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What are the functions of the kidney?

1. Filter the blood to remove waste as urine.
2. Major homeostatic regulator of water, salt, acid-base balance.
3. Produce hormones/enzymes (epo, renin, vitamin D)
4. Regulates blood pressure.


Describe the layers of the kidneys?

The outer layer is the cortex. Then there is the medulla (medullary pyramids). In the middle is the renal pelvis. The kidney is made up of multiple lobes. The lobe is made up of epithelial tubules called nephrons.


What is a nephron?

It is the functional unit of the kidney. There are 1 million per kidney. They are comprised of a blood filter attached to a tubular epithelium.


What is the blood filter?

It is the renal corpuscle (capillary tuft enveloped by the tubule).


Describe the tubules of the nephron?

There is the proximal, thin and distal tubules - where each of them are subdivided further.


What to the tubules drain into?

Collecting ducts.


Describe the sequence of urine flow?

1. Renal corpuscle.
2. Proximal convoluted tubule.
3. Proximal straight tubule.
4. Thin descending lib.
5. Thin ascending limb.
6. Thick ascending limb.
7. Distal convoluted tubule.
8. Connecting tubule.
9. Collecting duct.


What makes up the loop of henle?

1. Proximal straight tubule.
2. Thin descending limb.
3. Thin ascending limb.
4. Thick ascending limb.


How does the nephron work?

1. Blood filtration occurs in the renal corpuscle.
2. Proximal tubule is where bulk reabsorption occurs - 65% of the filtrate is immediately reabsorbed.
3. The remaining segments fine-tune salt levels, pH and/or extract water.


Describe the histology of the renal corpuscle?

The structure in the middle is the capillary tuft. The tuft is surrounded by epithelial cells (podocytes), and the capsule on the outside is surrounded by parietal epithelial cells (squamous epithelium).


What do the podocytes and the parietal epithelium do at the vascular pole?

They are continuous with each other.


How is the blood filtered in the glomerulus?

1. Fenestrated endothelium.
2. Basement membrane.
3. Podocytes - foot processes (form interdigitations).


What is the function of the blood filter?

1. Physical barrier.
2. Charge-selective barrier: restricts things like cells from passing into our urine i.e. RBC.


Describe the glomerular capillary endothelium?

The endothelial cells are lined with a glycocalyx coat (protein that is covered in negative charges). There are holes which allow small molecules through.


Describe the basement membrane?

It is quite thick and made up of collagen and negatively charged proteoglycans. The inner core acts as a physical barrier, but the two outer layers act as chase barriers.


Describe podocytes?

There are primary processes and secondary processes come off the podocyte and they interdigitate. Between the processes there is a slit membrane (protein bridge) and the podocytes are covered in a glycocalyx coat.


Describe the slit membrane?

Transmembrane complexes from both the foot processes that forms a protein bridge. It acts as a fine filter. It too is covered in a glycocalyx coat.


What happens if there is a defect in the glomerular filter?

This will lead to the filter to be leaky. As you age podocytes decrease in number.


What is podocyte effacement?

The foot processes aren't well formed (no indigitation structure).


Describe mesangial cells?

These attach to the basement membranes, endothelial cells or to themselves. They are inside the capillary tuft. They are like smooth muscle cells, they hold the structure together and they provide a supportive/contractile role (to maintain tuft shape). They produce extracellular matrix.


What happens in glomerulosclerosis?

So much pressure and damage to pdocytes, that mesangial cells have a hold time holding it together. To compensate they secrete more extracellular matrix, over time the deposition is so sever that is causes scarring of the glomerulus - it also causes constriction of the capillaries, thus the capillaries can no longer filter properly. This disease is known as focal segmental glomerulosclerosis or it can be quite wide spread if the disease is very well developed. Could end up with renal failure or high blood pressure.


What happens in diabetic nephropathy?

Lots of scarring occurs - high purple staining.


What is the juxtaglomerular apparatus?

It is made up of:
1. Thick ascending limb - it lines close to the renal corpuscle.
2. Macula dense - these are special cells in the wall of the thick ascending limb. It senses salt concentration in the filtrate.
3. JG cells - modified smooth muscles cells in the wall of the afferent arteriole that secretes renin.

This structure makes sure that the kidney filters at the right rate.


What are the functions of the JG apparatus?

1. If there is a high distal tubular NaCl concentration, it will induce afferent arteriolar vasoconstriction.
2. If there is low distal tubular NaCl concentration is will induce renin release.


Describe the proximal tubule cells?

It is cuboidal epithelium. There is a prominent brush border (microvilli) which increases the surface area for transporters. These cells undergo pinocytosis where they undergo the uptake of droplets with macromolecules e.g. proteins. There are lateral processes and inholdings (interdigitation) which act to increase the surface area.


Describe thin limb cells?

These cells are made up of squamous epithelium. Their role is to be apart of the loop of henle (water reabsorption). The nuclei bulge into the lumen.


Describe the distal tubule cells?

Structurally similar to proximal tubule cells except there is no brush border (however there is short microvilli). Fine tuning salt, pH, and urine concentration. The epithelium is cuboidal cells, there is interdigitating lateral processes and in-foldings (like proximal tubule cells). There is no pinocytosis.


Describe the collecting duct?

As the collecting duct moves closer to the base of the medullary pyramid it gets bigger. Start off with cuboidal to columnar epithelium.


Describe the higher organisation of the kidney?

The kidney is contained within the renal capsule (tough fibrous bag). The kidney is made up of lobes. It is divided into a cortex and medulla layer. The medulla is made up of conical structures called medullary pyramid. The apex of the pyramid is known as renal papillae (where collecting ducts empty). The papilla is enveloped by dilations of the ureter called calyces. The cortex contains medullary rays.