Urinary System


I.          Background

A.        Functions

1.         Filtration

            a.         Toxins

            b.         Wastes

            c.         Excess ions

2.         Secretion

3.         Regulate chemical makeup of blood

4.         Gluconeogenesis

5.         Regulate blood pressure

            a.         Renin

6.         RBC production

            a.         Erythropoietin

7.         Metabolize vitamin D to its active form

8.         Urine storage and transport

            a.         Bladder

            b.         Ureters

            c.         Urethra


II.         Anatomy


A.        External

1.         Located in the superior lumbar region

            a.         T12 – l3

2.         Right lower than left

3.         Convex laterally

4.         Renal hilus

            a.         Leads into internal space

            b.         Ureters, blood vessels, lymphatics and nerves join kidney

5.         Adrenal gland

            a.         Superior surface

            b.         Endocrine, not urinary

6.         Support tissue


            a.         Renal capsule

                        i.          Prevents invasive infection

            b.         Adipose capsule

                        i.          Posterior

                        ii.          Protects and anchors

            c.         Renal fascia

                        i.          Anchors kidney

                        ii.          Dense connective tissue


B.         Internal Anatomy


1.         Three regions

            a.         Cortex

            b.         Medulla

            c.         Pelvis

2.         Cortex

            a.         Superficial

            b.         Renal columns

                        i.          Inward extensions

                        ii.          Separate medullary pyramids

3.         Medulla

            a.         Deep to cortex

            b.         Medullary pyramids

                        i.          Base faces cortex

                        ii.          Apex (papillae) point internally

                        iii.         Parallel bundles of urine collecting tubules

4.         Pelvis

            a.         Tube continuous with ureter

            b.         Calyces

                        i.          Enclose papillae

                        ii.          Collect urine


III.       Nephrons



A.        Functions

1.         Blood-processing

2.         Collecting ducts

            a.         Convey urine

                        i.          Nephrons to renal pelvis

B.         Types of nephrons

1.         Cortical

            a.         Located primarily in cortex

            b.         Only a small pat of loop of Henle enters medulla

2.         Juxtamedullary

            a.         Located near cortex/medulla border

            b.         Produce concentrated urine

            c.         Loop of Henle invades medulla


C.        Structure

1.         Renal capsule

            a.         Glomerulus

                        i.          Capillaries

                        ii.          Fenetrated endothelium

                        iii.         Porous

                        iv.         Protein-free fluid—filtrate


            b.         Glomerular capsule (Bowmans’s)

4.         PCT—proximal convoluted tubule

            a.         Reabsorbs water and solutes from filtrate

            b.         Secretes substances into filtrate


5.         Loop of Henle

            a.         Descending limbs

            b.         Ascending limbs

            c.         Thick and thin segments

6.         DCT—distal convoluted tubule

            a.         Secretion primarily

7.         Collecting ducts

            a.         Receive filtrate from nephrons

8.         Capillary beds


            a.         Afferent arterioles

                        i.          Run throughout renal cortex

                        ii.          High pressure

                        iii.         Force fluids and solutes out of blood

            b.         Peritubular capillaries


                        i.          Reabsorb solutes and water

                        ii.          Cortical Nephrons

            c.         Efferent arterioles

                        i.          Exit glomerulus

            d.         Vasa recta

                        i.          JM nephrons

                        ii.          Straight connection between efferent arterioles  

9.         JGA—juxtaglomerular apparatus

            a.         DCT lies against afferent arteriole

                        i.          Modified

                        ii.          Mechanoreceptor

            b.         Secrete rennin (JG cells)

            c.         Macula densa

                        i.          DCT cells

                        ii.          Osmoreceptors

                        iii.         Respond to changes in solute concentration


10.       Filtration membrane


            a.         Fenestrated endothelium

                        i.          Glomerular capillaries

            b.         Podocytes of glomerular capsule

                        i.          Attach to basement membrane

                        ii.          Clefts between neighboring podocytes

                        iii.         Slit pores allow filtrate to enter capsular space but not macromolecules

            c.         Basement membrane

                        i.          Gel-like

                        ii.          Repels plasma proteins

                        iii.         Selects based on charge


IV.       Kidney Physiology

A.        Glomerular filtration

1.         Characteristics

            a.         Passive

            b.         Non-selective

            c.         Hydrostatic pressure

                        i.          Forces fluid through glomerulus

                        ii.          Significantly higher than capillary beds

                        iii.         55 mm Hg (vs. 18)

            d.         Small molecules move freely

                        i.          Water, glucose, nitrogenous wastes

            e.         Large molecules pass with difficulty

                        i.          Maintains the colloid osmotic pressure of glomerular blood

                        ii.          Not all water is lost from blood

2.         NFP—net filtration pressure


NFP = HPg – (OPg + HPc); 55 – (30 + 15) = 10 mm Hg


HPg = hydrostatic pressure of the glomerulus

OPg = osmotic pressure of the glomerulus

HPc = hydrostatic pressure of the capsule


3.         GFR—glomerular filtration rate

            a.         Volume of filtrate per minute

            b.         Factors affecting GFR

                        i.          Surface area available for filtration

                        ii.          Membrane permeability

                        iii.         NFP

            c.         GFR is directly proportionate to NFP

                        i.          Factors affecting NFP also affect GFR

                        ii.          Arteriole pressure


B.         Factors affecting GFR

1.         Intrinsic factors

            a.         Renal autoregulation

                        i.          Myogenic mechanisms

                        ii.          Tubuloglomerular feedback

            b.         Myogenic mechanisms

                        i.          Vascular smooth muscle contracts when stretched

                        ii.          Increasing systemic BP causes afferent arterioles to constrict

                        iii.         Prevent glomerular BP from increasing too high

            c.         Tubuloglomerular feedback mechanism

                        i.          Macula densa cells of JGA

                        ii.          Slow filtration or low osmotic pressure causes vasodilation of afferent arterioles

                        iii.         Rapid filtration or high osmotic pressure causes vasoconstriction of afferent arterioles

2.         Extrinsic controls

            a.         Neural and hormonal

            b.         Neural

                        i.          SNS

                        ii.          Vasoconstriction causes reduced blood flow to nephrons

                        iii.         Reduced flow decreases filtrate formation

                        iv.         Constriction indirectly activates renin-angiotensin mechanism (see below)

            c.         Hormonal

                        i.          Renin-angiotensin mechanism

                        ii.          Decreased BP causes rennin to be released

            d.         Renin-angiotensin mechanism

                        i.          JGA releases renin

                        ii.          Renin acts on angiotensinogen

                        iii.         Angiotensinogen causes angiotensin II to be formed

                        iv.         Angiotensin II is a vasoconstrictor; BP increases

                        v.         Angiotensin II also acts on the kidney tubule

                        vi.         Na+ reabsorption increases

                        vii.        Angiotensin II causes aldosterone to be released from the adrenal cortex

                        viii.       Aldosterone causes Na+ to be reclaimed from the kidney tubule

                        ix.         In sum, hormonal factors increases BP which increases GFR

3.         Factors that cause renin release

            a.         Reduced stretch of JG cells

                        i.          Decreased BP reduces stretch

            b.         Activated macula densa cells stimulates JG cells

            c.         LG cells are stimulated via B1-adrenergic receptors

4.         Other factors

            a.         Protagladins

                        i.          Vasodilators

                        ii.          Effects offset by SNS and angiotensin II

            b.         NO—vasodilator

            c.         Adenosine

                        i.          Constricts renal vasculature

            d.         Endothelium

                        i.          Vasoconstrictor


C.        Tubular reabsorption

1.         Trans-epithelial process

            a.         Mostly transcellular

                        i.          Cells are tight-junctioned

                        ii.          Limited paracellular movement

            b          Can be active or passive

2.         Na+ movement

            a.         Passive into tubule cells from filtrate

            b.         Actively transported from tubule cells into interstitial space

                        i.          Na+/K+ pump

                        ii.          Creates osmotic gradient

            c.         Diffuses from interstitial space into capillary (peritubular)





3.         Water, ions and nutrients

            a.         Osmotic gradient created by active transport of Na+



            b.         Water flows out along osmotic gradient

                        i.          Loss of water increases solute concentration

                        ii.          Concentration gradient then favors the movement of other solutes

4.         Secondary active transport

            a.         Na+/K+ pump creates a gradient that favors the movement of Na+ from the lumen into the tubule

            b.         Other substances are carried by the same carrier that moves Na+






Osmosis (Diffusion)








Descending Loop of





Ascending Loop of














Collecting Duct

Na+, K+, HCO3-

Cl-, H+




D.        Tubular secretion

1.         Movement of unwanted substances from plasma into filtrate

2.         Functions

            a.         Dispose of substances not already in filtrate

            b.         Eliminate substances that have been reabsorbed by passive processes

                        i.          Urea and uric acid

            c.         Eliminate excess K+

            d.         Control blood pH

3.         Occurs primarily in PCT

            a.         Drugs

            b.         NH4+

            c.         H+

4.         DCT

            a.         H+

            b.         Some drugs

5.         Collecting ducts

            a.         Cl-

            b.         HCO3-

            c.         NH4+


E.         Counter-current mechanism


1.         Descending loop of Henle

            a.         Permeable to water

            b.         Impermeable to solutes

            c.         Osmolarity of interestial fluid increases along the descending loop

                        i.          Water flows down osmotic gradient

                        ii.          Osmolarity of filtrate increase

2.         Ascending loop of Henle

            a.         Impermeable to water

            b.         Permeable to NaCl

                        i.          Passive in the thin segment

                        ii.          Active in the thick segment

            c.         Osmolarity of filtrate decreases

3.         Collecting duct

            a.         Urea passively moves into the interstitial space in the medullary region



4.         Blood vessels freely permeable to water and NaCl

            a.         Water movement parallels water movement in the loop of Henle



V.        Formation of urine

A.        Dilute  


VI.       Control of Bladder Function