Urinary bladder is constitutes the excretory system along with a pair of kidneys, a pair of ureters and urethra. This is today's topic for notes of biology class 12 .
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| Human Excretory System diagram showing Urinary Bladder |
It is a large pear shaped sac. It has thin and distensible walls. It is found in the pelvic region in the abdominal cavity of humans.The transitional epithelium of its inner lining makes it distensible. It has a well developed muscular layer which is known as detrusor muscle.It has three layers of muscles which are smooth. These are made up of -
1.Longitudinal fibres (inner and outer layers)
2.Circular fibres (middle layer)
The urinary bladder is divided into two parts. The upper part of the bladder is called its body.It is broad. It helps in the storage of urine. The lower part of the bladder is called its neck.It is narrow.It leads into the urethra.
Trigone or trigonum vesical is a smooth triangular area which is present at the base of the bladder. It lies between the openings of ureters and urethra. The bladder generally remains closed with the help of two sphincters present on its neck .The contraction of these two sphincters closes the bladder while they relax to open the bladder when urine has to be passed out. Out of these two sphincters, the outer sphincter is voluntary and is made up of skeletal muscles while the inner sphincter is involuntary and is made up of smooth muscles. The urinary bladder is innervated by both sympathetic and parasympathetic muscles. Urinary bladder helps in the storage of urine. Normally it can store about 0.5 to 1 litre of urine.
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