Urinary System

Nestor T. Hilvano, M.D., M.P.H.

Learning Objectives 1. List the functions of the kidneys. 2. Identify the external and internal features of the kidney. 3. Trace the flow of blood through the kidney.4. Describe the nephron and state the function of each segment. 5. Describe the glomerular filtration membrane and explain the

factors that influence glomerular filtration rate. 6. Name the major nitrogenous waste and identify their sources.7. Describe how the tubules reabsorb solutes. 8. Describe how the tubules secrete solutes from the blood into

the tubular fluid. 9. Explain how the ADH and aldosterone regulate the volume

and concentration of urine. 10. Describe the composition and properties of urine. 11. Describe the functional anatomy of the ureters, urinary

bladder, and urethra.12. Discuss micturation reflex and the regulation of urination.

Urinary System

• Major Functions 1. Excretion (removal)2. Elimination (discharge)3. Homeostatic regulation

- blood volume and solute concentration, acid base balance

• What are the organs of urinary system? • What is micturation?

Anatomy of Kidney• Position

– level of T12 to L3

• Shape– lateral surface - convex; medial - concave

• CT coverings– Renal fascia: outer layer of dense fibrous tissue, binds

to abdominal wall – Perinephric fat capsule: cushions kidney– Renal (fibrous) capsule: encloses kidney

Anatomy of Kidney• ____ – outer part, reddish brown and

granular• ____ - inner part, consist of triangular

structures called ____.• ____ - consists of pyramid, overlying

cortex, and renal columns; site of urine production

a. renal lobe c. renal cortex b. renal medulla d. renal pyramid

Nephron and Collecting System• Nephron – functional/structural unit of kidneys (urine

formation); consists of: 1. Renal corpuscle- made up of Bowman’s

capsule and glomerulus; function = filtration2. PCT- reabsorb organic nutrients, water, and

electrolytes3. Loop of Henle- descending loop= reabsorb

water; ascending loop=reabsorb salts 4. DCT- last segment; function= secrete waste

products, reabsorb water • Collecting system: Collecting (ducts) tubules,

papillary duct, minor calyx, major calyx, renal pelvis, ureter, urinary bladder, urethra

Nephrons

• Cortical nephrons (85%)– short nephron loops– efferent arterioles branch off peritubular capillaries

• Juxtamedullary nephrons (15%)– long nephron loops– efferent arterioles give rise to vasa recta

Renal corpuscle• Bowman capsule• Glomerulus• Filtration is passive (metabolic waste, excess ions,

and small solutes) • Filtration membrane consists of fenestrated

endothelium, lamina densa, and filtration slits • Why plasma proteins are not filtered under normal

condition?

Renal Tubule• Proximal convoluted tubule

– Reabsorption of organic nutrients, ions, and water from filtrate

• Nephron loop; descending and ascending limbs– thick segment = active transport of salts out of the

tubule– thin segment = water permeable (out)

• Distal convoluted tubule– active secretion of ions, drugs, acids, and toxins into

tubular fluid for excretion – selective reabsorption of sodium, calcium, water

Juxtaglomerular Complex

• JG cell= secrete renin• Macula densa = monitor salinity• Mesangial cell= release erythropoitin• Damage to JG complex – interfere w/

hormonal control of BP

Glomerular Filtration Rate (GFR)

• Basic process of urine formation: filtration, reabsorption, and secretion (excretion)

• Filtrate formed per minute

• GFR = NFP x Kf = 125 ml/min or 180 L/day– filtration coefficient (Kf) depends on permeability and surface

area of filtration barrier

• 99% of filtrate reabsorbed, 1 to 2 L of urine per day excreted

Effects of GFR Abnormalities GFR, urine output rises dehydration,

electrolyte depletion GFR wastes reabsorbed (azotemia

possible)• GFR controlled by adjusting glomerular blood

pressure – Autoregulation (local response): GFR –

paracrine secretion - constriction of afferent arteriole - GFR

– Autonomic regulation (sympathetic control) – Hormonal regulation (kidneys): renin and

angiotensin

DCT and Collecting Duct• involved in salt/water balance; acid/base balance• Function = fluid reabsorption is variable, regulated

by hormonal action• Aldosterone effects

BP renin release angiotensin II formation (in the capillaries of lungs)

– angiotensin II stimulates adrenal cortex– adrenal cortex secretes aldosterone

• promotes Na+ reabsorption promotes water reabsorption urine volume maintains BP

DCT and Collecting Duct

• Effect of ADH– dehydration stimulates hypothalamus – hypothalamus stimulates posterior pituitary to

release ADH water reabsorption urine volume

• ADH hyposecretion results to diabetes insipidus

DCT and Collecting Duct

• Atrial natriuretic peptide (ANP) – atria secrete ANP in response to BP– has four actions:

1.dilates afferent arteriole, constricts efferent arteriole - GFR

2. inhibits renin/angiotensin/aldosterone pathway

3. inhibits secretion and action of ADH

4. inhibits NaCl reabsorption

• Promotes Na+ and water excretion, urine volume, blood volume and BP

Control of Water Loss

• Producing hypotonic urine– NaCl reabsorbed by cortical CD– water remains in urine

• Producing hypertonic urine– dehydration ADH CD’s water

permeability– more water is reabsorbed– urine is more concentrated

Countercurrent Multiplier: Nephron loop

• Recaptures NaCl and returns it to renal medulla• Descending limb

– reabsorbs water – concentrates tubular fluid

• Ascending limb – reabsorbs Na+, K+, and Cl-

– tubular fluid becomes hypotonic

Countercurrent Exchange System

• Formed by vasa recta– provide blood supply to medulla

• Descending capillaries– water diffuses out of blood– NaCl diffuses into blood

• Ascending capillaries– water diffuses into blood– NaCl diffuses out of blood

Composition of Urine• Appearance – clear yellow; yellow color due to urobilin

pigment (generated in kidneys from urobilinogens)• Odor – varies; bacteria degrade urea to ammonia• Specific gravity - ranges from 1.003 -1.030

• Osmolarity - ranges from 855 to 1,335 mOsm/L• pH - range: 4.5 - 8, usually 6.0• Chemical composition: 93% to 97% water, 5% solutes by

volume (urea, NaCl, KCl, creatinine, uric acid)• Urine volume – 1 to 2 L/day

• Polyuria > 2L/day; Oliguria < 500 mL/day;

Anuria - 0 to 100 mL/day• Hematuria- presence of blood in the urine

Diuretics• Effects

– ____ urine output– ____ blood volume

• Uses– hypertension and congestive heart failure

• Mechanisms of action– ____ GFR– ____ tubular reabsorption a. Increase

b. Decrease

Ureter and Urinary Bladder• Ureter (about 25 cm long)

– 3 layers• mucosa• muscularis • Adventitia

___ Where is the common site of obstruction of urine

flow?• Urinary bladder- located in pelvic cavity, posterior to

pubic symphysis

- Histologic features of bladder = thick muscular layer

(detrusor muscle); capacity (500 ml. moderately full; 800 ml. maximum)

___ What is urinary trigone?

Male Urethra• Male Urethrao 18 cm longo Internal urethral sphinctero External urethral sphinctero 3 regions: _______, _______, _______

• Female Urethrao 3 to 4 cm long

Micturition• 200 ml urine in bladder, stretch receptors send signal to

sacral spinal cord (S2-S4)• Signals ascend to cortex• Signals descend to

– stimulate parasympathetic neurons • Result = urge to urinate

– Contraction of detrusor muscle – relaxation of internal urethral sphincter

• External urethral sphincter – voluntary control• How incontinence may develop in elderly?

Homework (Self-review) 1. Define the following: kidney, ureters, urinary bladder,

urethra, glomerular filtration, tubular reabsorption, tubular secretion, nephron, renal lobe, nephrolithiasis, micturation, incontinence, polyuria, and anuria.

2. Name the structure involves in the following: filtration of solutes and water, reabsorption of salt and secretion of K+, countercurrent multiplier, countercurrent exchange system, temporary storage of urine, urine formation, release renin.

3. Discuss the effect/s of renin- angiostensin- aldosterone on GFR, tubular reabsorption, and urine output.

4. Identify the structures in renal cortex and medulla.

5. Identify parts of nephron and collecting system.

6. Describe the normal properties of urine.