 Urinary bladder has two main functions, storage of urine and elimination of urine. Since kidneys continuously form urine, the urinary bladder should have the characteristic to store the urine and when it is filled it should have the ability to empty itself. So, maturation is the process by which urinary bladder empties itself when it becomes filled. This function of the bladder of storage of urine and elimination of the urine is best understood by seeing a relationship between the intravacycle pressure and the intravacycle volume what we is known as systematogram. So, here in this graph x axis shows intravacycle volume in ML and the y axis shows intravacycle pressure in centimeters of water. So, you see as the urine starts filling in the urinary bladder which is depicted by increase in this volume what happens the pressure in the bladder rises a bit but afterwards the pressure is almost constant and then again after a certain value in this case 400 ml again the pressure starts rising. So, there are different components of this graph this initial rise in pressure with increase in the volume is the 1a this is the 1b and again the rise is part 2. For voiding to occur there should be increase in the intravacycle pressure and you see when is the intravacycle pressure rising here after 150 ml and here after 400 ml. So, first urge of voiding occurs at 150 ml and maturation reflex starts at around 400 ml but here this flat portion depicts the storage ability of the urinary bladder. So, first let's see what makes this storage of urine possible in the urinary bladder. Well, it is explained by Laplace law. See 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 2 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. And the wall by its tendency to come back to its normal shape it will cause increase in the pressure inside the spherical organ. 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 lengthy also. So, whenever we are considering a narrow lengthy 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 a internal urethral sphincter. There is no separate urethral sphincter anatomical urethral sphincter per se in urinary bladder. Okay, now let's see how the mixturation will occur. See, when the bladder filling of approximately 400 ml of urine has happened, there is a stretching of the wall of the urinary bladder and this bladder wall has a stretch receptors. These stretch receptors become activated because of the stretch. Now, the affluent from these stretch receptors reach to the sacral cord at the level of S2, S3, S4 from where the efferents arise and supply the smooth muscle of the urinary bladder which we call as tetrusor muscle. So, this is the basic mixturation reflex. There is a receptor, there are afferents, there is a center which is at the level of the spinal cord and there are efferents supplying the tetrusor muscle. The nerve which is responsible for this is the pelvic nerve. So, the afferents also go via pelvic nerve and the efferents which is basically the parasympathetic nerves, they are also coming via the pelvic nerves. Also, pudental nerve arise from the sacral part of the spinal cord and supply the external urethral sphincter and keep it contracted. So, whenever there is initiation of the reflex, the activity in the parasympathetic nerve increases causing contraction of the tetrusor muscle while the activity in this pudental nerve decreases causing relaxation of the external urethral sphincter which facilitates this stretch reflex. Okay, so this is the simplest mixturation reflex but you should remember that this mixturation reflex is okay in infants but later on in life after three years of age we develop a control over this mixturation reflex. That means the voiding of the urine as well as the stopping of the mixturation can be voluntarily controlled. This is done by some higher centers which are present at the level of midbrain and ponds. So, this ponds has a mixturation center which we call as pontine mixturation center while midbrain has an inhibitory center. So, this reflex which we were talking about is under inhibitory control of this midbrain center which is which we can say as a storage center as well. So, whenever we get the urge of mixturation this storage center inhibits the reflex because the mixturation should start only when a suitable place is available for mixturation isn't it? So, it is kept inhibited by this storage center but there is another center which we spoke of that is the ponds mixturation center. So, this center actually facilitates this reflex and these centers in turn are under the inhibitory influence of the cortex. So, whenever there is a desire to mixturate the inhibitory influence on the mixturation center is removed and that on the storage center is increased. So, obviously it will increase the facilitation of the reflex and lead to voiding of the urine. On the other hand if there is a desire to delay the mixturation or stop the mixturation in that case the inhibitory influence over the midbrain storage center is removed thus increasing the storage of the urine or inhibiting the reflex. There is also a cortical influence on this pudendol which is supplying the external urethra sphincter. So, that even if the bladder pressure increases too much due to excessive filling of the urine maybe 800 ml 1 liter then voluntarily we can keep the external urethra sphincter contracted. So, that's how bladder maintains its function of storage as well as elimination of the urine. 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.