Ch 18 Urinary System

Essentials of Anatomy and Physiology, Seventh Edition
Valerie C. Scanlon and Tina Sanders
Copyright © F.A. Davis Company 2015
Chapter 18
The Urinary System

The urinary system consists of two kidneys, two
ureters, the urinary bladder, and the urethra.
Function of the kidneys—to form urine to excrete waste
products and to regulate the volume, electrolytes, and pH
of the blood and tissue fluid
The other organs of the urinary system—

Kidneys
Location: retroperitoneal on either side of the backbone,
partially protected by—
The renal fascia and adipose tissue—
Hilus—an indentation on the medial side
The ureter emerges from the hilus, as does the renal
vein; the renal artery enters it.
Question: Name another major organ with a hilus; does it have the same
anatomic purpose?

Answer
Each lung has a hilus, an indentation on the medial side. The
anatomic purpose is the same. For the lung, the pulmonary
artery and the primary bronchus enter at the hilus, and the
pulmonary veins emerge.

Kidney—internal structure—a kidney has two
layers of tissue and a medial cavity.
Renal cortex—the outer tissue layer, made of renal corpuscles
and convoluted tubules:
Renal medulla (pyramids)—the inner tissue layer, made of
loops of Henle and collecting tubules:
Renal pelvis—a cavity formed by the expanded end of the
ureter within the kidney:
The extensions are calyces:

The Nephron—the functional unit of the kidney;
consists of a renal corpuscle and a renal tubule
Renal corpuscle—consists of a glomerulus surrounded by
Bowman’s capsule
Glomerulus—a capillary network that extends from an
afferent arteriole to an efferent arteriole:

The Nephron (continued)
Bowman’s capsule—the end of a renal tubule, encloses a
glomerulus:
Outer layer—not permeable:
Inner layer—made of podocytes—

The Nephron (continued)
Renal tubule: Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting tubule
The collecting tubules unite—
Peritubular capillaries—arise from the efferent arteriole,
surround all parts of the renal tubule

Peritubular capillaries
Interlobular veins
Arcuate veins
Interlobar veins
Renal vein
Question: Into which vein do the two
renal veins empty?
Blood Vessels of the Kidney (in sequence)
Abdominal aorta
Renal artery
Interlobar arteries
Arcuate arteries
Interlobular arteries
Afferent arterioles
Glomeruli
Efferent arterioles

Answer
The two renal veins empty into the inferior vena cava.

Blood Vessels of the Kidney (continued)
Essential for the formation of urine from blood plasma are
capillaries. The blood vessels of the kidney include two sets
of capillaries.
Questions: Name the two sets of capillaries. What are they sites for (that is, what
happens in capillaries)?

Answers
The two sets of capillaries are the glomeruli and the peritubular
capillaries.
These capillaries are the sites of exchanges between the blood
and surrounding tissues, or blood and the renal filtrate.

Formation of Urine
The formation of urine involves three major processes:
Glomerular filtration
Tubular reabsorption
Tubular secretion

Formation of Urine (continued)
Glomerular filtration—takes place from the glomerulus to
Bowman’s capsule
Blood pressure in the glomerulus is relatively high:
The podocyte layer of Bowman’s capsule is very permeable.
The fluid in Bowman’s capsule is called renal filtrate.
In the filtrate—
Remaining in the blood—

Glomerular filtration is not selective in terms of usefulness—
Glomerular filtration rate (GFR)—100 to 125 mL per minute
Changes in blood flow through the kidney will change the GFR:

Tubular reabsorption—takes place from the filtrate in the
renal tubules to the blood in the peritubular capillaries—
useful materials in the renal filtrate are returned to the
blood.
Approximately 99% of the filtrate will be reabsorbed. Most
reabsorption takes place in the proximal convoluted
tubules.
Question: The cells of the proximal convoluted tubules have a membrane
modification that increases their surface area. Name this membrane
modification.

Answer
The cells of the proximal convoluted tubules have microvilli
that increase the surface area of the cell membranes and
make reabsorption more efficient.

Mechanisms of reabsorption:
Active transport—
Passive transport—
Osmosis—
Pinocytosis—

Tubular secretion—takes place from the blood in the
peritubular capillaries to the filtrate in the renal tubules—
unwanted materials can be excreted in urine.
Creatinine—
Metabolites of medications—
H+
ions—

Hormones that affect reabsorption (see Table 18–1):
Aldosterone—
Atrial natriuretic peptide—
Antidiuretic hormone—
Parathyroid hormone—

The Kidneys and Acid–Base Balance
The kidneys have the greatest capacity to compensate for pH
changes
If body fluids are becoming too acidic –
The kidneys will excrete:
The kidneys will retain:
If body fluids are becoming too alkaline –
The kidneys will excrete:
The kidneys will retain:

Other Functions of the Kidneys
Secretion of renin by juxtaglomerular cells
Stimulus: blood pressure decreases
Renin begins the renin-angiotensin mechanism (see Table 18–2)
Angiotensin II—
Secretion of erythropoietin
Stimulus: hypoxia (hypoxemia)
Question: What is the function of erythropoietin?

Answer
Erythropoietin increases the rate of red blood cell production in
the red bone marrow.
Question: What vitamin is activated by the kidneys?

Answer
Vitamin D is converted from inactive forms to the active form in
the kidneys.

Elimination of Urine—the function of the ureters, urinary
bladder, and urethra
Ureters—each extends from the hilus of a kidney to the
lower, posterior side of the urinary bladder
Urine is kept moving toward the bladder by waves of
peristalsis—

Elimination of Urine (continued)
Urinary bladder—a reservoir for temporary storage of urine;
below the peritoneum, behind the pubic bones
In women:
In men:
Mucosa—transitional epithelium—
Rugae—
Trigone—

Urinary bladder—detrusor muscle—a spherical muscle that
forms the wall of the bladder
Contraction—
Internal urethral sphincter—formed by detrusor muscle
fibers around the opening of the urethra
Contraction—
Relaxation—

Urethra—takes urine from the bladder to the exterior
In women: 1 to 1.5 inches in length
In men: 7 to 8 inches in length:
Prostatic urethra
Membranous urethra
Cavernous urethra
External urethral sphincter—the skeletal muscle of the
pelvic floor:

Elimination of Urine—Urination Reflex
Stimulus: stretching of the detrusor muscle—
Sensory impulses  spinal cord
Motor impulses (parasympathetic) from spinal cord 
detrusor muscle
Effectors: Detrusor muscle—
Internal urethral sphincter—
External urethral sphincter—

Characteristics of Urine (see Table 18–3)
Amount per 24 hours: 1 to 2 liters
Factors that cause oliguria—
Factors that cause polyuria—
Color—the typical yellow color is called “straw” or “amber.”

Characteristics of Urine (continued)
Specific gravity: a measure of the dissolved materials in urine
—1.010 to 1.025. The specific gravity of distilled water is
1.000.
The higher the specific gravity—
The lower the specific gravity—
Specific gravity is an indicator of the concentrating ability
of the kidneys—

pH—the pH range of urine is 4.6 to 8.0, with an average pH
of 6.0.
A vegetarian diet—
A high-protein diet—
Composition of urine—95% water, 5% salts and waste
products

Nitrogenous wastes excreted in urine:
Urea—
Creatinine—
Uric acid—

Wrap-Up Question
Name the part or aspect of the urinary system described.
1. Form the inner layer of Bowman’s capsule
2. Hormone that increases excretion of K+
ions
3. Increase surface area in proximal convoluted tubule
4. Vessels around the renal tubule
5. Secrete renin when BP decreases
6. Capillaries where renal filtration takes place
7. Reservoir for urine
8. Water is reabsorbed

Answers
1. Form the inner layer of Bowman’s capsule—podocytes
2. Hormone that increases excretion of K+
ions—aldosterone
3. Increase surface area in proximal convoluted tubule—
microvilli
4. Vessels around the renal tubule—peritubular capillaries
5. Secrete renin when BP decreases—juxtaglomerular cells
6. Capillaries where renal filtration takes place—glomeruli
7. Reservoir for urine—urinary bladder
8. Water is reabsorbed—osmosis

Ch 18 Urinary System

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