cell.

Step I

In the first step of the ornithine cycle, the ammonia produced in the mitochondria is converted into carbamoyl phosphate with the help of an enzyme called carbamoyl phosphate synthetase I. The reaction of this step is given below:

NH3 + CO2 + 2ATP → Carbamoyl phosphate + 2ADP + Pi

Step II

In order to create citrulline, the carbamoyl group must be transferred from carbamoyl phosphate to ornithine in the second step. Ornithine transcarbamoylase (OTC), an enzyme, catalyses this process. The reaction of this step is given below:

Carbamoyl phosphate + Ornithine → Citrulline +Pi

Citrulline is thus created and released into the cytosol to use in the remaining phases of the cycle.

Step III

An enzyme known as argininosuccinate synthetase catalyses the third step by utilising citrulline and ATP to create a citrullyl-AMP intermediate, which then combines with an amino group from aspartate to make argininosuccinate. The reaction is given below:

Citrulline + ATP + Aspartate → Argininosuccinate + AMP + PPi

Step IV

In this step, argininosuccinate is broken down into fumarate and arginine with the help of an enzyme, argininosuccinate lyase. The reaction is represented as follows:

Argininosuccinate → Fumarate + Arginine

Step V

This is the last step of the ornithine cycle in which arginine is hydrolysed to form urea and ornithine with the help of an enzyme called arginase. The reaction is given below:

Arginine → Urea + Ornithine

                                                                Fig: Ornithine cycle

Analysis of urine for disorders

Urine analysis aids in the clinical diagnosis of a variety of metabolic disorders as well as renal dysfunction. 

Common disorders

The following are the common disorders associated with urine formation. 

                                 Fig: Common disorders

Glycosuria

Presence of a high amount of reducing sugars, such as glucose, galactose, lactose, fructose etc., in the urine is called glycosuria. Here the level of reducing sugars increases above 0.25 mg/mL. 

This indicates that the person is suffering from diabetes mellitus.

Ketonuria

Presence of large amounts of ketone bodies (acetoacetate and 3-beta-hydroxybutyrate) in urine is called ketonuria. This is seen in the case of people with type 1 diabetes mellitus. Ketone bodies are end products of fatty acid breakdown. These are completely broken down in the liver and hence small amounts are present normally in urine. 

Alkaptonuria

Alkaptonuria is a very rare inheritable genetic disorder. Here the body cannot fully break down amino acids like tyrosine and phenylalanine. This leads to the accumulation of homogentisic acid in the body. In such individuals with alkaptonuria the urine turns black on exposure to air. Thus this disease is also called black urine disease.

Proteinuria

Presence of a high amount of proteins in urine. This condition is a sign of kidney damage.

Haematuria 

Presence of blood cells in urine. The blood in urine indicates a serious disorder. There are two types of hematuria:

Gross hematuria

It is a condition in which a person can see blood in his or her blood.

Microscopic hematuria

It is a condition in which the blood cells are visible in urine only under the microscope.

Uremia

Presence of excess urea in the urine. It occurs when the kidney is no longer capable of filtering out urea from the urine. This indicates the end stage of renal disease. It is harmful to the body. In this case urea can be removed by haemodialysis. 

Pyuria

It is the condition of urine that occurs when it contains white blood cells or pus. 

Dysuria

It is the medical term given to pain or discomfort caused while urinating. This can be caused by urinary tract infections. 

Practice Problems

Q 1. Why is human urine pale yellow in colour?

a. Presence of urea
b. Presence of urochrome
c. pH of urine is slightly acidic
d. Presence of nitrogen ions

Answer: The human urine is pale yellow in colour due to the presence of urochrome pigment. It is a fluid with a distinct odour. The pH of human urine is slightly acidic due to vigorous hydrogen ion secretion. Hence, the correct option is b.

Q 2. Determine the pH of urine?

a. Neutral
b. Basic
c. Slightly acidic
d. Highly alkaline

Answer: An adult human excretes 1 to 1.5 litres of urine every day on average. Urine is a light yellow coloured watery liquid with a distinct odour. Due to vigorous hydrogen ion secretion, it is mildly acidic (pH - 6.0). Hence, the correct option is c.

Q 3. If human urine is compared with human blood, it is _______________.

a. Hypotonic
b. Isotonic
c. Hypertonic
d. All of the above

Answer: The concentration of human urine is four times that of human blood. The osmolarity of blood is 300 mOsm/L, while the osmolarity of urine produced by the kidneys is 1200 mOsm/L. In comparison to human blood, human urine is hypertonic. Hypertonic refers to a solution with a higher concentration of solute than ordinary solutions. Hence, the correct option is c.

Q 4. Determine the correct statement about micturition.

a. Process of release of urine
b. Central nervous system initiating the contraction of urinary bladder
c. Central nervous system initiating the relaxation of urethral sphincter
d. All of the above

Answer: Micturition is defined as the process in which urine is expelled out from the body. The excretory system, which is found in both animals and humans, is responsible for removing waste products from the body. Therefore, we can say that the process of discharging urine from the urinary bladder is known as micturition. Hence, the correct option is a.Q

Q 5. Is micturition the same as the voiding phase?

Answer: Micturition is also known as the voiding phase of the bladder. It is a short-lasting phase. In this phase, the urge to urinate arises and the urinary bladder eliminates the urine through urethra.

Q 6. State whether the micturition is under voluntary or involuntary control?

Answer: Micturition occurs involuntarily in infants and young children till the age of 3 to 5 years. After this age it is regulated voluntarily. This voluntary release of urine from the human body is regulated by the neural mechanism that involve the centres in the brain and the spinal cord. 

Q 7. What is the mechanism of micturition?

Answer: When the urinary bladder is filled with urine, it stimulates stretch receptors to send signals to the central nervous system or spinal cord. In response, CNS sends a voluntary signal to the smooth muscles via motor neurons. This results in the contraction of smooth muscles of the urinary bladder and relaxation of the urethral sphincter, causing urine to be released.

Q 8. From where, does the micturition reflex initiate?

Answer: The micturition reflex is believed to be initiated from the stretch receptors. These stretch receptors are present in the wall of the urinary bladder.

FAQs

Q 1. Which sphincter is under voluntary control?
Answer: The external urethral sphincter is made of skeletal muscle. Hence it is under voluntary control of the somatic nervous system.

Q 2. What do you mean by the urine colour chart? 
Answer: The colour of urine is pale yellow due to the presence of the pigment called urochrome. If the colour changes to darker yellow or orange, it shows that the person is getting dehydrated. Orange-colored urine also indicates a serious liver condition. Darker brown colour urine indicates severe dehydration. It can also be caused by foods or medication.

                                                             Fig: Urine colour chart

Q 3. How can a person stimulate urination?
Answer: The condition of stimulation urination arises when a person goes for medical tests and they need a urine sample. The urination can be stimulated in the following ways:

• Tap the area present between the navel and pubic bone.
• Bend forward while sitting on the toilet pot.
• Placing your hand in warm water also can trigger the release of urine.

Q 4. When a person is in a feared or scared situation, why do they urinate?
Answer: Stress usually prompts the fight, fright or flight reaction, which releases hormones that interfere with the normal chemicals that keep the bladder relaxed, causing it to contract. Due to this, some people may feel the urge to urinate or may even urinate unintentionally.

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