Micturition and the nature of urine · Biology Class 11–12 | StemPy Lab
Course 15.3·Lesson 1 of 3
Micturition and the nature of urine
Urine from the nephrons runs through the ureters into the bladder, where it is stored until a voluntary signal from the brain empties it. This lesson explains the micturition reflex and what the colour, the volume and the contents of urine can tell a doctor.
about 25 min1 simulation1 practice question
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Where the urine goes
The nephrons pour their urine into the renal pelvis. It runs down the ureter to the urinary bladder, where it is stored. The bladder is a muscular bag. Its wall is made of smooth muscle, so it can stretch as it fills. This part of the chapter, micturition, is not in the NEET syllabus text. NCERT prints it, and school boards ask it.
Micturition
The urine stays in the bladder until the central nervous system (CNS) gives a voluntary signal. The steps follow.
The bladder fills, and its wall is stretched.
Stretch receptors in the wall of the bladder send signals to the CNS.
The CNS passes motor messages back to the bladder.
The smooth muscles of the bladder contract, and the urethral sphincterrelaxes at the same time.
The urine is released through the urethra.
The release of urine is called micturition. The neural mechanism that causes it is the micturition reflex.
A diagram of the micturition reflex. A box for the central nervous system is at the top. Two kidneys are at the sides, and a thin ureter runs down from each to a round urinary bladder in the middle. A urethra leaves the bladder at the bottom, with a ring of muscle, the urethral sphincter, at its start. Small green dots in the wall of the bladder are stretch receptors. A dashed green arrow runs from the bladder up to the central nervous system and is labelled as the sensory signal that the bladder is stretched. Red arrows run from the central nervous system down to the muscle of the bladder and to the sphincter, and are labelled as the motor signal: the bladder muscle contracts and the sphincter relaxes.
What urine is like
An adult human passes on average 1 to 1.5 litres of urine a day. A healthy urine has these properties.
It is a light yellow, watery fluid.
It has a characteristic odour.
It is slightly acidic, with a pH of about 6.0.
About 25 to 30 g of urea are excreted each day.
Many conditions can change the urine. A doctor tests the urine to find out about the body. This is the analysis of urine. It helps in the diagnosis of many metabolic disorders and of the malfunctioning of the kidney.
Finding in the urine
Name
What it suggests
Glucose
glycosuria
diabetes mellitus
Ketone bodies
ketonuria
diabetes mellitus
A very large volume of dilute urine, without glucose, is a sign of a lack of ADH. This is diabetes insipidus, which you met in the second course.
An extra. In physiology books, the urine can also be tested for protein and for blood. Protein or blood in the urine can point to a disease of the kidney, such as the inflammation of the glomeruli. This is not in NCERT.
The rate of the filling
The urine forms in the kidneys all the time, at about the same rate. So the bladder fills slowly through the day. The next simulation shows how long it takes to collect a chosen volume.
You should find that a larger volume takes longer to collect, and that a person who forms more urine in the day fills the bladder sooner.
Try it in Python
The program works out the rate at which urine forms and the volume in the bladder at different hours.
Practise with new numbers
Common mistakes
Saying that the bladder makes urine. It only stores it. The nephrons make the urine.
Saying that the ureter stores urine. It carries the urine from the kidney to the bladder.
Saying that the urethral sphincter contracts to release the urine. It relaxes. The bladder muscle contracts.
Saying that the urine of a healthy person is alkaline. It is slightly acidic.
Saying that glucose in the urine is normal. It points to diabetes mellitus.
Remember these
Micturition: release of urine. The neural mechanism is the micturition reflex.
Stretch receptors send signals to the CNS. The CNS sends motor messages: the bladder muscle contracts, and the urethral sphincter relaxes.
Urine: 1 to 1.5 L a day, light yellow, slightly acidic (pH about 6.0), 25 to 30 g of urea a day.
Glycosuria and ketonuria point to diabetes mellitus.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Name the process of the release of urine, and the mechanism that causes it.
What starts the micturition reflex?
What happens to the bladder muscle and to the urethral sphincter in micturition?
Give the volume, the pH and the urea of the urine of an adult in a day.
What do glycosuria and ketonuria suggest?
References
NCERT (2023). Biology, Textbook for Class XI, chapter 16, Excretory Products and Their Elimination.
Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 25, The Urinary System.
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