20.1 INTRODUCTION

The major function of the urinary system is to remove metabolic wastes from blood, and direct them out of the body. In doing so, blood homeostasis is maintained, as well. The organs of the urinary system include the kidneys, ureters, urinary bladder, and urethra.

See Fig. 20.1 and Fig 20.2, page 776.

20.2 KIDNEY

A. The term renal refers to the kidney.

B. Location of Kidney:

Fig 20.1, page 776 - Fig 20.3, page 777.

1. high on posterior abdominal wall

2. retroperitoneal

3. right kidney lies just below left. Why?

C. Structure of Kidney:

Fig 20.4a, page 778.

1. Renal capsule = tough fibrous shell around kidney.

2. Renal cortex = outer portion of kidney.

3. Renal medulla = inner portion of kidney.

4. Renal pyramids = cone shaped masses of tissue in renal medullae.

5. Ureter = tube leading from away from kidney.

6. Renal pelvis = superior end of ureter which is expanded to form a funnel shape.

7. Major calyces = divisions of renal pelvis (2-3 tubes).

8. Minor calyces = divisions of major calyces.

D. Functions of Kidney:

1. to remove metabolic wastes from blood and excrete them to outside in urine.

2. maintenance of blood homeostasis:

a. regulation of RBC formation (hormone erythropoietin)

b. blood pressure (enzyme renin)

c. blood volume (hormone ADH)

d. blood composition

e. blood pH

20.2 KIDNEY

E. Blood flow through Kidney:

1. Macroscopic = Fig 20.5, page 778 and Fig 20.6, page 780.

2. Microscopic = Fig 20.10, page 783, and Fig 20.14, page 803.

3. Summary Fig 20.15, page 786, however note that interlobular vessels are termed “cortical radiate” vessels in this figure.

Aorta

↓

Renal Artery (to each kidney)

↓

Interlobar arteries (between each pyramid)

↓

Arcuate arteries (between medulla & cortex)

↓

Interlobular arteries (within cortex)

↓

Afferent arteriole (leading to glomerulus)

↓

Glomerular capillaries (site of filtration)

↓

Efferent arteriole (leading away from glomerulus)

↓

Peritubular capillaries/vasa recta (around renal tubule)

↓

Interlobular veins (within cortex)

↓

Arcuate veins (between cortex and medulla)

↓

Interlobar veins (between pyramids)

↓

Renal vein (from each kidney)

↓

Inferior vena cava

20.2 KIDNEY

F. The functional unit of a kidney = the nephron.

Fig 20.4c, page 778.

1. Structure: A nephron is composed of a renal corpuscle and a renal tubule.

a. Renal Corpuscle = glomerulus (specialized capillaries which serve as filtration unit) within Bowman's capsule.

b. Renal Tubule =

1. proximal convoluted tubule

2. descending loop of Henle

3. ascending loop of Henle

4. distal convoluted tubule

5. collecting duct

○ Each collecting duct empties into a minor calyx, which leads to a major calyx and into the renal pelvis.

Refer to Fig 20.7 page 781 to see a scanning electron micrograph of glomeruli within the renal tubules, and Fig 20.11, page 783, illustrating light micrographs of the kidney, which you will observe in lab.

G. Juxtaglomerular Apparatus (JGA) = point of contact between the afferent arteriole and distal convoluted tubule (DCT).

See Fig. 20.12, page 784.

1. Macula Densa = cells in DCT in contact with afferent arteriole.

2. Juxtaglomerular cells = specialized smooth muscle cells in afferent arteriole.

a. The JGA is very important in the regulation of glomerular filtration rate. (see below)

H. Cortical and Juxtamedullary Nephrons

1. 80% of nephrons are found mostly in the cortex and are therefore termed “cortical nephrons”.

2. 20% of nephrons have a Loop of Henle that extend deep into the renal medulla and are therefore termed “juxtamedullary nephrons”.

a. Very important in the production of concentrated urine at times of dehydration, as will be discussed later.

b. See Figure 20.13 page 785.

9.3 URINE FORMATION

A. Introduction

1. The nephrons function to remove wastes from blood and regulate water and electrolyte concentrations.

2. Urine is the end-product of these functions.

3. Urine formation involves three major steps including

a. glomerular filtration

b. tubular reabsorption

c. tubular secretion

4. Urine excreted = Glomerular Filtration + Tubular Secretion – Tubular Reabsorption

B. Glomerular Filtration

Fig 20.16 and Fig 20.17, page 787.

1. The fenestrated glomerular capillaries (See Fig 20.9, page 782) filter water and dissolved materials (remember plasma components) from blood.

a. This "filtrate" is collected in Bowman's Capsule.

b. Proteins are not filtered out of blood!

2. Filtration Pressure

See Figure 20.17 page 787.

a. Filtration is due to a force net filtration pressure

1. outward + 60 mmHg called glomerular hydrostatic pressure (GHSP) inside glomerular capillaries.

2. inward - 18 mmHg in Bowman’s capsule.

3. inward – 32 mmHg colloid osmotic pressure in glomerular capillaries.

4. Net Filtration Pressure = 10 mmHg outward

3. Glomerular Filtration Rate (GFR)

a. Kidneys produce =125 ml fluid per minute (glomerular filtration rate/GFR)

1. Most of this is reabsorbed in proximal convoluted tubule.

2. See Fig 20.18, page 789.

4. Control of Glomerular Filtration Rate (GFR)

a. Normal GFR is approximately 125ml/minute (70 kg adult male, both kidneys) primarily due to glomerular hydrostatic pressure (GHSP). GFR remains relatively constant through two mechanisms, which include:

1. Autoregulation by vasomotor center in medulla

2. Renin-angiotensin system

9.3 URINE FORMATION

B. Glomerular Filtration

5. Control of Glomerular Filtration Rate (GFR)

a. Autoregulation (AR) by Vasomotor Center

1. The vasomotor center in the medulla regulates arteriole smooth muscle allowing for AR.

2. Under normal conditions, the parasympathetic autonomic nervous system (ANS) maintains AR through vasoconstriction of the afferent arteriole to decrease GFR when elevated or vasoconstriction of the efferent arteriole to increase GFR when low.

3. AR can be overridden by the sympathetic ANS during significant volume loss or gain.

○ A large blood volume loss, which markedly decreases blood pressure causes vasoconstriction of afferent arterioles, decreasing GFR, which decreases urine output to conserve water.

○ A large blood volume gain, which markedly increases blood pressure causes vasodilation of afferent arterioles, increasing GFR, which increases urine output to eliminate the excess water.

9.3 URINE FORMATION

B. Glomerular Filtration

5. Control of Glomerular Filtration Rate (GFR)

b. Renin-Angiotensin System See Fig 20.20, page 790.

1. A decrease in blood volume (BV) causes a decrease in blood pressure (BP), which in turn decreases the glomerular hydrostatic pressure (GHSP) and GFR.

2. Receptors in the Juxtaglomerular Apparatus (JGA) detect this decrease in two ways:

○ Baroreceptors in the JG cells of the afferent arteriole (AA) detect a decrease in stretch and secrete the enzyme renin.

○ Chemoreceptors in the macula densa cells in the distal convoluted tubule (DCT) detect a decrease in the levels of sodium (Na⁺), potassium (K⁺), and chloride (Cl^-), and further stimulate the JG cells of the AA to secrete the enzyme renin.

a. In blood, renin converts the plasma protein, angiotensinogen to angiotensin I.

b. Primarily in the lungs where endothelial capillaries produce angiotensin converting enzyme (ACE), ACE converts angiotensin I to Angiotensin II.

e. Angiotensin II targets four sites that work together to maintain sodium balance, water balance and blood pressure (which directly affects GFR).

o The efferent arterioles vasoconstrict, increasing GHSP, which directly increases GFR back to normal.

o The adrenal cortex secretes the hormone aldosterone, which targets the DCT, causing reabsorption of Na⁺ (and H₂0). This increases BV, which increases BP, which increases GHSP, and restores GFR back to normal.

o The posterior pituitary gland secretes the antidiuretic hormone (ADH), which targets the DCT, causing reabsorption H₂0. This increases BV, which increases BP, which increases GHSP, and restores GFR back to normal.

o The hypothalamus triggers thirst, which increases fluid intake. This increases BV, which increases BP, which increases GHSP, and restores GFR back to normal.

9.3 URINE FORMATION

C. Tubular Reabsorption:

See Fig 20.21a, page 791.

1. Tubular secretion is defined as the process by which substances are transported from the glomerular filtrate (through the walls of the renal tubule) to blood in the peritubular caps.

2. Most occurs in the PCT through the process of active transport

a. Reabsorption of water = osmosis.

See Fig 20.22, page 793.

3. Reabsorbed substances pass from the lumen of the renal tubule through the epithelial cells (PCT) and into the lumen of a peritubular capillary where they are returned to bloodstream.

4. Reabsorbed substances include:

a. glucose

b. amino acids

c. water

d. ions (sodium, chloride, phosphate, sulfate, potassium)

e. others (creatine, lactic acid. citric acid, urea, uric acid, ascorbic acid)

5. Substances that remain in filtrate become concentrated as water is reabsorbed.

D. Tubular Secretion:

See Fig 20.21b, page 791 and Fig 20.23, page 794.

1. Tubular secretion is the process by which substances are transported from the blood in the peritubular caps into the DCT.

2. Tubular secretion maintains ion concentrations in blood (i.e. if the blood is high in K⁺, K⁺ will be secreted into urine).

3. Tubular secretion allows for secretion of metabolic wastes (see below)

9.3 URINE FORMATION

E. Regulation of Urine Concentration and Volume

See Fig 20.24, page 795, through Fig 20.26, page 797.

1. The hormone anti-diuretic hormone (ADH) promotes the reabsorption of water through the DCT and collecting ducts, preventing excessive amounts of water from being lost in the urine. This negative feedback mechanism prevents dehydration.

a. See table 20.3, page 796.

2. Countercurrent Mechanism – juxtamedullary nephrons actively resorb NaCl in the ascending loop causing hypertonic medullary interstitial fluid

3. Countercurrent Multiplier – descending loop loses water to hypertonic medulla, further increasing tonicity of medulla

4. Vasa recta ensures NaCl stays in the medulla

5. ADH opens water channels (aquaporin) in DCT and CD that allows water to be resorbed by osmosis, due to the osmotic pressure set up by the juxtamedullary nephrons

F. Excretion of Wastes

1. Wastes are by-products of metabolism:

a. urea from amino acid metabolism

○ Plasma concentration reflects protein in diet.

○ enters tubules through glomerular filtration

○ 50% is passively reabsorbed in PCT.

○ 50% is excreted in urine.

○ A countercurrent mechanism with urea helps reabsorb water.

b. uric acid from nucleotide metabolism

○ enters tubules through glomerular filtration

○ Most is reabsorbed by AT.

○ Some is secreted into urine.

G. Urine Composition

1. 95% water

2. other 5% includes:

a. urea

b. uric acid

c. trace amino acids

d. electrolytes

e. drugs

9.3 URINE FORMATION

H. Renal Clearance

1. Rate at which a particular chemical is removed from the plasma

a. Inulin Clearance Test

b. Creatinine Clearance Test

c. Paraminohippuric Acid Test

2. Used to calculate GFR and efficiency by comparing blood and urine values

g. Urine Formation Summary

Use Table 20.1, page 788 to compare the levels of various substances present in plasma, glomerular filtrate, and urine.

URINE FORMATION SUMMARY TABLE (Keyed at the end of this outline)

Major Step in Urine Formation

Location in Nephron

Substances Transported and Mode of Transport

From where to where?

(i.e. from blood to glomerular filtrate)

20.4 ELIMINATION OF URINE

A. Ureters are small tubes that carry urine from each kidney to the urinary bladder through peristaltic movements.

1. 25 - 39 cm in length See Fig 20.1 and 20.2, page 776.

2. retroperitoneal

3. Three layers:

a. Inner mucosa = transitional epithelium.

b. Middle muscularis = inner circular layer of smooth muscle and outer longitudinal layer (peristalsis).

c. Outer serosa = fibrous CT.

See Fig 20.27, page 799.

B. Urinary Bladder See Fig 20.28, page 799 and Fig 20.29, page 800.

1. Location: within pelvic cavity

behind symphysis pubis

2. Structure:

a. hollow, distensible, muscular organ

b. lined by transitional epithelium

c. detrusor muscle = 3 layers of smooth muscle

d. covered by fibrous CT

3. Function: storage of urine

C. The Urethra is a tube that carries urine from the urinary bladder to the outside.

See Fig 20.31 and Fig 20.32, page 801.

1. Length depends on sex:

a. female = 4 cm

b. male = 20 cm

2. Histology depends on sex:

a. female = 3 layers

b. males = 2 layers

20.4 ELIMINATION OF URINE

D. Micturition = the process by which urine is expelled from urinary bladder to outside.

1. Micturition reflex center is in sacral spinal cord.

2. Parasympathetics cause detrusor muscle to contract in response to stretch of the urinary bladder.

3. External urethra sphincter (skeletal muscle) is last “doorway” to pass, and therefore micturition can be (and usually is) inhibited until released by conscious control.

E. Starts at glomerulus where glomerular filtrate is collected in

Bowman's capsule

See Table 20.5, page 802. PCT

loop of Henle

DCT

collecting duct (urine)

minor calyx

major calyx

renal pelvis

ureter

(peristalsis)

urinary bladder

(micturition)

urethra

outside

Use this flow chart as a review by adding key points where they belong (i.e. reabsorption, reabsorption of water under influence of ADH, secretion, etc.)

20.5 LIFE-SPAN CHANGES

A. As one ages, kidneys, ureters, and urethra changes occur, however nephrons are so numerous that the following changes are essentially masked.

1. The kidneys become grainy and scarred.

2. GFR decreases significantly as glomeruli atrophy, fill with connective tissue, or unwind.

3. Fat accumulates on the exterior of the renal tubules, making them asymmetric.

a. Reabsorption and secretion become slow or impaired.

b. The rate of drug clearance decreases.

4. Cardiovascular changes lead to decreased rates through urinary system.

5. The kidney:

a. slows in its response to changes.

b. is less efficient at activating Vitamin D.

B. Finally, elasticity of urinary organs declines.

a. Changes in urination patterns result.

Other Interesting Topics Concerning the Urinary System

A. A Medical Mystery. See introduction on page 775 re: Aristolocholic acid nephropathy.

B. Continuous ambulatory peritoneal dialysis. See box on 779.

C. Development of specialized glomerular cells. See box on page 781.

D. BUN – Blood Urine Nitrogen. See box on page 788.

E. Diuretics. See box on 796.

F. Infants and dehydration. See box on page 798.

G. Urinalysis: Clues to Health. See Clinical Application 20.4, page 803.

Homeostatic Imbalances/Disorder of the Urinary System

A. Chronic Renal Failure. See Clinical Application 20.1, page 779.

B. Glomerulonephritis. See Clinical Application 20.2, page 782

C. Shock. See box on page 789.

D. HUS – Hemolytic Uremic Syndrome. See box on page 790.

E. Glycosuria. See box on page 792.

F. Nephrotic Syndrome. See Clinical Application 20.3, page 793.

G. Nephritic Syndrome. See last paragraph of CA 20.3, page 793.

H. Renal Cell Carcinoma. See box on page 793.

I. Gout. See box on page 797.

J. Cystitis/Ureteritis. See box on page 799.

K. Kidney Stones. See box on page 799.

L. Developmental Abnormalities of the Urinary System. See Table 20.6, page 802.

M. Incontinence. See box on page 803.

INNERCONNECTIONS OF THE URINARY SYSTEM. See page 804.

CHAPTER SUMMARY – see pages 805-807.

CHAPTER ASSESSMENTS – see pages 807-808.

INTEGRATIVE ASSESSMENTS/ CRITICAL THINKING – see page 808.

Urine Formation SUMMARY TABLE

Major Step in Urine Formation

glomerular filtration

tubular reabsorption

tubular secretion

Location in Nephron

glomerulus

primarily through proximal convoluted tubule (PCT)

primarily through distal convoluted tubule (DCT)

Substances Transported and Mode of Transport

All plasma constituents except proteins (i.e. glucose, amino acids, water, ions, creatine, lactic acid, urea, uric aid, ascorbic acid, etc)

glucose, amino acids, water, ions, creatine, lactic acid, urea, uric acid, ascorbic acid, etc.

excess ions, trace amino acids, urea, uric acid, drugs

From where to where?

(i.e. from blood to glomerular filtrate)

from blood in glomerulus to “filtrate” in Bowman’s capsule

from “filtrate” in PCT to blood in peritubular capillaries

from blood in peritubular capillaries to “urine” in DCT