KIDNEY GROSS ANATOMY

General

1. Location – retroperitoneal; 12^th thoracic to 3^rd lumbar vertebrae

2. Size – 150 grams; 12 cm long

3. Shape – bean shaped

External anatomy

1. Protective layers – total of 3

-a. Renal capsule – transparent fibrous layer; attached to kidney; infection barrier

-b. Adipose capsule – attaches to posterior wall; trauma protection

-c. Renal fascia – dense fibrous connective tissue; adrenals; to other structures

2. Hilus – indentation in the medial surface; vessels, nerves, lymphatics

3. Renal sinus – space interior to hilus; vessels, nerves, etc.

Internal anatomy

1. Renal cortex – outer portion; lighter color; granular appearance

2. Renal medulla – darker appearance; inner part

-a. Renal columns – extensions of the cortex; separates medulla into pyramids

-b. Renal pyramids – cone shaped partitions of the medulla; base to cortex

-c. Papilla – apex of each kidney; faces minor calyces

-d. Collecting tubules – microscopic; give pyramids a stripped appearance

-e. Lobe – each pyramid with its cortical tissue cap; usually 8

2. Pelvis – funnels shaped tube; connects kidney to ureter

-a. Major calyces – two to three branches of the pelvis

-b. Minor calyces – branches of the major calyx; enclose papillae of pyramids

-c. Smooth muscle – calyces, pelvis, ureter; peristalsis

Blood supply

1. Arterial supply 1/4^th cardiac output delivered to kidneys every minute

-a. Renal arteries – branch of aorta; branch before entering renal hilus

-b. Segmental arteries – about 5 for each kidney; enter hilus

-c. Lobar arteries – segmental branches in sinus

-d. Interlobar arteries – branches of lobar; between pyramids to cortex

-e. Arcuate arteries – runs along medullary cortical border

-f. Interlobular arteries – supply cortex; afferent arterioles come from these

2. Venous drainage – similar but no lobar or segmental veins

-a. Renal vein – empties into the vena cava

Nerve supply

1. Renal plexus – celiac plexus; sympathetic; vasomotor; blood to glomerulus

THE NEPHRON

Nephron

1. Glomerulus – ball of capillaries

-a. Afferent arteriole – blood to glomerulus

-b. Efferent arteriole – blood from glomerulus

2. Glomerular (Bowman’s) capsule – encompass each glomerulus

-a. Visceral layer – direct contact with glomerulus; modified epithelia; filtration

-b. Parietal layer – outer layer; holds in filtrate but not involved with filtration

-c. Glomerular space – space between the visceral and parietal layers

3. Renal corpuscle – collective name for the glomerulus and Bowman’s capsule

4. Filtrate – formed in the renal corpuscle; no cells or proteins

5. Renal tubule – from capsule to collecting duct; three named parts

-a. Proximal convoluted tubule – immediately after the glomerular capsule

-b. Loop of Henle – descending limb; ascends limb

-c. Distal convoluted tubule – back in the cortex; to collecting duct

6. Collecting duct – filtrate from many nephrons; medulla stripped appearance

Nephron histology

1. Glomerulus – endothelium is fenestrated; many pores

2. Visceral layer – specialized epithelial cells

-a. Podocytes – branching epithelial cells

-b. Pedicels – foot processes of podocytes

-c. Filtration slits – gaps between pedicels from adjacent podocytes

3. Parietal layer – simple squamous epithelium

4. Proximal convoluted tubule – simple cuboidal; absorption and secretion

-a. Microvilli – greatly increase surface area

5. Loop of Henle – histological changes reflect function

-a. Descending limb – from PCT to deep medulla

-b. Ascending limb – from deep medulla to DCT

-a. Thin segment – simple squamous; freely permeable to water; descending limb

-b. Thick segment – simple cuboidal to columnar; most of ascending limb

Types of nephrons

1. Cortical nephrons – 85%; most of it in medulla

2. Juxtamedullary nephron – invades medulla; more extensive loop of Henle

-a. Concentrated urine – will result from this one

Microvasculature

1. Afferent arteriole – branches from the interlobular artery; feeds glomerulus

2. Efferent arteriole – leaves glomerulus; to peritubular capillaries and vasa recta

3. Peritubular capillaries – reabsorption water and solutes from tubule; secretion

4. Vasa recta – cling to long loops of loop of Henle; concentrated urine formation

Juxtaglomerular apparatus

1. Juxtaglomerular apparatus – initial DCT lies against afferent and efferent

2. Juxtaglomerular cells – arteriole smooth muscle cell; afferent and efferent

-a. Mechanoreceptors – sense the blood pressure

-b. Renin – granules; degranulate when low blood pressure

3. Macula densa – chemoreceptors; osmoreceptors; in DCT

4. Function – regulation of blood pressure and glomerular filtration rate

Filtration membrane

1. Glomerular endothelium – fenestrated; water and small molecules; no cells

2. Basement membrane – fused basal lamina of 2 layers; physical and ionic barrier

3. Visceral layer – filtration slits between pedicels of the podocytes

KIDNEY PHYSIOLOGY: URINE FORMATION

Processes of urine formation

1. Glomerular filtration – NFP; rate; and control

2. Tubular reabsorption –mechanisms and substances; regions

3. Tubular secretion - substances; control of pH

4. Urine concentration – loop of Henle

Glomerular filtration: net filtration pressure

1. GBHP – hydrostatic pressure pushing blood out of glomerulus; 55 mm

2. CHP – hydrostatic pressure pushing back into glomerulus; 15 mm

3. BCOP – osmotic pressure pulling fluid back into blood; 30 mm

4. COP – osmotic pressure pulling fluids into the capsule; insignificant

5. NFP – GBHP – CHP – BCOP = 10 mm

Glomerular filtration: glomerular filtration rate

1. GFR – 125 ml / minute

2. Total surface area – equal to the surface of the skin

3. NFR – directly proportional to GFR; effect NFR, effect GFR

a. High blood pressure – increase HPg; increase NFP; increase GFR

b. Dehydration – increase OPg; decrease NFP; decrease GFR

GFR regulation: tubuloglomerular mechanism (autoregulation)

1. Macula densa – DCT osmoreceptors; monitor salinity and filtrate flow rate

-a. High GFR – high salinity and filtrate flow; paracrine constrict afferent arteriole

-b. Low GFR – low salinity and filtrate flow; paracrine dilates afferent arteriole

2. JG cells – signaled by macula densa during low osmolarity and filtrate flow

-d. Renin – secrete in response to low blood pressure

GFR regulation: myogenic control (autoregulation)

1. Systemic blood pressure – changes

-a. Increases – increases stretch of smooth muscle in afferent; constrict

-b. Decreases – decreases stretch of smooth muscle in afferent; dilate

GFR regulation: sympathetic control

1. Stress – stimulates sympathetic and adrenal medulla; epinephrine

2. Afferent arteriole – constricts; GFR and urine formation reduced

3. Redirects blood – from kidneys to skeletal muscle, heart, and brain

GFR regulation: renin-angiotensin mechanism

1. JG cells – also release renin when blood pressure drops; less stretch

2. Renin – antgiotensinogen to angiotensin I to angiotensin II

3. Angiotensin II – has multiple effect

-a. Vasoconstriction – widespread; increases blood pressure

-b. Efferent arterioles – are constricted more; maintains GFR

-c. Aldosterone – adrenal cortex to release; ↑ Na and water reabsorption, ↑ BP

-d. Thirst – stimulated; water intake increases blood pressure

Proximal convoluted tubules: tubular reabsorption

1. Tubular reabsorption – almost all nutrients; not all absorption here

2. Peritubular capillaries – ↑concentration of proteins; ↓pressure and velocity

3. Solvent drag – water pulled into peritubular capillaries takes solutes with

4. Transcellular route – luminal and basolateral membranes of tubule cells

5. Paracellular route – pass between epithelial cells

6. Sodium –simple diffusion at luminal; active transport at basolateral

7. Glucose – 2^nd active transport with sodium at luminal end; facilitated at b.l. end

8. Amino acids – same as glucose

9. Water – follows sodium; osmosis

-a. obligatory water reabsorption – in PCT constant rate; can not change

10. Electrolytes – Cl, K, Mg, some Ca, paracellular; Sulfate, nitrates, phosphate pass

11. Proteins – pinocytosis; broken down to amino acids in tubular cell

12. Nitrogenous waste – some are actually reabsorbed

-a. Urea – about half is reabsorbed; blood leaves with half concentration

-b. Uric acid – almost all reabsorbed; secreted latter

-c. Creatinine – none absorbed; out the body

Proximal convoluted tubule: tubular secretion

1. Waste removal – uric acid, urea, creatinine

2. Acid-base balance – hydrogen ion and bicarbonate ion

Loop of Henle

1. Electrolyte reabsorption – 25% sodium, potassium and chloride ions

2. Water – about 20%

3. Thick segment – water does not leave; electrolytes actively pumped from lumen

4. Urine concentration – the most important job of the loop of Henle

Distal convoluted tubule and collecting duct

1. Aldosterone – adrenal cortex; direct and indirect stimulation

-a. Direct stimulation – low Na or high K; adrenal cortex

-b. Indirectly – low blood pressure from low sodium; renin angiotensin

-c. Principle cells – aldosterone causes more sodium channels

-d. Sodium – moves out of lumen; water and chloride ion follow

-e. Potassium – secreted into the lumen

-f. Urine – contains less NaCl and water and more K

2. Atrial natriuretic factor – atrial myocardium of the heart

-a. High BP – or BV causes the release by the heart

-b. Sodium channels – are closed, more sodium and water lost; lower BP and BV

Collecting duct

1. Cortex – where the collecting duct starts; fluid from many nephrons

2. Urine concentration – the prevention of excess water loss

Control of concentration

1. Collecting duct – differences in permeabilities and osmolarities

-a. Extracellular fluid – osmolarity nearly four times greater at papilla than cortex

-b. Medullary portion – freely permeable to water, not salt

2. Dehydration – couple of effects on urine concentration

-a. GFR – slows, through the tubules, more time for water to be absorbed

-b. ADH – secreted when dehydrated; ↑number of water channels in collecting ducts

3. Counter current multiplier – loop of Henle responsible for this

-a. Descending limb – permeable to water; impermeable to solutes;↑ osmolarity

-b. Thick segment – of ascending limb; pumps out solutes; not water; ↓ osmolarity

-c. Inner medulla – interstitial fluid has osmolarity similar to loop of Henle

-d. Urea – collecting duct ↑permeable; helps maintain ↑osmolarity of deep medulla

4. Counter current exchanger – vasa recta has an ascending and descending limb

-a. Descending – water leaves, salt enters; osmolarity mirrors ICF

-b. Ascending – water enters, salt leaves; osmolarity decreases

Formation of dilute urine

1. ADH – is not released

2. Principal cells – impermeable to water under low ADH

Formation of concentrated urine

1. ADH – released due to dehydration

2. Principal cells – freely permeable to water; leaves collecting duct

Renal clearance

1. Renal clearance – the volume of plasma cleared of a substance by kidneys/ 1 min.

2. Inulin – standard 125 ml/min; not reabsorbed or secreted

3. Reabsorbed substances – less than inulin to zero for glucose, Na, Cl

4. Secreted substances – creatinine; greater than inulin; 140 ml/min

Urine: physical characteristics

1. Color – clear to pale to deep yellow

-a. Urochrome – byproduct of heme breakdown

-b. Color – can change due to food, drugs, or disease

-c. Infection – can result in cloudy urine

2. Odor – fresh, slightly aromatic; old ammonia due to bacteria

3. pH – usually slightly acidic at pH 6; range from 4.5 to 8.0

-a. Diet – high protein, acidic; vegetarian, alkaline

4. Specific gravity – 1.0035 as compared to 1.0 distilled water; solutes

Urine: chemical composition

1. Nitrogenous waste – urea, uric acid, and creatinine

2. Normal constituents – greater-lesser: urea, sodium, potassium, phosphate, sulfate

3. Minor constituents – calcium, magnesium, and bicarbonate

4. Diagnosis – certain diseases

-a. Glucosuria – glucose; diabetes mellitus;

-b. Proteinuria (albuminuria) – heart disease, hypertension, kidney failure

-c. Ketonuria – starvation or diabetes mellitus

-d. Hematuria – erythrocytes; trauma, stones, infection, neoplasm

-e. Pyuria – pus, leukocytes; urinary tract infection

OTHER URINARY TRACT ORGANS

Ureters

1. Renal pelvis – continuous with it

2. Retroperitoneal – run medially; behind peritoneum

3. Posterior bladder wall – ureters enter; distal end close off as bladder fills

4. Histology – lumen; three layers

-a. Mucosa – transitional epithelium; continuous with bladder

-b. Muscularis – inner longitudinal; outer circular; last part, outer longitudinal

-c. Adventitia – fibrous connective tissue outer layer

5. Peristalsis – intensity of waves are determined by rate of urine formation

-a. Neural control – para- and sympathetic innervation; little role

-b. Smooth muscle stretch – local responses play most important role

6. Renal calculi – stones; calcium, magnesium, or uric acid crystallize

-a. Predisposition – infection, alkaline urine; cranberry juice, urine acidification

-b. Treatment – surgery; shock wave lithotripsy (ultrasonic waves)

Urinary bladder

1. Retroperitoneal – anterior to rectum, males; to vagina and uterus

2. Trigone – triangular base; three openings

3. Histology – made up of 3 layers

-a. Mucosa – transitional epithelium

-b. Detrusor muscle – inner and outer longitudinal; middle circular layer

-c. Adventitia – connective tissue covering; superior surface peritoneum

4. Rugae – folds in mucosa; disappear as bladder fills

5. Bladder capacity – 300 ml (no pressure); 500 ml (moderate); 1 L (optimal)

Urethra

1. Urethra – muscular tube; urine from bladder to outside

2. Mucosa – transitional (near bladder) to pseudostratified to stratified squamous

3. Internal urethral sphincter – detrusor; smooth muscle; involuntary control

4. External urethral sphincter – skeletal muscle; voluntary control

5. Female – 5 cm; opening between clitoris and vaginal opening

6. Male – 20 cm; three named parts

-a. prostatic urethra – runs through the prostate

-b. Membranous urethra – urogenital diaphragm

-c. Spongy (penile) urethra – through the penis

7. External urethral orifice – end of penis, men; closer to body, females

Micturition (voiding or urination)

1. Stretch receptors – in detrusor activated at 200 ml

2. Visceral reflex arc – for smooth muscle

-a. Afferent fibers – impulses to sacral spinal cord

-b. Pelvic splanchic – efferent fibers; detrusor contracts; internal sphincter relaxes

3. Voluntary control – can open or close external;not reflex subsides; another 200 ml

4. Incontinence – in post toddlers can have many causes; stress during pregnancy