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