Introduction to the vertebrate kidney:

The kidney is vertebrate excretory organ evolved for adaptation on land. It carries out function of excretion and Osmoregulation. Kidneys function in the important manner in maintenance of internal environment of body. Mammalian kidney is generally considered typical example of structure and function of typical vertebrate kidney.

Development of Kidney:

Process by which kidney develops is explained as nephrogenesis. Mammalian kidney develops from intermediate mesoderm. Kidney development proceeds by the series of 3 successive stages, each marked by development of more advanced pair of kidneys: pronephros, mesonephros, and metanephros.

Pronephros:

During around 22 days of gestation period in humans, paired pronephroi seem towards cranial end of intermediate mesoderm. In this region, epithelial cells arrange themselves in series of tubules known as nephrostomes and link laterally with pronephric duct, that doesn't reach outside of embryo. Therefore, pronephros is considered nonfunctional in mammals as it can't excrete waste from embryo.

Mesonephros:

Each pronephric duct grows towards tail of embryo, and in doing so induces intermediate mesoderm in thoracolumbar area to become epithelial tubules known as mesonephric tubules. Each mesonephric tubule obtains the blood supply from branch of aorta, ending in capillary tuft analogous to glomerulus of definitive nephron. Mesonephric tubule forms capsule around capillary tuft, for filtration of blood. This filtrate flows through mesonephric tubule and is poured in continuation of pronephric duct, now known as mesonephric duct or Wolffian duct. Nephrotomes of pronephros degenerate while mesonephric duct extends towards most caudal end of embryo, eventually attaching to cloaca. Mammalian mesonephros is like kidneys of aquatic amphibians and fishes.

Metanephros:

In fifth week of gestation, mesonephric duct develops the outpouching called as ureteric bud, near its attachment to cloaca. This bud, also known as metanephrogenic diverticulum, grows posteriorly and towards head of embryo. Elongated stalk of ureteric bud, metanephric duct, later forms ureter. As cranial end of bud extends in intermediate mesoderm, it suffers series of branchings to form collecting duct system of kidney. It also forms major and minor calyces and renal pelvis. Portion of intermediate mesoderm in contact with tips of branching ureteric bud is called as metanephrogenic blastema. Signals released from ureteric bud induce differentiation of metanephrogenic blastema in renal tubules. As renal tubules grow, they come in contact and join with linking tubules of collecting duct system, forming the continuous passage for flow from renal tubule to collecting duct. At the same time, precursors of vascular endothelial cells start to take their position at tips of renal tubules. Cells distinguish into cells of definitive glomerulus.

Parts of Mammalian Excretory System:

Kidneys:

These are paired structures there in abdominal cavity, protected by last two pairs of ribs (floating ribs). In humans, kidneys are metanephric, mesodermal in origin, created from nephrostome of early embryo. These are dark red, bean shaped with the concavity on inner side called as hilus, from where blood vessels, lymphatic vessels and nerves enter or leave kidneys. Right kidney is somewhat lower than left kidney to accommodate liver. Every kidney is covered by thin fibrous capsule of connective tissue, called renal capsule. Around the capsule, there is another layer of fat, adipose capsule that is again enclosed by an outer layer of fibrous connective tissue, renal fascia. All the layers protect kidneys from infections and injuries. Inside kidney is divided in 2 parts - outer dark cortex and inner light medulla. Cortex has Malpighian body, proximal convoluted tubule, distal convoluted tubule and comparatively small part of loop of Henle. Medulla is divided in number of conical regions known as renal pyramids or medullary pyramids that are made of nephrons and blood vessels. Every renal pyramid terminates into pointed structure, renal papillae towards pelvis side. In between 2 renal pyramids, cortex invaginates in medulla forming renal columns of Bertini. Renal pyramids are linked with minor calyces, that join together to form major calyces.

Ureters:

These are muscular, thick, narrow tubes leaving kidney from renal pelvis, run behind through hilus and opens in urinary bladder. Every ureter is approx 25-30cm long and carries urine from kidneys to urinary bladder by peristalsis.

Urinary Bladder:

Ureters open in urinary bladder that is muscular sac lying in pelvic cavity. Urinary bladder stores urine temporarily and opens in urethra whose opening is secured by 2 pairs of circular sphincters - internal sphincter, composed of smooth muscles and external sphincter, composed of skeletal muscles. Internal sphincter is under involuntary control and external sphincter is under voluntary control of nervous system.

Urethra:

It is muscular tube that extends from neck of urinary bladder and leads to external opening of urinary tract. In males, urethra is long (20 cm approx.) and opens at tip of penis by urinogenital aperture and carries both semen and urine. Urethra of females is short (4 cm approx.) and opens by urethral opening, in front of vaginal aperture. It holds only urine.

Functions of Kidney:

Excretory function:

Excretion of waste products: The kidneys excrete the variety of waste products made by metabolism, comprising nitrogenous wastes: urea (from protein catabolism) and uric acid (from nucleic acid metabolism).

Homeostatic function:

Acid-base balance: Kidneys regulate pH, by eliminating H ions concentration known as augmentation mineral ion concentration, and water composition of blood. By exchanging hydronium ions and hydroxyl ions, blood plasma is maintained by kidney at the neutral pH 7.4. Urine, conversely, is acidic at pH 5 or alkaline at pH 8. pH is maintained through 4 main protein transporters: NHE3 (a sodium-hydrogen exchanger), V-type H-ATPase (an isoform of hydrogen ATPase), NBC1 (a sodium-bicarbonate cotransporter) and AE1 (anion exchanger that exchanges chloride for bicarbonate).

Blood pressure:

Sodium ions are handled in homeostatic process involving aldosterone that increases sodium ion absorption in distal convoluted tubules. When blood pressure becomes low, proteolytic enzyme known as Renin is secreted by cells of juxtaglomerular apparatus (part of distal convoluted tubule) that are sensitive to pressure. Renin acts on blood protein, angiotensinogen, converting it to angiotensin I (10 amino acids). Angiotensin I is then converted by Angiotensin-converting enzyme (ACE) in lung capillaries to Angiotensin II (8 amino acids), that stimulates secretion of Aldosterone by adrenal cortex, that then affects kidney tubules. Aldosterone stimulates the increase in reabsorption of sodium ions from kidney tubules that causes increase in volume of water which is reabsorbed from tubule. This increase in water reabsorption increases volume of blood that finally raises blood pressure.

Plasma volume:

Any rise or drop in blood osmotic pressure because of lack or excess of water is detected by hypothalamus that notifies pituitary gland via negative feedback. The lack of water causes posterior pituitary gland to secrete antidiuretic hormone, that results in water reabsorption and increase in urine concentration. Tissue fluid concentration therefore returns to mean of 98%.

Hormone secretion:

Kidneys secrete the variety of hormones, comprising erythropoietin, urodilatin and vitamin D. When individual is bled or becomes hypoxic, haemoglobin synthesis is improved, and production and release of RBCs from bone marrow (erythropoesis) are increased, brought about by increased level of erythropoetin. In adults, approx 85% of erythropoetin comes from kidneys and 15% from liver. In foetal and neonatal life, major site of erythropoetin production and erythropoesis is liver but in child, erythropoetin production is taken over by kidney and erythropoesis is taken over by bone marrow.

Structure and Function of Nephron:

Nephron forms basic structural and functional unit of kidney. Mainly, nephron is liable for regulation of water and soluble substances by filtering blood, reabsorbing what is required and excreting rest as urine. Every human kidney has about one million nephrons. Nephrons eradicate wastes from body, regulate blood volume and pressure, manage levels of electrolytes and metabolites, and regulate blood pH. Its functions are very important to life and are controlled by endocrine system by hormones like aldosterone, antidiuretic hormone, and parathyroid hormone. Every nephron is made up of initial filtering component, renal corpuscle and tubule specialized for reabsorption and secretion, renal tubule. Renal corpuscle filters out large solutes from blood, delivering water and small solutes to renal tubule for alteration

1) Malphigian body:

Made up of glomerulus and Bowman's capsule, renal corpuscle or Malphigian body is starting of nephron. It is initial filtering component of nephron.

Glomerulus:

Glomerulus is capillary tuft which receives its blood supply from afferent arteriole of renal artery. Glomerular blood pressure gives driving force for water and solutes to be filtered out of blood and in space prepared by Bowman's capsule. Remainder of blood, essentially blood plasma, not filtered into glomerulus passes in narrower efferent arteriole. It then moves in vasa recta, that are collecting capillaries intertwined with convoluted tubules through interstitial space, and through which reabsorbed substances also enter. This then combines with efferent venules from other nephrons in renal vein, and rejoins main bloodstream.

Bowman's capsule:

It is also known as glomerular capsule; it encloses glomerulus and is made up of visceral inner layer and parietal outer layer, both formed by simple squamous epithelial cells. Fluids from blood in glomerulus are collected in Bowman's capsule (that is, glomerular filtrate) and further processed along nephron to form urine.

2) Renal tubule:

Flow of renal tubule is as follows:

Proximal Convoluted Tubule (PCT):

The proximal convoluted tubule as a part of nephron can be separated in initial convoluted portion and following straight, descending portion. Fluid in filtrate entering proximal convoluted tubule is reabsorbed in peritubular capillaries, comprising about two-thirds of filtered salt and water and all filtered organic solutes (mainly glucose and amino acids).

Loop of Henle:

Loop of Henle (also called as nephron loop) is U-shaped tube which comprises of descending limb and ascending limb. It starts in cortex, receiving filtrate from proximal convoluted tubule, expands in medulla, and then returns to cortex to empty in distal convoluted tubule. Its main role is to concentrate salt in interstitium, tissue surrounding loop. It is separated in descending and ascending limbs:

Descending limb:

Its descending limb is permeable to water but entirely impermeable to salt, and therefore only indirectly contributes to concentration of interstitium. As filtrate descends deeper in hypertonic interstitium of renal medulla, water flows freely out of descending limb by osmosis until tonicity of filtrate and interstitium equilibrate. Longer descending limbs permit more time for water to flow out of filtrate; so longer limbs build filtrate more hypertonic than shorter limbs.

Ascending limb:

Dissimilar descending limb, ascending limb of Henle's loop is impermeable to water, vital characteristic of countercurrent exchange mechanism used by loop. Ascending limb actively pumps sodium out of filtrate, generating hypertonic interstitium which drives countercurrent exchange. In passing by ascending limb, filtrate grows hypotonic as it has lost much of its sodium content. This hypotonic filtrate is passed to distal convoluted tubule in renal cortex.

Distal convoluted tubule (DCT):

Distal convoluted tubule is not similar to proximal convoluted tubule in structure and function. Cells lining tubule have many mitochondria to create enough energy (ATP) for active transport to occur. Many ion transports occurring in distal convoluted tubule is controlled by endocrine system. In presence of parathyroid hormone, distal convoluted tubule reabsorbs more calcium and excretes more phosphate. When aldosterone is there, more sodium is reabsorbed and more potassium excreted. Atrial natriuretic peptide causes distal convoluted tubule to excrete more sodium. Additionally, tubule as well secretes hydrogen and ammonium to regulate pH.

3) Collecting tubule:

Every distal convoluted tubule delivers its filtrate to system of collecting ducts, primary segment of which is collecting tubule. Collecting duct system starts in renal cortex and expands deep in medulla. As urine travels down collecting duct system, it passes by medullary interstitium that has high sodium concentration as result of loop of Henle's countercurrent multiplier system.

Although the collecting duct is usually impermeable to water, it becomes permeable in presence of antidiuretic hormone (ADH). As much as 3/4 of water from urine can be reabsorbed as it leaves collecting duct by osmosis. Therefore levels of ADH find whether urine will be concentrated or diluted. Dehydration results in the increase in ADH, whereas water sufficiency results in low ADH permitting for diluted urine.

Lower portions of collecting duct are too permeable to urea, permitting some of it to enter medulla of kidney, therefore maintaining high ion concentration (that is very significant for nephron). Urine leaves medullary collecting ducts through renal papilla, emptying in renal calyces, renal pelvis, and lastly in bladder via ureter. As it has different embryonic origin than rest of nephron, collecting duct is at times not considered part of nephron.

Collecting tubules from nephrons join together to form larger collecting duct that again join to create still larger ducts known as ducts of Bellini. Latter runs through renal pyramids and opens in renal pelvis.

4) Juxtaglomerular apparatus:

Juxtaglomerular apparatus happens near site of contact between thick ascending limb and afferent arteriole. It has 3 components:

i) Macula densa. The tightly packed area of cells, comprising renin granular cells; macula densa verifies chemical composition of fluid in distal convoluted tubule.

ii) Juxtaglomerular cells. These are specialized smooth muscle cells in wall of afferent arteriole and are site of renin synthesis and secretion and therefore play vital role in renin-angiotensin system.

iii) Extraglomerular mesangial cells. These couple to arteriole and contain no specific function.

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