The Urinary System

Chapter 18

Pgs 547-573

Overview

• Introduction• The Organization of the

Urinary System• The Kidneys

– Superficial and sectional anatomy

– The nephron– Blood supply to the kidneys

• Basic Principles of Urine Production– Filtration at the glomerulus– Reabsorption and

secretion along the renal tubule

– Control of kidney function

• Urine Transport, Storage, and Elimination– The ureters and urinary

bladder– The urethra– The micturition reflex and

urination• Fluid, Electrolyte, and

Acid-Base Balance– Fluid and electrolyte

balance– Acid-base balance

Functions of the Urinary System

• Remove organic wastes generated by cells

• Regulates blood volume and blood pressure

• Regulates plasma concentrations of ions

• Helps to stabilize blood pH

• Controls valuable nutrients

Basic Principles of Urine Formation

• Process involves excretion and elimination of dissolved solutes (3 metabolic wastes):– Urea

• Most abundant organic waste (21 grams/day)• Produced during break down of amino acids

– Creatinine• Generated during breakdown of creatine

phosphate (1.8 g/day)

– Uric acid• Breakdown and recycling of RNA (480 mg/day)

Three Distinct Processes of Urine Production

• Filtration– Bp forces water across filtration membrane

• Depends on solute size– Renal corpuscle across cap walls of glomerulus

• Reabsorption– Removal of water and solute molecules from filtrate after enters renal tubule– Selective process

• Simple diffusion or carrier proteins• Water passive (osmosis)

– Water and solutes reenter circ at peritubular caps and vasa recta– Primarily at PTC

• Secretion– Transport of solutes across tubular epith into filtrate– Necessary because:

• Filtration does not force all dissolved materials out of plasma– Blood entering peritubular caps may still contain undesirable substances

– Loop of Henle and collecting system (water, sodium, potassium lost to urine)• All processes create fluid very different from other body fluids

Filtration at the Glomerulus: Filtration Pressure

• Net force promoting filtration is filtration pressure

• Higher than capillary blood pressure elsewhere in body– Result of difference in diameter of afferent

and efferent arterioles• Which one do you think would have a smaller

diameter?

Filtration Pressure

• Very low (10 mm Hg)• If glomerular blood pressure drops, kidney

filtration will stop– Minor changes in blood pressure:

• Reflexive vasodilation/constriction of arterioles– Automatic or due to SNS

– Serious drop in bp can reduce or stop filtration

• Kidneys most sensitive to bp than any other organ– Control many homeostatic mechanisms for regulating

blood pressure and blood volume

Filtration at the Glomerulus: The Glomerular Filtration Rate

• Glomerular filtration– Process of filtrate production at the glomerulus

• Glomerular filtration rate (GFR)– Amount of filtrate produced in the kidneys each

minute– Averages 125 mL/min– 99% of filtrate reabsorbed

• Very important process– Inability to reclaim water can quickly cause death by

dehydration

DCT and Aldosterone

• DCT cells actively transport sodium ions out of tubular fluid in exchange for potassium or hydrogen ions

• Pumps regulated by aldosterone• Aldosterone secretion occurs:

– In response to circulating ACTH from anterior pituitary– In response to elevated potassium ion concentrations

in extracellular fluid• The higher the aldosterone levels, the more

sodium that is reclaimed and the more potassium that is lost

DCT and Antidiuretic Hormone (ADH)

• Controls the amount of water that is reabsorbed

• Absence of ADH:– DCT and collecting ducts impermeable to

water

• Higher the ADH, the greater the water permeability and the more concentrated the urine

Properties of Normal Urine

• pH: 4.5-8

• Water content: 93-97%

• Volume: 1200 mL/day

• Color: clear yellow– What does dark yellow urine indicate?

• Odor: varies with composition

• Bacterial content: sterile

The Control of Kidney Function

• Regulated in 3 ways:– Local, automatic adjustments

• in glomerular pressures • through changes in diameters of afferent and

efferent arterioles

– Activities of SNS– Effects of hormones

• Make complex, long-term adjustments in bp and blood vol

– Stabilize GFR by regulating transport mechanisms and water permeabilities in DCT and collecting duct

Local Regulation of Kidney Function

• Change in diameter of afferent and efferent arterioles and glomerular capillaries

• Can compensate for minor changes in bp• Ex: ↓ blood flow and ↓ glomerular

pressure will trigger:– ________ of the afferent arteriole and

glomerular capillaries and– ________ of the efferent arteriole

Sympathetic Activation and Kidney Function

• Autonomic regulation primarily through SNS• Serves to shift blood away from kidneys

– Affect on GFR?

• Direct effects on kidney function– Powerful constriction of afferent arterioles

• ↓ GFR, slows production of filtrate– Why is that important?

– Can override local regulation in sudden crisis• Acute fall in bp, heart attack• When done, GFR returns to normal

Sympathetic Activation

• Indirect effects– When changes region pattern of blood

circulation, blood flow to kidneys affected• Ex: dilation of bv in hot weather shunts blood

away from kidneys– Glomerular filtration declines temporarily

Hormonal Control of Kidney Function

• Angiotensin II• ADH• Aldosterone• Atrial Natriuetic Peptide (ANP)• Secretion of angiotensin II,

ADH, aldosterone integrated by renin-angiotensin system

Renin-Angiotensin System

• Glomerular pressures can remain low due to:– Decrease in blood volume– Fall in systemic bp– Blockage of renal artery

• Then juxtaglomerular apparatus releases enzyme renin

Renin → angiotensinogen → angiotensin I → angiotensin II

• Angiotensin II is a powerful vasoconstrictor

Renin-Angiotensin System• Angiotensin II has following effects:

– Peripheral capillary beds• Brief but powerful vasoconstriction

– Elevates bp in renal arteries

– Nephron• Triggers contraction of efferent arterioles

– Elevates glomerular pressures and filtration rates

– CNS• Triggers release of ADH

– Simulates reabsorption of water and sodium ions • Stimulates hypothalamus

– Thirst sensation

– Adrenal gland• Stimulates secretion of aldosterone

– Stimulates sodium reabsorption along DCT and collecting system• Stimulates secretion of epinephrine and norepinephrine

– Sudden, dramatic increase in systemic bp

ADH

• Increases water permeability of DCT and collecting duct– Stimulates reabsorption of water from tubular fluid

• Causes thirst sensation • Release occurs:

– Under angiotensin II stimulation– Independently

• Hypothalamus neurons stimulated by ↓ in bp or ↑ in solute concentration of circulating blood

Aldosterone

• Stimulates reabsorption of sodium ions and secretion of potassium ions in DCT and collecting duct

• Primarily occurs:– Under angiotensin II stimulation– In response to rise in potassium ion

concentration of blood

Atrial Natriuretic Peptide (ANP)

• Oppose renin-angiotensin system• Released by atrial cardiac muscles when bp and blood

volume too high• Affects on kidney:

– Decrease in rate of sodium ion reabsorption in DCT• Increased sodium ion loss in urine

– Dilation of glomerular capillaries• Increased filtration and urinary water loss

– Inactivation of renin-angiotensin II system • Inhibition of renin, aldosterone, ADH secretion

• Net result:– Increased loss of sodium ions– Increase in vol of urine produced– Combination lowers blood vol and bp

The Micturition Reflex and Urination

• Process of urination or micturition coordinated by micturition reflex

• Stretch receptors stimulated as bladder fills• Increased impulses in afferent sensory fibers:

– Brings parasympathetic motor neurons in sacral spinal cord to threshold

– Stimulates interneurons to relay sensation to cerebral cortex (conscious awareness)

• Urge to urinate when bladder contains 200 mL of urine

Micturition Reflex and Urination

• Both internal and external sphincters must be relaxed– External under voluntary control

• When external relaxes so does internal

Fluid, Electrolyte, and Acid-Base Balance

• Fluid Balance– Amount of water gained each day = to amount lost – Involves regulating content and distribution of water in ECF and ICF– Cells and tissues cannot transport water so reflects control of electrolyte

balance• Electrolyte Balance

– Gain electrolytes from food and drink; lose in urine, sweat, feces– Balance exists when net gain = net loss

• Involves balancing absorption rates• Acid-Base Balance

– Production of H+ = loss– pH of body fluids within normal limits– Body produces acids so prevention in reduction primary problem– Lungs and kidneys

Fluid Balance

• Water Loss • Water Gain

Fluid Shifts

• Water movement between ECF and ICF• Occur rapidly, reach equilibrium within min to hrs• Occur in response to changes in osmotic

concentration (osmolarity) of ECF– ECF more concentrated (hypertonic) than ICF

• Water moves from cells to ECF until equil reached

– ECF more dilute (hypotonic) than ICF• Water moves from ECF into cells and vol of ICF will increase

accordingly

Electrolyte Balance

• Important because:– A gain or loss of electrolytes can cause a gain or loss in water– The concentrations of individual electrolytes affect a variety of

cell functions

• Will discuss sodium and potassium b/c:– They are major contributors to osmotic concentration of ECF and

ICF• Most common problems with electrolyte balance caused by

imbalance between sodium gains and losses

– Have direct effects on normal functioning of living cells• Problems with potassium balance less common but more

dangerous

Sodium Balance

• Amount of Na+ in ECF represents balance between absorption in digestive tract and excretion– Excretion in:

• Urine– Primary

» Kidneys most important site (aldosterone and ANP)• Sweat

• If intake or output rate changes, corresponding gain or loss of water occurs– Water follows salt!!!– Ex:

• High salt meal will not raise [sodium ion] of bodily fluids– Sodium chloride crosses digestive epith and osmosis brings additional

water into ECF» Reason why people with ↑ bp not supposed to eat high salt diet

(dietary salt will be absorbed and blood vol and bp will increase)

Potassium Balance

• Primary cation of ICF (98% of potassium in body)

• Concentration in ECF represents balance between:– Rate of potassium ion entry across diges epith

• Proportional to amount in diet

– Rate of loss into urine• Strongly affected by aldosterone

– Reabsorption of sodium from filtrate in exchange for potassium ions from ISF

– High potassium levels in ECF = high aldosterone = additional loss of potassium in urine

Acid-Base Balance

• pH of body fluids represent balance between acids, bases, and salts in solution

• Maintained at 7.35-7.45– Any deviation dangerous– [H+] changes:

• Disrupt stability of cell membranes• Alters protein structure• Changes activities of important enzymes

– Cannot survive with pH below 6.8 or above 7.7

Acid-Base Balance

• pH below 7.35 = acidosis• pH above 7.45 = alkalosis• Affect all systems but nervous system and

cardiovascular very sensitive to fluctuations– Severe acidosis deadly b/c:

• CNS function deteriorates– Individual becomes comatose

• Cardiac contractions grow weak and irregular– Symptoms of heart failure

• Peripheral vasodilation – Dramatic drop in bp; circulatory collapse

• Problems with acidosis more common– Why?

Acids in the Body

• Carbonic acid (H2CO3) important

– Lungs: carbonic acid breaks down into CO2 + H2O

• CO2 diffuses into alveoli

– Peripheral tissues: CO2 in solution interacts with H2O

• Forms H2CO3 which dissociates into hydrogen ion and bicarbonate

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

• Reaction occurs spontaneously and rapidly• Carbonic anhydrase

Buffers and Buffer Systems

• Metabolic acids must be controlled by buffers• Buffers

– Dissolved compounds that can provide or remove hydrogen ions• Stabilize pH of solution

– Include weak acids (hydrogen ion donors) and weak bases (hydrogen ion acceptors)

• Buffer system– Consists of combination of weak acids and its dissociated

products• H+ and an anion

– 3 major systems:• Protein buffer system• Carbonic acid-bicarbonate buffer system• Phosphate buffer system

Protein Buffer System

• Contributes to regulation of pH in ECF and ICF

• Depend on ability of amino acids to respond to changes in pH by accepting or releasing hydrogen ions– ↑ pH, carboxyl group (--COOH) of a.a.

dissociates and releases a hydrogen ion

– ↓ pH, amino group (--NH2) accepts additional hydrogen ions (forms –NH3

+)

Protein Buffer System

• Plasma proteins and hemoglobin contribute to buffering capabilities of blood

• ISF contains extracellular protein and amino acids that help regulate pH

• ICF contains structural and functional proteins– Prevent change in pH when organic acids

produced by cellular metabolism (lactic acid)

Carbonic Acid-Bicarbonate Buffer System

• Important buffer system in ECF• Carbonic acid acts as weak acid; bicarbonate acts as weak base• Net effect:

CO2 + H2O ↔ H+ + HCO3-

• Hydrogen ions removal will be replaced through combo of water and carbon dioxide

• Hydrogen ions added will be removed through formation of water and carbon dioxide

• Primary role is to prevent pH changes caused by metabolic acids• Hydrogen ions released through dissociation of the acids combine with

bicarbonate and form water and carbon dioxide• Carbon dioxide excreted at lungs

• Can cope with large amounts of acids • Body fluids contain an abundance of bicarbonate ions (bicarbonate

reserve)

Phosphate Buffer System

• Weak acid (anion): dihydrogen phosphate (H2PO4

-)

H2PO4- ↔ H+ + HPO4

2-

• In ECF plays supporting role in regulating pH– Many more bicarbonate ions than phosphate ions

• Very important in ICF– High concentration of phosphate ions

Maintaining Acid-Base Balance

• Buffer systems only provide temporary solution– Hydrogen ions have been tied up but not eliminated

• Must be removed from body fluids

• maintenance of acid-base balance involves controlling hydrogen ion losses and gains

• Respiratory and renal mechanisms support buffer systems by:– Secreting or absorbing hydrogen ions– Controlling excretion of acids and bases– Generating additional buffers when necessary

Respiratory Contributions to pH Regulation

• Respiratory compensation– Change in respiratory rate that helps to stabilize pH– Occurs when ph outside normal limits

• Respiratory activity has direct effect on carbonic acid-bicarbonate buffer system– Increasing or decreasing rate of respiration alters pH by lowering

or raising PCO2

• Changes in PCO2 have direct effect on concentration of hydrogen ions in plasma

– ↑ PCO2, ↓ pH

• ↑ PCO2 stimulates carotid and aortic bodies (chemoreceptors)– Increase in resp rate, more carbon dioxide loss at lungs, ↑ PCO2

returns to normal

Renal Contributions to pH Regulation

• Renal compensation– Change in rates of hydrogen ion and bicarbonate ion

secretion or absorption by kidneys in response to change in plasma pH

• Normal conditions: body generates hydrogen ions through production of metabolic acids– Hydrogen ions released must be excreted in urine to

maintain balance• Glomerular filtration puts hydrogen ions and carbon dioxide

into filtrate• Kidney tubules modify pH of filtrate by secreting hydrogen

ions or reabsorbing bicarbonate ions