Figure 25.13

Copyright © 2010 Pearson Education, Inc. Figure 25.13

Activetransport

Passivetransport

Peri-tubular

capillary

2

4

4

3

31

1 2 43

Filtratein tubulelumen

Transcellular

Paracellular

Paracellular

Tight junction Lateral intercellular space

Capillaryendothelialcell

Luminalmembrane

Solutes

H2O

Tubule cell Interstitialfluid

Transcellular

Basolateralmembranes

1 Transport across the luminal membrane.2 Diffusion through the cytosol.

4 Movement through the interstitial fluid and into the capillary.

3 Transport across the basolateral membrane. (Often involves the lateral intercellular spaces because membrane transporters transport ions into these spaces.)

Movement via thetranscellular route involves:

The paracellular routeinvolves: • Movement through leaky tight junctions, particularly in the PCT.

Copyright © 2010 Pearson Education, Inc.

Reabsorption of Nutrients, Water, and Ions

• Na+ reabs provides means for reabsorbing most other substances

• Organics are reabs by secondary active transport

• Transport maximum (Tm) reflects number of carriers in renal tubules available

• When carriers are saturated, excess of that substance is excreted


Reabsorption of Nutrients, Water, and Ions

•Water is reabs by osmosis (obligatory water reabsorption), aided by water pores called aquaporins-ADH

• Cations and fat-soluble substances follow by diffusion


1 At the basolateral membrane, Na+ is pumped into the interstitial space by the Na+-K+

ATPase. Active Na+ transport creates concentration gradients that drive:

2 “Downhill” Na+ entry at theluminal membrane.

4 Reabsorption of water byosmosis. Water reabsorptionincreases the concentration of the solutes that are left behind. These solutes can then be reabsorbed asthey move down their concentration gradients:

3 Reabsorption of organic nutrients and certain ions by cotransport at the luminal membrane.

5 Lipid-solublesubstances diffuse by the transcellular route.

6 Cl– (and other anions), K+, and urea diffuse by the paracellular route.

Filtratein tubulelumen

GlucoseAmino acidsSome ionsVitamins

Lipid-solublesubstances

Nucleus

Tubule cell

Paracellularroute

Interstitialfluid

Peri-tubular

capillary

Tight junction

Primary active transport

Passive transport (diffusion) Secondary active transport

Transport protein

Ion channel or aquaporin

Cl–, Ca2+, K+

and otherions, urea

Cl–

3Na+

2K+

3Na+

2K+

K+

H2O

Na+

6

5

4

3

2

1


Reabsorptive Capabilities of Renal Tubules and Collecting Ducts

• Mechanism of aldosterone

• Targets collecting ducts and DCT

• Promotes synthesis of Na+ and K+ channels


Regulation of Urine Concentration and Volume

• Osmolality

• Number of solute particles in 1 kg of H2O

• Reflects ability to cause osmosis


Regulation of Urine Concentration and Volume

• Osmolality of body fluids

• Expressed in milliosmols (mOsm)

• The kidneys maintain osmolality of plasma at ~300 mOsm,


Countercurrent Mechanism

• Occurs when fluid flows in opposite directions in two adjacent segments of the same tube

• Filtrate flow in the loop of Henle (countercurrent multiplier)

• Blood flow in the vasa recta (countercurrent exchanger)


Countercurrent Mechanism

• Role of countercurrent mechanisms

• Establish and maintain an osmotic gradient (300 mOsm to 1200 mOsm) from renal cortex through the medulla

• Allow the kidneys to vary urine concentration


Cortex

Medulla


Formation of Concentrated Urine

• Depends on medullary osmotic gradient and ADH

• ADH triggers reab of H2O in collecting ducts

• Facultative water reabsorption occurs in presence of ADH so that 99% of H2O in filtrate is reabs


Diuretics

• Chemicals that enhance the urinary output

• Osmotic diuretics: substances not reabsorbed, (e.g., high glucose in a diabetic patient)

• ADH inhibitors such as alcohol

• Substances that inhibit Na+ reabs and obligatory H2O reabsorption ie. caffeine and many drugs

Copyright © 2010 Pearson Education, Inc. Figure 25.18a

Cortex

Outermedulla

Innermedulla

(a)

(b)

(c)

(e)

(d)

Na+ (65%)GlucoseAmino acids

H2O (65%) and many ions (e.g.Cl– and K+)

300

Milliosmols

600

1200

Blood pH regulation

H+,NH4

+

HCO3–

Somedrugs

Active transport(primary or secondary)Passive transport

(a) Proximal convoluted tubule: • 65% of filtrate volume reabsorbed • Na+, glucose, amino acids, and other nutrients actively transported; H2O and many ions follow passively • H+ and NH4

+ secretion and HCO3– reabsorption to

maintain blood pH (see Chapter 26) • Some drugs are secreted

Copyright © 2010 Pearson Education, Inc. Figure 25.18b

H2O

(b) Descending limb of loop of Henle • Freely permeable to H2O • Not permeable to NaCl • Filtrate becomes increasingly concentrated as H2O leaves by osmosis

(a)

(b)

(c)

(e)

(d)

Cortex

Outermedulla

Innermedulla

300

Milliosmols

600

1200


Copyright © 2010 Pearson Education, Inc. Figure 25.18c

Na+

Urea

Cl–

Na+

Cl–

K+

(c) Ascending limb of loop of Henle • Impermeable to H2O • Permeable to NaCl • Filtrate becomes increasingly dilute as salt is reabsorbed

(a)

(b)

(c)

(e)

(d)

Cortex

Outermedulla

Innermedulla

300

Milliosmols

600

1200


Copyright © 2010 Pearson Education, Inc. Figure 25.18d

Na+; aldosterone-regulatedCa2+; PTH-regulatedCl–; follows Na+

(d) Distal convoluted tubule • Na+ reabsorption regulated by aldosterone • Ca2+ reabsortion regulated by parathyroid hormone (PTH) • Cl– cotransported with Na+

(a)

(b)

(c)

(e)

(d)

Cortex

Outermedulla

Innermedulla

300

Milliosmols

600

1200


Copyright © 2010 Pearson Education, Inc. Figure 25.18e

Blood pHregulation

Urea;increasedby ADH

Na+

K+

H+

HCO3–

NH4+

H2O regulatedby ADH

Regulated byaldosterone:

(e) Collecting duct • H2O reabsorption through aquaporins regulated by ADH • Na+ reabsorption and K+ secretion regulated by aldosterone • H+ and HCO3

– reabsorption or secretion to maintain blood pH (see Chapter 26) • Urea reabsorption increased by ADH

(a)

(b)

(c)

(e)

(d)

Cortex

Outermedulla

Innermedulla

300

Milliosmols

600

1200

Active transport(primary or secondary) Passive transport


Renal Clearance

• Volume of plasma cleared of a particular substance in a given time

• Renal clearance tests are used to

• Determine GFR

• Detect glomerular damage


Renal Clearance

RC = UV/P

RC = renal clearance rate (ml/min)

U = concentration (mg/ml) of substance in urine

V = flow rate of urine formation (ml/min)

P = conc of same substance in plasma


Renal Clearance

• For any substance freely filtered and neither reabsorbed nor secreted by the kidneys (e.g., insulin),

RC = GFR = 125 ml/min

• If RC < 125 ml/min, substance is reabsorbed

• If RC = 0, substance is completely reabsorbed

• If RC > 125 ml/min, substance is secreted (most drug metabolites)

• Duck vs Beaver


Lumen

AdventitiaCircularlayerLongitudinallayer

TransitionalepitheliumLaminapropria


Renal Calculi

• Kidney stones form in renal pelvis

• Crystallized calcium, magnesium, or uric acid salts

• Larger stones block ureter, cause pressure and pain in kidneys

• May be due to chronic bacterial infection, urine retention, Ca2+ in blood, pH of urine


Urinary Bladder

• Trigone

• Triangular area outlined by openings for ureters and urethra

• Infections tend to persist in this region


Urinary Bladder

• Layers of the bladder wall

1. Transitional epithelial mucosa

2. Thick detrusor muscle (three layers of smooth muscle)

3. Fibrous adventitia (peritoneum on superior surface only)


Urinary Bladder

• Collapses when empty; rugae appear

• Expands and rises superiorly during filling without significant rise in internal pressure

Copyright © 2010 Pearson Education, Inc. Figure 25.21b

Ureter

Trigone

Peritoneum

Rugae

Detrusor muscle

Bladder neck

Internal urethralsphincterExternal urethralsphincterUrogenital diaphragm

Urethra

External urethralorifice

Ureteric orifices

(b) Female.


Micturition

• Urination or voiding

• Three simultaneous events

1. Contraction of detrusor muscle by ANS

2. Opening of internal urethral sphincter by ANS

3. Opening of external urethral sphincter by somatic nervous system


Somatic motornerve activity

External urethralsphincter opens

Sympatheticactivity

Parasympatheticactivity

Urinary bladderfilling stretches

bladder wall

Spinalcord

Promotes micturitionby acting on all threespinal efferents

Inhibits micturitionby acting on all three spinal efferents

Allow or inhibit micturitionas appropriate

Brain

Simplespinalreflex

Spinalcord

Inhibits

Parasympathetic activitySympathetic activitySomatic motor nerve activity

Pontine micturitioncenter

Pontine storagecenter

Higher braincenters

Detrusor musclecontracts; internalurethral sphincter

opens

Afferent impulsesfrom stretch

receptors

Micturition

Figure 25.13

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