 urinary bladder doesn't act like an elastic material rather it is a plastic material that means it can change in the shape. So as urine starts filling in the bladder initially there is a rise but then it stretches it relaxes causing increase in the radius of the bladder. To understand this let's consider Laplace Law. See Laplace Law says pressure in a spherical organ is equal to two times the tension on the wall divided by the radius of the spherical organ. So whenever filling is occurring inside a spherical organ we expect that it would stretch the wall if it is elastic it will tend to stretch the wall and this will create tension in the wall right and the wall by its tendency to come back to its normal shape it will cause increase in the pressure inside the spherical organ correct but in case of bladder what happens that with filling of the urine because of the relaxation of the bladder wall there is increase in the radius also. So increase in tension is counterbalanced by increase in the radius which is the denominator hence pressure does not change. So this can happen up to a certain extent until that critical level of 400 ml 450 ml is reached. So this makes the bladder apt for a storage of urine. Okay but there is one more thing you see the neck of the bladder it is very narrow that means radius is very small and it is little bit linty also. So whenever we are considering a narrow linty tube this laplace law also considers the length of the tube. So here it becomes as into L as well. So what we are saying is that because of more length and less radius here in the neck region of the bladder pressure is quite high. So lesser pressure in the urinary bladder per se and higher pressure in the neck region of the bladder will prevent the elimination of the urine. Isn't it the bladder pressure should become more than that of the neck for flow to start. So it is because of this laplace law only that this neck region of the urinary bladder acts as an internal urethral sphincter. There is no separate urethral sphincter anatomical urethral sphincter per se in urinary bladder. Well thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel Physiology Open. Thank you.