Chapter 15: The Urinary System
Urinary System Regulates Body Fluids
Excretion: processes that remove waste and excess materials from
the body
Digestive System: excretes food residues and waste produced by
the liver
Respiratory System (lungs): excretes carbon dioxide
Integumentary System (skin): excretes water and salt
Urinary System (kidneys): excretes nitrogenous wastes, excess
solutes and water
The Kidneys Regulate Water Levels:
To maintain homeostasis,
Water input = water output
Kidneys adjust water output as necessary
-water input: food, drink, metabolism
-water output: lungs, skin, feces, kidneys
-kidneys modify output based on intake and loss
-output varies from ½ liter/day to 1 liter/hour
The Kidneys Regulate Nitrogenous Wastes and Other Solutes:
Protein metabolism produces nitrogenous wastes
Initially, NH (ammonia) is produced during breakdown of amino
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acids
Liver detoxifies NH , producing urea
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Urea is transported from liver to kidneys for disposal
Other solutes regulated by kidneys
-Sodium, chloride, potassium, calcium, hydrogen ions, creatinine
Organs and Urinary System:
Kidneys
–Principal organ of urinary system
–Cortex: outer portion of the kidney
–Medulla: inner region of the kidney
–Renal pelvis: hollow space in center of kidney where urine
collects
Ureters
–Muscular tubes that transport urine from kidneys to bladder
Urinary bladder
–Three layers of smooth muscle, lined with epithelial cells
–Stores urine (600–1,000 ml)
Urethra
–Carries urine from bladder to outside of body
–Two sphincters control urination
The Internal Structure of the Kidney:
Nephron: functional unit of the kidney
–Two functional parts:
–Tubule
–Associated blood supply
1 million nephrons per kidney
Each nephron consists of a long thin hollow tube (tubule) plus
associated blood supply
Role of nephrons: remove approximately 180 liters of fluid from the
blood daily, and return most of it, minus the wastes that are excreted
Nephron structure
–Glomerular capsule: cuplike end of nephron tubule surrounding
glomerulus (network of capillaries)—this is where filtration
occurs
–Four distinct regions of tubule
– Proximal tubule: extends from glomerular capsule to
renal medulla
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– Loop of Henle: extends into medulla then back into
cortex
– Distal tubule
– Collecting duct: shared by several nephrons, empties
into renal pelvis
Special Blood Vessels Supply the Tubule:
Renal artery supplies the kidney
Blood vessels associated with tubules
–Arterioles
–Afferent: enters the glomerular capsule
–Efferent: leaves the glomerular capsule
–Capillaries
–Glomerular: network within the glomerular capsule
–Peritubular: surround proximal and distal tubule
–Vasa recta: parallels the loop of Henle
Renal vein: collects filtered blood from kidneys
Formation of Urine: Filtration, Reabsorption, and Secretion:
Renal artery supplies the kidney
Blood vessels associated with tubules
–Arterioles
–Afferent: enters the glomerular capsule
–Efferent: leaves the glomerular capsule
–Capillaries
–Glomerular: network within the glomerular capsule
–Peritubular: surround proximal and distal tubule
–Vasa recta: parallels the loop of Henle
Renal vein: collects filtered blood from kidneys
Glomerular Filtration Filters Fluid from Capillaries
Filters protein-free plasma fluid from capillaries into glomerular
capsule
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Large volume filtration, yet highly selective
–Impermeable to large proteins and cells
Filtration is driven by high blood pressure in glomerular capillaries
Rate of filtration is regulated in two ways
–Resting rate under local control that adjusts diameter of afferent
arterioles
–Stress causes sympathetic nervous system to reduce blood flow
to kidneys
Tubular Reabsorption Returns Filtered Water and Solutes to
Blood:
One hundred percent of filtered glucose, amino acids, and bicarbonate
and 50% of urea are reabsorbed
Most tubular reabsorption occurs in proximal tubule
Water reabsorption
–Sixty-five to seventy percent occurs in proximal tubule
–Twenty-five percent occurs in loop of Henle
–Less than 10% occurs in distal tubule and collecting duct—but
this is where water excretion is regulated
Brush border of microvilli on proximal tubule cells facilitate
reabsorption
Reabsorption process starts in proximal tubule
1. Sodium moved by active transport out of tubule cell into interstitial
fluid (on capillary side)
2. Sodium diffuses from interstitial fluid into capillary
3. This establishes a concentration gradient favoring facilitated diffusion
of sodium from lumen of tubule into proximal tubule cell
4. Chloride passively accompanies sodium (balanced charge)
5. Water diffuses through aquaporins from lumen to proximal tubule cell
to interstitial fluid to capillary
6. Movement of sodium provides energy to cotransport glucose and
amino acids from tubule into surrounding cells
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7. Glucose, amino acids then diffuse to the interstitial fluid and capillaries
Tubular Secretion Removes Other Substances from Blood:
Involves the movement of materials from the peritubular capillaries or
vasa recta to the tubule
Purpose:
-Regulation of chemical levels in body
-Excretion of harmful chemicals
Substances secreted:
-Penicillin, cocaine, marijuana, pesticides, preservatives,
hydrogen ions, ammonium, potassium
Producing a Dilute Urine: Excreting Excess Water:
Kidneys respond to excess water by excreting it
Mechanism:
-distal tubile is impermeable to water, so water is not reabsorbed
here
-NaCl is reabsorbed without the concurrent reabsorption of water
-high-volume dilute urine is produced
Producing Concentrated Urine: Conserving Water
Too little water can lead to lower blood volume, declining blood
pressure, risk of dehydration of body cells
Kidneys respond by conserving water and producing a more
concentrated urine
Mechanism:
-mediated by ADH (antidiuretic hormone) from the posterior
pituitary gland
-ADH increases permeability of the collecting ducts to water and
increases conservation of water
Urination Depends on a Reflex
Micturition reflex: neural reflex that enables emptying of the bladder
–Responds to stretch receptors in bladder wall
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–Internal urethral sphincter
–Smooth muscle
–External urethral sphincter
–Skeletal muscle, under voluntary control
Brain can override the micturition reflex and control the timing of
urination
Voluntary control becomes increasingly difficult as the bladder gets
very full
Kidneys Maintain Homeostasis in Many Ways:
Contribute to maintenance of water balance
Contribute to maintenance of salt balance
Secrete an enzyme involved in the control of blood volume and blood
pressure
Maintain acid−base balance and blood pH
Regulate red blood cell production via erythropoietin
Activate an inactive form of vitamin D
ADH (Antidiuretic Hormone) Regulates Water Balance
Involves the following organs:
–Hypothalamus: synthesizes ADH
–Posterior pituitary gland: releases ADH
–Kidneys: respond to ADH
Negative feedback loop regulates solute concentration of the blood
–Involves increasing or reducing ADH secretion, which will modify
water reabsorption by kidneys
–Involves increasing or decreasing thirst
ADH Regulates Water Balance
If blood solute concentration is too high (water concentration too low),
ADH released
–ADH causes:
–Increase in permeability of collecting duct to water
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–Increase in water reabsorbed by kidney
–Decrease in urine production
–Increase in thirst
If blood solute concentration is too low (water concentration too high),
ADH secretion is reduced
–Decrease in permeability of collecting duct to water
–Decrease in water reabsorbed by kidney
–Increase in urine production
Decrease in thirst
Diuresis: high urine flow rate
Diuretic: any substance that increases the formation and excretion of
urine
–Lasix (furosemide): medication that reduces blood volume and
blood pressure
–Used in treatment of congestive heart failure and
hypertension
–Caffeine: inhibits sodium reabsorption
–Alcohol: inhibits ADH release
Aldosterone Regulates Salt Balance
Blood volume control is dependent on salt balance
Aldosterone: adrenal hormone that regulates sodium excretion
–Mechanism: increases Na reabsorption from distal tubule and
+
collecting duct
Aldosterone secretion is controlled by the renin-angiotensin system
The Renin-Angiotensin System Controls Blood Volume and
Blood Pressure
Aldosterone release is stimulated indirectly by decreased blood
volume or blood pressure
Decreased blood volume/blood pressure causes release of renin
(enzyme) from juxtaglomerular apparatus (region where afferent and
efferent arterioles are in close contact with distal tubule)
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Renin cleaves inactive angiotensinogen (produced by liver), releasing
angiotensin I
Angiotensin converting enzyme (ACE) in lungs converts antiotensin I
(inactive peptide) to angiotensin II (biologically active peptide)
Effects of angiotensin II:
–Constricts arterioles, which raises blood pressure
–Stimulates release of aldosterone from adrenal glands
–Aldosterone: increases sodium reabsorption by distal tubules
and collection ducts
ACE inhibitors: medication for blood pressure control
Inhibit angiotensin converting enzyme (ACE) in the lungs
–Block normal production of angiotensin II
–Aldosterone concentration falls
–Sodium and water excretion increase
–Blood volume reduced slightly
–Blood vessels dilate, lowering blood pressure
Atrial Natriuretic Hormone Protects Against Blood Volume Excess
Another controller of renal sodium excretion
High blood volume stretches atria of heart
Atria secrete ANH (atrial natriuretic hormone) in response to
stretching
ANH inhibits Na reabsorption in distal tubules and collecting ducts
+
Na+ excretion increases
Water follows the Na+
Effect of ANH is opposite to that of aldosterone
Kidneys Help Maintain Acid-Base Balance and
Blood pH
Blood pH must stay between 7.35 and 7.45
pH regulated by kidneys, buffers, lungs
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Role of kidneys in pH maintenance:
–Reabsorption of filtered bicarbonate
–Excretion of acid as ammonium (NH )
4+
Erythropoietin Stimulates Production of Red Blood Cells
Decrease in amount of oxygen is detected by certain cells throughout
the kidney
O2 sensitive cells in kidney secrete hormone, erythropoietin, in
response to decrease in oxygen
Erythropoietin triggers increase in red blood cell production in the
bone marrow
Kidneys Activate Vitamin D
Exposure of skin to sunlight causes production of an inactive form of
vitamin D from a precursor found in the skin
Inactive form of vitamin D is transported to liver, where it is modified
Inactive form of vitamin D is then converted to active form by kidneys
Conversion to active vitamin D in kidneys is influenced by activity of
PTH (parathyroid hormone)
Disorders of the Urinary System
Kidney stones
–Crystallized minerals
–Block urine flow
Urinary tract infections (UTI)
–Usually caused by bacteria
–More common in women than men because of shorter urethra
–If untreated, bladder infections may ascend to involve kidneys
Acute renal failure
–Short-term impairment, may be reversible
–Potential causes: sustained very low blood pressure, large kidney
stones within renal pelvis, infections, transfusion reactions,
severe injury, toxin exposure, drug reactions
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Chronic renal failure
–Also known as end stage renal disease (ESRD)
–ESRD: long-term, irreversible damage leading to >60% reduction
in functioning nephrons
–Patients may have <10% normal filtering capacity
–Results when
Renal tubular cells do not receive the nutrients they need
Glomerular filtration is blocked for too long
–Diabetes may lead to diabetic nephropathy, which often
progresses to ESRD
Dialysis Cleanses Blood Artificially
Dialysis: attempts to duplicate function of healthy kidneys
CAPD: continuous ambulatory peritoneal dialysis
–Can be done at home
–Uses peritoneal cavity for waste and ion removal
–Risk of infection
Hemodialysis
–Requires several visits/week to a dialysis center
–Blood is circulated through a kidney machine
Dialysis Cleanses Blood Artificially
Problems with dialysis
–Dialysis cannot achieve complete homeostasis of ions and
wastes
–Dialysis does not replace renal hormones
Kidney Transplants Are a Permanent Solution to Renal Failure
Best hope for many chronic renal failure patients
Improvements in transplant protocols/processes have improved
outcomes
–Better tissue-matching techniques
–Improved anti-rejection medications
–National data banks
Shortage of donated kidneys
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Urinary Incontinence Is a Loss of Bladder Control
Develops with age due to aging bladder muscles
More common in women
–Pregnancy and childbirth
–Hormonal changes after menopause
–Men: incontinence most often associated with enlarged prostate
or prostate cancer
Treatments
–Bladder training and various exercises
–Management of fluid intake
–Medications
–Surgery
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