Urinary/Renal Study Guide

how do the kidneys maintain the body’s internal environment?

• Regulating total water volume and total solute concentration in water

• Regulating ion concentrations in extracellular fluid (ECF)

• Ensuring long-term acid-base balance

• Excreting metabolic wastes, toxins, drugs 

• Producing renin (regulates blood pressure) and erythropoietin (regulates RBC production)

ureters

transport urine from kidneys to urinary bladder

urinary bladder

temporary storage reservoir for urine

urethra

• transports urine out of body

location of kidneys

• Retroperitoneal, in the superior lumbar region

  • Located between T₁₂ and L₅

• Right kidney is crowded by liver, so is lower than left

• Adrenal (suprarenal) gland sits atop each kidney

• Ureters, renal blood vessels, lymphatics, and nerves enter and exit at hilum

3 layers of tissue surrounding kidney

renal fascia, perirenal fat capsule, fibrous capsule

renal fascia

Anchoring outer layer of dense fibrous connective tissue

Perirenal fat capsule

fatty cushion

fibrous capsule

• Transparent capsule that prevents spread of infection to kidney

3 regions of internal kidney

renal cortex, renal medulla, renal pelvis

renal cortex

1. granular-appearing superficial region

renal medulla

1. deep to cortex, composed of cone-shaped medullary (renal) pyramids 

renal pyramids

Broad base of pyramid faces cortex

Papilla, tip of pyramid, points internally

Renal pyramids are separated by renal columns, inward extensions of cortical tissue

lobe

• medullary pyramid and its surrounding cortical tissue; about eight lobes per kidney

renal pelvis

Funnel-shaped tube continuous with ureter; contains minor and major calyces

minor calyces

Cup-shaped areas that collect urine draining from pyramidal papillae

major calyces

• Areas that collect urine from minor calyces 

• Empty urine into renal pelvis

urine flow

• Renal pyramid 🡪 minor calyx 🡪 major calyx 🡪 renal pelvis 🡪 ureter

renal arteries

deliver about one-fourth (1200 ml) of cardiac output to kidneys each minute

arterial flow

• renal 🡪 segmental 🡪 interlobar 🡪 arcuate 🡪 cortical radiate (interlobular)

venous flow

• cortical radiate 🡪 arcuate 🡪 interlobar 🡪 renal veins

nephrons

• are the structural and functional units of the kidney that form urine

2 main parts of nephrons

renal corpuscle and renal tubule

renal corpuscle

• composed of glomerulus and glomerular capsule

2 parts of the renal corpuscle

glomerulus, and glomerular capsule

glomerulus

• Tuft of highly porous capillaries

  • Allows for efficient filtrate formation (Filtrate: plasma-derived fluid that renal tubules process to form urine)

glomerular capsule

• Also called Bowman’s capsule: cup-shaped, hollow structure surrounding glomerulus

  • Extensions terminate in foot processes that cling to basement membrane

    • Filtration slits between foot processes allow filtrate to pass into capsular space

renal tubule/3 major parts

epithelial cells; proximal convoluted tubule, nephron loop, distal convoluted tubule

distal convoluted tubule

Distal, farthest from renal corpuscle; drains into collecting duct; Cuboidal cells with very few microvilli 

• Function more in secretion than reabsorption

• Confined to cortex

proximal convoluted tubule (PCT)

• Cuboidal cells with dense microvilli that form brush border

  • Increase surface area

  • Also have large mitochondria

• Functions in reabsorption and secretion

  • Confined to cortex

nephron loop

• Formerly called loop of Henle

• U-shaped structure consisting of two limbs 

  • Descending limb: Thin segment

  • Ascending limb 

    • Thick ascending limb

collecting ducts

• receive filtrate from many nephrons

• Run through medullary pyramids

  • Give pyramids their striped appearance

  • Ducts fuse together to deliver urine through papillae into minor calyces2

2 cell types of collecting ducts

principle cells and intercalated cells

principle cells

• Sparse with short microvilli, Maintain water and Na⁺ balance

intercalated cells

Cuboidal cells with microvilli,  help maintain acid-base balance of blood

the Three processes are involved in urine formation and adjustment of blood composition:

glomerular filtration, tubular reabsorption, and tubular secretion

gl0merular filtration

1. produces cell- and protein-free filtrate

tubular reabsorption

1. selectively returns 99% of substances from filtrate to  blood in renal tubules and collecting ducts (what it needs to keep)

tubular secretion

1. selectively moves substances from blood to filtrate in renal tubules and collecting ducts (what it needs to get rid of to fine tune levels)

outward pressures that affect filtration

• Forces that promote filtrate formation

  • Hydrostatic pressure in glomerular capillaries (HP_gc) is essentially glomerular blood pressure

    • Chief force pushing water, solutes out of blood

    • Quite high: 55 mm Hg

      • Compared to ~ 26 mm Hg seen in most capillary beds

      • Reason is that efferent arteriole is a high-resistance vessel with a diameter smaller than afferent arteriole

inward pressures that affect filtration

• Forces inhibiting filtrate formation:

  • Hydrostatic pressure in capsular space (HP_cs): filtrate pressure in capsule

  • Colloid osmotic pressure in capillaries (OP_gc): “pull” of proteins in blood

• Net filtration pressure (NFP): sum of forces

  • 55 mm Hg forcing out minus 45 mm Hg opposing = net outward force of 10 mm Hg

  • Pressure responsible for filtrate formation

    • Main controllable factor determining glomerular filtration rate (GFR)

glomerular filtration rate

volume of filtrate formed per minute by both kidneys; is directly proportional to net filtration pressure, total surface area available for filtration, and filtration membrane permeability

gfr affects systemic blood pressure by

• Increased GFR causes increased urine output, which lowers blood pressure, and vice versa

• Goal of extrinsic controls: maintain systemic blood pressure

  • Nervous system and endocrine mechanisms are main extrinsic controls

myogenic mechanism

• Local smooth muscle contracts when stretched

  • Increased BP causes muscle to stretch, leading to constriction of afferent arterioles

    • Restricts blood flow into glomerulus

    • Protects glomeruli from damaging high BP

  • Decreased BP causes dilation of afferent arterioles

  • Both help maintain normal GFR despite normal fluctuations in blood pressure

tubuloglomerular feedback mecahnism

• Flow-dependent mechanism directed by macula densa cells

  • Respond to filtrate’s NaCl concentration

• If GFR increases, filtrate flow rate increases

  • Leads to decreased reabsorption time, causing high NaCl levels in filtrate

  • Feedback mechanism causes constriction of afferent arteriole, which lowers NFP and GFR, allowing more time for NaCl reabsorption

  • Opposite mechanism for decreased GFR

extrinsic controls: neural and hormonal mechanisms

• Purpose of extrinsic controls is to regulate GFR to maintain systemic blood pressure

• Extrinsic controls will override renal intrinsic controls if blood volume needs to be increased

• Sympathetic nervous system

  • Under normal conditions at rest

    • Renal blood vessels dilated

      • Renal autoregulation mechanisms prevail

renin angiotensin aldosterone mechanism

• Main mechanism for increasing blood pressure

• Three pathways to renin release by granular cells

  1. Direct stimulation of granular cells by sympathetic nervous system

  2. Stimulation by activated macula densa cells when filtrate NaCl concentration is low

     1. Reduced stretch of granular cells

tubular reabsorption

• quickly reclaims most of tubular contents and returns them to blood

• Selective transepithelial process

  • Almost all organic nutrients are reabsorbed

  • Water and ion reabsorption is hormonally regulated and adjusted

Includes active and passive tubular reabsorption

in tubular reabsorption substances can follow two routes

transcellular and paracellular

sodium transport across the basolateral membrane

• Na⁺ is most abundant cation in filtrate

• Transport of Na⁺ across basolateral membrane of tubule cell is via primary active transport 

• Na⁺-K⁺ ATPase pumps Na⁺ into interstitial space

• Na⁺ is then swept by bulk flow into peritubular capillaries

• Na⁺ reabsorption by primary active transport provides energy and means for reabsorbing almost every other substance

secondary active transport of tubular reabsorption

• Electrochemical gradient created by pumps at basolateral surface give “push” needed for transport of other solutes

  • Organic nutrients reabsorbed by secondary active transport are cotransported with Na⁺ Ex:  Glucose, amino acids, some ions, vitamins

passive reabsorption of water

• Movement of Na⁺ and other solutes creates osmotic gradient for water

  • Water is reabsorbed by osmosis

passive tubular reabsorption of solutes

• Solute concentration in filtrate increases as water is reabsorbed, creating gradients

  • Fat-soluble substances, some ions, and urea will follow water into capillaries down their concentration gradients

tubular secretion

• is reabsorption in reverse

• Occurs almost completely in PCT

• Selected substances are moved from peritublar capillaries through tubule cells out into filtrate

  • K⁺, H⁺, NH₄⁺, creatinine, organic acids and bases

  • Substances synthesized in tubule cells also are secreted (example: HCO₃^–)

  • Tubular secretion is important for removing unwanted substances such as urea, uric acid, drugs, metabolites, and excess ions.

diuretics

chemicals that enhance urinary output

osmotic diuretics

substance not reabsorbed, so water remains in urine

urine composition

• 95% water and 5% solutes

• Nitrogenous wastes 

  • Urea (from amino acid breakdown): largest solute component

  • Uric acid (from nucleic acid metabolism)

  • Creatinine (metabolite of creatine phosphate)

• Other normal solutes found in urine

  • Na⁺, K⁺, PO₄^3–, and SO₄^2–, Ca²⁺, Mg²⁺ and HCO₃^–

  • Abnormally high concentrations of any constituent, or abnormal components such as blood proteins, WBCs, and bile pigments, may indicate pathology

ureters

• slender tubes that convey urine from kidneys to bladder

  • Begin at L₂ as continuation of renal pelvis

• Enter base of bladder through posterior wall

  • As bladder pressure increases, distal ends of ureters close, preventing backflow of urine 

urinary bladder anatomy

• Muscular sac for temporary storage of urine

• Retroperitoneal, on pelvic floor posterior to pubic symphysis

  • Males: prostate inferior to bladder neck

  • Females: anterior to vagina and uterus

• Has openings for ureters and urethra

  • Trigone

    • Smooth triangular area outlined by openings for ureters and urethra

sphincters

internal and external urethral sphincter

internal urethral sphincter

• Involuntary (smooth muscle) at bladder-urethra junction

external urethral sphincter

• Voluntary (skeletal) muscle surrounding urethra as it passes through pelvic floor

3 simultaneous events for micturation

1. Contraction of detrusor by ANS

2. Opening of internal urethral sphincter by ANS

3. Opening of external urethral sphincter by somatic nervous system

• Reflexive urination (urination in infants)

• Pontine control centers mature between ages 2 and 3

  • Pontine storage center inhibits micturition

regulation of water intake

• Governed by hypothalamic thirst center

  • Hypothalamic osmoreceptors detect ECF osmolality and are activated by:

    • Increased plasma osmolality of 1–2%

    • Dry mouth

    • Decreased blood volume or pressure

      • Angiotensin II or baroreceptor input

influence of adh hormone

• Water reabsorption in collecting ducts is proportional to ADH release

• Decreased ADH leads to dilute urine and drop in volume of body fluids

• Increased ADH leads to concentrated urine, due to reabsorption of water, causing increased volume of body fluids

  • Hypothalamic osmoreceptors sense ECF solute concentration and regulate ADH accordingly

electrolyte balance

• usually refers only to salt balance even though electrolytes also include acids, bases, and some proteins

influence of aldosterone and angiotensin !!

• When aldosterone concentrations are high:

  • Na⁺ is actively reabsorbed in DCT and CT and water follows, so ECF volume increases

• When aldosterone concentrations are low:

  • Na⁺ is not actively reabsorbed and is lost to urine, along with increased loss of water

normal pH of body fluids

• Arterial blood: pH 7.4

• Venous blood and interstitial fluid: pH 7.35

  • ICF: pH 7.0

concentration of hydrogen ions is regulated by 3 mechanisms

• Chemical buffer systems

  • Rapid, first line of defense

•  Brain stem respiratory centers

  • Acts within 1–3 minutes

•  Renal mechanisms

  • Most potent, but require hours to days to effect pH changes