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 somatic and autonomic nervous systems. Such a process is necessary for ridding the body of waste products and excess fluid by means of maintaining homeostasis.

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.

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.

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.

Composition of Human Urine

A typical adult excretes 1 to 1.5 L of urine 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: Behavioral therapies, medications, surgery

Urinary Retention

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

Treatment: Catheterization, medications, surgery

Overactive Bladder

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

Diagnostic Tools

Diagnostic tools include:

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

Treatment and management includes:

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.

Micturition NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Mechanism of Micturition

• Disorders related to Micturition

Practice Questions for NEET

Q1. Which specific action occurs in the urinary system during micturition?

1. Urethra relaxes

2. Ureter contracts

3. Ureter relaxes

4. Urethra contracts

Correct answer: 1) Urethra relaxes

Explanation:

When the bladder is full and ready to release urine, the relaxation of the urethra is crucial. It allows the urethral muscles to loosen and open up, creating a wider pathway for the urine to flow from the bladder to the outside of the body. This relaxation of the urethra is coordinated with the contraction of the bladder muscles to create a coordinated and controlled release of urine. Without the relaxation of the urethra, it would be difficult for urine to pass through and be expelled from the body.

Hence, the correct answer is option 1) Urethra relaxes.

Q2. What will happen if the stretch receptors of the urinary bladder wall are removed?

1. Micturition will continue

2. Urine will continue to collect normally in the bladder

3. There will be no micturition

4. Urine will not collect in the bladder

Correct answer: 1) Micturition will continue

Explanation:

Steps involved in micturition
This signal is initiated by the stretching of the urinary bladder as it gets filled with urine. The stretch receptors in the bladder wall detect the increase in volume and send signals to the brain, signaling the need to urinate. The brain then sends signals to the detrusor muscle to contract, while the external sphincter muscle relaxes, allowing urine to be expelled. If the stretch receptors are removed from the bladder, the brain will not receive the signals to trigger urination, leading to a loss of control over the micturition process.

Hence, the correct answer is option 1) Micturition will continue.

Q3. The counter-current mechanism involves ____?

1. Vasa recta and PCT

2. Vasa recta and DCT

3. Vasa recta and loop of Henle

4. Vasa recta and collecting duct

Correct answer: 3) Vasa recta and loop of Henle

Explanation:

The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of the vasa recta is also in a counter-current pattern.

This mechanism helps to maintain a concentration gradient in the medullary interstitium, thereby concentrating the filtrate( urine).

Hence, the correct answer is option 3) Vasa recta and loop of Henle.

Also Read:

• MCQs on Micturition

• Human Excretory System

• Ultrafiltration

Recommended Video on Micturition