16.6. Micturition

Urine formed by the nephrons is ultimately carried to the urinary bladder where it is stored till a voluntary signal is given by the central nervous system (CNS). This signal is initiated by the stretching of the urinary bladder as it gets filled with urine.

The process of micturition begins when urine, produced in the kidneys, flows down the ureters and fills the urinary bladder. As the bladder fills, it stretches, which activates stretch receptors in its walls. These receptors then send signals to the central nervous system, alerting it that the bladder is full and it is time to release the urine.

Think of the bladder like a balloon. When you blow air into a balloon, it expands. Similarly, as the bladder fills with urine, it expands. Just as you feel the urge to release the air when a balloon is full, our body lets us know when the bladder is full and it’s time to urinate.

In response, the stretch receptors on the walls of the bladder send signals to the CNS. The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter causing the release of urine.

Once the CNS receives the signals from the stretch receptors, it activates the smooth muscles in the bladder wall, causing them to contract. This contraction generates pressure to push the urine down toward the urethra. At the same time, the sphincter muscles around the urethra relax, creating an opening through which urine can exit the body. This coordinated action is what we refer to as the micturition reflex.

Imagine starting a song at a concert. As the music begins (the signal), the band (bladder muscles) starts playing, while the sound engineer (urethral sphincters) unplugs the mic to let the music flow out. This teamwork between the band and sound engineer is like the bladder contracting and the sphincters relaxing.

An adult human excretes, on an average, 1 to 1.5 litres of urine per day. The urine formed is a light yellow coloured watery fluid which is slightly acidic (pH-6.0) and has a characteristic odour.

The output of urine averages between 1 to 1.5 liters daily, which can vary based on hydration and dietary factors. The light yellow color comes from a pigment called urochrome. The pH of urine is typically around 6.0, making it slightly acidic, which is important for limiting the growth of bacteria in the urinary tract.

Consider how the color of lemonade changes with the addition of sugar. Similarly, urine’s color can change based on what you consume. Staying hydrated makes your urine clear and light, while dehydration makes it darker, just like how adding concentrate to lemonade makes it more vibrant in color.

On an average, 25-30 gm of urea is excreted out per day. Various conditions can affect the characteristics of urine. Analysis of urine helps in clinical diagnosis of many metabolic disorders as well as malfunctioning of the kidney.

Urea is a waste product that results from protein metabolism and is typically excreted in urine. The average amount of urea excreted daily ranges from 25 to 30 grams, which can be influenced by dietary protein intake. A routine analysis of urine can reveal abnormalities, such as glucose or ketone bodies, which may indicate health issues like diabetes mellitus.

Just like a chef tastes a dish to ensure it has the right flavor profile, doctors analyze urine to check for signs of underlying health issues. For instance, if they find too much sugar in the urine (like tasting too much salt in food), it can signal that a patient might have diabetes.

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