chapter 26

Chapter 26 The

Urinary System

Copyright © John Wiley & Sons, Inc. All rights reserved.

The Urinary System The urinary systems consists of the kidneys, ureters,

bladder, and urethra, along with its associated nerves and

blood vessels.

The system maintains homeostasis by:

Regulating blood volume, pressure, pH, and

concentration (osmolarity) of electrolytes (Na+, K+,

Ca 2+, Cl-, HPO4-3, Mg2+, HCO3

-)

Reabsorbing glucose and excreting wastes

Releasing certain hormones like renin and EPO


The kidneys are bean-shaped organs located just above the

waist between the peritoneum and the posterior wall of

the abdomen (in the retroperitoneal space).

They are partially protected

by the eleventh and

twelfth pairs of ribs.

Because of the position

of the liver, the right

kidney is slightly

lower than the left.

Renal Anatomy


A ureter (approximately 25 cm long)

originates near an indented area of each

kidney called the hilum and travels to

the base of the bladder.

From the bladder, the

urethra (4 cm in length in

women and 24 cm in length

in men) allows urine to

be excreted.

Renal Anatomy


Renal Anatomy A frontal section through the kidney reveals two distinct

regions of internal anatomy, the cortex and medulla.

The main function of the cortex

is filtration to form urine.

The main function of

the medulla is to

collect and

excrete urine.


The renal pyramids consists of 8 to 18 conical subdivisions

within the medulla that contain the kidney’s

secreting apparatus and tubules.

Renal columns are

composed of lines of

blood vessels and

fibrous material

which allows the

cortex to be better anchored.

Renal Anatomy


The renal papilla is the location where the medullary pyramids empty urine into cuplike structures called minor and major calyces. 8-18 minor calyces and 2-3 major calyces receive urine from the papilla of one renal pyramid.

Renal Anatomy


From the major calyces, urine drains into a single large

cavity called the renal pelvis and then out through

the ureter.

The hilum expands into

a cavity within the

kidney called the renal

sinus, which contains part

of the renal pelvis, the calyces,

and branches of the renal blood

vessels and nerves.

Renal Anatomy


The ureters transport urine from the renal pelvis of the

kidneys to the bladder using peristaltic waves,

hydrostatic pressure and gravity to move the urine.

There is no anatomical

valve at the

opening of the

ureter into

the bladder.

Renal Anatomy


The urethra is a small tube leading from the internal

urethral orifice in the bladder floor to the exterior.

In males, it

is also used

to discharge

semen.

Renal Anatomy


Renal Blood Flow The renal artery and renal vein pass into the substance of

the kidney (the parenchyma) at the hilum.

Arterial blood enters via the renal artery and exits the

renal vein.


Blood follows the path depicted in the

graphic to the right:

Renal Blood Flow


The Nephron The nephron is the functional unit of the kidney.

It is a microscopic

structure composed

of blood vessels and

tubules that collect

the filtrate which

will ultimately

become urine.


The Nephron Each nephron receives one afferent arteriole, which

divides into a tangled, ball-shaped capillary network

called the glomerulus.

The glomerular

capillaries then

reunite to form an

efferent arteriole that

carries blood out of

the glomerulus.


The Nephron Glomerular capillaries are unique among capillaries in

the body because they are positioned between two

arterioles, rather than between an arteriole and a venule.

There are venules in the kidney, but they come later.

The Renal Corpuscle consists of two structures:

The glomerular capillaries

The glomerular capsule (Bowman’s capsule) – a double-

walled epithelial cup that surrounds the glomerular

capillaries.


Bowman’s capsule consists of visceral and parietal layers.

The visceral layer is made of modified simple squamous

epithelial cells called podocytes. The many foot-like

projections of these cells (pedicels) wrap around the

single layer of endothelial cells of the glomerular

capillaries and form the inner wall of the capsule.

The parietal layer of the glomerular capsule is a simple

squamous epithelium and forms the outer wall of the

capsule.

The Nephron


The epithelium of the visceral

and parietal layers of the

renal corpuscle form

festrations (pores)

which act as a

filtration (dialysis)

membrane.

The Nephron


Fluid filtered from the glomerular capillaries enters

Bowman’s space, (the space between the two layers of the

glomerular capsule), which is the lumen of the urinary

tube.

The Nephron


Blood plasma is filtered through the glomerular capillaries

into the glomerular capsule.

Filtered fluid passes into

the renal tubule, which

has three main sections:

◦the proximal convoluted

tubule (PCT)

◦the loop of Henle

◦the distal convoluted

tubule (DCT)

The Nephron


The distal convoluted tubules of several nephrons empty

into a single collecting duct.

Collecting ducts

unite and converge

into several hundred

large papillary ducts

which drain into the

minor calyces, major calyces,

renal pelvis, and ureters.

The Nephron


The Nephron The first part of the loop of Henle (the descending limb)

dips into the renal medulla. It then makes a hairpin turn and returns to the renal cortex as the ascending limb. The descending limb of the loop of Henle is composed of

a simple squamous epithelium. The ascending limb of the loop may be either “thin”

(composed of a simple squamous epithelium) or “thick” (composed of simple cuboidal to low columnar cells). ◦ Some nephrons contain both thick and thin ascending

limbs.


The Nephron Based on the length of the loop of Henle and the presence

of thin segments in the ascending limb, nephrons can be

sorted into two populations:

cortical and juxtamedullary.

Nephrons with long loops of

Henle enable the kidneys to

create a concentration gradient

in the renal medulla and to

excrete very dilute or very

concentrated urine.


The Nephron Cortical nephrons make up about 80–85% of the 1 million

microscopic nephrons that comprise each kidney.

Their renal corpuscles are located in the outer portion of

the cortex, with short loops of Henle that penetrate only

a small way into the medulla.

The ascending limbs of their loops of Henle consist of

only a thick segment, lacking any thin portions.

Nephrons with short loops receive their blood supply

from peritubular capillaries that arise from efferent

arterioles.


The Nephron


The Nephron The other 15–20% of the nephrons are juxtamedullary

nephrons .

Their renal corpuscles lie deep in the cortex, close to

the medulla, and they have long loops of Henle that

extend into the deepest region of the medulla.

The ascending limbs of their loops of Henle consist of

both thin and thick segments.

Nephrons with long loops receive their blood supply

from the vasa recta that arise from peritubular

capillaries before becoming peritubular venules.


Juxtamedullary Nephron


In each nephron, the final part of the ascending limb of

the loop of Henle makes contact with the afferent

arteriole serving that renal corpuscle. Because the

columnar tubule cells in this region are crowded together,

they are known as the macula densa.

Alongside the macula densa, the wall of the afferent

arteriole contains modified smooth muscle fibers called

juxtaglomerular (JG) cells.

◦ Together with the macula densa, they constitute the

juxtaglomerular apparatus (JGA).

The Nephron


The Nephron As we will shortly see, the JGA helps regulate blood

pressure within the kidneys.


Renal Physiology The 3 basic functions performed by nephrons and

collecting ducts are:

Glomerular filtration - pressure forces filtration of

waste-laden blood in the glomerulus. The glomerular

filtration rate (GFR) is the amount of filtrate formed in

all the renal corpuscles of both kidneys each minute.

Tubular reabsorption – the process of returning

important substances from the filtrate back to the body.

Tubular secretion – the movement of waste materials

from the body to the filtrate.


Glomerular Filtration Glomerular filtration is the formation of a protein-free

filtrate (ultrafiltrate) of plasma across the glomerular

membrane.

Only a portion of the blood plasma delivered to the

kidney via the renal artery is filtered.

Plasma which escapes filtration, along with its protein

and cellular elements, exits the renal corpuscle via the

efferent arteriole, perfuses the tubular capillary beds,

and is eventually collected in the renal venous system.


Glomerular Filtration In the average adult, the body’s entire extracellular fluid

volume is filtered about 12 times per day.

Since we cannot afford to lose that amount of fluid, the vast majority must be reclaimed, with just a small portion being excreted in the urine.


The daily composition of plasma, filtrate, and urine are compared.

Glomerular Filtration Total

Amount in Plasma

Amount in 180 L of filtrate (/day)

Amount returned to

blood/d (Reabsorbed)

Amount in Urine (/day)

Water (passive) 3 L 180 L 178-179 L 1-2 L

Protein (active) 200 g 2 g 1.9 g 0.1 g

Glucose (active) 3 g 162 g 162 g 0 g

Urea (passive) 1 g 54 g 24 g (about 1/2)

30 g (about 1/2)

Creatinine 0.03 g 1.6 g 0 g (all filtered)

1.6 g (none reabsorbed)


Glomerular Filtration Filtration is controlled by 2 opposing hydrostatic forces

and 2 opposing osmotic forces at the glomerular

membrane called Starling forces.

Blood hydrostatic pressure (55mmHg) is the main force

that “pushes” water and solutes through the filtration

membrane (promotes filtration).

Capsular hydrostatic pressure (15 mmHg) is exerted

against the filtration membrane by fluid in the capsular

space (opposes filtration).


Glomerular Filtration Starling forces continued

Blood osmotic (oncotic) pressure (30 mmHg) is the

pressure of plasma proteins “pulling” on water (opposes

filtration).

Normally very little protein escapes through the

filtration membrane making capsular oncotic pressure a

negligible force except in certain disease states.

Net Filtration = Blood Hydrostatic Pressure – Blood

Osmotic Pressure – Capsular Hydrostatic Pressure

Net Filtration = 55-30-15 = 10 mmHg


Glomerular Filtration

Normally, 3 Starling forces are at work in glomerular filtration


Regulation of the GFR is critical to maintaining

homeostasis and is regulated by an assortment of local and

systemic mechanisms:

Renal autoregulation occurs when the kidneys

themselves regulate GFR.

Neural regulation occurs when the ANS regulates renal

blood flow and GFR.

Hormonal regulation involves angiotensin II and atrial

natriuretic peptide (ANP).



Renal autoregulation of GFR occurs by two means:

Stretching in the glomerular capillaries triggers

myogenic contraction of smooth muscle

cells in afferent arterioles (reduces GFR).

Pressure and flow monitored in the

macula densa provides tubuloglomerular

feedback to the glomerulus, causing the

afferent arterioles to constrict (decreasing

blood flow and GFR) or dilate (increasing

blood flow and GFR) appropriately.



Neural regulation of GFR is possible because the renal

blood vessels are supplied by sympathetic ANS fibers that

release norepinephrine causing vasoconstriction.

Sympathetic input to

the kidneys is most

important with extreme

drops of B.P. (as occurs

with hemorrhage).



Two hormones contribute to regulation of GFR

Angiotensin II is a potent vasoconstrictor of both

afferent and efferent arterioles (reduces GFR).

A sudden large increase in BP stretches the cardiac atria

and releases atrial

natriuretic peptide (ANP).

◦ ANP causes the

glomerulus to relax,

increasing the surface

area for filtration.



Filtration slit Pedicel of podocyte

Fenestration (pore) of glomerular endothelial cell

Basal lamina

Lumen of glomerulus

(b) Filtration membrane

TEM 78,000x

(a) Details of filtration membrane

Filtration slit Pedicel

Fenestration (pore) of glomerular endothelial cell: prevents filtration of blood cells but allows all components of blood plasma to pass through

Podocyte of visceral layer of glomerular (Bowman’s) capsule

1



Basal lamina

Lumen of glomerulus


TEM 78,000x




Basal lamina of glomerulus: prevents filtration of larger proteins


1

2



Basal lamina

Lumen of glomerulus


TEM 78,000x




Basal lamina of glomerulus: prevents filtration of larger proteins

Slit membrane between pedicels: prevents filtration of medium-sized proteins


1

2

3

The Filtration Membrane


Glomerular Filtration (Interactions Animation)

Renal Filtration

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NET FILTRATION PRESSURE (NFP) =GBHP – CHP – BCOP = 55 mmHg 15 mmHg 30 mmHg = 10 mmHg

GLOMERULAR BLOOD HYDROSTATIC PRESSURE (GBHP) = 55 mmHg

Capsular space

Glomerular (Bowman's) capsule

Efferent arteriole

Afferent arteriole

1

Proximal convoluted tubule


CAPSULAR HYDROSTATIC PRESSURE (CHP) = 15 mmHg


Capsular space


Efferent arteriole

Afferent arteriole

1 2



BLOOD COLLOID OSMOTIC PRESSURE (BCOP) = 30 mmHg

CAPSULAR HYDROSTATIC PRESSURE (CHP) = 15 mmHg


Capsular space


Efferent arteriole

Afferent arteriole

1 2

3


Pressures That Drive Glomerular Filtration


Tubular Reabsorption Tubular reabsorption is the process of returning important

substances (“good stuff”) from the filtrate back into the

renal interstitium, then into the renal blood vessels... and

ultimately back into the body.


Tubular Reabsorption The “good stuff” is glucose, electrolytes, vitamins, water,

amino acids, and any small proteins that might have

inadvertently escaped from the blood into the filtrate.

Ninety nine percent of the glomerular filtrate is

reabsorbed (most of it before the end of the PCT)!

To appreciate the magnitude of tubular reabsorption,

look once again at the table in the next slide and

compare the amounts of substances that are filtered,

reabsorbed, and excreted in urine.


Tubular Reabsorption Total

Amount in Plasma

Amount in 180 L of filtrate (/day)

Amount returned to

blood/d (Reabsorbed)

Amount in Urine (/day)

Water (passive) 3 L 180 L 178-179 L 1-2 L

Protein (active) 200 g 2 g 1.9 g 0.1 g

Glucose (active) 3 g 162 g 162 g 0 g

Urea (passive) 1 g 54 g 24 g (about 1/2)

30 g (about 1/2)

Creatinine 0.03 g 1.6 g 0 g (all filtered)

1.6 g (none reabsorbed)


Reabsorption into the interstitium has two routes:

Paracellular reabsorption is a passive process that occurs

between adjacent tubule

cells (tight junctions do

not completely seal off

interstitial fluid from

tubule fluid.)

Transcellular reabsorption

is movement through an

individual cell.

Tubular Reabsorption


It is a tremendous feat to reabsorb all of the nutrients and

fluid we must to survive, while still filtering out,

concentrating and excreting toxic substance.

To accomplish this, the kidney establishes a

countercurrent flow between the filtrate in the limbs of

the Loops of Henle and the blood in the peritubular

capillaries and Vasa Recta.

◦ Two types of countercurrent mechanisms exist in the

kidneys: countercurrent multiplication and

countercurrent exchange.



Countercurrent multiplication is the process by which a

progressively increasing osmotic gradient is formed in the

interstitial fluid of the renal medulla as a result of

countercurrent flow.

Countercurrent exchange is the process by which solutes

and water are passively exchanged between the blood of

the vasa recta and interstitial fluid of the renal medulla as

a result of countercurrent flow.

This provides oxygen and nutrients to the renal medulla

without washing out or diminishing the gradient.



Both mechanisms contribute to reabsorption of fluid and

electrolytes and the formation of concentrated urine.



Tubular Reabsorption Reabsorption of fluids, ions, and other substances occurs

by active and passive means.

A variety of symporters and antiporters actively

transport Na+ , Cl– , Ca2+, H+, HCO3– , glucose, HPO4

2– ,

SO42– , NH4

+, urea, all amino acids, and lactic acid.

Reabsorption of water can be obligatory or facultative,

but it always moves by osmosis down its concentration

gradient depending on the permeability of the tubule

cells (which varies between the PCT, the different

portions of the loop of Henle, DCT, and collecting ducts).


Tubular Reabsorption Obligatory reabsorption of water occurs when it is

obliged to follow the solutes as they are reabsorbed (to

maintain the osmotic gradient).

Facultative reabsorption describes variable water

reabsorption, adapted to specific needs.

It is regulated by the effects

of ADH and aldosterone on

the principal cells of the renal

tubules and collecting ducts.


This graphic depicts the formation of a dilute urine, mostly

through obligatory

reabsorption of water.

Compare this process to

the one depicted on the

next slide where urine is

concentrated by the action

of ADH on the DCT and

collecting ducts of

juxtamedullary nephrons.



Urine can be

up to 4 times

more

concentrated

than blood

plasma.


Copyright © John Wiley & Sons, Inc. All rights reserved. (b) Recycling of salts and urea in the vasa recta (a) Reabsorption of Na+CI– and water in a long-loop juxtamedullary nephron

Glomerular (Bowman’s) capsule

Afferent arteriole

Efferent arteriole

Glomerulus

Distal convoluted tubule


Symporters in thick ascending limb cause buildup of Na+ and Cl–

Interstitial fluid in renal medulla

300

1200

1000

800

Osmotic gradient

600

400

H 2 O H 2 O

H 2 O

200

1200

980

600 780

400 580

200 380

300

100

Loop of Henle 1200 Concentrated urine

300

300

320

400

600

800

1000

1200

800

H2O

Urea

Papillary duct

Collecting duct

300

500

700

900

1100

1200

400

800

1000

600

Na+CI– Blood flow

Flow of tubular fluid

Presense of Na+-K+-2CI–

symporters Interstitial fluid in renal cortex

320

Juxtamedullary nephron and its blood supply together

Vasa recta

Loop of Henle

H2O

H2O

H2O

H2O

H2O

H2O

H2O

1

H2O

H2O

Na+CI–

Na+CI–

H2O

Na+CI–

H2O

Na+CI–

(b) Recycling of salts and urea in the vasa recta (a) Reabsorption of Na+CI– and water in a long-loop juxtamedullary nephron


Afferent arteriole

Efferent arteriole

Glomerulus





300

1200

1000

800

Osmotic gradient

600

400

H 2 O H 2 O

H 2 O

200

1200

980

600 780

400 580

200 380

300

100


300

300

320

400

600

800

1000

1200

800

H2O

Urea

Papillary duct

Collecting duct

Countercurrent flow through loop of Henle establishes an osmotic gradient

300

500

700

900

1100

1200

400

800

1000

600

Na+CI– Blood flow




320


Vasa recta

Loop of Henle

H2O

H2O

H2O

H2O

H2O

H2O

H2O

1

2

H2O

H2O

Na+CI–

Na+CI–

H2O

Na+CI–

H2O

Na+CI–



Afferent arteriole

Efferent arteriole

Glomerulus





300

1200

1000

800

Osmotic gradient

600

400

H 2 O H 2 O

H 2 O

200

1200

980

600 780

400 580

200 380

300

100


300

300

320

400

600

800

1000

1200

800

H2O

Urea

Papillary duct

Collecting duct


Principal cells in collecting duct reabsorb more water when ADH is present

300

500

700

900

1100

1200

400

800

1000

600

Na+CI– Blood flow




320


Vasa recta

Loop of Henle

H2O

H2O

H2O

H2O

H2O

H2O

H2O

1

2

3

H2O

H2O

Na+CI–

Na+CI–

H2O

Na+CI–

H2O

Na+CI–



Afferent arteriole

Efferent arteriole

Glomerulus





300

1200

1000

800

Osmotic gradient

600

400

H 2 O H 2 O

H 2 O

200

1200

980

600 780

400 580

200 380

300

100


300

300

320

400

600

800

1000

1200

800

H2O

Urea

Papillary duct

Urea recycling causes buildup of urea in the renal medulla

Collecting duct


Principal cells in collecting duct reabsorb more water when ADH is present

300

500

700

900

1100

1200

400

800

1000

600

Na+CI– Blood flow




320


Vasa recta

Loop of Henle

H2O

H2O

H2O

H2O

H2O

H2O

H2O

1

2

3

4

H2O

H2O

Na+CI–

Na+CI–

H2O

Na+CI–

H2O

Na+CI–


Tubular Reabsorption (Interactions Animation)

Water Homeostasis

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Tubular Reabsorption If higher than normal amounts of a substance are present

in the filtrate, then the renal threshold for reabsorption of

that substance may be surpassed.

When that happens, the substance cannot be reabsorbed

fast enough, and it will be excreted in the urine.

For example, the renal [reabsorption] threshold of

glucose is 180-200mg/dl. When this level is exceeded (as

in diabetes mellitus), the glucose is said to “spill” into

the urine (meaning a substance which is not normally

present in urine begins to appear).


Tubular Secretion Tubular secretion is the movement of substances from the

capillaries which surround the nephron into the filtrate.

It occurs at a site other than the filtration membrane (in

the proximal convoluted tubule, distal convoluted

tubule and collecting ducts) by active transport.

The process of tubular secretion controls pH.

Hydrogen and ammonium ions are secreted to decrease

the acidity in the body, and bicarbonate is conserved.

◦ Secreted substances include H+, K+, NH4+, and some

drugs; the amount often depends on body needs.


Tubular Secretion Maintaining the body’s proper pH requires cooperation

mainly between the lungs and the kidneys.

The lungs eliminate CO2.

◦ Provides a rapid response (minutes)

The kidneys eliminate H+ and NH4+ ions and conserve

bicarbonate.

◦ This is a slower response (hours-days).

The alimentary canal (digestive), and integumentary

system (skin) provide minor contributions.


Five hormones affect the extent of Na+, Cl–, Ca2+, and

water reabsorption as well as K+ secretion by the renal

tubules. These hormones, all of which are key to

maintaining homeostasis of not only renal blood flow

and B.P., but systemic blood flow and B.P., are:

angiotensin II

antidiuretic hormone (ADH)

aldosterone

atrial natriuretic peptide (ANP)

parathyroid hormone (PTH)

Hormones and Homeostasis


We have already mentioned the effect of ANP on GFR.

ADH is released by the posterior pituitary in response to

low blood flow in this part of the brain.

ADH affects facultative water reabsorption by

increasing the water permeability of principal cells in

the last part of the distal convoluted tubule and

throughout the collecting duct.

◦ In the absence of ADH, the apical

membranes of principal cells

are almost impermeable to water.



Secretion of the hormones angiotensin II and aldosterone

are tied to one another: When blood volume and blood

pressure decrease or the sympathetic NS is stimulated, the

walls of the

afferent arterioles

are stretched

less, and the

cells of the JGA

secrete renin.



Renin clips off a 10-amino-acid peptide called

angiotensin I from angiotensinogen, which is synthesized

by hepatocytes. By clipping off two more amino acids,

angiotensin converting enzyme (ACE) converts

angiotensin I to angiotensin II, which is the active form

of the hormone. Angiotensin II has 3 main effects:

1. Vasoconstriction decreases GFR.

2. It increases blood volume by increasing reabsorption

of water and electrolytes in the PCT.

3. It stimulates the adrenal cortex to release aldosterone.



Aldosterone stimulates the principal cells in the

collecting ducts to reabsorb more Na+ and Cl– and secrete

more K+. The osmotic consequence of reabsorbing more

Na+ and Cl– is that more

water is reabsorbed,

which increases blood

volume and blood

pressure.



The events of

filtration,

absorption, and

secretion are

summarized in

this graphic.

Summary of Renal Function


Increases in GFR usually result in an increase in urine

production. However, as we have seen, the mechanisms

which control electrolyte and

water reabsorption in the

various parts of the nephron

and collecting ducts are subject

to many complex controls.

Normal urine output (UOP)

is 1-2 L/d.

Urine


A urinalysis analyzes the physical, chemical and

microscopic properties of urine.

Water accounts for 95% of total urine volume.

The solutes normally present in urine are filtered and

secreted substances that are not reabsorbed.

If disease alters metabolism or kidney function, traces of

substances normally not present or normal constituents

in abnormal amounts may appear (bacteria, albumin

protein, glucose, white blood cells, red blood cells to

name a few).

Urine


In addition to a urinalysis, two blood tests are commonly

done clinically to assess the adequacy of renal function.

Blood urea nitrogen (BUN) measures nitrogen wastes in

blood from catabolism and deamination of amino acids.

Creatinine levels appear in the blood as a result of

catabolism of creatine phosphate in skeletal muscle.

◦ The serum creatinine test measures the amount of

creatinine in the blood, which increases in states of

renal dysfunction.

Urine

chapter 26

Documents

renal anatomy

copyright john wiley

renal pelvis

renal artery

renal papilla

renal pyramids

renal blood vessels

renal columns