Nephron Information

There are a few things that we haven't done together on the nephrons in lab... So, I'd like to cover those items here.

Did you know that there are about 1,000,000 nephrons in a single kidney???

First of all, we have been talking about glomeruli and tubules in a nephron. Officially, we can talk about each nephron as having two regions: the renal corpuscle and the renal tubule. The renal corpuscle is the term for the glomerular capsule plus the glomerulus (a.k.a., glomerular capillaries) within it. The renal tubule is all the rest of the nephron and includes the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.

If you take a look at this drawing (I don't have a scanner at home, so it is a bad photo... sorry!), you will see a few things that we had not talked about before. Let's start at the glomerular region. The glomerular capsule, in order to receive filtrate, is an enclosed space. This is possible because the glomerular capsule is made up of two layers of cells. The outer layer (or parietal layer) is made up of regular simple squamous epithelium; you have seen these cells in lab. nephron.jpg (22476 bytes) The inner layer (or visceral layer) is made up of bizarre cells called podocytes that are tightly apposed to the glomerular capillaries. These podocytes wrap the capillaries, but somewhat incompletely.

The podocytes are highly branched cells. Its branches are called pedicels. The pedicels of adjacent podocytes contact one another, but a bit loosely. The gaps in their contacts are referred to as slit pores. These pores facilitate glomerular filtration. This is described more in the filtration page.

We discussed in lab that the arteriole leading into the glomerulus is called the afferent arteriole to distinguish it from the arteriole that leaves the glomerulus, the efferent arteriole. Both are considered arterioles... structurally they are both arterioles rather than venules, but also, the efferent arteriole leads into the second capillary bed. This second capillary bed is actually called the peritubular capillary system. The portion of the peritubular capillaries that surrounds the loop of Henle is called the vasa recta (that is noted in the figure above). You will see that it is involved in assisting the reabsorption taking place in the kidneys.

Another feature of the nephron I have drawn is the juxtaglomerular apparatus. It turns out that every nephron has a contact region between its afferent arteriole and its distal convoluted tubule. At this contact region, the cells of both the afferent arteriole and the distal convoluted tubule are specialized. You will see that this region is important in the secretion of a hormone called renin.

explain.jpg (12580 bytes) I have numbered the tubules in the above figure, in the order in which fluid moves through the tubules. As fluid moves through, different things are occurring within the tubules and its environment in order to filter the blood. The processes that are occurring in each of these regions are noted in this listing (another figure) to the right. You can use this as a guide as you progress through the pages that follow.

Finally, your book defines two populations of nephrons: cortical nephrons and juxtamedullary nephrons. The cortical nephrons, as you would expect from their name, are almost entirely embedded within the cortex of the kidney. They have shortish loops of Henle that do not really extend far into the medulla. By contrast, the juxtamedullary nephrons have longs loops of Henle that do extend far into the medulla. The glomeruli of the juxtamedullary nephrons are typically close to the medulla (which is how these got their name). What does this matter? Why is it even mentioned that these two populations of nephrons exist? Because, in order for the kidneys to reabsorb a lot of water, the medullary pyraminds have to have a solute concentration gradient across them... this is generated by the long loops of Henle that extend all the way into the medullary pyramids! You will probably find that this statement seems vague now, but will clear up as you progress through the web pages.