L#26 - Nephrons
Nephrons:
are the structural and functional units that form urine
there are more than 1 million nephrons PER kidney
the 2 main parts of a nephron are the renal corpuscle and renal tubules
Each renal corpuscle is composed of a tuft of capillaries (the glomerulus), surrounded by a glomerular (Bowman’s) capsule
the glomerular capillaries are fenestrated to increase permeability — allowing the formation of solute-rich, but protein-free filtrate
Glomerular (Bowman’s) Capsule
The glomerular capsule has a parietal layer that forms the outer capsular structure and an inner visceral layer surrounding glomerular capillaries, consisting of podocytes that allow filtrate to pass into the space within the glomerular capsule
The renal tubule begins at the glomerular capsule as the proximal convoluted tubule
continues thru the nephron loop (loop of Henle)
and turns into a distal convoluted tubule
before emptying into a collecting duct
Renal tubules
The wall of the proximal convoluted tubule has dense microvilli to increase surface area for absorption from — and secretion to — the urine
The nephron loop (Loop of Henle) has a thin descending limb and an ascending limb that has both thin (simple squamous epithelium) and thick (simple cuboidal epithelial) segments
The distal convoluted tubule is similar to the proximal convoluted tubule
except the cells almost entirely lack microvilli**
Collecting ducts
The collecting duct contains principal cells w/ sodium and potassium ion channels and aquaporins to reabsorb water and intercalated cells that control pH by secreting or absorbing H+ or bicarbonate
The collecting ducts collect filtrate from many nephrons
and extend thru the renal pyramid
to the renal papilla, where they empty into a minor calyx.
The collecting ducts give the pyramids their striped appearance
2 classes of nephrons
85% are cortical nephrons, which are located almost entirely within the cortex and have a short nephron loop (for the loop of Henle) — not as good at making concentrated urine
15% are juxtamedullary nephrons located near the cortex-medulla junction and have a long nephron loop (for the loop of Henle) — good at making concentrated urine (helps to stay hydrated)
The renal tubule of each nephron is closely associated w/ 2 capillary beds: the glomerulus and the peritubular capillaries (cortical nephrons) or vasa recta (juxtamedullary nephrons)
Neuron capillary beds
The glomerulus is specialized for filtration is fed and drained by an afferent and efferent arteriole, which serves to maintain the high pressure in the glomerulus needed to favour filtration
Peritubular capillaries are low-pressure, porous capillaries that surround adjacent renal tubules to absorb solutes and water from the tubule cells
The vasa recta arise from the efferent arterioles near juxtamedullary nephrons and run parallel to the longest nephron loops
Juxtaglomerular apparatus
The juxtaglomerular apparatus (juxtaglomerular complex) is a structural arrangement b/w the afferent arteriole and the distal convoluted tubule
The macula densa cells in the distal convoluted tubule use chemoreceptors to monitor NaCl content of filtrate
Granular cells, derived from the wall of the arterioles, act as mechanoreceptors that monitor blood pressure and house secretory vesicles that contain the enzyme renin
Physiology of the kidney
3 processes are involved in urine formation and adjustment of blood composition:
1 - Glomerular filtration — produces cell-free and protein-free filtrate
2 - Tubular reabsorption — selectively returns 99% of substances from filtrate to blood in renal tubules and collecting ducts
3 - Tubular secretion — selectively moves substances from blood to filtrate in renal tubules and collecting ducts
Reabsorption and secretion
Different regions of the nephron tubules and collecting ducts secrete or absorb different molecules
Reabsorption and secretion depends on specific transport proteins and channels in the epithelial cell membranes at different parts of the nephron
Glomerular filtration
Glomerular filtration is a passive (not requiring metabolic energy), non-selective process in which hydrostatic pressure forces fluids thru the glomerular membrane
No reabsorption occurs within the capillaries of the glomerulus
The filtration membrane is a porous membrane that allows free passage of water and solutes smaller than plasma proteins
The filtration of membrane
The filtration membrane consists of 3 layers:
1 - The fenestrated endothelium of the glomerular capillaries
2 - A basement membrane consisting of negatively charged glycoproteins that inhibit the filtration of large or negatively charged molecules
3 - The food processes (pedicels) of podocytes of the visceral layer of the glomerular capsule
Glomerular filtration:
1 - Fenestration (pore) of glomerular endothelial cell — prevents filtration of blood cells but allows blood plasma components to pass thru
2 - Basement membrane of glomerulus — prevents filtration of larger or neg. charged molecules
3 - Slit membrane b/w podocyte foot processes — prevents filtration of medium0sized molecules
Net filtration pressure (NFP)
The forces that drive glomerular filtration are similar to the forces that cause bulk flow in blood capillaries
Fluid (blood plasma) is squeezed out of the glomerulus and into the Bowman’s capsule because there is higher NFP inside the glomerulus
The glomerular blood hydrostatic pressure (BHP) — essentially glomerular blood pressure, is the primary force pushing water and solutes out of the blood across the filtration membrane
Hydrostatic and osmotic pressures
There is also capsular hydrostatic pressure (CHP) which is the fluid pressure inside the Bowman’s capsule and blood colloid osmotic pressure (BCOP) from blood solutes in glomerular capillaries
both of these forces oppose the glomerular blood hydrostatic pressure (BHP) and try to move fluid back into the glomerulus capillaries
Blood hydrostatic pressure (BHP) in the glomerulus is so high that the net filtration pressure (NFP) is out of the glomerulus and into the capsule