(Transcribed from Dr. Kalliecharan’s lecture, 30 Mar 2000 by Brian Buschman)
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The kidneys are covered by three protective layers, the renal fascia, adipose capsule and the renal capsule. Structurally it has a cortex, medulla, capsule and a hillus. The hillus is the entry/exit point for the artery, vein, nerves and lymphatics.
The cortex is composed of an outer cortical zone, inner juxtamedullary zone and has extensions as renal columns. The medulla is made up of pyramids.
The renal pyramids drain into minor calyces that drain into major calyces that join to form the renal sinus.
The nephron’s primary function are secretion, absorption and filtration. They begin at the renal corpuscle made of the Bowman’s capsule and glomerulus. Nephrons come as cortical and juxtamedullary. The juxtamedullary nephrons are right next to the medulla and are responsible for the hypertonicity of urine.
The renal corpuscle has an unique design. It’s outer layer, the parietal layer of Bowman’s capsule, is made of simple squamous epithelia. It also has the visceral layer made of the endothelium of renal capillaries and the secondary pedicles of the podocytes. The filtration slits are made by the fenestrations in the capillaries and regulated by the placement of the podocytes. The barrier also includes a thick basal lamina lined with heparin sulfate. The charge of the heparin sulfate prevents the passage of negatively charged molecules through the fenestrae.
Mesangial cells are cells that are placed on the walls of the capillaries which are believed to function in cleaning the fenestrations to keep filtration working properly, that is to get rid of all the clogs in the system.
The proximal convoluted tubule (PCT) has a simple cuboidal epithelium with a brushed boarder.
The loop of Henley has three parts:
1) The large part of the descending limp which, in section, appears exactly like the PCT.
2) The bottom of the loop is small, thin walled and made of simple squamous epithelia.
3) The large part of the ascending limb is exactly like the distal convoluted tubule (DCT).
The simple squamous part of the loop of Henley is easily mixed up with the endothelium of the surrounding capillaries. The capillaries can be identified because of the blood cells in the middle.
The DCT is made of a simple cuboidal epithelia.
The DCT will empty into the collecting duct. As you go from the DCT to the collecting duct the cells progress from simple cuboidal to simple columnar.
Near the arterial entrance to the renal corpuscle are the macula densa cells in the endothelium. Nearby are the juxtaglomerular (JG) cells which are modified smooth muscle cells that secrete rennin. The secretion of renin is in response to stimulation by sympathetic nerves.
Signals from the carotid sinus tell the brain the blood pressure is high and signal the JGA to release renin. Renin cleaved angiotensinogen into angiotensin I. The endothelial cells of the lungs contain angiotensin converting enzyme (ACE) that converts angiotensin I into angiotensin II. Angiotensin II stimulates the release of aldesterone from the zona glomerulosa of the adrenals. Aldesterone increases NA2+ reabsorption. Water follows and results in increased blood pressure.
Blood enters the kidney via renal arteries which branch into interlobar arteries that run between the pyramids. At the top of the pyramids they branch out to give arcuate arteries. The arcuate arteries will then branch into interlobular arteries that are perpendicular to the arcuate arteries. From the interlobular arteries comes the afferent arteriole which branched to supply a renal corpuscle. Inside the corpuscle it will give the capillaries of the glomerulus which rejoin to form the efferent arteriole. The efferent arteriole branches into the peritubular capillaries which function in secretion and reabsorption. Finally form the venules and leave.
The ureters have three layers:
1) Mucosa that has:
a. An inner mucous membrane
b. Transitional epithelium with a lamina propria.
c. Mucus that is secreted from the mucosa to protect the epithelia from damage from urine.
2) Muscularis externa. The proximal two-thirds has an inner longitudinal layer and an outer circular muscle layer. The distal one-third has an inner longitudinal, middle circular and outer longitudinal muscle layer.
3) It’s outer layer is an adventitia.
The bladder contains the same three layers as the distal third of the ureter.
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