Kidney Komponents

More information about kidneys than you ever wanted to know!
Or if you did want to know it, here it is. This is copied from my lab report on the renal system. Click the title link to see some excellent slides of various parts of the kidney along with an in-depth tutorial from the University of Texas cellular biology graduate student program.

Renal Cortex
The renal cortex is the outer portion of the kidney between the renal capsule and the renal medulla. The renal cortex forms a shell around the medulla. Its tissues dip into the medulla between adjacent renal pyramids to form renal columns. It contains renal corpuscles and renal tubules, except for those portions of the loop of Henle which descend into the renal medulla. It also contains blood vessels and cortical collecting ducts. The granular appearance of the cortex is due to the random arrangement of tiny tubules associated with nephrons. The renal cortex is the part of the kidney where ultrafiltration occurs.

Renal Medulla
The renal medulla is the innermost part of the kidney. It is split up into cone-shaped masses of tissue called renal pyramids, whose bases are directed toward the convex surface of the kidney, and the apices of which form the renal papillae. Each pyramid together with the associated overlying cortex forms a renal lobe. The tip of each pyramid, called a papilla, empties into a calyx, and the calices empty into the renal pelvis.
The renal medulla also contains blood vessels. Blood enters into the kidney via the renal artery, which then splits to form the arcuate arterioles. The arcuate arterioles in turn branch into interlobar arterioles, which finally reach the glomeruli.

Renal Pyramids
Renal pyramids, also known as malpighian pyramids, are the cone-shaped masses contained in the renal medulla. The renal medulla is made up of 8 to 18 renal pyramids. The broad base of each pyramid faces the renal cortex. Its apex, or papilla, points internally. The pyramids appear striped because they are formed by straight parallel segments of nephrons.

Bases of Pyramids
The broad outer portion of a renal pyramid that lies next to the cortex. Also known as basis pyramidis renis.

Renal Papilla
The papillae are small conical projections along the wall of the renal sinus. They have openings through which urine passes into the calyces.

Renal Columns
Tissue between the renal pyramids that allows for support of the renal cortex. The columns consist of blood vessels, urinary tubes, and fibrous material.

Renal pelvis
The renal pelvis is the funnel-shaped proximal part of the ureter, located approximately in the center of the kidney. It is the point of convergence of two or three major calyces. Each renal papilla is surrounded by a branch of the renal pelvis called a calyx. The major function of the renal pelvis is to act as a funnel for urine flowing to the ureter.

Calyces
The calyces surround the apex of the renal pyramids. There are minor and major calyces. Urine passes through a papilla at the apex into a minor calyx, then travels into a major calyx before passing through the renal pelvis into the ureter. Peristalsis of the smooth muscle of pace-maker cells in the walls of the calyces propels urine through the renal pelvis.

Glomerulus and Bowman's capsule
The glomerulus is the main filter of the nephron. It is a semipermeable, twisted mass of tiny tubes through which blood passes, allowing water and soluble wastes to pass through and be excreted out of the Bowman's capsule as urine. The filtered blood passes out of the glomerulus into the efferent arteriole to be returned through the medullary plexus to the intralobular vein.
The Bowman's capsule contains the primary glomerulus. Blood is transported into the Bowman's capsule from the afferent arteriole, which branches off of the interlobular artery. Within the capsule, the blood is filtered through the glomerulus and exits via the efferent arteriole. Meanwhile, the filtered water and aqueous wastes are passed from the Bowman's capsule into the proximal convoluted tubule.
Here is the best drawing I've seen of the inside of a glomerulus.

Filtration membrane
The filtration membrane is formed from the endothelial cells of the capillaries, basement membrane, and visceral epithelium of the Bowman’s capsule. It is composed of three layers:
Fenestrated endothelium of the glomerular capillaries
Visceral membrane of the glomerular capsule (podocytes)
Basement membrane composed of fused basal laminae of the other layers

Podocytes
Podocytes are cells of the visceral epithelium in the kidneys. They form a crucial component of the glomerular filtration barrier. Structural features of podocytes indicate a high rate of vesicular traffic. Many coated vesicles and pits can be seen along the basolateral domain of podocytes. Within their cell bodies, podocytes have a well-developed endoplasmic reticulum and a large Golgi apparatus, indicative of a high capacity for protein synthesis and post-translational modifications. There are also a large number of multivesicular bodies and other lysosomal components within the podocytes, indicating high endocytic activity.
Adjacent podocytes interlock to cover the basal lamina of the glomerular capillaries. There are thin filtration slits left between the podocytes. The slits are covered by diaphragms, which are composed of numerous cell-surface proteins, including nephrin, podocalyxin, and P-cadherin. These proteins ensure that large macromolecules such as serum albumin and gamma globulin remain in the bloodstream. Small molecules such as water, glucose, and ionic salts pass through the slit diaphragms and form an ultrafiltrate, which is further processed by the nephron to produce urine.
Disruption of the slit diaphragms or destruction of the podocytes can lead to massive proteinuria, whereby large amounts of protein are lost from the blood. An example of this occurs in Finnish-type Nephrosis, a congenital disorder caused by a mutation in the nephrin gene. This defect causes neonatal proteinuria leading to end-stage renal failure.
Information gathered primarily from http://en.wikipedia.org/wiki/Podocyte

Juxtaglomerular Apparatus
The juxtaglomerular apparatus is a structure consisting of the macula densa, mesangial cells, and juxtaglomerular cells. Juxtaglomerular cells, also known as JG cells, or granular cells, are the site of renin secretion.
JG cells are found in the afferent arterioles of the glomerulus and act as an intra-renal pressure sensor. Lowered pressure leads to secretion of rennin, which increases systemic blood pressure via the renin-angiotensin system.
The macula densa senses fluid flow rate and sodium chloride concentration in the distal tubule of the kidney and secretes paracrine vasopressor, which acts on the adjacent afferent arteriole to decrease glomerular filtration rate.
Mesangial cells regulate blood flow in the glomerulus and monitor sodium and chloride levels in the distal convoluted tubules. These cells communicate with the afferent arteriole and can cause vasoconstriction, decreasing the blood flow and GFR if necessary.

Peritubular Capillaries
Peritubular capillaries are the tiny blood vessels beside the nephrons, allowing reabsorption and secretion between blood and the inner lumen of the nephron. Ions and minerals to remain in the body are reabsorbed into the peritubular capillaries through active transport, secondary active transport, or transcytosis. Ions to be excreted as waste are secreted from the capillaries into the nephron and sent to the bladder. The majority of exchange through the peritublar capillaries occurs because of chemical gradients, osmosis, and Na+ pumps.

Distal Convoluted Tubule
The distal convoluted tubule is the portion of a nephron between the loop of Henle and the collecting duct system. It is partly responsible for the regulation of potassium, sodium, calcium, and pH.
The DCT regulates pH by absorbing bicarbonate and secreting H+ protons into the filtrate. Sodium and potassium levels are controlled by secreting K+ and absorbing Na+.
Sodium absorption by the distal tubule is mediated by the hormone aldosterone. Aldosterone increases sodium reabsorption. Sodium and chlorine reabsorption are also mediated by a group of four kinases called WNK kinases.
The distal convoluted tubule also participates in calcium regulation by absorbing Ca2+ in response to parathyroid hormone.
Histologically, cells of the DCT can be differentiated from cells of the proximal convoluted tubule by looking for these features:
DCT cells do not have an apical brush border
DCT cells are less eosinophilic than proximal cells
DCT cells have less cytoplasm
DCT cells are more likely to have visible nuclei
Information primarily gathered from http://en.wikipedia.org/wiki/Distal_convoluted_tubule

Proximal Convoluted Tubule
The proximal convoluted tubule is the longest (14mm) and widest (60µm) part of the nephron. It is lined with epithelial cells containing microvilli and numerous mitochondria. The most distinctive characteristic of the proximal tubule is its brush border. In the PCT, over 80% of the filtrate is reabsorbed into the tissue fluid and returned to the blood. This ensures that all necessary materials that were filtered out of the blood, such as glucose and amino acids, are now returned.

Thin (descending) Loop of Henle
The descending limb of the Loop of Henle has low permeability to ions and urea, while being highly permeable to water. The ascending limb of the LOH is impermeable to water. The net effect is for sodium chloride to leave the ascending limb and to enter the descending limb, having first passed through the renal medullary interstitium. Water is readily reabsorbed from the descending limb by osmosis, increasing the concentration of the urine. Osmolality can reach up to 1200 mOsmol/kg by the end of the descending limb.

Vasa Recta Capillary
The Vasa recta, or straight vessels, are bundles of thin vessels which carry blood into and out of the medulla. The Vasa recta eventually return blood to arcuate veins.

Thick (ascending) Loop of Henle
The ascending limb of the LOH is impermeable to water. As the fluid passes through the ascending limb, it becomes increasingly dilute as the sodium chloride is removed. Thus, the fluid entering the distal convoluted tubule is hypotonic (150 mmol/l).
Sodium, potassium (K+) and chloride (Cl-) ions are reabsorbed by active transport. K+ is passively transported along its concentration gradient through a K+ channel in the basolateral aspect of the cells, back into the lumen of the ascending limb. This K+ "leak" generates a positive electrochemical potential difference in the lumen. The electrical gradient causes more reabsorption of Na+, as well as other cations such as magnesium (Mg2+) and calcium Ca2+.
Information primarily gathered from http://en.wikipedia.org/wiki/Loop_of_Henle

Collecting Ducts
There are several components of the collecting duct system, which includes the connecting tubules and cortical and medullary collecting ducts. With respect to the renal corpuscle, the connecting tubule is the most proximal part of the collecting duct system. It is adjacent to the distal convoluted tubule, which is the most distal segment of the renal tubule. Connecting tubules from several adjacent nephrons merge to form cortical collecting tubules, and these may join to form cortical collecting ducts. Connecting tubules of some juxtamedullary nephrons may arch upward, forming an arcade.
The cortical collecting ducts receive filtrate from multiple connecting tubules and descend into the renal medulla to form medullary collecting ducts. Medullary collecting ducts are divided into outer and inner segments, the latter reaching deeply into the medulla. The terminal portions of these ducts are the papillary ducts, which end at the renal papilla and empty into a minor calyx.
Each component of the collecting duct system contains two cell types: intercalated cells and a segment-specific cell type. For the connecting tubules, this specific cell type is the connecting tubule cell; for the collecting ducts, it is the principal cell. The inner medullary collecting ducts contain an additional cell type, the inner medullary collecting duct cell.
The principal cell mediates the collecting duct's influence on sodium and potassium balance via sodium and potassium channels located on the cell's apical membrane. Intercalated cells come α and β varieties and participate in acid-base homeostasis. The α-intercalated cells secrete acid via an apical H+-ATPase and H+/K+ exchanger in the form of hydrogen ions and reabsorb bicarbonate via a basolateral Cl-/HCO3- exchanger. Damage to the α-intercalated cell's ability to secrete acid can result in distal renal tubular acidosis.
Similarly, β-intercalated cells secrete bicarbonate via an apical Cl-/HCO3- and reabsorb acid via a basal H+-ATPase. Because of their contribution to acid-base homeostasis, the intercalated cells play important roles in the kidney's response to acidosis and alkalosis.
The collecting duct system plays a role in electrolyte and fluid balance through reabsorption and excretion, which are regulated by the hormones aldosterone and antidiuretic hormone. The collecting duct system is the last component of the kidney to influence the body's electrolyte and fluid balance. It accounts for 4-5% of the kidney's reabsorption of sodium and 5% of reabsorption of water. During extreme dehydration, over 24% of the filtered water may be reabsorbed in the collecting duct system.
The collecting duct system regulates electrolytes, including chloride, potassium, hydrogen ions, and bicarbonate. The variable reabsorption of water and, depending on fluid balances and hormonal influences, the reabsorption or secretion of sodium, potassium, hydrogen, and bicarbonate ion continues here.
The wide variation in water reabsorption levels of the collecting duct system reveals its dependence on hormonal activation. The collecting ducts, particularly the outer medullary and cortical collecting ducts, are largely impermeable to water without the presence of ADH, or vasopressin. In the absence of ADH, excess water in the renal filtrate is allowed to enter the urine, promoting diuresis. When ADH is present, aquaporins allow for the reabsorption of water, inhibiting diuresis.
Information (and copying of unusual alpha-numeric characters) found at http://en.wikipedia.org/wiki/Collecting_duct_system