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The Urinary System and Excretion
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Lecture Outline
The Urinary System
o Excretion – the removal of metabolic wastes from the body
o Kidneys are the primary organ of excretion
o Four functions essential to homeostasis:
• Excretion of metabolic wastes• Preservation of water-salt balance• Maintenance of acid-base balance• Secretion of hormones
The Urinary System
• Excretion of metabolic wastes Urea
By-product of amino acid metabolism Ammonia combines with carbon dioxide to
produce urea Less toxic than ammonia
Creatinine – result of the metabolic breakdown of creatine phosphate
Uric acid Result from the breakdown of nucleotides Insoluble Gout occurs if excess uric acid collects in the
joints
The Urinary System
• Preservation of water-salt balance Blood volume is closely associated with the salt
balance of the body Salts have the ability to cause osmosis The more salts there are in the blood, the greater the
blood volume Kidneys also maintain other ions in the blood
Potassium Bicarbonate Calcium
Maintenance of blood pressure Kidneys influence cardiac output regulating blood
volume Kidneys influence peripheral resistance by producing
renin Activates angiotensin Angiotensin constricts blood vessels
The Urinary System
• Maintenance of acid-base balance Kidneys excrete hydrogen ions from the blood into
the urine Kidneys reabsorb bicarbonate ions and return
them to the blood
• Secretion of hormones Release renin when blood supply decreases Erythropoietin
Released when the oxygen-carrying capacity of the blood is reduced
Stimulates red blood cell production Kidneys help activate vitamin D
Precursor to calcitrol Promotes calcium absorption
The Urinary System
o Organs of the Urinary System• Kidneys
Paired organs Located in the lumbar region Retroperitoneal – covered by the parietal
peritoneum Held in place by the renal fascia Covered by the renal capsule Hilum
Renal artery enters Renal vein and a ureter exit
The Urinary System
• Ureters Extend from the kidneys to the bladder Wall layers:
Mucosa (mucous membrane) – inner layer Smooth muscle – middle layer Fibrous connective tissue – outer layer
• Urinary Bladder In the pelvic cavity Stores urine Three openings:
Two for the ureters One for the urethra Trigone – area at the base of the bladder outlined by the openings
Detrusor muscle Middle layer of circular fibers Two layers of longitudinal muscle
Mucosa of transitional epithelium Sphincters
Internal sphincter around the opening to the urethra External sphincter composed of skeletal muscle
The Urinary System
• Urethra Extends from the urinary bladder to an
external opening In males
Averages 20 cm when penis is not erect Encircled by the prostate gland Carries urine during urination and sperm during
ejaculation In females
About 4 cm long Prone to bacterial infection
The Urinary System
o Urination (micturition)• Stretch receptors
in the bladder are stimulated when the bladder is full
• Nerve impulses from the spinal cord cause the bladder to contract and the sphincters to relax
Anatomy of the Kidney and Excretion
o Three regions:• Renal cortex• Renal medulla• Renal pelvis
o Anatomy of a Nephron (renal tubules)
• Afferent arteriole• Glomerulus• Efferent arteriole• Peritubular capillary network
Anatomy of the Kidney and Excretion
• Parts of a Nephron Renal corpuscle
Glomerular capsule (Bowman’s capsule) Glomerulus
Renal tubule Proximal convoluted tubule (PCT) Loop of Henle Distal convoluted tubule (DCT) Collecting duct
Anatomy of the Kidney and Excretion
o Urine Formation• Glomerular Filtration
Whole blood enters the afferent arteriole and the glomerulus
Glomerular blood pressure causes blood to be filtered
Water Nitrogenous wastes Nutrients Salts (ions)
Nonfiltered components leave the glomerulus by the efferent arteriole
Formed elements (blood cells and platelets) Plasma proteins
Anatomy of the Kidney and Excretion
• Tubular Reabsorption Involve both passive and active processes Occurs in the PCT Molecules and ions are reclaimed from
the filtrate Water Nutrients (i.e. glucose and amino acids) Required salts (ions)
Every substance has a maximum rate of transport
Anatomy of the Kidney and Excretion
• Tubular Secretion Substances are removed from the blood
Hydrogen ions Potassium ions Creatinine drugs
Active process Occurs in the DCT
Regulatory Functions of the Kidneys
o Fluid and Electrolyte Balance• Total water intake should equal total
water loss• How water enters the body
Liquids Foods By-product of metabolism
• How water exits the body Urine Sweat Exhaled air Feces
Regulatory Functions of the Kidneys
• Reabsorption of Water Reabsorption of salt leads to the
reabsorption of water Most reabsorbed in the PCT Excretion of hypertonic urine is
dependent on the reabsorption of water from:
The loop of Henle The collecting duct
Antidiuretic Hormone (ADH) Causes water to be reabsorbed at the collecting
duct Decreases the amount of urine
Regulatory Functions of the Kidneys
• Reabsorption of Electrolytes The electrolytes
Sodium Potassium Bicarbonate ion Other ions
Calcium Phosphate ions
The kidneys Greater than 99% of filtered sodium is reabsorbed
back into the blood 67% is reabsorbed at the PCT 25% is reabsorbed at the ascending limb of the loop
of Henle The rest is reabsorbed from the DCT and the
collecting duct
Regulatory Functions of the Kidneys
Aldosterone Promotes the excretion of potassium Promotes the reabsorption of sodium and water Kidneys release renin when blood pressure is
too low to promote glomerular filtration Renin converts angiotensinogen to
angiotensin I Angiotensin I is converted to angiotensin II Angiotensin II stimulates the release of
aldosterone Atrial natriuretic hormone (ANH)
Secreted by the atria of the heart when blood volume is high
Inhibits the secretion of renin and aldosterone Promotes the excretion of sodium and water
Regulatory Functions of the Kidneys
• Diuretics Chemicals that increase the flow of urine Examples:
Alcohol Caffeine Diuretic drugs
Regulatory Functions of the Kidneys
o Acid-Base Balance• Acid-Base Buffer Systems
Buffer – a chemical that can take up excess hydrogen or hydroxide ions
An important buffer in the blood is a combination of carbonic acid and bicarbonate ions
• Respiratory Regulation of Acid-Base Balance
Increasing the breathing rate rids the body of hydrogen ions
Breathing readjusts the proportion of carbonic acid and bicarbonate ions in the blood
Regulatory Functions of the Kidneys
• Renal Regulation of Acid-Base Balance
Can rid the body of a wide range of acidic and basic substances
Slower acting than other two systems Have a more powerful effect on pH Reabsorb bicarbonate ions and excrete
hydrogen ions as needed to maintain a normal pH
Ammonia and phosphate ions are buffers for hydrogen ions in the urine
Problems with Kidney Function
o Water accounts for about 95% of urine volume
o Remaining 5% consists of electrolytes and various solutes
o Urine usually does not contain proteins and blood cells
o Urinalysis is an examination of the physical, chemical, and microscopic properties of the urine
o Composition of urine changes if disease has altered body metabolism or if kidney function is abnormal
Problems with Kidney Function
Problems with Kidney Function
o Infections• Urethritis• Cystitis• Pyelonephritis
o Glomerular damage• May allow large substances to be
filtered Albumin, white blood cells, and possibly
red blood cells Detected by a urinalysis
• Extensive damage can cause uremia
Problems with Kidney Function
• Hemodialysis Utilizes either an artificial kidney machine
or continuous ambulatory peritoneal dialysis
Dialysate – dialysis solution Used to extract waste products, toxic
chemicals, and drugs from patient’s blood Used to add needed substances to the
patient’s blood Three- to six-hour treatment twice a week
Problems with Kidney Function
• Replacing a Kidney A functioning kidney from a donor is
received by a patient with renal failure Possibility of organ rejection One-year survival rate is 97% if the
kidney is from a relative and 90% if it is from a nonrelative
Patient must take anti-rejection medication for the rest of his or her life
Effects of Aging
o Total renal function may decrease to only 50%
o Kidneys decrease in size and have fewer nephrons
o Kidney stones are more commono Infections are more commono Enlargement of the prostate in
maleso Incontinence
Homeostasis
o Excretion• Sweat glands• Liver• Lungs• Kidneys are the primary organ of excretion
o Kidneys are the primary organs of homeostasis• Water-salt balance• Acid-base balance
o The kidneys assist the endocrine and cardiovascular systems by producing erythropoietin
o Regulation of blood calcium levels assist the skeletal, nervous, and muscular systems