Anatomy and Physiology of Human's Kidney

Kidney is a couple organ inside the abdomen, its form is like a nut, dark red color. Its location is in the abdomen, close to posterior vertebra, inside the peritoneum and back muscle. Kidney has adrenal gland on it, but that gland has different function with kidney especially in urine elimination. Kidney structure consist of fibrous tissue that cover the kidney from the outside, and cortex as the outter layer, and medulla as the inner layer. Kidney structure consist of 1.000.000 nephron as kidney function unit.

Every nephron consist capillary (Malpighi body or glomerulus) and tubules. Kidney has an important role in fluid and electrolyte balance. Kidney filters waste product from the blood  to be urine.

Human has a pair kidney, the right side and left side. The right side of kidney is lower than the left side, because there is a liver organ in it. Kidney located in posterior abdomen, outside peritoneum cavity. Medial side of kidney is hilum where the artery and vein supply kidney with blood, lymph fluid, nerves, and ureters that transport urine to bladder. Kidney consist of fibrous capsule to protect kidney structures.

Anatomy

Kidney is a couple organ, its form is like a nut with dark red color, the length about 12,5 cm and its tick is 2,5 cm (about fist size). Every kidney has weight about 125 to 175 grams for man and 115 to 155 grams from woman.

Location

Kidney’s location is in upper area, that is in posterior abdomen wall that is close to last ribs. This organ is retroperitoneal organ and located between back muscle and peritoneum cavity. Every kidney has an adrenal gland on it.

Kidney Location in the Abdomen

in normal condition, left kidney is higher than right kidney about 1,5 to 2 cm because of  liver anatomy. Every kidney has three wraps, fascia renal as the outter cover, perirenal fat tissue, and fibrous capsule. Those kidney wrapper is to cover the kidney and maintain the kidney structure still in its position.

Kidney Internal Structure

1. Kidney internal structures are hilum, kidney sinus, pelvis, parenchim, medulla, and cortex. We will know the kidney internal structure one by one.
2. Hilum is a kidney’s medial corvature.
3. Sinus is a cavity consist of fat tissue as an entrance of hilum. Sinus form complex entrance and exit for urether, renal arthery, vena arthery, nerves, and lymph.
4. Pelvic is proximal urether expandtion. This pelvic tip become 2 to 3 mayor calixes, calixes is a small cavity that reach glandulers where urine is filtered. Every single calix will has  8 to 18 branches called minor calix.
5. Kidney parenchim is kidney tissue that cover kidney sinus. This tissues consist of two part, medulla and cortex. Medulla consist of triangula masses called pyramide. Every narrow tip of pyramide (papilla) entered to minor calix and  penetrated urine storage duct. Cortex consist of tubulus and nephron blood vessels, cortex is among pyramides.
6. Kidney can be divided to kidney lobus, every lobus consist of one pyramide and cortex tissue that is covered it.

Nephron structure

Nephrone is kidney functional unit. One kidney consist 1 to 4 million nephrons. One of the nephrones consist vasculer component (capulary) and tubuler component. Nephron consist of glomerulus, Bowman capsul, conturtuse tubulus, ansa henle, distal tubulus, and storage ductus.

Glomerulus

Glomerulus is first structure in nephrone, it is like a capilary rolls that is arranged by vasa aferen blood supply and resupply through vas eferen. Glomerulus is surronded by Bowman capsul, capsul Bowman is like double epithelium capsul. Its structure is covered by endotelium cells and basal membrane. Epithelium cells is in basal membrane, and other endotel in different place. Glomerulus and Bowman capsul work together to form kidney corpuskel.

Bowman capsul visceral layer is internal epithelium layers. Liseran layer cells is modified to be podosit (foot like cells), those cells is special epithelium cells around glomerulus capilary. Every podosit cells sick to glomerular surface through primer prosesus that consist of secondary procesus called pedickel (small foot)

Pedicle integrate with same prosesu from  other podosit. Narrow space between pedicle called filtration slits (pore), its space is about 25nm. Every pore is covered by membran, and it will be possible for some molecul exchange. Glomerular filtration barier is tissue barrier that divide blood in glomerular capiler from inner space of Bowman capsul. This barier consist of endothelium, lamina basalis, and filtration slits.

Parietal layer of Bowman capsul form kidney corpuscle. In kidney corpuscle vasculer, arteriola aferen flows  to glomerulus and arteriol eferen out from glomerulus. In kidney corpuscle urinary, glomerulus filtrate the blood flow that come in.

Proximal contur tubulus lenght is about 15 mm and complex. There is coboit epithelium cells that is rich of micro vilus (brush border) and expand lumen surface. Ansa henle. Proximal tubulus goes to ansa henle desenden that cut kidney medula and back to upper tublus, acendent tubulus ansa henle.

Cortex Nephrone is outside the cortex. This nephrone has small curve that cut 1/3 medulla. Jukstamedular nephrone is near to medulla. This nephron has long curve that cut to pyramid. Distal tubulus is so long about 5 mm and form last nephron segment. This tubulus connected to aferen arteriol wall. The tubulus cells that is touched  arteriol has modified cells called macula densa. Macula densa has functions in chemoreseptor and stimulate natrium ion.

Arteriole wall consist of modified smooth muscle called jukstaglomlular cell. This cells are stimulated by blood  pressure to produce renin. Macula densa, jukstaglomelular, and  mesangium cells work together to form jukstaglomelural apparatus, and  it will regulate blood pressure.

Storage duct. Because every storage duct s is close to cortex, so those tubuluses will reach distal tubulus. Storage ductus form is big, straight, and wide. Storage tubulus form bigger tube that excrete urine to bigger place (calix minor and calix mayor), and from kidney pelvic, urine will flow to urether and store in bladder.

Filtration, Reabsorption, and Secretion

Glomerular Filtration

Filtration is a process that filter blood and fluid exchange  in certain pressure in Bowman capsul. This filtrations are helped some factors: glomerular capiler membran is more permeable than other capiler in the body so the filtration will work so fast. Blood pressure in glomerular capiler is stronger than other capilary pressures because eferen arteriol is smaller than aferen arteriole.

Filtration Mechanism

Hidrostatic pressure (blood), glomeruler pushes fluid and sulable substances out from blood to Bowman capsul.  Hidrostatic pressure is from Bowman capsul. This pressure tend to push fluid out from capsul into glomerulus. Osmotic pressure in glomerulus is made from plasm protein, this pressure is the pressure that pull fluid from Bowman capsul. Effective filtration force (EEPI) is netto forece. This pressure is pressure between hidrostatic and osmotic pressure.

EFP= (Hydrostatic) – (capsul pressure) + (osmotic)

Glomerular filtration rate is filtration total that is formed per minute in all nephron. In man, GFR is about 125 ml/minutes or 180 L in 24 hours, and 110 ml/minutes for woman.

Factors that Influence GFR

1. Effective filtration force.

GFR is equal with EFR and force changes will influence GFR. Arteriol constriction determine kidney blood flow and glomerular hidrostatic pressure. Aferen contriction decrease blood flow and decrease filtration. Eferen contriction increase the filtration in glomerulus lead to higher GFR.

2. Sympathies stimulation

Sympatis impuls stimulation such as like stress will make arteriol aferen constrict and decrease the blood filtration in glomerulus and GFR will lower.

3. Urinary obstruction

Urinary obstruction caused by kidney stone or urther stone will increase hydostatic in Bowman capsul and decreae GFR.

4. Liver disease or low protein diet

Low protein diet will or liver disease will decrease osmotic pressure so GFR will increase.

5. Some kidney diseases

Kidney disease can increase glomerular capilary permeability and lead to GFR in high filtration.

6. Glomerular filtrat composition

Filtrat in Bowman capsul is identic with filtrate plasm in water and sulable substance with low molecular, such as glucose, chloride, natrium, fosfat, urea, uric acid, and creatinin. Albumin plasm can be filtrated, but it is almost reabsorbed and normally absent in urine. White blood cells and infiltrated protein, if this present in urine indicate for infection.

Tubules Reabsorption

Tubules reabsorption is to reabsorb the useful substances in primer urine in proximal tubulus. Almost filtrate (90%) effectively reabsorbed in kidney tubulus by passive difution, transport active, or fasilitated difution. About 85% natrium chloride, water, glucose, amino acid in glomerular filtrat are absorbed in proximal tubulus, although reabsorbtion occur in whole nephrone. Tubulus reabsorbtion are natrium, chlor ion and other negative ion, glucose, fructose, and amino acid, water, urea, and other anorganic ions.

Secretion

Tubular secretion mechanism is an active process that move substances out from the body, its process is in peritubular capilary through tubular fluid to excrete in urine. Hydrogen, kalium, ammonium, creatinin, and hypurat acid, and certain drugs (penisilin) are actively secreted into tubulus.

Hydrogen ion and ammonium are exchanged with natrium ion in distal tubulus. Tubular secretion that is active toward hydrogen ion and ammonium helps pH plasm regulation and acid base balance in the body. Tubular secretion is a mechanism that important to excrete waste product or chemical substances.

Kidney Auto Regulation

Autoregulation mechanism is to prevent kidney blood supply and GFR abnormality. This autoregulation is occur about 80 to 180 mmHg in blood pressure. If this range (normally about 100 mmHg) is increase, aferen arteriole will constrict to reduce kidney blood supply and reduce GFR.  Otherwise if the blood pressure is decrease, so the GFR will increase. Thus, mayor abnormalitis can be prevented.

Autoregulation involve reseptor feedbacek in arteriole wall and apparatus jukstaglomerular. Beside autoregulation mechanism, changes in artery pressure can increase GFR. There are many glomerular filtrate that is produced in a day, only small changes can increase urine output.

Although artery pressure cause clear changes in urine output, this pressure can decrease to 75 mmHg or in high pressure about 160 mmHg, it only can change in small portion in GFR. This effect called GFR autoregulation. It is important because nephron need optimal GFR, only 5% abnormalities in it, it will be a big problem and lead to excessive urine output.

GFR Autoregulation Mechanism-Tubuloglomerulus Feedback

Every nephrone has combination mechanism in GFR autoregulation. That combination are between aferen arteriole vasodilator feedback and eferen arteriol vasoconstrictore feedback. This combination feedback called tubuloglomerulus feedback.

Jukstaglomerulus complex ilustrate jukstaglomerulus complex that show distal tubulus pass a corner between aferen and eferen arterioles. Epithelium cells in this distal tubulus that contact with arteriol is more solid than other tubulus cells and it called makladensa. In distal tubulus, macula densa location is in tubulus distal segment, near to asendenansa henle. Smooth muscle cells in both aferen and eferen arteriole consist of granula where it contact with macula densa. This cell called jukstaaglomerulus (JG cell) and this granule consist of inactive renin.

Arteriole Vasodilator Feedback Mechanism.

Low GFR will reabsorb excessive chloride in tubulus so that decrease chloride ion consentration in macula densa. Otherwise, decrease number of chloride ions will make macula dense to dilate aferen arteriol.

In detail, low GFR causes chloride consentration decrease and make aferen arteriole dilate. This condition will increase blood flow to glomerulus and increase glomerulus pressure. It will increase GFR back to normal.

Eferen Arteriole Vasoconstrictor Feedback Mechanism

Small number of chloride ion in macula densa will make jukstaglomerulus cell release renin and this will produce angiotensin. Then angiotensin constrict eferen arteriole because it is more sensitive toward angiotensin 2 than aferen arteriole.

Now we can describe eferen arteriole vasoconstrictor that help in maintaining constant GFR. Low GFR make chloride ions reabsorbtion in macula densa. Then small chloride ions consentration will stimulate JG cells release renin and their granula. Renin stimulate angiotensin 2 production and it will constrict eferen arteriol, it lead to glomerulus pressure. And the last, GFR will back to normal condition.

Kidney Blood Supply Autoregulation

If artery pressure changes in minutes, so kidney blood supply and GFR will be regulated at the same time. Aferen arteriol vasodilator feedback mechanism makes kidney autoregulation, if kidney blood supply is low, so glomeerulus pressure and GFR will decrease. Feedback mechanism makes aferen arteriole will dilate to regulate GFR back to normal and dilatation will increase blood flow back to normal although in low artery pressure.

Renin-Angiotensin System

Renin is a hormon that is producted by kidney. The function of this hormon is to regulate blood supply when ischemia occur. Renin is syntesied and released from jukstaglomerulus cells. Renin – angiotensin mechanism is not only maintain normal glomerulus filtration, but also  maintain urea excretion.

In hypotention condition, it need to protect water and salt and excrete waste product, urea is the most common waste product. We have already talked about urea excretion is equal with GFR. Eferen arteriole vasoconstriction mechanism can maintain glomerulus filtration, low artery pressure, urea that will be excreted in urine almost reach normal. So hypotenstion (65 to 70 mmHg) will not make urea retention.

Because of angiotensin is produced in kidney, it will circulate in whole body during artery hypotention, so this condition will make water, ions (natrium, chloride, kalium) retention. So, it will maintain water and other ions although urea is excreated.

Perhaps angiotensin makes water and ion conservation with this mechanism. It will increase arteriol resistance that will decrease kidney blood supply and then decrease peritubulus capiler pressure. Otherwise, it can increase water and electrolytes reabsorbtion from tubulus system.