Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson on filtration. In this lesson today, we will be discussing filtration and its role in urine production. So filtration is the first step in urine production. And the process of urine formation ensures that only necessary products are excreted from the body. And these steps of urine formation occur in the nephrons, which are found in the kidneys. So these nephrons filter blood, and they filter water and solutes out of the blood, and then help to form urine.
So there's three steps to urine formation. And it includes reabsorption and secretion. So filtration's the first step, followed by reabsorption and then secretion. But today, we're going to be focusing our attention on this first step of filtration here.
So basically, right here what I have is a picture of a nephron. Now filtration takes part in only a specific part of the nephron, which is here in the Bowman's capsule. OK. So we have right here our afferent arterioles.
So basically, what happens in filtration is that these afferent arterioles here are very, very narrow. So what happens is that as blood is delivered towards the nephron through the afferent arterioles, it's actually delivered at a high pressure because of how narrow these afferent arterioles are. So this high pressure will force water and solutes from the blood-- from the blood in these afferent arterioles-- into the Bowman's capsule of the nephron, which is this bulb structure right here.
So the solutes that it could force out of the blood are things like glucose, sodium, urea, vitamins, amino acids, et cetera. So they're small solutes that are being forced out of the blood and into the Bowman's capsule because of this high pressure that they're delivered at. So the water and solutes that are pushed out of the blood would then flow into the proximal tubule, which is this area right here. And that's where the next stage of urine formation occurs, reabsorption. But we're not going to get into that too much in this lesson here.
So then the larger solutes in the blood, such as blood cells or platelets, they will actually end up staying in the blood, and then will follow the arteriole as it weaves around the nephron here. So they will stay in the blood and they will not be pushed out of the blood. But those smaller solutes, like the glucose, sodium, urea, vitamins, amino acids, will be pushed out of the blood and then will move into the nephron to start the next step of urine formation. So this lesson has been an overview on filtration as the first step in urine formation.