Kidney
-The kidneys are large, reddish, bean-shaped organs situated retroperitoneal on the posterior abdominal wall.
-The kidney, embedded in peri-renal fat. It has:
1. A convex border situated laterally.
2. A concave border, the hilum facing medially. Branches of the renal artery and vein, lymph vessels, and ureter pierce the kidney at its hilum. The ureter is expanded at this region, forming the renal pelvis.
-The renal pelvis is divided into two or three major calyces. Several smaller branches, the minor calyces, arise from each major calyx.
-The kidney is invested by a thin, loosely adhering capsule, consisting mainly of dense, irregular collagenous connective tissue with occasional elastic fibers and smooth muscle cells.
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The hemisected view of the kidney shows that the kidney is formed of:
1- An outer cortex which is dark brown and granular due to the presence of malpighian corpuscles.
2- An inner medulla which consists of 10-18 conical or pyramidal structures, the medullary pyramids. They appear striated due to the presence of straight tubules.
-From the base of each medullary pyramid, parallel arrays of tubules, the medullary rays (collecting tubules), penetrate the cortex.
-The apex of the medullary pyramids, known as the renal papilla, points toward the hilum. The apex is perforated by 20 or so openings of the ducts of Bellini; this sieve-like region is known as the area cribrosa. The apex is surrounded by a cup-like minor calyx.
-Neighboring pyramids are separated from each other by material resembling the cortex, the cortical columns (of Bertin).
The renal lobe: Is a portion of a kidney consisting of a renal pyramid and the renal cortex above it.
The renal lobule: It consists of a medullary ray in the center and the surrounding cortical tissue. It is
formed of those nephrons that are served by a common collecting duct.
The functional unit of the kidney is the uriniferous tubule.
origin, the nephron and the collecting tubule
ng tubule, and multiple collecting tubules join in the deeper aspect of the medulla to form larger and larger ducts. The largest of these ducts, the ducts of Bellini, perforate the renal papilla at the area cribrosa.
Nephron
-The nephron consists of four distinct parts:
1. The renal corpuscles
2. Proximal convoluted tubule 3. Loop of Henle.
4. Distal convoluted tubule
-There are two types of nephrons, depending on the location of their renal corpuscles and the length of their Henle's loop.
Shorter cortical nephrons, which do not extend deep into the medulla.
longer juxtamedullary nephrons, whose renal corpuscle is located in the cortex and whose tubular parts extend deep into the medulla.
1-The renal corpuscles
-Each renal corpuscle consists of a tuft of capillaries, the glomerulus, surrounded by a double-walled epithelial capsule called glomerular (Bowman's) capsule.
-Each renal corpuscle has 2 poles:
Vascular pole, where the afferent arteriole enters and the efferent arteriole leaves.
Urinary pole, where the proximal convoluted tubule begins.
Bowman'scapsule:
-walled epithelial capsule called glomerular (Bowman's) capsule formed of:
The external layer forms the outer limit of the renal corpuscle and is called the parietal layer of Bowman's capsule and composed of simple squamous epithelial cells.
The internal layer (the visceral layer) of the capsule envelops the capillaries of the glomerulus composed of epithelial cells that become modified and are known as podocytes.
space, which receives the fluid filtered through the capillary wall and the visceral layer.
Podocytes:
large cells have numerous long cytoplasmic extensions, primary (major) processes.
arranged in an orderly fashion.
by interdigitating with pedicels from neighboring major processes of different podocytes.
width, known as filtration slits, remain between adjacent pedicels.
are not completely open; instead, they are covered by a thin slit diaphragm that extends between neighboring pedicels and acts as a part of the filtration barrier.
The cell body of the podocyte houses the irregularly shaped nucleus as well as rough endoplasmic reticulum (RER), Golgi apparatus, and numerous free ribosomes. Pedicels cytoplasm is devoid of organelles but house microtubules and microfilaments.
Glomerular capillaries:
glomerulus is composed of tufts of fenestrated capillaries supplied by the afferent glomerular arteriole and drained by the efferent glomerular arteriole.
diaphragm.
pores are large, ranging between 70 and 90 nm in diameter; hence, these capillaries act as a barrier only to elements of the blood and to macromolecules whose effective diameter exceeds the size of the fenestrae.
ls of the glomerular capillaries and the podocytes that cover their external surfaces is a thick basement membrane.
- and podocyte basal laminae.
yers:
1- The electron dense central lamina densa.
2- Less electron-dense layers, the laminae rarae on either side of the lamina densa. Lamina rara interna, between the endothelial cells of the capillary and the lamina densa, and the lamina rara externa, between the lamina densa and the visceral layer of Bowman's capsule.
