Micturition: Definition, Physiology, Mechanism & Human Urine

Micturition: Definition, Meaning, Topics, Stages, Properties, Problems, Process

Edited By Irshad Anwar | Updated on Jul 02, 2025 06:44 PM IST

Micturition definition: Micturition is the process by which urine is released from the urinary bladder. This process is important to the body in the removal of waste products formed through metabolic processes, maintaining fluid and electrolyte balance. The knowledge of micturition helps in understanding the anatomy and physiology of the urinary system, neural control of urine release, and the role of kidney functions in excretion. In this article, micturition, the anatomy of the urinary system, the physiology of micturition, human urine, the mechanism of micturition, and disorders related to micturition are discussed. Micturition is a topic of the chapter in Excretory Products and Their Elimination in Biology.

This Story also Contains

What is Micturition?
Anatomy of the Urinary System
Physiology of Micturition
Human Urine
Mechanism of Micturition
Disorders Related to Micturition

Micturition: Definition, Meaning, Topics, Stages, Properties, Problems, Process

What is Micturition?

Micturition, also known as urination, is a process whereby the urinary bladder empties urine through the urethra to the outside. This involves the coordinated contraction of the detrusor muscle in the bladder and relaxation of the urethral sphincters, which are under the control of the autonomic and somatic nervous systems. Such a process is necessary for ridding the body of waste products and excess fluid by means of maintaining homeostasis.

Also Read:

Anatomy of the Urinary System

The human excretory system includes :

Kidneys: Filter blood and produce urine.

Ureters: Carry urine from the kidneys to the bladder.

Urinary Bladder: Stores the urine till it is excreted.

Urethra: Excretes urine from the bladder out of the body.

Urinary System

Physiology of Micturition

The physiology of urine formation is discussed below:

Urine Formation

Filtration: The blood gets filtered in the kidneys to form urine.

Reabsorption: The urine formation involves the reabsorption of useful substances back into the blood.

Secretion: The waste products are secreted into the urine.

Micturition Physiology

Storage of Urine

The urinary bladder is a balloon-shaped muscular organ. The organ can hold, several times, up to 16 ounces of urine for 2 to 5 hours. Its opening is closed by circular sphincter muscles preventing leakage.

Human Urine

A typical adult excretes 1 to 1.5 L of pee daily on average. Normal human urine is a pale yellow liquid that is mostly composed of water (95%) and solid waste (5%). Its pH is almost 6, making it slightly acidic.

Urine analysis can be used to diagnose a variety of endocrine problems. For instance, the presence of ketone bodies and glucose in the urine can be used to identify diabetes in a patient. It is therefore a crucial component of clinical diagnosis.

Mechanism of Micturition

This is achieved via the perfectly coordinated activity of the nervous system and urinary organs. The micturition reflex, through stretch receptors in the bladder, signals the brain to begin the voiding process.

The stages of the micturition process are discussed below:

Storage Phase

Bladder Filling: During this phase, the urine that forms from the kidneys moves down the ureters and flows into the bladder. The bladder walls (detrusor muscle) are relaxed, and the increased quantities of urine do not push the pressure inside upward.

Sphincter Control: The internal urethral sphincter is an involuntary muscle and the external urethral sphincter is a voluntary muscle and the membranes will remain contracted and prevent the leakage of urine. It also helps in urinary continence, allowing the urine to collect within the bladder until an appropriate time for voiding.

Nervous System Involvement: The sympathetic nervous system is most active during the storage phase. It constantly stimulates the detrusor muscle to be relaxed until such time that storage is no longer appropriate and the internal urethral sphincter stays contracted. The frontal lobe of the brain and the brain stem continuously inhibit the urge to urinate until it is appropriate to do so.

Voiding Phase (Micturition Reflex)

Detection of Bladder Fullness: As the bladder meets a particular level of fullness, stretch receptors within the wall of the bladder sense this increase in volume. A signal from the stretch receptors is transmitted via the pelvic nerves to the spinal cord and then to the brain, notifying the central nervous system that it is now full.

Urge to Urinate: When the brain receives the signals, it then becomes apparent that there is a need for urination. The urge usually becomes stronger as the bladder continues filling, along with the activation of stretch receptors.

Brain Response: When urination is necessary, the brain sends impulses through the parasympathetic nervous system to the bladder and the urethra. The parasympathetic nerves stimulate the contraction of the detrusor muscle and the relaxation of the internal urethral sphincter, hence preparing the body to urinate.

Voluntary Control: The external urethral sphincter is controlled by the will and relaxes during this stage, thus urine flows from the bladder through the urethra and out of the body.

Muscle Actions: It ensures smooth and complete emptying of the bladder through a coordinated series of muscle actions that include contraction of the detrusor muscle and relaxation of both the internal and external urethral sphincters.

Disorders Related to Micturition

The common disorders related to micturition are:

Urinary Incontinence

Types and Causes: Stress incontinence, urge incontinence, and overflow incontinence

Treatments Available: Behavioral therapies, medications, surgery

Urinary Retention

Causes and Symptoms: Blockages, nerve problems and weak bladder muscles

Treatments Available: Catheterization, medications, surgery

Overactive Bladder

Symptoms and Management: Increased frequency, urgency, drugs, and behavioural therapy.

Diagnostic Tools and Tests

Urinalysis: Analysis of the urine composition.

Urodynamic Studies: The study of the process of functioning with storing and passing out urine through the bladder and the urethra.

Imaging Techniques: Some are imaging studies like ultrasound, MRI, and CT that represent the existence of the structure of urinary tract passages.

Treatment and Management

Bladder Training

Techniques of increasing the capacity of the bladder.

Pelvic Floor Exercises

Strengthen muscles governing urination.

Pharmaceuticals

Medicines are commonly prescribed with their mechanisms.

Surgical Interventions

Their indication, procedures, success rates, and risks.

Also Read:

Urine Formation and Osmoregulation	Counter Current Mechanism
Dialysis	Disorders of the Excretory System
Glomerular Filtrationion Rate	Ammonotelism

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Frequently Asked Questions (FAQs)

1. What is Micturition?

It is the process where urine is expelled from the urinary bladder.

2. What is Neural Control of Micturition?

The neural control of micturition is provided by the micturition reflex, the conductance that carries the signal between the bladder and the brain.

3. What are some common disorders associated with micturition, and what are their treatments?

Common disorders include incontinence of urine, urinary retention, and overactive bladder, treated by behavioural therapies, medications, and sometimes surgery.

4. What are commonly used diagnostic tests in the assessment of problems associated with micturition?

Nowadays, urinalysis, urodynamic studies, and imaging techniques with ultrasound and MRI find everyday applications.

5. What are recent developments on the therapeutic front - treatment of micturition disorders?

New drugs, new minimally invasive surgical procedures; many clinical trials are still on.

6. Describe the process of urine formation in kidney

Urine formation in the kidneys involves three main steps: filtration, reabsorption, and secretion. Blood is filtered in the glomerulus to remove waste and excess substances, useful materials are reabsorbed into the bloodstream, and additional waste products are secreted into the tubules, forming urine that collects in the renal pelvis for excretion.

7. What is the role of pelvic floor muscles in micturition?

Pelvic floor muscles play a crucial role in micturition. They support the bladder and urethra, and their contraction helps maintain continence by keeping the urethra closed. During urination, these muscles relax to allow urine flow. Weak or dysfunctional pelvic floor muscles can lead to urinary incontinence or difficulty emptying the bladder completely.

8. How does the cystourethrogram help in understanding micturition problems?

A cystourethrogram is an X-ray examination of the bladder and urethra during urination. It helps visualize the anatomy and function of the lower urinary tract, revealing issues like vesicoureteral reflux, urethral strictures, or incomplete bladder emptying. This test is valuable for diagnosing structural or functional problems affecting micturition.

9. How does pregnancy affect micturition?

Pregnancy can significantly affect micturition. As the uterus grows, it puts pressure on the bladder, reducing its capacity and leading to more frequent urination. Hormonal changes can also increase urine production. Additionally, pregnancy and childbirth can weaken pelvic floor muscles, potentially leading to stress incontinence.

10. What is residual urine and why is it important?

Residual urine is the amount of urine left in the bladder after urination. A small amount (less than 50 ml) is normal, but larger amounts can indicate problems with bladder emptying. High residual urine volumes can lead to urinary tract infections, bladder stones, or kidney problems, making it an important diagnostic measure.

11. How do benign prostatic hyperplasia (BPH) and micturition relate?

Benign prostatic hyperplasia (BPH), or enlarged prostate, can significantly affect micturition in men. As the prostate enlarges, it can compress the urethra, making it difficult to start urination, maintain a strong stream, or empty the bladder completely. This can lead to symptoms like frequent urination, urgency, and nocturia. In severe cases, BPH can cause urinary retention, requiring medical intervention.

12. What happens if the micturition reflex is damaged?

Damage to the micturition reflex can lead to various urinary problems. These may include urinary incontinence (inability to control urination), urinary retention (inability to empty the bladder completely), or frequent urination. The specific symptoms depend on where in the reflex pathway the damage occurs.

13. How does aging affect micturition?

Aging can affect micturition in several ways. The bladder may become less elastic, reducing its capacity. Weakened pelvic floor muscles can lead to incontinence. Prostate enlargement in men can obstruct urine flow. Additionally, neurological changes can affect bladder control and sensation.

14. What is nocturnal enuresis and how does it relate to micturition?

Nocturnal enuresis, commonly known as bedwetting, is involuntary urination during sleep. It occurs when the brain doesn't respond to signals from the full bladder during sleep. This condition is common in children and usually resolves with age as the nervous system matures and nighttime bladder control improves.

15. How do urinary tract infections (UTIs) affect micturition?

Urinary tract infections can significantly affect micturition. They often cause frequent urges to urinate, even when the bladder isn't full. UTIs can also cause a burning sensation during urination, difficulty starting urination, or inability to empty the bladder completely. These symptoms occur due to inflammation of the urinary tract.

16. How does caffeine consumption affect micturition?

Caffeine is a diuretic, meaning it increases urine production. It also stimulates bladder muscle contractions. As a result, consuming caffeine can increase the frequency and urgency of urination. This is why people often need to urinate more frequently after consuming coffee or other caffeinated beverages.

17. How does the nervous system control micturition?

The nervous system controls micturition through a complex interplay between the autonomic and somatic nervous systems. The micturition reflex is initiated when stretch receptors in the bladder wall are activated, sending signals to the spinal cord and brain. The brain then decides whether it's appropriate to urinate and sends signals back to either inhibit or facilitate bladder emptying.

18. How does the urinary sphincter work in micturition?

The urinary sphincter is a ring of muscle that controls the flow of urine. There are two sphincters: the internal sphincter (controlled involuntarily) and the external sphincter (under voluntary control). During micturition, both sphincters relax to allow urine to flow out of the bladder and through the urethra.

19. What role does the detrusor muscle play in micturition?

The detrusor muscle is the main muscle of the bladder wall. During the filling phase, it relaxes to allow the bladder to expand. During micturition, it contracts to expel urine from the bladder. The coordination between the detrusor muscle and the urinary sphincters is crucial for proper bladder function.

20. How does the bladder know when it's full?

The bladder has stretch receptors in its walls that detect when it's expanding. When the bladder reaches about half its capacity, these receptors send signals to the brain, creating the first urge to urinate. As the bladder continues to fill, the urge becomes stronger.

21. What is the micturition reflex?

The micturition reflex is the involuntary neural mechanism that triggers urination. It's initiated when stretch receptors in the bladder wall are activated by filling. This sends signals to the spinal cord and brain, which can then trigger bladder contraction and sphincter relaxation if urination is appropriate.

22. What is the normal bladder capacity and how does it vary?

The normal adult bladder capacity is about 400-600 ml, but it can vary widely between individuals. The first urge to urinate usually occurs when the bladder contains about 150-200 ml. Factors like age, body size, and medical conditions can affect bladder capacity.

23. What is urinary continence and how is it maintained?

Urinary continence is the ability to control urination. It's maintained by the coordinated action of the bladder muscles, urinary sphincters, and nervous system. The sphincters remain contracted while the detrusor muscle is relaxed, preventing involuntary urine leakage.

24. What is the role of antidiuretic hormone (ADH) in micturition?

Antidiuretic hormone (ADH) doesn't directly control micturition, but it affects urine production. ADH increases water reabsorption in the kidneys, reducing urine volume. This indirectly affects micturition by influencing how quickly the bladder fills.

25. What is the role of the parasympathetic nervous system in micturition?

The parasympathetic nervous system is primarily responsible for bladder emptying. When activated, it causes the detrusor muscle to contract and the internal urethral sphincter to relax, initiating micturition. This system is crucial for the voiding phase of the micturition cycle.

26. How do diuretics affect micturition?

Diuretics increase urine production by promoting water and salt excretion by the kidneys. This leads to more frequent urination as the bladder fills more quickly. While diuretics don't directly affect the micturition reflex, they significantly impact the filling phase of the micturition cycle.

27. What are the main stages of micturition?

The main stages of micturition are: 1) Filling phase - where urine accumulates in the bladder, 2) Storage phase - where the bladder expands to accommodate more urine, 3) Voiding phase - where the bladder contracts and urine is expelled, and 4) Empty phase - where the bladder is completely emptied.

28. What is the difference between voluntary and involuntary micturition?

Voluntary micturition is when a person consciously decides to urinate and relaxes the external urethral sphincter. Involuntary micturition occurs when the bladder empties without conscious control, such as in infants or in some medical conditions. As we develop, we gain the ability to voluntarily control micturition.

29. How does the sympathetic nervous system contribute to micturition?

The sympathetic nervous system plays a role in urine storage. It relaxes the detrusor muscle and contracts the internal urethral sphincter, promoting urine retention in the bladder. During micturition, sympathetic activity decreases to allow bladder emptying.

30. How does the brain control the timing of micturition?

The brain, particularly the pontine micturition center in the brainstem, receives signals from the bladder about its fullness. It then integrates this information with other factors (like social appropriateness) to decide whether to initiate micturition. If it's not an appropriate time, the brain can suppress the micturition reflex.

31. How does the position of the body affect micturition?

Body position can affect micturition. Standing or sitting upright uses gravity to help empty the bladder more completely. Lying down can make it more difficult to fully empty the bladder. This is why urologists often recommend urinating while standing or sitting for better bladder emptying.

32. What is micturition and why is it important?

Micturition, also known as urination, is the process of emptying the urinary bladder. It's important because it helps eliminate waste products and excess water from the body, maintaining fluid balance and removing potentially harmful substances.

33. What is nocturia and how does it relate to normal micturition patterns?

Nocturia is the need to wake up one or more times during the night to urinate. While occasional nocturia can be normal, frequent nocturia disrupts normal sleep patterns and can indicate underlying issues. Normal micturition patterns typically allow for uninterrupted sleep, with the bladder able to store urine produced overnight until morning. Nocturia can be caused by excessive fluid intake before bed, sleep disorders, or medical conditions affecting urine production or bladder function.

34. What is the difference between polyuria and frequent urination?

Polyuria refers to excessive urine production (typically more than 3 liters per day), while frequent urination (or frequency) refers to urinating more often than usual. Polyuria usually leads to frequency, but frequency can occur without polyuria if the bladder is sensitive or irritated, causing the urge to urinate even when not much urine is present.

35. What is meant by "toilet training" in the context of micturition?

Toilet training is the process of teaching young children to control their bladder and bowel movements and use the toilet. In terms of micturition, it involves helping children recognize the sensation of a full bladder, voluntarily hold urine, and consciously initiate micturition at appropriate times and places. This process represents the development of voluntary control over the micturition reflex.

36. What is the significance of post-void residual volume in assessing micturition?

Post-void residual (PVR) volume is the amount of urine left in the bladder after urination. It's an important measure in assessing micturition efficiency. A high PVR (typically >50-100 ml) can indicate incomplete bladder emptying, which may be due to bladder muscle weakness, nerve problems, or urinary tract obstruction. Consistently high PVR volumes increase the risk of urinary tract infections and can lead to bladder and kidney problems over time.

37. What is the difference between stress incontinence and urge incontinence?

Stress incontinence is urine leakage that occurs with physical stress on the bladder, such as coughing, sneezing, or exercising. It's often due to weakened pelvic floor muscles. Urge incontinence, on the other hand, is a sudden, intense urge to urinate followed by involuntary loss of urine. It's typically caused by overactive bladder muscles or neurological issues.

38. What is the role of urodynamic studies in assessing micturition disorders?

Urodynamic studies are a group of tests that evaluate how well the bladder, urethra, and sphincters store and release urine. These tests measure factors like bladder pressure, urine flow rate, and sphincter activity during filling and voiding. They're crucial for diagnosing complex micturition disorders and guiding treatment decisions.

39. How does spinal cord injury affect micturition?

Spinal cord injuries can severely disrupt micturition by interrupting the neural pathways between the brain and the bladder. Depending on the level and completeness of the injury, this can lead to various problems such as urinary retention, incontinence, or detrusor-sphincter dyssynergia (uncoordinated contraction of the bladder and sphincter muscles).

40. How do anticholinergic medications affect micturition?

Anticholinergic medications can significantly affect micturition by blocking the action of acetylcholine, a neurotransmitter involved in bladder contraction. These drugs relax the detrusor muscle, increase bladder capacity, and reduce the frequency and urgency of urination. They're often used to treat overactive bladder but can sometimes lead to urinary retention as a side effect.

41. How does chronic constipation affect micturition?

Chronic constipation can affect micturition in several ways. The buildup of stool in the rectum can put pressure on the bladder, reducing its capacity and potentially leading to frequent urination or urgency. Straining during bowel movements can weaken pelvic floor muscles over time, potentially contributing to stress incontinence. Additionally, the nerves controlling bowel and bladder function are closely related, so dysfunction in one area can affect the other.

42. What is the role of cystometry in understanding micturition disorders?

Cystometry is a urodynamic test that measures bladder pressure and volume during filling and emptying. It helps assess bladder function, detrusor muscle activity, and bladder compliance. This test is crucial for diagnosing conditions like overactive bladder, neurogenic bladder, or detrusor underactivity. By providing detailed information about bladder behavior, cystometry helps clinicians understand the underlying causes of micturition disorders and guide treatment decisions.

43. How does diabetes mellitus affect micturition?

Diabetes mellitus can affect micturition in several ways. High blood sugar levels can increase urine production (polyuria), leading to frequent urination. Over time, diabetes can damage nerves controlling the bladder (diabetic cystopathy), potentially causing difficulties in sensing bladder fullness or emptying the bladder completely. Diabetes also increases the risk of urinary tract infections, which can further disrupt normal micturition patterns.

44. How do different types of urinary catheters affect the micturition process?

Urinary catheters bypass the normal micturition process by directly draining urine from the bladder. Indwelling catheters (like Foley catheters) remain in place and continuously drain urine, eliminating the need for normal micturition. Intermittent catheters are inserted and removed several times a day, allowing for scheduled bladder emptying. While catheters can be necessary for managing certain conditions, long-term use can affect bladder muscle tone and increase the risk of urinary tract infections.

45. What is the relationship between fluid intake and micturition frequency?

Fluid intake directly affects micturition frequency. Increased fluid consumption leads to greater urine production and more frequent urination. Conversely, reduced fluid intake decreases urine volume and frequency. However, the relationship isn't always linear due to factors like the type of fluid consumed (e.g., caffeine can increase frequency more than water) and the body's homeostatic mechanisms that regulate water balance.

46. How does vesicoureteral reflux impact the micturition process?

Vesicoureteral reflux (VUR) is a condition where urine flows backward from the bladder into the ureters and sometimes to the kidneys. While it doesn't directly affect the act of micturition, VUR can lead to recurrent urinary tract infections and kidney damage. In severe cases, it may cause increased urinary frequency or urgency due to bladder irritation. Treatment of VUR is important to prevent long-term complications that could affect overall urinary system health.

47. What is the role of urethral pressure in maintaining continence between micturition events?

Urethral pressure plays a crucial role in maintaining urinary continence between micturition events. The urethra normally maintains a higher pressure than the bladder, creating a pressure gradient that keeps urine in the bladder. This is achieved through several mechanisms, including the internal and external urethral sphincters, the supportive structures around the urethra, and the mucosal seal of the urethral lining. When urethral pressure falls below bladder pressure, urine leakage can occur.