Kidney: Nephron/Uriniferous Tubule

Each kidney contains 1-4 million nephrons. 

A nephron is the functional unit of filtration. 
Each nephron has 4 main parts:

Renal Corpuscle, which can be further divided into a glomerulus and a Bowman's capsule 

Renal plasma filtration begins in the glomerulus, which is situated within the renal corpuscle. 
The glomerulus is a ball of intermeshed network of highly fenestrated capillaries that protrude from the junction of the afferent and efferent arterioles. (This junction, where the afferent arteriole divides into several glomerular capillaries, is the vascular pole of the renal corpuscle, as seen in this micrograph.) 
The 50-100 nanometer fenestrae make up the first, least discriminating part of the filter of the renal corpuscle. 
The plasma filtrate leaves the glomerulus through those fenestrae. 
It should be noted that the glomerulus is made up of endothelial cells and mesangial cells. 
The rest of the renal corpuscle is made up of Bowman's capsule. 
Bowman's capsule consists of epithelial cells that envelop the glomerular capillaries and form the outer limit of the renal corpuscle. 
 

Those cells which surround the glomerulus make up the visceral layer and those that delimit the outer edge of Bowman's space comprise the parietal layer. This capsule surrounds the capsular space, the area into which the plasma is filtered. 
The point at which the parietal layer of Bowman's capsule becomes the proximal tubule is called the urinary pole of Bowman's capsule. 
This specially-stained micrograph shows the urinary pole of the capsule opening into the proximal tubule. 

The visceral layer epithelial cells are called podocytes (Gr. pod=foot). 
The podocytes extend a network of primary and secondary processes which end as pedicels out and around the glomerulus. 
Between the pedicels, there exist slits which are about 20 nanometers wide. 
These slits are covered by a slit diaphragm, which comprises a part of the renal corpuscle's filter. 
The slit diaphragm is made mostly of an anionic sialoprotein (podocalyxin). 
The filtrate enters Bowman's space through these slits and their diaphragm. 

At higher magnification, one can appreciate how the pedicels interdigitate to create the slit pores.


2. Proximal Tubule

3. Loop of Henle, with thin and thick limbs

4. Distal Tubule 

From the distal convoluted tubule, the filtrate flows into a collecting duct. 

A nephron and the collecting duct which drains it are collectively called a uriniferous tubule.