Chapter 25 Urinary System Lecture 16 Marieb’s Human Anatomy and Physiology Marieb Hoehn.

Chapter 25Urinary System

Lecture 16

Marieb’s HumanAnatomy and

Physiology

Marieb Hoehn

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Lecture Overview

• Tubular reabsorption and secretion

• Regulation of urine concentration and volume

• Urea and uric acid excretion

• Renal clearance

• Elimination of urine

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Simplified Overview of Nephron Function

Renal corpuscle – provides the raw materials to the nephron for processing

PCT – reclaims those substances the body can use; gets rid of some things

DCT – gets rid of those substances the body doesn’t want or need; reabsorbs some more Na+, Ca2+

Collecting duct – provides the OPTION of reclaiming H2O or letting it pass out of the body

Obligatory H2O reabsorption…

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Overview of Renal Function• Filtration (based primarily on size; but some electrical)

– Occurs exclusively in renal corpuscle– Occurs across the filtration membrane

• Nutrient and fluid reabsorption– Primarily in the proximal convoluted tubule (PCT) – 65%

vol.– Rest occurs in nephron loop , DCT, and collecting tubule

• Active secretion occurs primarily– PCT– DCT

• Regulation of final volume and solute concentration of urine– Nephron loops (juxtamedullary nephrons)– Collecting ducts Good overview slide…

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Overview of Renal FunctionFigure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

You should know what is moved (red arrows indicate the most important items), and in what part of the nephron they are moved.

Keep in mind: Where Na+ goes, H2O and Cl- usually follow.

**See the Summary Table in your Study Guide for Exam 3

*

*

**

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Tubular Reabsorption and Secretion

Secretion – elimination of substances by the body that did not leave the blood

at the glomerulus (blood tubule)

Figure from: Hole’s Human A&P, 12th edition, 2010

Reabsorption – reclaiming of substances in filtrate by body (tubule blood)

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Tubular Reabsorption• Reabsorption by tubular cells is a selective process

– Diffusion– Osmosis– Carrier-mediated transport

• Facilitated diffusion• Active transport• Co- and counter-transport

– Occurs mainly in the proximal convoluted tubules (PCT)– Has a transport maximum, Tm, for most substances besides

Na+

• Tm is the rate at which solutes can be transported, e.g., 375 mg/min• Renal threshold is the plasma level (concentration) above which a

particular solute will appear in urine, e.g., 180 mg/dl

• Peritubular capillaries are well-suited for reabsorption– Low hydrostatic pressure– High degree of permeability– Higher colloid osmotic pressure due to filtration

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Renal Clearance (C)• the rate (ml/min) at which a substance is removed from the plasma; volume of plasma from which the kidneys clear a particular substance in a given time (usually 1 minute)

• C (ml/min) = U * V / P

[U = urine concentration, V = rate of urine formation, P = plasma concentration]

• tests of renal clearance (Cx)• inulin clearance test (standard; Cinulin = GFR = 125 ml/min)• creatinine clearance test (easy to do; C = 140ml/min)• paraminohippuric acid (PAH) test

• tests of renal clearance are used to calculate glomerular filtration rate

Examples:- Cx = 125 ml/min (125 ml of ‘x’ is removed from the plasma every min = 100%)- Cx = 60 ml/min (some reabsorption of ‘x’ is occurring)- Cx = 0 ml/min (complete reasbsorption of ‘x’ is occurring) - Cx = 630 ml/min (secretion of ‘x’ is occurring)

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Reabsorption in PCT

Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

65% of filtrate volume is reabsorbed in the PCT

All uric acid, about 50% of urea, and no creatinine is reabsorbed

8 mm Hg

COP

Tubular fluid

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Reabsorption in the PCTSubstance Mechanism of

ReabsorptionNotes

Na+ (Cl-) Primary Active Transport Na+ reabsorption is the driving force for most

other reabsorption

H2O Osmosis Closely associated with movement of Na+

(Obligatory water reabsorption)

Glucose Secondary Active transport Limited # of molecules can be handled

(Tm = 375 mg/min); attracts H20

Amino Acids Secondary Active transport Three different active transport modalities; difficult

to overwhelm

Other electrolytes Secondary Active transport

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Sodium and Water Filtration, Reabsorption, and Excretion

Because of the large volumes involved, small changes in tubular reabsorption amount to LARGE changes in excretion of Na+ and H2O (since “water follows salt”)


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Tubular Secretion

Tubular secretion (reabsorption in reverse) moves substances from the blood into the tubular lumen (urine)

1. Getting rid of substances not already in filtrate

2. Eliminating undesirable substances reclaimed by passive process, e.g., urea

3. Ridding body of excess K+

4. Controlling blood pH


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Secretion in the PCT and DCTIn the DCT potassium ions or hydrogen ions may be secreted in exchange for reabsorbed sodium ions. Reabsorption of Na+ in the DCT is increased by the hormone, aldosterone.

Other compounds are actively secreted as well, e.g., histamine, ammonia, creatinine, penicillin, phenobarbital.

Active Active and Passive


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Where have we been; we are we going?

Renal corpuscle – provides the raw materials to the nephron for processing

PCT – reclaims those substances the body can use

DCT – gets rid of those substances the body doesn’t want or need; reabsorb some more Na+, Ca2+

Collecting duct – provides the OPTION of reclaiming H2O or letting it pass out of the body

300 mOsm/L

*Note osmolarity of fluid in PCT

Obligatory H2O reabsorption…

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Overview of Facultative H2O Reabsorption

• Note that outflow of water from collecting duct is dependent upon the osmotic gradient in the medulla

Under influence of ADH

(Facultative water reabsorption)

Urea


Increasin

g concen

tration →

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The Loop of Henle (Nephron Loop)

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Crucial renal function is to keep the body fluids at about 300 mOsm (osmolarity of blood plasma) by varying the concentration of urine

The mechanism shown is called the “countercurrent multiplier”

SO HOW DOES THIS HELP?

Increasin

g concen

tration →

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The Countercurrent Multiplier

• Recall that all movement of H2O occurs passively by osmosis

• We would like some mechanism to concentrate urine– Excrete more H2O when body fluids are tending to become

hypotonic (more dilute)– Excrete less H2O when body fluids are tending to become

hypertonic (more concentrated)• Utilizes two factors

– Hypertonicity of the peritubular fluid, established by the countercurrent multiplier

– Variable permeability of the collecting ducts to H2O depending upon levels of ADH

• We will use the phrase, “Water follows salt (solute) when it can”

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Influence of ADH on H2O Reabsorption• Note that amount of water pulled out of collecting duct is dependent upon the osmotic gradient in the medulla that was established by the countercurrent multiplier.

Under influence of ADH

(Facultative water reabsorption)

Urea


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Vasa Recta of Juxtamedullary Nephrons

Recall that the vasa recta is present in juxtamedullary nephron loops (which give the kidneys the ability to produce a concentrated urine)

The vasa recta functions to

1. Deliver blood to medullary cells

2. Return reabsorbed solutes and water in the medulla to the general circulation without disrupting the medullary concentration gradient

(Countercurrent Exchanger)


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Urea and Uric Acid Excretion

Urea• product of amino acid catabolism (deamination)• plasma concentration reflects the amount or protein in diet• enters renal tubules through glomerular filtration• 50% reabsorbed• rest is excreted

Uric Acid• product of nucleic acid metabolism• enters renal tubules through glomerular filtration• 100% of filtered uric acid is reabsorbed• 10% secreted and excreted

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Summary of Events in the Nephron/Collecting Duct

1. Filtrate produced

2. Reabsorption of 65% of filtrate

3. Obligatory water reabsorption

4. Reabsorption of Na+ and Cl- by active transport

5,6. Facultative reabsorption of water

7. Absorption of solutes and water by vasa recta

(Aldosterone)

(Aldosterone)

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Diuretics

• Osmotic diuretics, e.g., mannitol, glucose• Drugs that block Na+/Cl- transport in PCT and DCT,

e.g., hydrochlorothiazide• High-ceiling/loop diuretics that reduce gradient along

nephron loop, e.g., furosemide (Lasix)• Aldosterone-blocking agents, e.g., spironolactone (K+

sparing), natriuretic peptides• ACE inhibitors, e.g., Captopril• Drugs with diuretic side-effects, e.g., alcohol (how?),

caffeine

A diuretic promotes the loss of water in the urine

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Urine• Urine composition varies depending upon

– Diet– Level of activity

• Major constituents of urine– H2O (95%) – Creatinine (remember, NONE of this is reabsorbed)– Urea (most abundant solute), uric acid– Trace amounts of amino acids– Electrolytes– Urochrome (yellow color), urobilin, trace of bilirubin

• Normal urine output is 0.6-2.5 L/day (25-100 ml/hr)• Output below about 25 ml/hour = kidney failure

(oliguria - anuria)

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Terms to know…

• Anuria – absence of urine

• Diuresis – increased production of urine

• Dysuria – difficult or painful urination

• Enuresis – uncontrolled (involuntary) urination

• Glycosuria (glucosuria) – glucose in the urine

• Hematuria – blood in the urine

• Oliguria – scanty output of urine

• Polyuria – excessive urine output

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Elimination of Urine

• nephrons• collecting ducts• renal papillae• minor and major calyces• renal pelvis• ureters• urinary bladder• urethra• outside world

Flow of Urine

Know this…

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Ureters• 25 cm long• extend downward posterior to the parietal peritoneum• parallel to vertebral column• in pelvic cavity, joins urinary bladder• peristaltic contractions

Mucous coat – transitional epithelium continuous with linings of the renal tubules and urinary bladderMuscular coat – smooth muscle in longitudinal and circular bundles; carries out peristalsis to move urine toward bladderFibrous coat (adventitia)– CT layer continuous with the renal capsule and peritoneum

Walls of the ureters:


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Urinary Bladder [Cyst(o)]

Four layers:

1. Mucous layer (transitional epi.)

2. Submucous coat

3. Muscular coat

4. Serous layer

Muscular layer runs in all directions (detrusor muscle) under parasympathetic control. Contraction compresses the bladder and causes urine to flow into urethra

Temporary reservoir for storage of urine

Note the internal sphincter at neck of bladder

Frontal section, anterior posterior


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Urinary Bladder and Urethra - Female

Urinary bladder is inferior to the uterus and is separated from the rectum by the vagina.

Note the short urethra (about 4 cm)

Note the passage of the urethra through the urogenital diaphragm that forms the external urethral sphincter

Pelvic region, midsagittal section


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Urinary Bladder and Urethra - Male

Base of the urinary bladder lies between the rectum and pubis symphysis.

Pelvic region, midsagittal section


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Urinary Bladder and Urethra - Male

Note the long urethra (about 18-20 cm). There are three sections to the male urethra:

- Prostatic urethra - Membranous urethra - Penile urethra

Note the passage of the urethra through the urogenital diaphagm that forms the external urethral sphincter


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The Pelvic Diaphragm (Female)

These are DEEP muscles that form the pelvic floor and extend between the pubic bones


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Cross Section of UrethraExtends from neck of bladder to exterior

Epithelium varies from

- Transitional at neck of bladder

- Stratified columnar at midpoint

- Stratified squamous near external urethral meatus


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Micturition (Urination) Reflex

• trigger = bladder distention & stimulation of stretch receptors• micturition center activated in sacral portion of spinal cord

• parasympathetic nerve impulses cause detrusor muscle to contract (short reflex) and internal urethral sphincter to open• need to urinate is sensed (spinal cord -> thalamus - > cortex) - urge to urinate at about 150-200 ml of urine - discomfort at about 300 ml of urine - maximum capacity of bladder is about 600-1000 ml

• voluntary (tonal) contraction of external urethral sphincter prevents urination and also closes the internal sphincter

• when decision is made to urinate, external and internal urethral sphincters relax, detrusor muscle contracts, and urine is expelled

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Micturition Reflex


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Life-Span Changes

• kidneys appear scarred and grainy • kidney cells die• by age 80, kidneys have lost a third of their mass• kidney shrinkage due to loss of glomeruli• proteinuria may develop• renal tubules thicken• harder for kidneys to clear certain substances• bladder, ureters, and urethra lose elasticity• bladder holds less urine

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Review

• Urine formation is a product of – Filtration– Tubular reabsorption– Tubular secretion

• Tubular reabsorption– Reclaims important substances in the filtrate– Takes place primarily in the PCT– Uses diffusion, osmosis, and carrier-mediated

transport

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Review

• Tubular secretion – Rids the body of substances that have not been

filtered or are present in excess– Takes place throughout the tubules– Is mainly dependent upon active transport

• Regulation of urine concentration and volume– Results from a combination of

• Countercurrent multiplier in loop of Henle

• Responsiveness of the DCT and collecting ducts to ADH and aldosterone

– Is critical to homeostasis

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Review

• Urine composition – Is variable– Depends upon both diet and activity– Consists of mostly water plus

• Creatinine

• Urea

• Uric acid

• Traces of amino acids

• Electrolytes

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Review• Elimination of Urine

– Know pathway of urine flow– Ureters transport urine from kidney to urinary

bladder– Urinary bladder

• is a temporary storage site for urine• has muscular coat (detrusor) under parasympathetic

control

– Urethra is the conduit for urine from the bladder to the exterior

• Varies in length between males (18-20 cm) and females (4 cm)

• Has different types of epithelium along its length

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Review

• Micturition (urination) reflex– Begins with distension of urinary bladder– Activates mild contraction of the detrusor

muscle– Sends impulses to higher brain centers– Requires voluntary relaxation of the external

urethral sphincter in order for urination to occur

Chapter 25 Urinary System Lecture 16 Marieb’s Human Anatomy and Physiology Marieb Hoehn.

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