Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson today on secretion. Today we will be taking a look at the process of secretion and its role in urine formation. Urine formation, as a whole, takes place in the kidneys. The nephrons of the kidneys will filter blood and produce urine.
There's three steps to urine formation that ensure only necessary products are excreted from the body. Secretion is actually that third step in urine formation. We first have filtration, which occurs here in the Bowman's Capsule. Then we have reabsorption, which occurs in the proximal tubule. And then secretion is the third step that we're going to talk about today.
Basically, in secretion, it starts at the proximal tubule, and then it'll occur at the other tubular parts of the nephron as it moves through. Substances such as hydrogen, potassium and urea at this point will move from the peritubular capillaries, here. They will actually move into the nephron, and join with urine that's already forming.
In reabsorption, water and extra solutes that haven't been reabsorbed back into the blood will become a part of urine. Then that hydrogen, potassium, and urea from the peritubular capillaries will join with that, forming urine, and move towards the collecting duct, which will then move the forming urine into the ureter, and into the urinary bladder.
So basically, secretion helps to maintain the body's acid base balance, and make sure that substances don't build up in the blood. As I mentioned, it starts at the proximal tubule and will work its way down through the Loop of Henley. And then that urine will move into the collecting duct here.
And then the collecting duct will deliver that urine to the renal pelvis of the kidney, and then it will be funneled into the ureter. If we take a look at this diagram over here to help explain a little bit better.
So we have the kidney. The kidney is where the nephrons are located within the kidney. The nephrons produce the urine. They deliver it to the collecting duct. The collecting duct moves it to the renal pelvis of the kidney, and then that funnels it into the ureter here. And then the ureter will funnel that urine down to the urinary bladder.
And it will be stored in the urinary bladder until it's excreted through the urethra. The urethra basically connects the urinary bladder to the outside, as urine is excreted. When urine is expelled from the urinary bladder through the urethra to the outside, we call that urination. And that's a reflex response.
So basically what happens is, as the bladder starts to fill with urine that it's storing, the tension across the muscles of the urinary bladder will increase, and then the internal sphincter-- we have a little internal sphincter here-- will relax. The bladder will then basically contract, and urine will be forced out through the urethra. So that's how the process of urination happens. This lesson has been an overview on the third step of urine formation, called secretion.