BIOL315 Chapter 26 - Huntingdon Collegejdaniels.huntingdon.edu/315/BIOL315 Chapter 26.pdf · The Urinary System Chapter 26 Overview • The urinary system has three major ... •

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The Urinary System

Chapter 26

Overview

• The urinary system has three major functions

• Excretion; the removal of organic wastes from body tissues

li i i h di h f h• Elimination; the discharge of wastes to the environment

• Homeostatic regulation of the volume and solute concentration of blood plasma

Overview

• Urinary system includes:

• The kidneys which produce urine

• The ureters; through which urine travels from kid bl ddkidneys to bladder

• The urinary bladder which stores urine

• The urethra which conducts urine to the exterior of the body

The Kidneys

• The kidneys lie on either side of the vertebral column, between vertebrae T12 and L3, with the left kidney lying slightly superior to the right

• Both the kidneys and associated adrenal glands are retroperitonealp

• The hilum, a prominent indentation, provides entry for the renal artery and renal nerves, and provides exit for renal veins and the ureter

• About 20-25% of total cardiac output of blood enters the kidneys each minute (~1.2L/m)

• Each kidney weighs about 150 grams

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Sectional Anatomy of The Kidneys• The kidney itself has two layers, an outer

cortex and an inner medulla

• The cortex is the superficial portion of the kidney, in contact with the outer renal capsuley, p

• The medulla consists of 6-18 conical structures called renal pyramids

• The base of each pyramid faces the cortex, while the tips of each pyramid (AKA renal papilla) project into the renal sinus

Sectional Anatomy of The Kidneys• Each pyramid has a series of fine grooves that

converge at the renal papilla

• Adjacent pyramids are separated by bands of cortical tissue called renal columns

• This divides the kidney into renal lobes, each consisting of an individual renal pyramid, the overlying renal cortex, and the adjacent renal columns

Sectional Anatomy of The Kidneys

• Urine production occurs in the nephrons of the renal lobes

• As urine is produced within each renal papilla, it is discharged into a minor calyxS l i l f j l• Several minor calyces merge to form a major calyx

• 2-3 major calyces fuse to form a large funnel shaped chamber, the renal pelvis

• The renal pelvis occupies the renal sinus and connects to the ureter, which drains the kidney and connects to the urinary bladder.

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• Renal arteries branch repeatedly– Renal artery

– Segmental artery

– Interlobar artery

– Arcuate artery

Blood supply of the kidneys

y

– Interlobular artery

– Afferent arterioles

• Afferent arterioles deliver blood to the glomerulus of the nephron

• Renal venules follow similar pattern ending with renal veins

The Nephron

• Each nephron consists of a renal corpuscle and renal tubule

• The renal corpuscle is a spherical structure consisting of a cup shaped chamber, g p p ,Bowman’s capsule, and a intertwining network of about 50 fenestrated capillaries, called the glomerulus

• The glomerulus projects into Bowman’s capsule

Renal Corpuscle

• The renal corpuscle is the site where filtration occurs

• During filtration blood pressure forces water and dissolved solutes out of the glomerularand dissolved solutes out of the glomerular capillaries and into the capsular space

• This essentially protein free solution is termed filtrate, and is similar to blood plasma

• From the corpuscle filtrate enters the renal tubule

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Renal Tubule

• The renal tubule is a long tubular pathway up to 50 mm in length

• It consists of the proximal convoluted tubule (PCT), The loop of Henle (nephron loop), and the distal convoluted tubule (DCT)

• The renal tubule is responsible for three critical functions– Reabsorbing all useful organic substrates that enter the

filtrate– Reabsorbing over 90% of the water entering the filtrate– Secreting into tubular fluid any waste products that failed

to enter the renal corpuscle via filtration

Renal Tubule

• The renal tubule has two convoluted segments, the proximal convoluted tubule and the distal convoluted tubule, separated by a U-shaped tube the loop of Henletube, the loop of Henle

• The convoluted tubules are in the cortex of the kidney and the loop of Henle extends partially or completely into the medulla

Renal Tubule

• As it travels along the tubule, the filtrate, now called tubular fluid, changes in composition to ultimately become urine

• Each nephron empties into a collecting duct• Each nephron empties into a collecting duct

• Each collecting duct receives tubular fluid from many nephrons

• Each collecting duct descends into the medulla and empties into a papillary duct, which empties into a minor calyx…

P959See alsoTable 26-1

Open staxFig 25.17p1148

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Nephrons

• About 85% of all nephrons are cortical nephrons located almost entirely within the superficial cortex of the kidney and having a short loop of Henle

• The remaining 15% are juxtamedullary nephrons which have long loops of Henle that extend far into the medulla

• Juxtamedullary nephrons allow the production of concentrated urine

The Renal Corpuscle

• The gross anatomy of the renal corpuscle consists of a spherical structure, Bowman’s capsule, within which is a capillary network, the glomerulus.

• The outer wall of Bowman’s capsule is a layer of p ysimple squamous epithelium called a parietal epithelium

• The parietal epithelium is continuous with the visceral epithelium, which covers the capillaries of the glomerulus

• The visceral epithelium consists of large cells with processes, or “feet,” that wrap around the glomerular capillaries

Th ll ll d d t ( d f t)

The Renal Corpuscle

• These cells are called podocytes (podos, foot)

• The pedicels (feet) of podocytes are separated by filtration slits through which materials pass out of the blood during filtration

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Functional anatomy of the renal tubule

• The proximal convoluted tubule (PCT) is lined with a simple cuboidal epithelium whose apical surfaces bear microvilli

• The PCT actively reabsorbs organic nutrients• The PCT actively reabsorbs organic nutrients, plasma proteins (if present), ions, and water from the filtrate and releases them into the peritubular fluid


• The Loop of Henle is divided into the descending limb and ascending limb

• Fluid in the descending limb moves towards the renal pelvis, fluid in the ascending limb toward the renal p gcortex

• Each limb has a thick segment and a thin segment

• Thick segments have a cuboidal epithelium

• Thin segment have a squamous epithelium

• The size of the lumen is unchanged


• The thick descending limb is essentially an extension of the PCT, it pumps sodium and chloride ions out of the tubular fluid

• The thick ascending limb also pumps sodium and chloride ions out of the tubular fluid

• The effect of this pumping is most notable in the medullary region adjacent to the long ascending limb where it creates an unusually high concentration of solutes in the peritubular fluid

• the thin descending limb is permeable to water but not solutes, so water moves out of the tubular fluid helping to concentrate the tubular fluid


• The distal convoluted tubule (DCT) differs from the PCT in that it has a smaller diameter and its epithelial cells lack microvilli

• The distal convoluted tubule actively secretes• The distal convoluted tubule actively secretes ions, toxins, drugs and selectively reabsorbs water, sodium, and calcium ions from tubular fluid

Renal Physiology

• Renal physiology results in the production of urine

• Urine production is designed to maintain homeostasis by regulating the volume and composition of blood

• This involves the excretion of solutes, specificallyThis involves the excretion of solutes, specifically metabolic waste products including:– Urea from the breakdown of amino acids (~21g/day)

– Creatinine generated by the breakdown of creatine phosphate (~1.8g/day)

– Uric acid from the breakdown of nitrogenous bases in DNA and RNA (~480 mg/day)

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Renal Physiology

• These waste products are dissolved in the bloodstream and can only be eliminated while dissolved in urine

• Accordingly, their elimination includes an unavoidable loss of water

• Since the kidneys are capable of producing urine with a concentration (1200-1400 mOsm/L) four times that of plasma (~300 mOsm/L) we are capable of limiting water loss.

• Were this not possible water loss would lead to death by dehydration in a matter of hours

Basic processes of urine formation

• In order to produce urine your kidneys rely on three processes

1. Filtration; in which blood pressure forces water and solutes across the walls of thewater and solutes across the walls of the glomerular capillaries

2. Reabsorption; the removal of water and solutes from the filtrate, often a selective process

Basic processes of urine formation

3. Secretion; the transport of solutes from the peritubular fluid into the tubular fluid, this is necessary because filtration does not force all the dissolve materials out of plasmathe dissolve materials out of plasma

• Secretion provides a backup for filtration and can further lower the plasma concentration of undesirable materials such as toxins and drugs

• Bulk filtration at the glomerulus is subsequently modified by diffusion, osmosis, and carrier mediated transport

• Carrier mediated transport includes– Facilitated diffusion

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Carrier Mediated Transport

– Active transport– Cotransport– Countertransport

• In carrier mediated transport the carrier proteins have a transport maximum (Tm) which determines renal threshold for the reabsorption of substances in tubular fluid

Renal Function (summary 982-983)

• Most regions of the nephron perform a combination of secretion and reabsorption

• General functions can be identifiedFilt ti l i l i th l l– Filtration occurs exclusively in the renal corpuscle

– Nutrient reabsorption occurs primarily along the PCT

– Active secretion occurs primarily at PCT and DCT

– Collecting Ducts regulate the final volume and solute concentration of the urine

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Filtration Pressures

• We’ve said that filtration occurs as blood pressure forces water and solutes across the walls of the glomerular capillaries

• The force behind filtration is glomerularThe force behind filtration is glomerular hydrostatic pressure (GHP), the blood pressure in the glomerular capillaries (~50 mm Hg)

• GHP pushes water and solute molecules out of the plasma and into the filtrate


• GHP is opposed by capsular hydrostatic pressure (CsHP, ~15 mm Hg)

• CsHP results from the resistance to the flow of filtrate along the nephron and conducting system

• Blood colloid osmotic pressure (BCOP, ~25 mm Hg), which tends to osmotically draw water out of the filtrate and results from proteins suspended in blood plasma, also opposes GHP


• The actual filtration pressure (FP) at the glomerulus is represented by the following equation

• FP = GHP- (CsHP + BCOP)

• 50 – (15 + 25) = 10 mm Hg

• This is the average pressure forcing water and solutes out of the glomerular capillaries and into the capsular space as filtrate.

• Problems that affect filtration pressure can seriously disrupt kidney function

Glomerular filtration

• The glomerular filtration rate (GFR) is the amount of filtrate produced by your kidneys each minute and averages about 125 ml/minute (180 lit /d )(180 liters/day)

• 99% of this volume is reabsorbed

• Filtration depends on adequate bloodflow to the glomerulus and the maintenance of normal filtration pressures

Glomerular filtration

• If GHP drops by 20%, to 40 mm Hg, filtration would cease (40 – (15 + 25) = 0 mm Hg)

• Three levels of control stabilize the glomerular filtration rate (GFR)filtration rate (GFR).

• Autoregulation, at the local level (glomerulus)• Hormonal regulation, initiated by the kidneys• Autonomic regulation, primarily by the

sympathetic division of the autonomic nervous system

Autoregulation

• Autoregulation is triggered by a reduction in blood flow and a decline in GHP

• Autoregulation triggers:– The dilation of the afferent arterioleThe dilation of the afferent arteriole– Relaxation of supporting cells and dilation of

glomerular capillaries– Constriction of the efferent arteriole

• Conversely, if GHP rises, the afferent arteriole constricts to reduce glomerular blood flow

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Hormonal Regulation

• GFR is regulated by hormones of the renin-angiotensin system

• A drop in blood pressure triggers renin release from the juxtaglomerular apparatusj g pp

• Renin activates angiotensin I & II which elevate blood pressure and volume

• Angiotensin I & II also stimulate the CNS causing increased thirst, ADH production, and increased sympathetic motor tone

Autonomic Regulation

• Most autonomic innervation of the kidneys consists of sympathetic postganglionic fibers

• Sympathetic activation produces powerful constriction of the afferent arterioles, decreasing GFR and slowing the production of filtrateand slowing the production of filtrate

• Sympathetic activation usually occurs in response to crisis (heart attack, blood loss) and overrides local regulatory mechanisms that act to stabilize GFR

• As the crisis passes, sympathetic tone decreases and the filtration rate returns to normal

Reabsorption and Secretion at the PCT

• The PCT normally reabsorbs 60-70% of the volume of filtrate produced by the renal corpuscle

• The functions of the PCT include– Reabsorption of organic nutrients ~99% of the

glucose, amino acids and other organic nutrients are reabsorbed via facilitated diffusion and cotransport

– Active reabsorption of ions, including sodium, potassium, bicarbonate, phosphate, and sulfate by ion pumps


– Reabsorption of water by osmosis (~108 L/Day). As the previously listed transport activities proceed the solute concentration of the peritubular fluid increases. This pulls water out of the tubular fluid by diffusion

– Passive reabsorption of ions. As active reabsorption and osmosis proceed, the relative concentration of other solutes (urea, Cl-, lipid soluble compounds) increases and these ions will diffuse into the peritubular fluid


– Secretion; although more important in the DCT, the active secretion of potassium and hydrogen ions, as well as other substances occurs in the PCT

Loop of Henle and countercurrent multiplication

• The loop of Henle will reabsorb about half of the water and about 2/3 of the sodium and chloride ions remaining in the tubular fluid

• This reabsorption is accomplished using the principle of countercurrent multiplication exchange

• Countercurrent refers to the fact that the exchange occurs between fluids moving in opposite directions

• Tubular fluid in the descending limb is moving in the opposite direction from tubular fluid in the ascending limb

• Multiplication refers to the fact that the effect of the exchange increases as movement of the fluids continues

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Loop of Henle and Countercurrent multiplication

• Recall that the thin descending limb of the loop of Henle is permeable to water but relatively impermeable to solutes

• And the thick ascending limb, which is g ,relatively impermeable to both water and solutes, contains active transport mechanisms that pump sodium and chloride from the tubular fluid into the peritubular fluid of the medulla

Loop of Henle and Countercurrent multiplication

• Countercurrent multiplication operates as follows:1. Sodium and Chloride are pumped out of the thick

ascending limb and into the peritubular fluid

2. This elevates the osmotic concentration around the thin d di li bdescending limb

3. Which results in an osmotic flow of water out of the thin descending limb, increasing the solute concentration within the thin descending limb

4. As this highly concentrated solution arrives in the thick ascending limb it accelerates the transport of sodium and chloride into the medulla (and we’re back to #1)

Openstax25.20 p1154

Benefits of Countercurrent Multiplication

• The countercurrent multiplication system in the nephon performs two services– It efficiently reabsorbs solutes and water before the

tubular fluid reaches the DCT and collectingtubular fluid reaches the DCT and collecting system

– It establishes a concentration gradient that permits the passive reabsorption of water from the tubular fluid in the collecting system (regulated by ADH)

Reabsorption and secretion at the DCT

• DCT performs final adjustment in the solute concentration and volume of the tubular fluid through active secretion or absorption

• Tubular cells actively reabsorb Na+ and Cl- in yexchange for potassium or hydrogen ions that are secreted

• Calcium is also reabsorbed• The DCT also actively secretes ions, toxins,

drugs

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Reabsorption and secretion along the collecting system

• Water and solute loss is regulated by aldosterone and ADH

• Sodium ions bicarbonate and urea areSodium ions, bicarbonate, and urea are reabsorbed

• Body fluid pH is controlled by secretion of hydrogen or bicarbonate ions in the collecting system


• The collecting ducts receive tubular fluid from numerous nephrons and carry it toward the renal sinus

• The amount of water and solute loss in the collecting system is regulated by aldosterone and ADHsystem is regulated by aldosterone and ADH

• Aldosterone controls the sodium pumps along the DCT and the proximal portion of the collecting system

• ADH controls the permeability of the DCT and collecting system to water.


• Reabsorption in the collecting system includes three important compounds

• Sodium ion reabsorption is regulated by aldosterone sensitive ion pumps that exchange Na+ in the tubular p p gfluid for Cl- in the peritubular fluid

• Bicarbonate is reabsorbed in exchange for Cl- in the peritubular fluid

• Urea which tends to diffuse out of the tubular fluid in the deepest portion of the medulla


• The collecting system is an important site for the control of body fluid pH by means of the secretion of bicarbonate and hydrogen ions

• If peritubular pH declines hydrogen ions are• If peritubular pH declines, hydrogen ions are secreted and bicarbonate reabsorbed

• If peritubular pH increases, hydrogen ions are reabsorbed and bicarbonate secreted

Control of urine volume and osmotic concentration

• Urine volume and osmotic concentration are regulated by controlling water reabsorption

• Water is reabsorbed by osmosis in the PCT and descending limb of the loop of Henleand descending limb of the loop of Henle

• The water permeabilities of these tissues cannot be adjusted and water reabsorption there represents obligatory water reabsorption

Control of urine volume and osmotic concentration

• In the DCT and collecting system the amount of water reabsorption can be precisely controlled by a process called facultative water reabsorption

• Such control is possible because these segments are relatively impermeable to water except in therelatively impermeable to water except in the presence of ADH

• This hormone triggers the appearance of water channels in the apical cell membranes and dramatically enhances osmotic water movement

• More ADH means more osmotic water movement

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Composition of normal urine

• The composition of the 1.2 Liters of normal urine produced each day varies with the metabolic and hormonal events of the body

• Urine composition reflects the filtration, absorption and secretion activity of the nephronsy p

• The concentration of urine depends on the osmotic movement of water across the walls of the tubules and collecting ducts

• Because composition and concentration vary independently, one can produce a large quantity of dilute urine, or a small quantity of concentrated urine and still excrete the same quantity of dissolved materials

See summaryText pages 982-983

Urine Transport, Storage, and Elimination

• Urine production ends with fluid entering the renal pelvis

• The rest of the urinary system transports, stores and eliminates the urinestores and eliminates the urine

• And consists of – Ureters

– Bladder

– Urethra

Ureters

• The ureters are a pair of muscular tubes that extend from the renal pelvis to the bladder

• Each ureter consists of three layersA i– An inner mucosa

– A middle muscular layer

– An outer connective tissue layer

• Peristaltic contractions of the muscular layer force urine toward the urinary bladder

Urinary Bladder

• The urinary bladder is a hollow, muscular organ that serves as the reservoir for the storage of urine

• During micturition contraction of detrusor muscle voids the bladder

• The mucosa lining the urinary bladder forms folds, or ruggae, that disappear as the bladder fills.

• The trigone, which funnels urine into the urethra, is the triangular area bounded by the ureteral openings and the entrance to the urethra

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The Urethra

• The urethra extends from the urinary bladder to the exterior of the body

• It passes through the urogenital diaphragm, or external urinary sphincterexternal urinary sphincter

• It differs in length and function in males and females

The Micturition Reflex and Urination

• Urination is coordinated by the micturition reflex

• This reflex is initiated by stretch receptors in wall of bladder

• The urge to urinate generally appears when theThe urge to urinate generally appears when the bladder contains around 200ml of urine

• Once bladder volume reaches about 500 ml the micturition reflex may force open the internal urethral sphincter

• This leads to a reflexive release of the external urethral sphincter and urination proceeds

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BIOL315 Chapter 26 - Huntingdon Collegejdaniels.huntingdon.edu/315/BIOL315 Chapter 26.pdf · The Urinary System Chapter 26 Overview • The urinary system has three major ... •