Kidney

Chapter 8 Urine formation by the Kidneys

functions of the kidneys 1.

2.

3. 4.

5. 6.

Excretion of metabolic waste products and foreign chemicals Regulation of water and electrolyte balances Regulation of acid-base balance Regulation of body fluid osmolarity and electrolyte concentrations Regulation of arterial pressure Secretion of hormones

The endocrine function of kidney 1. Renin 2. Erythropoietin(EPO) 3. 1,25-dihydroxycholecalciferol (1,25(OH)2-D3) 4. prostaglandins(PG) 5. kenins

§1 Physiological anatomy of the Kidney

1.1 General organization of the urinary system and the kidne Kidney: paired organs,about fist sized, 150 g, outside peritoneum against the back.

Kidney Anatomy

1.2 Nephron   

the basic functional unit of kidney 1 million nephrons in each kidney The kidney cannot regenerate new nephrons.

glomerulus

renal corpuscle Nephron

Bowman capsule

proximal tubule

renal tubule

thick segment of descending limb Loop of Henle thin segment of descending limb thin segment of ascending limb thick segment of ascending limb distal tubule

(afferent arteriole)

(efferent arteriole)

1.3 Cortical nephron and Juxtamedullary nephron

Juxtamedullary nephron cortical nephron Location

close to renal medulla

Proportion 10-15% Glomerular volume larger Loop of Henle long loop deep in medulla The ratio of the caliber 1:1 between afferent arteriole and efferent arteriole Vasa recta + Juxtaglomerular apparatus few Major functions concentrate and dilute urine

cortex 85-90% smaller short loop 2:1

more filtration ， reabsorption and secretion

1.4 Glomerular capillary membrane 1. Three major layers: (1) capillary endothelium (2) basement membrane (3) epithelium (podocytes) of visceral layer of Bowman’s capsule

fenestrae (fenestration

③

②

① capillary endothelium

epithelium

(1) capillary endothelium 



fenestrae(fenestrat ion) 70-90nm Not act as a major barrier for plasma proteins

(2) basement membrane 



Meshwork of collagen and proteoglycan fibrillae that have spaces( 28nm) Filter large amounts of water and small solutes, but effectively prevent filtration of plasma proteins

(3) epithelium (podocytes) 



 



surrounding the outer surface of the capillary basement membrane podocytes :long footlike processes pedicels slit pores(filtration slits) ： 25nm Provide some restriction to filtration

epithelium

2. The filterability of solutes is determined by their size and electrical charge 1. Mechanical barrier: the selective filter of moleculal weight(MW) : MW>69 thousand impermeable MW<69 thousandpermeable 2. Electrochemical barrier: the selective filter of electric charge (a layer of negative protein (saliva protein) located at the surface of filtration membrane) Molecule with positive charges permeable Molecule with negative charges impermeable

1.5 Juxtaglomerular apparatus Distribute in cortical nephron mainly. Consist of Juxtaglomerular cell, extraglomerular mesangial cell and Macula densa. Functions: Macula densa can perceive the change of Na+ concentrations in the distal convoluted tubule . Juxtaglomerular cell can release renin when given a suitable stimulus.

Macula densa is a specialized group of epithelial cells in the initial portion of the distal tubules that comes in close contact with the afferent and efferent arteriols. Juxtaglomerular cell is in the wall of the afferent and efferent arterioles, and can secrete renin. Extraglomerular mesangial cell: Phagocytosis ,contraction

↓ Arterial pressure

Macula densa feedback mechaniam

(-) ↓glomerular

hydrostatic (-) pressure

↓ GFR ↓ Macula densa NaCl

↑ renin ↑ Angiotensin II ↑ Efferent arteriolar resisance

↓ Afferent arteriolar resisance

Renal blood supply Renal artery→segmental arteries →interlobar arteries→arcuate arteries → interlobular arteries(radial arteries)→ afferent arterioles →glomerular capillaries characteristics of →efferent arterioles renal blood supply: two capillaries beds →peritubular capillaries → interlobular vein →arcuate vein →interlobar vein →segmental vein →renal vein.

Renal artery

interlobar arteries arcuate arteries

interlobular arteries

two capillaries beds 



glomerular capillaries: Higher hydrostatic pressure( about 60 mmHg) --- in favor of rapid fluid filtration ; peritubular capillaries: Lower hydrostatic pressure ( about 13 mmHg) ---in favor of rapid fluid reabsorption;

The formation of urine by Kidney 1.

glomerular filtration 2. tubular reabsorption 3. tubular secretion Concentration and dilution of urine

Peritubular capillary blood

Urinary excretion rate=filtration rate-reabsorption rate + secretion rate

Glomerular filtration  

The first step in urine formation when blood flows into the glomerular capillaries, the water bulk flow of protein-free plasma filtrate into Bowman’s capsule through the glomerular membrane

ultrafiltrate 

Most substances in the plasma(except protein)are freely filtrated,so that their concentrations in Bowman’s capsule are almost the same as in the plasma.



The ultra filtrate contains almost no protein because the glomerular membranes restrict the movement of such high-molecule-weight substance

GFR (glomerular filtration rate) the amount of ultra filtrate formed by two kidneys per minute. Normal value:125ml/min,180L/day Filtration fraction = GFR / Renal plasma flow Normal value:about 20% (125/660=19%) (about 20% of the plasma flowing through the kidney is filtered by the glomerular capillaries)

The GFR is determined by (1)Effective filtration pressure (EFP) and (2)glomerular capillary filtration coefficient(Kf) GFR= Kf ☓ EFP





Effective filtration pressure,EFP Represents the sum of the hydrostatic and colloid osmotic forces that either favor or oppose filtration. Forces favoring filtration:

Glomerular hydrostatic pressure(PG) Bowman’s capsule colloid osmotic pressure (πB)=0 

Forces opposing filtration ：

Bowman’s capsule hydrostatic pressure (PB) Glomerular capsule colloid osmotic pressure (πG)

Glomerular hydrostatic Pressure (60mmHg)

Glomerular colloid osmotic Pressure (32mmHg)

Bowman’s capsule pressure (18mmHg) Glomerular hydrostatic Pressure (10mmHg)(60mmHg)

EFP=

Bowman’s capsule pressure (18mmHg)

-

-

Glomerular colloid osmotic Pressure (32mmHg)

Determinants of GFR 

1. EFP: ↑ GFR ↑ 1) Glomerular hydrostatic pressure(PG) : (fig) 80~180mmHg, PG ↑ EFP <80 mmHg or ≻ 180mmHg, PG ↑ EFP ↑  GFR ↑ 2)Bowman’s capsule hydrostatic pressure (PB) : ↑ EFP↓ 2) Example: oppress and block of ureter 3)Glomerular capsule colloid osmotic pressure (πG): ↑  EFP ↓  GFR ↓ Example: drinking large quantities of water,decreased albumin; πG ↓  EFP ↑  GFR ↑ Example :dehydration 脱水

Determinants of GFR 2. Renal plasma flow (RPF) RPF ↑ the velocity of πG increase ↓  filtration areas↑ GFP ↑

Determinants of GFR 3. glomerular capillary filtration coefficient(Kf) Kf is the product of the permeability and filtering surface area of the capillaries. Kf ↑  GFR ↑

Example: diabetes mellitus( 糖尿病 ) thickness of glomerular membrane ↑  Kf ↓  EFP↓  GFR↓

Renal Blood Flow and it’s Regulation

Characteristics of RBF:

1. High blood flow: 1200ml/min: ¼ cardiac output 0.4 % of total body weight A high blood flow is necessary for glomerular filtration. 2.Distribution: cortex  94% outer medulla  5 - 6% inner medulla  <1% 。

3. Determinants of renal blood flow





△P = Renal artery pressure － Renal vein pressure R total renal vascular resistance

4. Physiological control of RBF and GFR 1. Autoregulation of the RBF and GFR （ 1 ） RBF is relatively constant when BP fluctuating between 80 ～ 180 mm Hg even if there are not regulations of nerve and humoral factors. (Figure) （ 2 ） Myogenic mechanism: Tubuloglomerular feedback: （ 3 ） significance: to maintain a relatively constant GFR to allow of renal excretion of water and solutes under normal conditions.

ΔP = Q= R

ΔP 8ηL ᅲᅲ R4

2. Nervous and Hormonal Regulation of RBF  



Nervous regulation: Essentially all the blood vessels of the kidneys, including both the afferent and efferent arterioles, are richly innervated of the renal sympathetic nerve fibers. defense reaction, brain ischemia, or severe hemorrhage  renal nerve (sympathetic nerve) activation  NE releasing α-adrenoceptor  afferent arterioles contraction  RBF↓ ， GFR ↓ (and the vessels of heart and brain dilate, blood flow↑)

 Hormonal

regulation:

Norepinephrine, Epinephrine ↑  afferent arterioles contract  RBF↓ ， GFR ↓ (the vessels of heart and brain dilate, blood flow↑ ) Significance: in emergency  reallocate of blood,and ensure the blood supply of brain and heart.

Hormone or autacoid GFR NE E Endothelin Angiotensin II NO Prostaglandins

effect on ↓ ↓ ↓ — or ↓ ↑ ↑

For many substances, reabsorption plays a much more important role than does secretion in determining the final urinary excretion rate

Reabsorption of Renal tubule and Collecting duct Conception: the process that some substances (such as water, solutes) of the tubular fluid are selectively reabsorbed from the tubules back into the blood. Tubular reabsorption is highly selective:  G,aa,K+ all to be reabsorbed  H O,Na+Cl-  most to be reabsorbed 2  Urea  part to be reabsorbed  Creatinine  not to be reabsorbed

Filtration,reabsorption,and excretion rate of different substances by the kidneys filterde reasorbed Glucose(gm/day) Bicarbonate(mEq/day) ›99.9 sodium (mEq/day) 99.4 chloride(mEq/day) 99.1 urea(gm/day) creatinine(gm/day)

Amount filtered

amount

amount

reabsorbed excreted

180 4,320

180 4,318

0

25,560

25,410

150

19,440

19,260

46.8 1.8

23.4 0

% of load 100

2

180 23.4 1.8

50 0

substance to be reabsorbed must be transported 



across the tubular epithelial membranes into the renal interstitial fluid through the peritubular capillary membrane back into the blood

The transporting pathways of substance through the renal tubular epithelial cells Transcellular pathway: through the cell membranes  Paracellular pathway: through the junctional spaces 

Mechanisms of Reabsorption 1. Passive transport 1). Down electrochemical gradient; 2). not require energy; 3). Mode:Diffusion,Osmosis,facilitated diffusion 4). Example:H2O

2. Active transport 1). Against an electrochemical gradient; 2). require energy; 3). Depend on carrier proteins that penetrate through the membrane 4). divided into two types: 



Primary active transport: coupled directly to an energy source(hydrolysis of ATP) Secondary active transport :coupled indirectly to an energy source(an ion gradient)

Primary active transport is linked to hydrolysis of ATP 

 

Importance: move solutes against an electrochemical gradient energy source: hydrolysis of ATP Example: sodium-potassium ATPase pump

Na+-K+ ATPase hydrolysis ATP  release energy  Transport Na+ out of the cell into the interstitium  Transport K+ from the interstitium into the cell The intracellular concentration of sodium is lower (chemical difference)  The cell interior is electrically negative than the outside (electrical difference) Favor Na+ to diffuse from the tubular lumen  into the cell through the brush border

Secondary active transport 



Co – transport: glucose-sodium transport amino acids -sodium transport phosphate -sodium transport Counter- transport: H+-Na+ transport

Glucose and Amino Acids are reabsorbed by secondary active transport 





They are actively transported across the apical cell membranes of the epithelial cells Their active transport depends on the sodium gradient across this membrane All other steps are passive

Co – transport of Glucose (or amino Acids) along with Sodium ions through The brush border of The tubular epithelial cells

GLUCOSE REABSORPTION HAS A TUBULAR MAXIMUM 



Threshold for glucose: the filtered load of glucose at which glucose first begins to appear in the urine Transport maximum: the maximum rate at which glucose can be reasorbed from the tubules

Passive water reabsorption by osmosis is coupled mainly to sodium reabsorption Solutes transported out of the tubule their concentrations inside the tubule their concentrations in the interstitium  create a concentration difference  cause water reabsorption by osmosis from the  tubular lumen to the renal interstitium  Prerequisite: the membrane is permeable to water 

Reabsorption of chloride,urea and other solutes by passive diffusion + Na reabsorption H2O reabsorption Lumen Negative Luminal Clpotential concentration Passive Clreabsorption

Luminal Urea concentration Passive Urea reabsorption

Reabsorption and secretion along different parts of the nephron

Proximal tubule 

reabsorb : about 65% of the filtered Na+, Cl- , HCO3- , K+ , H2O

essentially all the filtered glucose and amino acids  secrete: Organic acids,bases and H+

Solute and water in the loop of Henle 

The descending thin limb of the loop of Henle:

o

Highly permeable to water(20% filtered water reabsorbed) Moderately permeable to most solutes,including urea and sodium

o



The thick ascending limb of the loop of Henle:

o

Reabsorb about 25% of the filtered loads of Na+, Cland K+ (1Na+-2Cl--1K+ Co – transport ) Reabsorb large amounts of Ca2+, HCO3- and Mg2+

o o

impermeable to water(the tubule fluid in the ascending limb becomes very dilute)



The thin ascending limb of the loop of Henle: much lower reabsorptive capacity than the thick ascending limb

Distal tubule  o

o o

Early distal tubule: Has many of the same characteristics as the thick ascending loop of Henle Reabsorb Na+, Cl- , Ca2+, and Mg2+ impermeable to water and urea

late distal tubule and cortical collecting bubule

  o

o



Have similar functional characteristics Composed of two distinct cell types:

Principal cells: reabsorb Na+ and water( controlled by the level of ADH) secrete K+ Intercalated cells: reabsorb K+ secrete H+ (role: regulate acidbase balance)

impermeable to urea

Medullary collecting duct 

 

Reabsorb less than 10% of the filtered water and sodium( controlled by the level of ADH) permeable to urea secrete H+(effects: regulate acid-base balance)

Secretion of the Renal tubules and collecting duct:

Conception: the process that the epithelia of renal tubules and collecting duct secrete their metabolic products or substance of blood into the tubular lumen. Include: 1. The secretion of H+ 2. The secretion of K+ 3. The secretion of NH3

1. The secretion of H+ Position: mainly in proximal tubule Achieve by H+-Na+ antiport Can promote the reabsorption of NaHCO3 Other position:Distal tubule and collecting duct H+-K+ ATPase ， hydrogen ATPase Secreted H+ has three function: 　　　　 H+ + HCO3- 　→ H2CO3 H+ + NaHPO4- → NaH2PO4 H+ + NH3 → NH4+

2 . The secretion of K+: 



Most of the daily variation in K+ secretion is caused by changes in K+ secretion in the distal tubule and collecting duct Position: principal cells of distal tubule and collecting duct

Two steps of K+ secretion •

•

Uptake from the interstitium into the cell by Na+-K+ ATPase Passive diffusion from the interior of the cell into the tubular fluid though K+ channels

3 、 The secretion of NH3 







NH3 in the renal tubule is come from Glutamine deamination NH3 enter tubule by ways of diffusion or NH4 Na+ antiport The secretion of H+ may promote the secretion of NH3 significance ： promote the secretion of H+ and the reabsorption of NaHCO3 , so play an important role in keep the acid-base balance

The concentration and dilution of urine Hyperosmotic urine: lack of water → concentrated urine → osmotic pressure can up to 1200mmol/L Hypo-osmotic urine: excess water → dilute urine → osmotic pressure can down to 40mmol/L Isosmotic urine: may be renal failure

1. Dilution of urine Excess water in the body  extracellular fluid osmolarity ↓  ADH secretion ↓  reduces the permeability of the distal tubules and collecting ducts to water  Solutes of tubular fluid are reabsorbed, but water is not reabsorbed so much  lead to dilution of urine.

2. Concentration of urine Water deficit in the body  extracellular fluid osmolarity ↑  ADH secretion ↑  increase the permeability of the distal tubules and collecting ducts to water  large amounts of water is reabsorbed  lead to concentration of urine

Determinants of concentration and dilution The osmotic gradient in Medulla  water absorption force The level of ADH

ADH increases the permeability of the distal tubules and collecting ducts to water

COUNTERCURRENT MAKES THE OSMOTIC GRADIENT 

Outer medulla : 



Active reabsorption of NaCl at thick ascending limb of loop of Henle;

Inner medulla : 



Urea diffusing from collecting duct of inner medulla NaCl diffusing from thin ascending limb of loop of Henle;

THE OSMOTIC GRADIENT CONCENTRATES THE URINE WHEN ADH IS PRESENT

Regulation of urine formation Regulative pathway: filtration, reabsorption, and secretion. 1. Autoregulation glomerulotubular balance 

The ability of the tubules to increase reabsorption rate in response to increased tubular load, even though the percentage of glomerular filtrate reabsorbed in the proximal tubule remains relatively constant at about 65% （ constant 　 fraction reabsorption ）

glomerulotubular balance 

GFR:

125ml/min

150ml/min

The absolute rate of proximal tubular reabsorption: 81ml/min 97.5ml/min (65% of GFR) (65% of GFR)  Importance: help prevent overloading of the tubular segments when GFR increases.

2. Humoral regulation:  

ADH RAAS

  

 





Antidiuretic hormone, ADH

Also called vasopressin,VP a small peptide with 9 aa; Synthetic site: supraoptic nucleus and paraventricular nucleus of Hypothalamus Site of Storage: neurohypophysis (posterior pituitary) Site of action: the ADH receptor at distal tubule and collecting duct effects: increase the water permeability of distal tubule and collecting duct Results:More water is reabsorbed Urine volume is reduced Fluid is conserved in the body

Regulation factors of ADH 1). Extracellular fluid osmolarity: Water dificit  extracellular fluid osmolarity ↑  excite osmorecepter of Hypothalamus  excite ADH neurons  posterior pituitary release ADH ↑  water permeability of distal tubule and collecting duct↑  urine↓ Contrariwise , Extracellular fluid osmolarity ↓  urine ↑

2). Blood volume

Blood volume↑  Excite cardiopulmonary receptor  vagi excitation  Inhibit hypothalamus release ADH  Urine↓ Contrariwise urine ↑

3). Else Baroreceptor excitation  ADH ↓ Pain, nausea, vomit ,ANGⅡ → ADH ↑ Disease of supraoptic nucleus and paraventricular nucleus  ADH ↓↓, this phenomenon is called diabetes insipidus.

Renin-AngiotensinAldosteron System RAAS



Renin: Secreted by juxtaglomerular cells



Angiotensin II: the body’s most powerful sodiumretaining hormone



Aldosterone: Secreted by zona glomerulosa cells of adrenal cortex  Function: improve of sodium reabsorption ,water reabsorption and potassium secretion by distal tubule and collecting duct 

Directly stimulates sodium reabsorption, especially in the proximal tubules Increase the quantity of NE released by sympathetic nerves ending. Increased the vasoconstrictor tone of sympathetic vasoconstrictor center.

Aldosterone function: increases the reabsorption of sodium and the secretion of potassium in principal cells of the distal tubule, Then increase the reabsorption of water by osmosis

Mechanism of alsosterone effects

Regulation of secretion of renin 

  

Baroreceptors of afferent arterioles excite receptors of macula densa excite Renal sympathetic nerve excite PGE2,PGI2,NE and Adr  renin ↑

3. Nervous regulation 

Renal sympathetic nerve: NE 1). Contraction of renal vessel→ RBF↓ →GFR ↓ 2). Activate RAAS 3).improve the reabsorption of Na+,Cl- ,water by proximal tubule

Renal clearance Definition: The renal clearance of a substance is the volume of plasma that is completely cleared of the substance by the kidneys per unit time. the ability of the kidneys to "clear" or remove a specific substance from the blood. Clearance equation: C = U ×V / P （ ml/min ）



UX×V ＝ GFR × PX － RX ＋ SX

Urine Straw yellow ； Specific gravity: 1.015 - 1.025; Osmotic pressure : urine > plasma ； Acidity ： pH: 5.0 - 7.0 ; Normal quantity of urine of adults: 1000 - 2000ml/ 24h, 1500ml. >2500ml / 24h , called diuresis ； 100 - 500ml / 24h, called oliguria ； <100ml /24h, called anuria.

Urine Transportation, Storage, and Elimination 





Basic pathway:  urine flows through papillary ducts into the minor calyces → major calyces → renal pelvis → ureters → the urinary bladder →urethra Micturition is a two stage process involving the passive storage and the active voiding of urine. The nervous control to micturition includes the parasympathetic, sympathetic and somatic nervous systems.

THE URINARY BLADDER STORES THE URINE 





Gravity and peristaltic contractions propel the urine along the ureter Parasympathetic stimulation contracts the bladder and micturition results if the sphincters (internal and external urethral sphincters) relax The external sphincter is under voluntary control

Anatomy 

a. trigone: a triangle-shaped area bounded by the ureteral openings superiorly and the opening to the urethra inferiorly.                 b. mucosa: transitional epithelium + connective tissue of the lamina propria;  with rugae                 c. muscularis:  detrusor muscle; 3 layers of smooth muscle                 d. internal urethral sphincter: formed by circular fibers, around the opening to the urethra (smooth muscle)                 e. external urethral sphincter : skeletal muscle                 f. adventitia / serosa

micturition reflex ❶urine volume > 200-400 ml → bladder pressure↑ ❷ → excites sensory stretch receptors in bladder wall(the bladder feels "full" ) →conduct sensory signals by sensory nerve fiber of pelvic nerves ❸ → sacral micrurition centers(S2-S4) and brain ❹ → parasympathetic nerve fiber of pelvic nerves to the urinary bladder wall →the detrusor muscle contract and the internal urethral sphincter relax

→ urine enters posterior urethra → further excites sensory stretch receptors in posterior urethra and bladder wall → further increase in reflex contraction of bladder(self-regeneration,positive feedback) ❺ (meanwhile of ❹) → inhibit pudendal nerve under voluntary control → the external urethral sphincter  diastole ❻ →urinate

cystometrogram 

Cystometric study uses a device to pump water into the bladder. The device then measures the amount of fluid present in the bladder when you first feel the need to urinate, when you are able to sense fullness, and when your bladder is completely full.

Facilitation or inhibition of micturation by the brain Centers in the brain:  pons: strong facilitory and inhibitory centers in the brain stem  several centers located in the cebebral cortex: mainly inhibitory 

Higher center exert final control of micturition 





The higher centers keep the micturition reflex partially inhibited except when micturition is desired The higher centers can prevent micturition, even if the micturition reflex does occur,by continual tonic contraction of the external bladder sphincter until a convenient time presents itself. When it is time to urinate,the cortical centers can facilitate the sacral micturition centers to help initiate a micturition reflex and at the same time inhibit the external urinary sphincter so that urination can occur.

Reabsorption of Na+ (1). About 65 - 70% proximal tubule; 20%  loop of Henle; 10% distal tubule; 3%  collecting duct. (2). Filtration of Na+ > 500g/D, excretion of Na+ is 3 ~ 5 g/d, 99% of filtration is reabsorbed. (3). Pathway of reabsorption: Transcellular pathway Paracellular pathway

The net reabsorption of sodium ions involves at least three steps (1). Sodium diffuses across the luminal membrane into the cell down an electrochemical gradient established by the sodium pump on the basolateral side of the membrane. (2). Sodium is transported across the memebrane into interstitial fluid against an electrochemical gradient by the sodium pump. (3). Sodium is reabsorbed from the interstitial fluid into the peritubular capillaries by ultrafiltration.

The mechanism of Na+ reabsorption 



The first step of Na+ reabsorption in different tubular segments has it’s own characteristics. The second step and the third step of Na+ reabsorption in the renal tubules are similar.

The mechanism of Na+ reabsorption 1. Early Proximal tubule : Sodium enter into cells by way of: (1) Na+-H+ antiport; (2) Na+- G symport; Na+- aa symport; Na+- latic acid symport;

2. Later Proximal tubule: Sodium enter into cells by way of: (1). Na+-H+ antiport; (2). Paracellular pathway: 顺电势梯度被 动吸收

3.Thick segment of loop ascending limb

Sodium enter into cells by way of: 1- sodium, 2- chloride, 1- potasium symport End result of three kinds of ions: Na+ is pumped into interstitial fluid by sodium pump and reabsorbed ; Cl- diffuse into cell gap down electrochemical gradient through Cl- channels : K+ diffuse back to tubular lumen down electrochemical gradient by K+ channels; Furosemide can inhibit the symport of 1Na+2Cl-1K+ and play role in diuresis.

4. Early Distal tubule: Sodium enter into cells by way of:  

Na+- Cl- symport Thiazide （噻嗪类）

5. Later Distal tubule and Collecting Duct: 



Sodium diffuses across the apical membrane into the Principal cell （ Sodium channel ） Reaborsoption of Na+ at distal tubule and collecting duct regulated by Aldosterone;

Reabsorption of water:       



1. Quantity of reasorption:99% 2. Passive reabsorption: osmotic pressure 3. Position and siginificance: Proximal tubule: 65-70%; Accompanied by the reabsorption of NaCl Has nothing to do with whether the body lack water or not. Not regulated by hormones;



Distal tubule: 10%;



Collecting duct:10% ；



Relation to whether body lack water or not; Regulated by ADH and Aldosterone; Descending limb:10%;

 

Reabsorption of HCO3- : 

 



About 80 ~ 85 per cent in proximal tubule. Reabsorbed in a form of CO2 . Relate to the H+ secretion of tubular epithelium. Significance:maintain the acid- base balance of extracellular fluid.

Acetazolamide can inhibit the action of Carbonic Anhydrase, diuretic

Reabsorption of K+ : All the filtrated K+ is reabsorbed, and the K+ in the urine come from excretion of distal tubule and collecting duct . Position: most at proximal tubule Active transport.

Reabsorption of glucose:  



Position: proximal tubule. All the filtrated glucose is reabsorbed under normal condition. Secondary active transport, accompanied by the primary active transport of sodium .

Renal threshold for glucose: 



the maximal blood sugar concentration which can not result in glucosuria. Reasons: there is a limit to the amount of transporter proteins and binding site.

维持稳态的控制系统的组成要素

体液的组成

肾功能机制