Urine, as most of us know, forms in the kidneys. The kidneys contain millions of tiny functional units called nephrons. These perform filtration and concentrate the urine.
Everything, however, starts with the glomerulus, which is a tuft of capillaries through which waste products are filtered out.
Renal Circulation
- Blood that’s ready to be filtered comes in through the renal artery, then splits in the afferent arteriole.
- This then goes through the glomerulus and undergoes glomerular filtration
- “Unwanted” stuff goes into Bowman’s Capsule and the rest of blood keeps circulating
- The rest of the blood comes out of the glomerulus through the efferent arterioles through the peritubular capillaries and vasa recta
- Finally, these join up in the Renal Vein and goes into the Circulatory System
This work by Cenveo is licensed under a Creative Commons Attribution 3.0 United States (http://creativecommons.org/licenses/by/3.0/us/)
Glomerular Filtration
The glomerular capillary walls are designed like a sieve with 3 layers
- Layer 1: Endothelium
- Relatively large pores
- Allows solutes, plasma proteins, and fluids can pass through
- Filters out blood cells
- Layer 2: Basement membrane
- Made up of 3 layers and fused with the endothelial layer
- Prevent plasma proteins from being filtered out
- Solutes and fluids make it through
- Layer 3: Epithelium
- Contains podocytes that attach to the basement membrane by foot processes
- Wrap around the capillaries and leave slits known as filtration slits between them
- Thin diaphragm between the slits acts as a final filtration barrier
- Only allows smallest proteins and small solutes like glucose, ions, and urea
Fluid Movement through Glomerulus
- Governed by hydrostatic pressure and osmotic pressure
Fate of the Filtrate
This is where the nephrons come in. Glomerular filtration is efficient, but imperfect. It essentially sorts everything by size.
- If it’s small enough to make it through the Bowman’s Capsule, it will.
- While big substances like blood cells stay in the blood, some necessary things like ions and even water can make it into the filtrate
- The nephron’s job is to now send the needed things back into the blood (ie peritubular capillaries and vasa recta).