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What are the functions of the urinary system?

The function of the urinary system is the homeostasis of the blood. In particular, it is homeostasis of:
- plasma composition: by regulating the excretion of water, ions and organic waste products into the urine
- blood pressure: through the enzyme renin
- RBC content: through secretion of erythropoietin

The kidneys receive 25% of cardiac output.


Briefly, describe the nephron.

It is the functional unit of the kidney. It is where blood is filtered at the molecular level (dialysis) to produce urine while retaining cells and large proteins.
There about 1 million nephrons in each human kidney.


What is the function of the renal tubule?

In short, it adjusts the composition of the ultrafiltrate to recover nutrients, water, etc. and regulate plasma composition.


What is the function of the proximal convoluted tubule?

Its function is reabsorption from the ultrafiltrate:
- by active transport across the membrane of the cell: small molecules like Na+, glucose, amino acids
- by pinocytosis: macromolecules, especially proteins. These are broken down in lysosomes and returned to the blood.
- by passive flux: water, Cl-


Describe the structure of a cell of the proximal convoluted tubule.

It has long microvilli for a high surface area for reabsorption; it also has lytic enzymes on the surface to break down macromolecules.

Pinocytotic vesicles carrying macromolecules to lysosomes.

It has many lysosomes to break down and recycle macromolecules.

It has many mitochondria to fuel active transport (seen especially near the basolateral sodium pump).


What are the functions of the Loop of Henle?

Its functions are the reabsorption of water and salts from the filtrate - by passive flux across the epithelium, by osmosis and concentration gradients.


Describe the structure of an epithelial cell in the thin Loop of Henle.

It has a thin, squamous epithelium to allow passive fluxes.
It has a minimum of organelles.


What are the functions of the distal convoluted tubule and the thick ascending Loop of Henle?

It achieves homeostasis by regulated active transport and exchange of ions (Na+/ K+, H+/HCO3-).


Describe the structure of an epithelial cell of the distal convoluted tubule.

It has a cuboidal epithelium, which is thicker than squamous, to reduce passive fluxes and accommodate organelles.

It has a few, short, microvilli (unlike the PCT).

It has many mitochondria to fuel active transport. These are mainly basal and can show as a pale or striped basal area in H&E-stained sections.


What are the functions of the collecting duct and the collecting tubule?

It handles the transport of urine to the ureter.
It's also involved in water homeostasis as the passive reabsorption of water, regulated through epithelial permeability.


Describe the structure of an epithelial cell of the collecting duct.

It has a cuboidal to columnar epithelium so that it can prevent the passive flux of water (and urea, etc.).

It has specialised dense membranes at cell contacts (red). Their function is unclear - they're probably also helping to prevent passive flux.


What does the macula densa do?

It senses [Na+] in the DCT fluid. It appears to signal to juxtaglomerular cells.


What do juxtaglomerular cells do?

They release renin - more so in response to lower [Na+] in the DCT.
Renin indirectly increases vascular tone and sodium reabsorption.


What do Lacis cells do?

Their function is unknown.
They are also called extraglomerular mesangial cells.


Describe the transitional epithelium.

The transitional epithelium is a specialised stratified epithelium found only in ureters and the bladder. It is specialised to be impermeable to urine.

It changes appearance when stretching. It is somewhat like stratified squamous epithelium (SSE) when distended (full), but its apical cells are biggest and have much apical cytoplasm.
In the SSE, the basal cells are the biggest, while the apical cells are very flat.


There are plaques in specialised (urine-resistant) plasma membrane in the apical cells of the transitional epithelium.
Describe their function in a distended and contracted bladder.

DISTENDED (bladder full):
These impermeable, rigid membrane patches (plaques) protect apical cells from the toxic urine.

CONTRACTED (bladder empty):
The rigid plaques are invaginated, forming pits and vesicles in the cell, allowing the cell surface area to decrease.


What is cystitis?

Cystitis is the inflammation of the bladder, usually caused by a bladder infection.


Why are UTIs (urinary tract infections) quite common?

The transitional epithelium is highly impermeable, so the leukocytes of the immune system cannot readily penetrate it.


Why is it commoner in females?

The female urethra is shorter, so there is more risk of contamination (eg. from the anal region).


What would be the advice on UTI prevention?

Take plenty of fluids.

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