Anatomy of the Renal Tubule

The Renal Tubule

After filtrateThe fluid that is filtered from the blood into the nephron and will eventually become urine. is formed in the renal corpuscleThe structure in the nephron that consists of the glomerulus and Bowman’s capsule, where filtratio, it travels through the renal tubule. It then exchanges with the second capillary bed on the nephronThe functional unit of the kidney that filters blood and produces urine.. Remember that this capillary bed can be a peritubular capillary bed as pictured on the left. Alternatively, it can be a vast recta for a juxtamedullaryRefers to nephrons or structures located near the boundary between the renal cortex and medulla. nephron as pictured on the right. The pictures on the right of this slide depict how nephrons really look. For the remainder of this class, we will use a picture like the left one. This picture is not accurate to what nephrons look like in the kidney. This picture on the left is used for teaching about the renal tubule. It stretches the renal tubule out, allowing us to tell a story from left to right. Note how a nephron in reality kind of swings back on itself here at the nephron loop. Keep this in mind as we continue to draw nephrons. We will also talk about how filtrate is adjusted to make urineThe liquid waste excreted by the kidneys..

We are also going to stretch out a capillary for a better drawing.  This allows us to diagram the exchange that happens between the capillary and the nephron.  But, again, remember that these pictures with the capillary bed twisting and turning around the nephron are more accurate.  I like this diagram because it has the arcuate arteryArteries that curve over the base of the renal pyramids, supplying blood to the kidney cortex. and vein and even the cortical radiate arteriesSmall arteries that branch from the arcuate arteries and supply blood to the cortex. and venulesSmall veins that collect blood from capillaries and transport it to larger veins..  If you trace the route of the blood capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. on either nephron with your finger, you will enter the glomerulusA network of capillaries in the nephron where blood filtration occurs.. You will use the afferent arterioleThe small artery that carries blood into the glomerulus of the nephron.. Then, the peritubular capillary will lead back to the cortical radiate venule.  Same with this juxtamedullary nephron. Its capillary bed, called a vasa rectaCapillaries surrounding the loop of Henle in juxtamedullary nephrons that help maintain the medullar, is fed by the efferent arterioleThe small artery that carries blood away from the glomerulus after filtration. leaving the glomerulus. Then, it exchanges with the nephron. Finally, it is drained by this cortical radiate venule.

Parts Of The Renal Tubule

As filtrate flows through the renal tubule, it is exchanging substances with the capillary bed that surrounds it.  As filtrate flows through the tubules, the cell types change. The tubule’s ability to exchange certain substances also changes.  I feel confident in saying that all these cellsThe basic structural and functional units of life. are cuboidal. However, in this part of this loop, the tubule is made of squamous cells.

 This squiggly portion is called the proximal convoluted tubuleThe first part of the nephron tubule where most reabsorption of water, ions, and nutrients occurs.. It is near or proximalCloser to the point of attachment or origin. to the renal corpuscle. It is also curvy or convoluted.  Notice how these simple cuboidal cells of the PCT have villiFinger-like projections in the small intestine that increase surface area for absorption. on them.  We have seen villi before in the small intestine. It was adapted for a large surface area. This allowed for nutrients to be exchanged. 

After filtrate flows through the PCT, it enters into the nephron loop. This loop was once called the loop of Henle. We will call it the nephron loop. There is a descending loopThe portion of the nephron loop that is permeable to water but not solutes, leading to water reabsor on the side of the PCT and the ascending loopThe portion of the nephron loop that actively transports sodium and chloride out, making the medulla on this other side.  Notice how both sides of the loop have a thin and a thick section A cut or slice of the body or an organ for study..  This is due to the cells.  The thick section is made up of cuboidal cells and the bottom of the loop is made of squamous cells.  Remember that the nephron loops of juxtamedullary nephrons dip down into the medullary pyramids of the kidney. In contrast, cortical nephrons are almost entirely kept in the cortex of the kidney. 

As filtrate flows up this ascending arm, it enters into the DCT or the distal convoluted tubuleThe part of the nephron where final adjustments to ion and water balance occur under hormonal contro.  Notice how the diameter of the tubule increases in the DCT.  The cells also have villi, however not as many as the PCT.  The final part of the nephron tubule is the collecting ductA duct in the nephron that collects urine from multiple nephrons and adjusts water reabsorption..  This has an even large diameter because it collects filtrate from numerous nephrons.  The collecting duct dips into the medullary pyramid. It extends all the way to the renal papillaThe tip of a renal pyramid where urine drains into the minor calyx.. Here, urine weeps into the minor calyxA small cavity in the kidney where urine from a renal pyramid collects before moving to the major ca.  The collecting duct has two populations of cells. These cells have two very different purposes concerning pHA measure of hydrogen ion concentration in a solution.. They are also influenced by hormones on urine composition.

Processes of Urine Formation

Let’s very simply define the three processes that are required to make urine.  Let’s do this before we make it more complicated!  Remember that blood flowing through the kidney will encounter two capillary beds.  The first capillary bed is in the renal corpuscle.  The second capillary bed is either a peritubular capillary or a vasa recta, depending on the type of nephron.  It’s easy to see the first capillary bed, called the glomerulus. You can observe that it is fed by an afferent arteriole. It is drained by an efferent arteriole.  This efferent arteriole transitions into being the peritubular capillary or the vasa recta.  The capillary bed then twists, turns, and hugs the nephron, all the while exchanging with it.

When blood enters the glomerulus it is immediately filtered, separating everything solely by size.  Things larger than albuminA plasma protein that helps maintain osmotic pressure and transport substances. go to the efferent arteriole. Things smaller than albumin go to the capsular spaceThe space between the glomerular capsule and the glomerulus where filtrate collects. around the glomerulus. The capsular space is drained by the PCT.  This is really no different than the filtrationThe process by which fluid moves out of capillaries into surrounding tissues due to hydrostatic pre that goes on in the arteriole side of any capillary bed.  Substances are being pushed out of the glomerulus and into the capsular space via filtration.

As the filtrate flows in the nephron toward the collecting duct, blood flows in the capillary bed. The blood is headed to the cortical radiate venule. This venule drains the peritubular capillary or vasa recta.  As filtrate and blood flow past each other, they are allowing exchange between the filtrate and the blood.  The process that we would expect, reabsorptionThe process of fluid moving back into capillaries from surrounding tissues due to colloid osmotic p moves items from the tubule to the capillary.  This is similar to the reabsorption we observed on the venule side of capillary beds. It is driven by that colloid osmotic pressureThe pressure exerted by proteins (mainly albumin) in the blood that pulls water into the capillaries of the salty blood in the capillary. 

However, there is one more process present here in the kidney.  This is the process that moves things from the capillary bed to the renal tubule.  No, it’s not filtration, we already did that in the glomerulus.  This process is called secretionThe process of moving substances from the blood into the nephron tubule to be excreted in urine. and is specific to these capillary beds.  I think of secretion as the way to get rid of large things I don’t want.  In the glomerulus, large substances like toxins stay in the blood. They are too big to filter out.  But, I don’t want those toxins in my blood.  The kidney gets a second chance to remove those toxins here in the tubule using secretion.