The Urinary System

Chapter 26

Functions of the Urinary System

1. Removal of metabolic wastes (especially nitrogenous wastes e.g. urea & uric acid).

2. Water balance (and therefore blood pressure).

3. Control of electrolyte balance.

4. Control of pH.

5. Removal of toxins.

Anatomy of the Urinary System

• The Kidneys: the functional heart of the urinary system.

• The Ureters: pipeline from the kidneys to the bladder.

• The Urinary Bladder: holding tank of urine.

• The Urethra: avenue of relief

(word of the day “micturition” = voiding the bladder)

UrinarySystem

Components

The Kidneys

• Location: retroperitoneal against the dorsal wall of the abdominal cavity. The right kidney is slightly lower than the left.

• Size & weight: approximately 150 grams (about 5 ounces) each and 12 cm x 10 cm x 4 cm.

• Shaped like a bean (or are beans shaped like kidneys?)

The Position of

the Kidneys

Figure 26–2

Gross Anatomy

of the

Urinary System

Figure 26–3

The Structure of the Kidney

Figure 26–4

Blood Supply to Kidneys

• Kidneys receive 20–25% of total cardiac output

• 1200 - 1500 ml of blood flows through kidneys each minute

• Kidney receives blood through renal artery

Blood Supply to

the Kidneys

Figure 26–5

A slice of kidney

Medulla

Cortex

Glomeruli

Capsule

The Nephron: functional unit of the kidney

Afferent Arteriole

Glomeruli

Interlobular artery

Functional Anatomy

of Nephron

and Collecting System

Figure 26–6

Renal Corpuscle

The Nephron

and Collecting System

Table 26–1

Filtrate & Urine

flow

Cortical and Juxtamedullary Nephrons Figure 26–7

The renal corpuscle and the Juxtaglomerular apparatus

The renal filtration membrane:Podocytes and fenestrated capillaries

Filtration slits Pedicles

The filtration membrane

An Overview of Urine

Formation

Figure 26–9 (Navigator)

Blood pressure drops due to peripheral resistance

Filtration pressures: NFP must be positive for U2P

GFRGFR is “Glomerular Filtration Rate”.• It is directly proportional to NFP.• It is a measurement of FLOW in milliliters

per minute (ml/min).• If NFP drops more than 15% below 10

mmHg, GFR goes to 0.• If NFP goes up less than 30% above

normal, the kidneys can handle it without major compensatory mechanisms kicking in.

Creatinine Clearance Test

• Is used to estimate GFR

• A more accurate GFR test uses inulin:– which is not metabolized

Response to Reduction in

GFR

Figure 26–11

Tubular reabsorption

•Reabsorption of filtered solutes occurs in the Proximal Convoluted Tubules.•Most solutes are reabsorbed by secondary active transport with Na+. Does this look familiar?

4 Types of Carrier-Mediated Transport

1. Facilitated diffusion

2. Active transport

3. Cotransport

4. Countertransport

The Transport Maximum (Tm)• If nutrient concentrations rise in tubular

fluid:– reabsorption rates increase until carrier

proteins are saturated

• Concentration higher than transport maximum:– exceeds reabsorptive abilities of nephron– some material will remain in the tubular fluid

and appear in the urine

• Determines the renal threshold

Transport Activities

at the PCT

Figure 26–12 (Navigator)

Reabsorptionof sodium

Reabsorption of glucose

Reabsorption of electrolytes

and water

Reabsorption of ions in the Loop

of Henle

Reabsorption

• 60–70% of filtrate volume produced at glomerulus:– is reabsorbed before tubular fluid reaches

loop of Henle

The Loop of Henle

• Reabsorbs about 1/2 of water, and 2/3 of sodium and chloride ions remaining in tubular fluid by the process of countercurrent exchange

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 cortex

Multiplication

• Refers to effect of exchange:– increases as movement of fluid continues

Parallel Segments of Loop of Henle

• Are very close together, separated only by peritubular fluid

• Have very different permeability characteristics

Formation of dilute

urine

Countercurrent Multiplication and Concentration of Urine

Figure 26–13a (Navigator)

Countercurrent Multiplication and Concentration of Urine Figure 26–13b, c

Na+—K+/2 Cl— Transporter

• Each cycle of pump carries ions into tubular cell:– 1 sodium ion– 1 potassium ion– 2 chloride ions

Tubular Fluid at DCT

• Arrives with osmotic concentration of 100 mOsm/L:– 1/3 concentration of peritubular fluid of renal

cortex

• Rate of ion transport across thick ascending limb:– is proportional to ion’s concentration in tubular

fluid

Regional Differences

• More Na+ and Cl— are pumped into medulla:– at start of thick ascending limb– than near cortex

• Regional difference in ion transport rate:– causes concentration gradient within medulla

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

Urea and the Concentration Gradient (1 of 2)

• Thick ascending limb of loop of Henle, DCT, and collecting ducts:– are impermeable to urea

• As water is reabsorbed:– concentration of urea rises

Urea and the Concentration Gradient (2 of 2)

• Tubular fluid reaching papillary duct:– contains 450 mOsm/L urea

• Papillary ducts are permeable to urea:– concentration in medulla averages 450

mOsm/L

2 Benefits of Countercurrent Multiplication

1. 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

In the collecting

duct

Reabsorption in the Distal Convoluted Tubule & Collecting

Duct

Differences between

Solute Composition

in Urine and Plasma

Table 26–2

Summary of tubular

reabsorption/excretion

Renal autoregulation

Compensatory Mechanisms to maintain

GFR

A Pyelogram

Figure 26–17

After the Kidneys:

the bladder

& urethra

Male Figure 26–18a

Female Figure 26–18c

The HumanBladder:It can hold

a maximum of 800 –

1000 ml!

Histology

Physical characteristics of Urine

• Color - Clear to deep yellow, almost rusty, depending on concentration.

• Odor - Fresh urine is slightly aromatic, stale urine smells like downtown Tacoma on a Sunday morning.

• pH – range of 4.5 – 8 depending on diet. High protein leads to low pH (acid ash diet), vegetarian (alkaline ash) diet leads to high pH. Heavy vomiting and bacterial infection can also lead to alkaline urine.

• Specific gravity – Normal range is 1.001 – 1.030. Distilled water has a s.g. of 1.000. Anything solutes cause the specific gravity of a liquid to go up. Concentrated urine has a higher s.g. than dilute urine.

General Characteristics of Normal UrineTable 26–5

Typical Values

Obtained from

Standard Urinalysis

Table 26–6

Composition of UrineNormalUreaUric acidCreatinineNaKPhosphatesSulfatesBicarbonateCaMg

AbnormalGlucose “glycosuria”Proteins “proteinuria” or

“albuminuria”Ketones “ketonuria”Hemoglobin

“hemoglobinuria”Erythrocytes “hematuria”Bile pigments “bilirubinura”Leukocytes “pyruia”

Abnormally low output = oliguriaNo output = anuriaAbnormally high output = polyuriaDiuresis = increased urine outputDiuretic = substance that leads to diuresis

A Summary of Renal Function

Figure 26–16a

A Summary of Renal Function

Figure 26–16b

The Micturition ReflexFigure 26–20 (Navigator)

Incontinence

• Is the inability to control urination voluntarily

• May be caused by trauma to internal or external urethral sphincter

Age-Related Changes in Urinary System

• Decline in number of functional nephrons

• Reduction in GFR

• Reduced sensitivity to ADH

• Problems with micturition reflex

Stages of Renal Disease

• Stage 1: signs of kidney damage w/ GFR ≥ 90.

• Stage 2: signs of kidney damage w/ GFR 60 – 89.

• Stage 3: GFR 30 – 59.

• Stage 4: GFR 15 – 29.

• Stage 5: < 15

Renal ClearanceThe volume of plasma that is cleared of a particular substance in a given time (usually one minute).

RC = UV/PU = concentration of the substance in urine (mg/ml)V = flow rate of formation (ml/min)P = concentration of the substance in the plasma (mg/ml)High renal clearance values means that the substance is being effectively cleared, low values means that more is being reabsorbed. For some solutes low is good (glucose should be 0). For others, high RC would be expected (creatinine should be complete, urea should be about 80%).

The Excretory System

• Includes all systems with excretory functions that affect body fluids composition:– urinary system– integumentary system– respiratory system– digestive system

Kidney stonesRenal Calculi

“Well Mr. Osborne, I

don’t think that it’s kidney

stone after all”