The Urinary System

Regulate chemical composition of body fluids

Eliminates waste

Controls composition of bloods – ion levels and concentration

Help maintain PCO2 & acid/base balance [pH]

Help regulate blood pressure by secreting renin [renin-angiotensin system]

Contribute to metabolism

detoxify free radicals and drugs [with peroxisomes]

gluconeogenesis [during fasting]

produce erythropoietin – stimulates red blood cell production

activation of vitamin D [as calcitrol]

Metabolic waste – waste substance produced by the body [often lethal]

50% of N containing waste is urea [from protein – aa NH2 ammonia urea [by liver]

Uric acid – from nucleic acids

Creatinine – from creatine phosphate

BUN – typical = 10-20 mg/dL

Too high = azotemia [renal insufficiency]

Plasma creatinine increase above 1.5 mg/dL with decreased filtration normal = 0.6-1.2 mg/dL

Retroperitoneal in the superior lumbar region.

Extend from twelfth thoracic to third lumbar vertebra.

Right kidney is lower than left because it is crowded by the liver.

Renal fascia – outer layer of dense fibrous connective tissue that anchors the kidney to abdominal wall.

Adipose capsule – shock absorbing.

Renal capsule – fibrous cover that prevents kidney infection.

Cortex –outer - cortical zone and juxtamedullary zone

Medulla – renal pyramids [8-18]

Renal papillae – narrow ends of pyramids

Renal columns – between pyramids

Urine Flow – formed in nephrons papillary ducts minor calyx major calyx renal pelvis

~ one-fourth (1200 ml) of systemic cardiac output flows through the kidneys each minute.

Arterial flow into venous flow out of the kidneys follow similar paths.

Figure 25.3c

Renal Arteries segmental arteries interlobar arteries arcuate arteries cortical radial arteries afferent arterioles

Efferent arterioles also form vasa recta with deep juxtamedulary nephrons

Venules cortical radial veins arcuate veins interlobar veins segmental veins renal veins inferior vena cava

Figure 25.3c

NERVE SUPPLY -

Renal plexus of sympathetic division of ANS – to afferent & efferent arterioles [vasomotor nerves] - regulate flow and pressure

Figure 25.3c

Nephrons are the structural & functional units that form urine, consisting of:

Renal Corpuscle – glomerulus + Bowman’s capsule

Renal Tubule - PCT, loop of Henle, DCT

BOWMAN”S CAPSULE

Parietal layer – simple squamous epithelium.

Capsular space.

Visceral layer consists of modified, branching epithelial podocytes.

Functions – pressure filtration of blood – water and small solutes leave blood

vascular pole - blood in

urinary pole -urine out

VP

UP

Proximal convoluted tubule (PCT) – cuboidal epithelium with microvilli & mitochondria

Loop of Henle [nephron loop]:

Descending limb [thin] simple squamous epithelium – permeable to water [out], urea [in]; thick walls

Ascending limb [thick] – cuboidal to low columnar epithelium; thick at top, then thin

Distal convoluted tubule (DCT):

Principal cells:

Cuboidal cells without microvilli.

Help maintain water & salt balance.

Collecting Ducts - drains several DCT's

Combine to form papillary ducts calyces

Cuboidal epithelium, then columnar

All nephrons begin in the cortex. Where the loop of Henle reaches to determines type

Juxtamedullary nephrons:

Have loops of Henle that deeply penetrate medulla.

Cortical nephrons – 85% of nephrons:

Have loops of Henle that only slightly penetrate medulla.

Figure 25.5b

General

Glomerular filtrate - from plasma but with no protein

Tubular fluid - from PCT through DCT

Endothelium of glomerulus – open pores [fenestrations] – 70-90 nm diameter everything but cells and platelets pass through

Basal lamina [basement membrane] of glomerulus – serves as dialysis membrane –blocks large plasma proteins

Filtration slits - Endothelium of visceral layer of glomerular capsule – podocytes form filtration slits [spaces between pedicels] – negatively charged - repel anions -30 nm slit width

Glomerulus blood filtering depends on 3 main pressures –1 promotes, 2 oppose

Blood Hydrostatic Pressure [HPG] – about 60 torr – forces fluid out of capillaries

Capsular Hydrostatic Pressure [HPC] – about -18 torr – opposes –from fluid already in capsular space

Colloidal Osmotic Pressure of blood [OPC] about -32 torr – opposes

NFP = HPG – [HPC + OPC] = 55 – [15- 30]] = about 10 torr outward

The positive pressure moves fluid out of the glomerulus into Bowman’s capsule.

Pressure remains high throughout length so filtration continues. Especially sensitive to hypertension

Amount of filtrate formed in all areas of the renal corpuscles of both kidneys every minute

Directly related to pressures that determine NFP

Adult rate is about 125 mL/min – 180L/day [males]

If the GFR is too high:

Needed substances cannot be reabsorbed quickly enough and are lost in the urine.

If the GFR is too low:

Everything is reabsorbed, including wastes that are normally disposed of.

Three mechanisms control GFR:

Renal autoregulation (intrinsic system);

Neural controls;

Hormonal mechanisms (renin-angiotensin system).

Renal Autoregulation of GFR - blood flow autoadjustment

Myogenic Mechanism – Smooth muscle contracts when stretched reduces blood flow which reduces pressure downstream.

Tubuloglomerular feedback – negative feedback mechanism using the

Juxtaglomerular apparatus [respond to NaCl concentration]

juxtaglomerular cells - smooth muscle fibers of afferent arterioles – mechanoreceptors - dilate or constrict

with pressure change & secrete renin.

macula densa- chemoreceptors – at end of ascending limb tall crowded cells that monitor Na+ and Cl- concentration

DC Muller, Johns Hopkins School of Medicine

Neural Regulation – sympathetic

Norepinephrine causes vasoconstriction

Affects smooth muscles of vessels – low input dilation, high constriction

Renin-angiotensin II pathway- JG cells release renin in response to

1 – decreased delivery of fluid and NaCl to macula densa

2 – decreased stretching of JG cells

3 – increased rate of stimulation by renal sympathetic nerves

Angiotensin II is the active hormone that

produces constriction of arterioles to increase GBHP and raise GFR

stimulates secretion of aldosterone, which enhances reabsorption of Na+ [and water] by principal cells in collecting ducts

stimulates the thirst center of the hypothalamus

stimulates release of ADH which increases water reabsorption increase in blood volume higher BP

Every nephron has 2 capillary beds: Glomerulus & Peritubular

Each glomerulus is:

Fed by an afferent arteriole

Drained by an efferent arteriole

Peritubular beds are low-pressure, porous capillaries.

Vasa recta – long, straight efferent arterioles of juxtamedullary nephrons.

99% of materials move from filtrate back into peritubular capillaries or vasa recta

Solutes are reabsorbed by active or passive transport

Water is reabsorbed by osmosis = facultative water reabsorption

Small peptides and proteins are reabsorbed by pinocytosis

Most reabsorption occurs in PCT’s.

Na+ reabsorption- by facilitated diffusion, symporters & antiporters.

Reabsorption of water

Helps establish concentration gradients

Promotes reabsorption of other substances

Substances reabsorbed in PCT:

100% of filtered glucose, lactate & amino acids

90% of bicarbonate ions

65% of Na+ & water

50% of Cl & K+

Transport maximum (Tm):

Reflects the number of carriers in the renal tubules available

Exists for nearly every substance that is actively reabsorbed

When the carriers are saturated, excess of that substance is excreted

Removes materials from blood and adds them into filtrate

Function – to rid body of certain materials and help control blood pH.

Except for K+, the PCT is the main site of secretion.

Removes urea, uric acid, bile salts catecholamines, prostaglandins, morphine, penicillin, etc.

pH regulation - H+ - by intercalated cells; increasing HCO3

- reabsorption when pH is low

Allows for production of dilute or concentrated urine.

25-30% Na+& K+

35% Cl-

15% water

Variable absorption based on need - 2 cell types

Principal cells – have infolding of basement membrane – maintain water and Na balance. Sensitive to ADH and aldosterone

Intercalated Cells - very few – lots of mitochondria - can reabsorb K+ and secrete H+ to rid body of excess acid

Hormonal Influences

Aldosterone - renin angiotensin system

ADH - in response to dehydration and high osmolality - acts on collecting ducts,increases water absorption

Parathyroid hormone stimulates Ca2+ uptake and increases phosphate excretion

Atrial Natriuretic Peptide [increases GFR]

Secreted by atria of heart when muscle is stretched [high Bp]

Promotes excretion of water and Na+

Inhibits ADH secretion & antagonizes renin system

Reduces blood volume and BP

By end of DCT 95% of filtrate has been reabsorbed; 90% of water

Dilute Urine is hypotonic to blood plasma.

To produce dilute urine – just don’t remove any water after ascending loop of Henle

Dilution occurs in the absence of ADH – makes principal cells impermeable to water reabsorption

Concentrated urine is hypertonic to plasma.

ADH is present – water channels [aquaporins] form in principal cell membranes [increase water reabsorption]

More Complex than dilution.

Solute concentration is maintained by counter current mechanism

Based on anatomic arrangement of juxtamedullary nephrons & the vasa recta.

Get salinity gradient produced in ECF - very high at base.

Countercurrent Multiplier – loop of Henle - recaptures Na+ and returns it to deep medullary tissues keeping the gradient in place.

1. Descending – water leaves, Na+ & Cl- enter.

2. Ascending water enters, NaCl leaves.

Recycling of urea in renal medulla:

gets concentrated in tubules

diffuses out at collecting duct

into medulla

into tubular fluid in ascending loop of Henle - repeats.

Countercurrent exchanger – Vasa Recta - Blood flows in opposite direction from loop - Maintains a gradient – keep removing water and adding salt.

Appearance - clear, colorless to amber [pus, bacteria blood, etc. make cloudy and/or colored

Odor - slight - increases with standing due to bacterial ammonia production

Specific gravity - 1.001 - 1.028 [water is 1.000]

pH

Slightly acidic (pH 6) with a range of 4.5 to 8.2

Diet can alter pH

Osmolarity - 50 - 1200 mOsm/L

Composition - 95 water

1 -2L/da

Polyuria - excessive output, Oliguria - low, Anuria – none

Diabetes - 4 forms - I, II gestational and insipidus. In most - results from high sugar in tubule. Insipidus is from hyposecretion of ADH

Diuretics – enhance urinary output

Osmotic – not reabsorbed

ADH inhibitors [alcohol]

Na+ symporter inhibitors [Lasix]

The volume of plasma that is cleared of a particular substance in a given time.

Renal clearance tests are used to:

Determine the GFR

Detect glomerular damage

Follow progress of renal disease

RC = UV/P

RC = renal clearance rate

U = concentration (mg/ml) of the substance in urine

V = flow rate of urine formation (ml/min)

P = concentration of the same substance in plasma

1/kidney – retroperitoneal, 25 cm inches long

Valve-like region as enter bladder – prevents backflow

3 layer wall

mucosa [transitional epithelium]

muscularis –longitudinal, outer circular

adventitia – fibrous coat

Smooth, collapsible, muscular sac that temporarily stores urine.

In pelvic cavity

Trigone – triangular area outlined by openings for ureters and urethra.

3 layers: Transitional epithelial mucosa

Thick muscular layer

Serous coat

Storage – capacity 700-800mL

Muscular tube that drains urine from the bladder & conveys it out of the body.

Female much shorter than male.

The male urethra has 3 regions:

Prostatic

Membranous

Spongy

Stretch receptors stimulated by 200-400mL – signal to sacral spinal cord - some to

Sympathetic neurons that suppress urination others to

Parasympathetic - micturation reflex

Voluntary relaxation of external sphincter

Renal insufficiency = state in which kidneys cannot maintain homeostasis due to extensive destruction of nephrons

Causes - hypertension, infections, trauma, ischemia, poisoning, tubule blockage - protein, etc. atherosclerosis, glomerulonephritis

Dialysis - blood is removed and passed through a chamber with a semipermeable membrane - materials are removed from blood by diffusion.