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Human Anatomy, First Edition

McKinley & O'Loughlin

Urinary System

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General Structure and Functions of the Urinary System General Concept:

Waste products accumulate in blood Are toxic Must be removed to maintain

homeostasis Urinary System organs

remove waste products from the blood then from the body

Major homeostatic system

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General Structure and Functions of the Urinary System

Organs of the Urinary System: Kidneys Ureters Urinary Bladder Urethra

Primary organs: kidneys filter waste products from the bloodstream convert the filtrate into urine.

The Urinary Tract: Includes:

ureters urinary bladder urethra

Because they transport the urine out of the body.

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Functions of the Urinary System Removing waste products from the bloodstream. Storage of urine.

the urinary bladder is an expandable, muscular sac that can store as much as 1 liter of urine

Excretion of urine. Blood volume regulation.

the kidneys control the volume of interstitial fluid and blood under the direction of certain hormones

Regulation of erythrocyte production. as the kidneys filter the blood, they are also indirectly

measuring the oxygen level in the blood Erythropoietin (EPO): hormone produced by kidney

Released if blood oxygen levels fall Stimulates RBC production in red bone marrow

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Kidneys: Gross and Sectional Anatomy Retroperitoneal

Anterior surface covered with peritoneum Posterior surface against posterior

abdominal wall Superior pole: T-12 Inferior pole: L-3 Right kidney ~ 2cm lower than left Adrenal gland on superior pole

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Kidneys: Gross and Sectional Anatomy Sectioned on a coronal plane:

Renal Cortex - outer Renal Medulla - inner

Divided into renal pyramids and columns 8 to 15 per kidney

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Blood Supply to the Kidney About 20 to 25% of cardiac output to

kidneys

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Blood Supply to the Kidney Blood plasma is filtered across the

glomerulus into the glomerular space.

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Nephrons The functional filtration unit in the kidney. Consists of the following:

Renal corpuscle Glomerulus Glomerular capsule (Bowman’s capsule)

Proximal convoluted tubule (PCT) Nephron loop (loop of Henle)

Ascending loop of Henle Descending loop of Henle

Distal convoluted tubule (DCT) collectively called the renal tubule

In both kidneys: approximately 2.5 million nephrons. Are microscopic: measure about 5 centimeters in

length.

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Urine Formation Three processes Filtration

Renal corpuscle: forms filtrate From blood to tubule

Reabsorption Mostly PCT Water and salt: rest of nephron From tubule to blood

Secretion From blood to tubule

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Innervation of the Kidney innervated by a mass of autonomic nervous system

fibers called the renal plexus.

The renal plexus accompanies each renal artery enters the kidney through the hilum.

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Urinary Tract : Ureters long, fibromuscular tubes conduct urine from the kidneys to the urinary bladder. average 25 centimeters in length retroperitoneal. ureters originate at the renal pelvis extend inferiorly to enter the posterolateral wall of the

base of the urinary bladder. wall is composed of three concentric tunics.

mucosa muscularis adventitia.

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Urinary Tract – Urinary Bladder The urinary bladder:

expandable, muscular container serves as a reservoir for urine

positioned immediately superior and posterior to the pubic symphysis.

in females the urinary bladder is in contact with the uterus posterosuperiorly

and with the vagina posteroinferiorly. in males

it is in contact with the rectum posterosuperiorly and is immediately superior to the prostate gland.

is a retroperitoneal organ. when empty exhibits an upside-down pyramidal shape. Filling with urine distends it superiorly until it assumes an oval

shape.

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Urinary Tract – Urinary Bladder Trigone

posteroinferior triangular area of the urinary bladder wall formed by imaginary lines

connect the two posterior ureteral openings and the anterior urethral opening.

The trigone remains immovable as the urinary bladder fills and evacuates.

It functions as a funnel directs urine into the urethra as the bladder wall contracts

four tunics mucosa submucosa Muscularis: called the detrusor muscle Adventitia.

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Micturition (Urination) The expulsion of urine from the bladder. Initiated by a complex sequence of events called the

micturition reflex. The bladder is supplied by both parasympathetic and

sympathetic nerve fibers of the autonomic nervous system.

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Urethra Fibromuscular tube

exits the urinary bladder through the urethral opening at anteroinferior surface

conducts urine to the exterior of the body. Tunica mucosa: is a protective mucous membrane

houses clusters of mucin-producing cells called urethral glands.

Tunica muscularis: primarily smooth muscle fibers help propel urine to the outside of the body.

Two urethral sphincters: Internal urethral sphincter

restrict the release of urine until the pressure within the urinary bladder is high enough

External urethral sphincter and voluntary activities needed to release the urine are

activated.

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Urethra The internal urethral sphincter

involuntary (smooth muscle) superior sphincter surrounding the neck of the bladder,

where the urethra originates. a circular thickening of the detrusor muscle controlled by the autonomic nervous system

The external urethral sphincter inferior to the internal urethral sphincter formed by skeletal muscle fibers of the urogenital

diaphragm. a voluntary sphincter controlled by the somatic nervous system this is the muscle children learn to control when they

become “toilet-trained”

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Female Urethra Has a single function:

to transport urine from the urinary bladder to the vestibule, an external space immediately internal to the labia minora

3 to 5 centimeters long, and opens to the outside of the body at the external urethral orifice located in the female perineum.

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Male Urethra Urinary and reproductive functions:

passageway for both urine and semen Approximately 18 to 20 centimeters long. Partitioned into three segments:

prostatic urethra is approximately 3 to 4 centimeters long and is the most dilatable portion of the urethra

extends through the prostate gland, immediately inferior to the male bladder, where multiple small prostatic ducts enter it

membranous urethra is the shortest and least dilatable portion extends from the inferior surface of the prostate gland through the

urogenital diaphragm spongy urethra is the longest part (15 centimeters)

encased within a cylinder of erectile tissue in the penis called the corpus spongiosum

extends to the external urethral orifice

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Components of Urine

• Urine = 1-2 l /day

• 95% water

• + urea, creatinine, K+, ammonia, uric acid, Na+, Cl-, Mg2+, sulfate, phosphate & Ca2+

• Depends on diet and state of health

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Aging and the Urinary System Changes in the size and functioning of the kidneys begin at 30. Gradual reduction in kidney size. Reduced blood flow to the kidneys. Decrease in the number of functional nephrons. Reabsorption and secretion are reduced. Diminished ability to filter and cleanse the blood. Less aldosterone or antidiuretic hormone. Ability to control blood volume and blood pressure is reduced. Bladder decreases in size. More frequent urination. Control of the urethral sphincters—and micturition—may be lost.

Aging Kidneys shrink- decrease in capacity Thirst decreases dehydration urinary tract infections Males: prostate enlargement frequent

urination & slow flow Females: more prone to leakage of

external sphincter (incontinence) Both: nocturia