The kidneys are paired retroperitoneal structures, organized into a cortex (containing glomeruli, the cortical labyrinth, and medullary rays), the outer medulla (with an outer and an inner stripe), and an inner medulla which forms the renal papillae.
Glomerulus sits inside Bowman’s capsule. It consists of capillary loops with an afferent and efferent arteriole. Between the capillary loops are mesangial cells, which phagocytose material trapped in the filtration barrier, and contractile to close off glomerular loops.
Filtration barrier has 3 layers
1. Fenestrated capillary endothelium of glomerulus (blocks RBCs and plts)
2. Basement membrane (NOT a lipid bilayer; a glycoprotein gel)
3. Interdigitated podocyte foot processes, linked by slit diaphragms (modified tight junctions)
Bowman’s capsule gives off a tubule, which has a proximal part, then Henle’s loop (descending thin, ascending thin in long loops, ascending thick segments), the DCT, and then tubules merge in the collecting-duct system.
The JG apparatus is where the thick ascending limb (the ‘macula densa’) passes between the afferent and efferent arterioles. 3 cell types
1. Granular cells in the afferent arteriole, secrete RENIN
2. Mesangial cells
3. Macula densa cells (detect osmolality and flow rate of filtrate)
Maintaining homeostatic concentrations of important ions (Na, K, Mg, SO4, PO4, etc) and organic solutes (urea, uric acid, glucose)
Maintaining serum osmolality
Regulating ECF volume and blood pressure
Excreting foreign substances (drugs, toxins)
Performing gluconeogenesis
Regulating haematopoesis and maintaining HCT
Hydroxylation of 25-OHcholecalciferol