Chapter 26: The Urinary System

Chapter 26:

The Urinary System

Primary sources for figures and content:

Marieb, E. N. Human Anatomy & Physiology. 6th ed. San Francisco: Pearson Benjamin Cummings, 2004.

Martini, F. H. Fundamentals of Anatomy & Physiology. 6th ed. San Francisco: Pearson Benjamin Cummings, 2004.

Urinary System

• Components– Kidneys– Urinary Tract

•Ureters•Urinary

Bladder•Urethra

Figure 26–1

Functions of the Urinary System

1. Excretion by the Kidneys: – removal of organic wastes from body fluids

2. Elimination by the Urinary Tract:– discharge of waste products

3. Homeostatic regulation of plasma volume and solute concentrations by the kidneys:

– Blood volume, BP– Concentration of ions– Stabilize blood pH– Conserve nutrients– Assist liver: deamination, detoxification

Functions of the Urinary System

4. Other kidney functions: – Gluconeogenesis during starvation– Produce renin to regulate BP– Produce erythropoietin for RBC

production– Convert Vitamin D to calcitriol for calcium

absorption in the GI tract

The location and structures of the kidneys.

Gross Anatomy of the Urinary System

Figure 26–3

Kidneys• 1% body weight• Retroperitoneal, posterior abdominal wall• Adrenal gland anchored superior• 3 layers CT anchor kidneys

1. Renal capsule: - collagen fibers covering organ

2. Adipose capsule: - adipose cushion around the renal capsule

3. Renal fascia: - collagen fibers fused to renal capsule and deep fascia

of body wall and peritoneum

Kidneys

• Renal ptosis = floating kidney– Cause Starvation or injury– Result Kidney becomes loose from

body wall•Kidney could twist blood vessels or

ureters

The Structure of the Kidney

Figure 26–4

Kidney

• Hilum (Hilus):– Point of entry for renal artery and renal

nerves – Ureters enter and exit

• Hilus opens to renal sinus• Renal sinus lined with renal capsule

that is contiguous with outside

Kidney• Kidney has two layers

1. Cortex = superficial•Contact renal capsule•Houses filtration structures = nephrons

2. Medulla = 6-18 renal pyramids•Parallel bundles of collection tubules•Apex = papilla, points toward renal sinus

• Kidney divided into sections: renal lobes• Renal lobe =

– renal pyramid + surrounding cortex called renal columns– Lobe is site for urine production

Urine Production

• Nephron (cortex) collecting ducts (medulla) papilla minor calyx major calyx renal pelvis

• Renal pelvis– Fills majority of real sinus– Funnels urine into ureters

• Pyelonephritis– Inflammation of kidney– Infection usually enters from ureter and

spreads up through ducts to nephron

Blood Supply to the Kidneys

Figure 26–5

Blood Supply and Innervation to Kidney

• Receives 20-25% cardiac output• Highly vascularized, many capillaries involved in

filtration (nephrons)• Innervation from Renal Plexus controlled by ANS• Most is sympathetic to

1. Adjust rate of urine formation- Change BP and flow at nephron

2. Stimulate release of renin- Restricts water and sodium loss at nephron

Blood Supply and Innervation to Kidney

• Two important capillary beds associated with each nephron1. Glomerulus = filtration2. Peritubular capillaries:

- reclaim filtrate, concentrate urine• Both connected to arterioles only (not for

oxygen exchange)• Afferent arteriole capillary efferent

arteriole

Glomerulus

• Consists of 50 intertwining capillaries

• Blood delivered via afferent arteriole

• Blood leaves in efferent arteriole:– flows into peritubular capillaries– which drain into small venules– and return blood to venous system

The Nephron and Collecting System

Figure 26–6

Nephron

• Two types of nephrons– Cortical nephron = Majority

•In cortex, short loop of Henle– Juxtamedullary nephrons = 15%

•At cortex/medulla interface •Long loops of Henle•Important for water conservation and

concentrated urine

Cortical and Juxtamedullary Nephrons

Figure 26–7

Renal Corpuscle• Site of filtration• 2 parts

1. Glomerular capsule•Thin parietal epithelium, forms capsule around glomerulus

2. Glomerulus•Fenestrated capillaries covered by podocytes•Podocytes are on the visceral epithelium

– Wrapped around the capillaries, to create filtration slits on surface of capillaries

– Slits smaller than fenestrations in glomerular capillaries to restrict filtration of large molecules

The Renal Corpuscle

Figure 26–8

Renal Corpuscle

• Golmerulonephritis– Inflammation of glomeruli– Prevents filtration– Can be result of antigen/Ab

complexes trapped in filtration slits following allergy or blood infection

The Nephron and Collecting System

Figure 26–6

Renal Tubule• Reabsorption to process raw filtrate into urine• 3 parts

1. PCT (proximal convoluted tubules)•Simple cuboidal epithelium with microvilli•Reabsorbs organic nutrients, ions, water, small plasma

proteins from filtrate exiting glomerular capsule

2. Loop of Henle•Simple squamous epithelium

•Reabsorbs Na+, Cl-, H2O form filtrate

• Important to regulate volume and solute concentration of urine•Descending and ascending limbs

Renal Tubule3. DCT (distal convoluted tubules)

•Simple cuboidal epithelium•Flat surface•Four important functions

1. Secretion2. Reabsorb Na+ and Ca++ from filtrate3. Optional H2O reabsorption from filtrate under

hormonal control4. Contribute to formation of Juxtaglomerular

Apparatus

Table 26–1

Juxtaglomerular Apparatus (JGA)

• Consists of two cell types1. Endocrine cells of DCT = macula densa2. Granular cells of apparent arteriole =

Juxtaglomerular cells

• Together cells monitor blood and produce1. Renin: Enzyme, restricts Na+ and H2O at

nephron2. Erythropoietin: hormone, stimulates RBC

production

Collecting System

• Collecting ducts + papillary ducts = nephrons 1 collecting duct (renal pyramid) Many collecting ducts 1 papillary duct

• Final osmotic concentration of filtrate adjusted by collecting duct, after this urine is complete and exits kidney:– Papillary ducts (renal papilla) minor calyx

major calyx renal pelvis ureter• Polycystic Kidney Disease

– Genetic, cysts form that cause swelling of kidney tubules, compression reduces function

KEY CONCEPT

• Kidneys remove waste products from blood

• Nephrons are primary functional units of kidneys

• Kidneys help regulate:– blood volume and pressure– ion levels– blood pH

Which portion of the nephron is NOT in the renal cortex?

A. proximal convoluted tubule

B. distal convoluted tubule

C. collecting duct

D. loop of Henle

Why don’t plasma proteins pass into the capsular space under

normal circumstances?

A. glomerular capillary pores are too small

B. glomerular blood pressure is too low

C. glomerular filtration rate is too low

D. glomerular blood flow is too slow

Damage to which part of the nephron would interfere with the control of

blood pressure?

A. juxtaglomerular apparatus

B. Bowman’s capsule

C. PCT

D. loop of Henle

Renal Physiology

Renal Physiology

• Urinary system functions to regulate blood volume and concentration– Removes wastes and produce urine

• Filtrate =– Everything in blood plasma except

large proteins and cells• Urine = metabolic waste, 1% filtrate

Common Wastes1. Urea: from catabolism of amino acids2. Creatinine: from catabolism or damage of skeletal muscle

tissue– Creatine phosphate is energy storage of muscle

3. Uric acid: from recycling of RNA4. Urobilin: from breakdown of hemoglobin (yellow color)• All wastes excreted as solution in water• Loss of filtering toxic waste buildup

– death in a few days• Dialysis blood filtering machine used for patients with

kidney failure

Urine Formation

1. Glomerular Filtrations– Blood hydrostatic pressure forces water

and solutes through glomerular wall2. Tubular Reabsorption

– Selective uptake of water and solutes from filtrate

3. Tubular Secretion– Transport of wastes from capillaries to

tubules

An Overview of Urine Formation

Figure 26–9 (Navigator)

1. Glomerular Filtration

• Occurs through filtration membrane1.Fenestrated endothelium of glomerular

capillaries - Restricts cells

2.Podocytes (visceral epithelium of capsule)- Filtration slits restrict solutes protein

sized and larger

3.Fused basal lamina for both

1. Glomerular Filtration

• Filtrations depends on1. Large surface area2. High glomerular blood pressure3. Good permeability

• Glomerular Filtration Rate (GFR)– Amount of filtrate kidneys produce/minute– ~125 ml/min 180L/day– 99% reabsorbed, 1% lost as urine– Drop in blood pressure = decrease GFR

• Decrease 15% BP = 0 GFR

Glomerular Filtration

• Filtration is passive but all small solutes escape e.g. glucose, amino acids, etc.

Figure 26–10

Regulation of Filtrations3 Levels of GFR Control

1. Autoregulation (local level)2. Hormonal regulation (initiated by

kidneys)3. Autonomic regulation (by

sympathetic division of ANS)

1. Autoregulation of GFR• Functions to maintain constant GFR with normal blood

pressure fluctuations in systemic arteriole pressureA. Reduced blood flow/BP triggers

• Dilation of afferent arteriole and glomerular capillaries• Constriction of efferent arteriole• All functions to INCREASE PRESSURE at the

glomerulus to INCREASE GFR

B. High blood flow/BP triggers• Constriction of afferent arteriole and glomerular

capillaries• Dilation of efferent arteriole• All functions to DECREASE PRESSURE at the

glomerulus to DECREASE GFR

2. Hormonal Regulation

• Extrinsic regulation aimed at maintaining systemic blood pressureA. Renin = Enzyme released by

juxtaglomerular apparatus in response to:1. Decline in BP in kidney2. Decline in osmotic concentration of

filtrate3. Direct sympathetic stimulation

2. Hormonal RegulationA. Renin =

•Renin activates angiotensin in blood to form Angiotensin II which triggers

1. Arteriole constriction to elevate BP2. Secretion of aldosterone from adrenal glands

»Aldosterone promotes sodium reabsorption in kidney tubules

3. Thirst4. Release of ADH from pituitary (ADH

promotes water uptake in tubules)•Effect =

– Increase blood volume– Decrease urine production

2. Hormonal Regulation

B. Natriuretic Peptide =•Hormone released in response to stretching

in heart or aorta (increase blood volume)•Triggers

1. Dilation of afferent arteriole2. Constriction of efferent arteriole

•Effect = – Increase GFR– Increase Urine Production– Decrease blood volume

3. Autonomic Nervous System Regulation

• Sympathetic causes vasoconstriction– Decrease GFR– Decrease Urine Production– Prolonged sympathetic stimulation can

cause hypoxia of kidneys and waste accumulation in blood

Response to Reduction

in GFR

Figure 26–11

KEY CONCEPT

• Glomeruli produce about 180 L of filtrate per day (70 times plasma volume)

• Almost all fluid volume must be reabsorbed to avoid fatal dehydration

What nephron structures are involved in filtration?

A. glomerular capillaries, lamina densa, and filtration slits of the podocytes

B. filtration slits of the podocytes, PCT

C. PCT, lamina densa, descending loop of Henle

D. glomerular capillaries, PCT

What occurs when the plasma concentration of a substance

exceeds its tubular maximum?

A. glomerular blood pressure increases

B. filtration shuts down

C. excess is excreted in urine

D. glomerular osmotic pressure decreases

How would a decrease in blood pressure affect the GFR?

A. increase

B. decrease

C. cause random fluctuations

D. no effect due to glomerular compensation

2. Reabsorption and 3. Secretion

• Reabsorption: – recovers useful materials from filtrate

• Secretion: – ejects waste products, toxins, and

other undesirable solutes

2. Tubular Reabsorption

• Transport proteins in renal tubule cells• Return substances from filtrate to plasma• When carrier proteins are saturated by the

substance they carry the renal threshold for that substance has been reached. All additional amounts of that substance will be lost in urine– E.g. Glycosuria = glucose in urine

•Glucose levels in blood/filtrate exceed renal threshold


• PCT reabsorption– PCT reabsorbs 60-70% of filtrate1.Reabsorption of 99% of organic nutrients by

facilitated diffusion and cotransport2.Passive reabsorption of ions by diffusion3.Selective reabsorption of ions by active transport

- Ion pumps controlled by hormones4.Reabsorption of water by osmosis

- Water follows ions


• Loop of Henle reabsorption– Functions to concentrate filtrate

– Reabsorbs half remaining water and 2/3 Na+ and Cl- by countercurrent multiplications•Ascending limb pumps ions from filtrate to

medulla•High ion concentration then causes water to

move by osmosis out of descending limb

Countercurrent Multiplication

• Is exchange that occurs between 2 parallel segments of loop of Henle: – the thin, descending limb– the thick, ascending limb

• Countercurrent = Refers to exchange between tubular fluids moving in opposite directions:– fluid in descending limb flows toward renal

pelvis– fluid in ascending limb flows toward

medulla/cortex• Multiplication = Refers to effect of exchange:

– increases as movement of fluid continues

Countercurrent Multiplication and Concentration of Urine

Figure 26–13a (Navigator)

Countercurrent Multiplication and Concentration of Urine

The Thin Descending Limb

• Is permeable to water, impermeable to solutes• As tubular fluid flows along thin descending limb:

– osmosis moves water into peritubular fluid– leaving solutes behind– osmotic concentration of tubular fluid increases

• Normal Maximum Solute Concentration– Of peritubular fluid near turn of loop of Henle:

1200 mOsm/L• The Concentration Gradient of the Medulla

– 2/3 (750 mOsm/L) from Na+ and Cl—: pumped out of ascending limb

• Remainder from urea

2 Benefits of Countercurrent Multiplication1. Efficiently reabsorbs solutes and

water:– before tubular fluid reaches DCT and

collecting system

2. Establishes concentration gradient:– that permits passive reabsorption of

water from tubular fluid in collecting system


• DCT reabsorption– Aldosterone promotes Na+ uptake and K+ loss

via sodium potassium pump– Parathyroid hormone and calcitriol promote

Ca++ uptake– ADH stimulates water uptake

3. Tubular Secretion

• Selectively removes solutes from blood delivers them to filtrate

1. Dispose of drugs and wastes that were not filtered2. Eliminate wastes that were reabsorbed3. Rid body of excess K+ 4. Control blood pH: Remove H+

•CO2 + H2O H2CO3 H+ + HCO3-

**Bicarbonate ions used to buffer blood pH but H+ must be secreted into filtrate

• Secretion carried out mostly by DCT, but some also occurs in collecting ducts

KEY CONCEPT

• Reabsorption involves diffusion, osmosis, channel-mediated diffusion, and active transport

• Many processes are independently regulated by local or hormonal mechanisms

• The primary mechanism governing water reabsorption is “water follows salt”

• Secretion is a selective, carrier mediated process

What effect would increased amounts of aldosterone have on the K+

concentration of urine?

A. increase

B. decrease

C. no effect

D. impossible to predict

What effect would a decrease in the Na+ concentration of filtrate have on

the pH of tubular fluid?

A. higher

B. lower

C. no effect

D. impossible to predict

How would the lack of juxtamedullary nephrons affect the volume and osmotic concentration of urine?

A. increase volume; decrease osmotic concentration

B. decrease volume; decrease osmotic concentration

C. increase volume; increase osmotic concentration

D. decrease volume; increase osmotic concentration

Why does a decrease in the amount of Na+ in the distal convoluted tubule lead

to an increase in blood pressure?

A. Because it increases renin production.

B. Because it decreases water content in blood.

C. Because it increases filtration rate.

D. Because it increases water loss through kidneys.

Control of Water Volume• Control of Water Volume

– Obligatory water reabsorption occurs by osmosis in PCT and descending loop of Henle •Cannot be prevented

– Facultative water reabsorption can occur in DCT and collecting ducts•Usually impermeable

– ADH causes formation of water channels by triggering insertion of aquaporin proteins in cell membrane of DCT and collecting ducts

– Aquaporins allow more osmosis to concentrate urine and conserve water

The Effects of ADH on the DCT and Collecting Duct


Control of Water Volume• Control of Water Volume

1. Diuretics = Substance that cause water loss•Osmostic diuretics

– Substances that cannot be reabsorbed and thus take water with then

•Hypertension and edema meds– Prevent Na+ uptake– Water follows salt

•Alcohol– Inhibits ADH preventing facultative water

reabsorption

Control of Water Volume

• Control of Water Volume2. Diabetes insipidus = not enough ADH

•Produce large quantities of dilute urine•Up to 24 L/day, normal = 1.2 L/day

3. Anuria = low urinary output•Less than 150 ml/day•Usually due to events that block filtration

– Nephritis– Immune reactions– Crushing injuries

Urine Transport, Storage, Elimination

• Urine Transport, Storage and Elimination– Urine production and modification

•Renal tubules and collecting system– Once in renal pelvis Urine Complete excreted via

ureters, bladder, urethra– Nephrolithiasis = Blockage of urinary passage

•E.g. Calculi (kidney stones)– Crystallized deposits of calcium, magnesium, or uric

acid– Form in renal pelvis, can become lodged in ureters– Large ones may need disruption by a lithotripter

Ureters

• Ureters– Connect renal pelvis to urinary bladder– Wall layers

1. Mucosa, with transitional epithelium2. Muscularis, with two layers of smooth

muscle3. Adventitia, attaches to posterior body wall

– Contractions occur every 30 sec to force urine toward bladder

Urinary Bladder• Urinary Bladder

– Wall folded into rugae when empty – expands– Wall layers

•Mucosa with transitional epithelium•Muscularis with 3 layers of smooth muscle = detrusor

muscle1. Contraction causes expulsion of urine from bladder2. Detrusor muscle thickened around urethral opening to

create the internal urethral sphincter»Provides involuntary control over release of urine

3. Adventitia = Fibrous, Anchors bladder to pelvic floor

Urethra• Urethra

– Single tube, connects bladder to environment– Lined with pseudostratified columnar

epithelium– Passes through band of skeletal muscle that

forms external urethral sphincter•under voluntary control•relaxation results in micturition

Micturition Reflex

• When bladder contains ~ 200ml urine1. Stretch receptors triggered2. Signal conscious awareness of pressure 3. Stimulates contraction of detrusor muscle

• Voluntary maintenance of contracted external urethral sphincter prevents urination1. Detrusor will relax2. Opening will open internal urethral sphincter3. Urination will occur

• Continued increase in urinary volume will repeatedly trigger reflex

Micturition Reflex

• If volume exceeds ~500 ml– Forced relaxation of internal and external urethral

sphincters will result in non-voluntary urination/micturition

• Incontinence – Inability to voluntarily control urine excretion– Due to:

• loss of muscle tone• Damage to sphincters• Damage to nerves or control centers in brain

The Micturition Reflex


Age Related Changes

1. Decline in functional nephrons2. Reduction in GFR

– Damage or decrease blood flow

3. Reduced sensitivity to ADH = dilute urine

4. Problems with micturition – Incontinence– Urinary retention, enlarged prostate

What effect would a high-protein diet have on the composition of urine?

A. increased urea

B. increased potassium

C. increased fluid volume

D. A and C are correct

An obstruction of a ureter by a kidney stone would interfere with the flow of

urine between which two points?

A. ureter and urethra

B. renal medulla and renal pelvis

C. renal medulla and urethra

D. renal pelvis and urinary bladder

The ability to control the micturition reflex depends on your ability to

control which muscle?

A. urogenital diaphragm

B. internal urinary sphincter

C. external urinary sphincter

D. coccygeus

Urinalysis

• Is the analysis of a urine sample:– an important diagnostic tool

• Includes color and appearance of urine

General Characteristics of Normal Urine

Table 26–5

Typical Values Obtained from Standard Urinalysis

Table 26–6

A Summary of Renal Function

Figure 26–16a

A Summary of Renal Function

Figure 26–16b

Step 1: Glomerulus

• Filtrate produced at renal corpuscle has the same composition as blood plasma:– without plasma proteins

Step 2: Proximal Convoluted Tubule (PCT)

• Active removal of ions and organic substrates:– produces osmotic water flow out of

tubular fluid– reduces volume of filtrate– keeps solutions inside and outside

tubule isotonic

Step 3: PCT and Descending Limb

• Water moves into peritubular fluids, leaving highly concentrated tubular fluid

• Reduction in volume occurs by obligatory water reabsorption

Step 4: Thick Ascending Limb

• Tubular cells actively transport Na+ and Cl— out of tubule

• Urea becomes higher proportion of total osmotic concentration

Step 5: DCT and Collecting Ducts

• Final adjustments in composition of tubular fluid

• Osmotic concentration is adjusted through active transport (reabsorption or secretion)

Step 6: DCT and Collecting Ducts

• Final adjustments in volume and osmotic concentration of tubular fluid

• Exposure to ADH determines final urine concentration

Organs for the Conduction and Storage of Urine

Figure 26–18a


Figure 26–18b


Figure 26–18c

Organs that Collect and Transport Urine

Figure 26–19

SUMMARY• Urinary system functions:

– excretion– Elimination

• Kidneys• Urine• Urination (micturition) • Nephron• Renal corpuscle• Renal tubule• Filtrate• Tubular fluid• Collecting system• Loop of Henle

SUMMARY

• Urine formation• Filtration• Reabsorption• Secretion• Glomerulus filtration• Countercurrent multiplication• Ureters• Urinary bladder• Urethra• Micturition reflex

Documents

Chapter 26: The Urinary System