Urinary

Hydrogen ion secretion

• Hydrogen ions generated by dissociation of carbonic acid

• Are secreted by sodium ion–linked countertransport

in exchange for Na+ in tubular fluid

• Bicarbonate ions diffuse into bloodstream to help prevent

changes in plasma pH

• Acidifies tubular fluid

• Elevates blood pH

• Accelerates when blood pH falls

• pH of blood decreases in

• Metabolic acidosis (lactic acidosis) - can develop after

exhaustive muscle activity

• Ketoacidosis - can develop in starvation or diabetes

mellitus

Control of blood pH

• Accomplished by H+ removal and bicarbonate ion

production at kidneys

• Aldosterone stimulates H+ secretion

• Prolonged aldosterone stimulation can cause

alkalosis

• Abnormally high blood pH

• In response to acidosis

• PCT and DCT deaminate amino acids

• Ties up H+ and yields ammonium ions (NH4

+) and

bicarbonate ions (HCO3

–)

• Ammonium ions are pumped into tubular fluid

• Bicarbonate ions enter bloodstream via peritubular

fluid

• Benefits of tubular deamination

• Provides carbon chains for catabolism

• Generates bicarbonate ions to buffer plasma

Reabsorption and secretion in collecting system

• Collecting ducts

• Receive tubular fluid from many nephrons

• Carry it toward renal sinus

• Hormones regulate water and solute loss

• Aldosterone controls sodium ion pumps

• Actions are opposed by ANP

• ADH controls permeability to water

• Secretion is suppressed by ANP

Reabsorption and Secretion – Collecting System

• Reabsorption in collecting system

• Sodium ion reabsorption - Na+ exchanged for K+

• Bicarbonate ion reabsorption - HCO3

– exchanged for Cl–

• Urea reabsorption - by diffusion

• Secretion in collecting system

• Controls pH of body fluids

• Low pH in peritubular fluid

• Carrier proteins pump H+ into tubular fluid and

reabsorb bicarbonate ions

• High pH in peritubular fluid (uncommon)

• Collecting system secretes bicarbonate ions and

pumps H+ into peritubular fluid

Reabsorption and Secretion

• The Control of Urine Volume and Osmotic Concentration

• Through control of water reabsorption

• Water is reabsorbed by osmosis in:

• Proximal convoluted tubule

• Descending limb of nephron loop

• Water Reabsorption

• Occurs when osmotic concentration of peritubular fluid

exceeds that of tubular fluid

• 1–2 percent of water in original filtrate is recovered

• During sodium ion reabsorption in distal convoluted

tubule and collecting system

Reabsorption and Secretion

• Obligatory Water Reabsorption

• Is water movement that cannot be prevented

• Usually recovers 85 percent of filtrate produced

• Facultative Water Reabsorption

• Controls volume of water reabsorbed along DCT and

collecting system

• 15 percent of filtrate volume (27 liters/day)

• Segments are relatively impermeable to water

• Except in presence of ADH

Reabsorption and Secretion

• ADH

• Hormone that causes special water channels (aquaporins)

to appear in apical cell membranes

• Increases rate of osmotic water movement

• Higher levels of ADH increase:

• Number of water channels and water permeability of

DCT and collecting system

• Without ADH:

• Water is not reabsorbed and all fluid reaching DCT is

lost in urine producing large amounts of dilute urine

• The Hypothalamus continuously secretes low levels of ADH

• DCT and collecting system are always permeable to water and

at normal ADH levels - Collecting system reabsorbs 16.8

liters/day (9.3 percent of filtrate

The Function of the Vasa Recta

• To return solutes and water reabsorbed in medulla

to general circulation without disrupting the

concentration gradient

• Some solutes absorbed in descending portion do

not diffuse out in ascending portion

• More water moves into ascending portion than is

moved out of descending portion

• Carries water and solutes out of medulla

• Balances solute reabsorption and osmosis in

medulla

Osmotic Concentration

• Blood entering the vasa recta:

• Has osmotic concentration of 300 mOsm/L

• Increases as blood descends into medulla

• Involves solute absorption and water loss

• Blood flowing toward cortex:

• Gradually decreases with solute concentration of

peritubular fluid

• Involves solute diffusion and osmosis

Seven Steps of Renal Function

• Step 1 Glomerulus

• Filtrate produced at renal corpuscle has the same

composition as blood plasma (minus plasma proteins)

• Step 2 Proximal Convoluted Tubule (PCT)

• Active removal of ions and organic substrates

• Produces osmotic water flow out of tubular fluid;

reduces volume of filtrate and keeps solutions

inside and outside tubule isotonic

• Step 3 PCT and Descending Limb

• Water moves into peritubular fluids, leaving highly

concentrated tubular fluid and reduction in volume

occurs by obligatory water reabsorption

Summary of Renal Function

• Seven Steps of Renal Function

• Step 4 Thick Ascending Limb

• Tubular cells actively transport Na+ and Cl– out of

tubule; urea accounts for higher proportion of total

osmotic concentration

• Step 5 DCT and Collecting Ducts

• Final adjustments in composition of tubular fluid

• Osmotic concentration is adjusted through active

transport (reabsorption or secretion)

• Step 6 DCT and Collecting Ducts

• Final adjustments in volume and osmotic

concentration of tubular fluid

• Exposure to ADH determines final urine

concentration

Seven Steps of Renal Function

• Step 7 Vasa Recta

• Absorbs solutes and water reabsorbed by nephron

loop and the ducts

• Maintains concentration gradient of medulla

• Urine Production

• Ends when fluid enters the renal pelvis

The Composition of Normal Urine

• Results from filtration, absorption, and secretion activities of

nephrons

• Some compounds (such as urea) are neither excreted nor

reabsorbed

• Organic nutrients are completely reabsorbed

• Other compounds missed by filtration process (e.g.,

creatinine) are actively secreted into tubular fluid

• A urine sample depends on osmotic movement of water

across walls of tubules and collecting ducts

• Is a clear, sterile solution; yellow color (pigment urobilin)

• Generated in kidneys from urobilinogens

• Urinalysis, the analysis of a urine sample, is an important

diagnostic tool

Urine Production

• A healthy adult produces:

• 1200 mL per day (0.6 percent of filtrate)

• With osmotic concentration of 855–1335 mOsm/L

• Diuresis

• Is the elimination of urine

• Typically indicates production of large volumes of urine

• Diuretics

• Are drugs that promote water loss in urine

• Diuretic therapy reduces:

• Blood volume

• Blood pressure

• Extracellular fluid volume

Filtrate modification and urine production

• End when fluid enters renal pelvis

• Urine transport, storage, and elimination

• Take place in urinary tract

• Ureters, urinary bladder, urethra

• Structures

• Minor and major calyces, renal pelvis, ureters,

urinary bladder, and proximal portion of urethra

• Are lined by transitional epithelium

• That undergoes cycles of distention and

contraction

The Micturition Reflex and Urination

• As the bladder fills with urine:

• Stretch receptors in urinary bladder stimulate sensory fibers in

pelvic nerve and stimulus travels from afferent fibers in pelvic

nerves to sacral spinal cord

• Efferent fibers in pelvic nerves:

• Stimulate ganglionic neurons in wall of bladder

• Postganglionic neuron in intramural ganglion stimulates detrusor

muscle contraction

• Interneuron relays sensation to thalamus

• Projection fibers from thalamus deliver sensation to cerebral cortex

• Voluntary relaxation of external urethral sphincter causes relaxation

of internal urethral sphincter

• Begins when stretch receptors stimulate parasympathetic

preganglionic motor neurons

• Volume >500 mL triggers micturition reflex

Infants

• Lack voluntary control over urination

• Corticospinal connections are not established

• Incontinence

• Is the inability to control urination voluntarily

• May be caused by trauma to internal or external

urethral sphincter

Age-Related Changes

• Decline in number of functional nephrons

• Reduction in GFR

• Reduced sensitivity to ADH

• Problems with micturition reflex

• Sphincter muscles lose tone leading to

incontinence

• Control of micturition can be lost due to a stroke,

Alzheimer’s disease, and other CNS problems

• In males, urinary retention may develop if enlarged

prostate gland compresses the urethra and

restricts urine flow