The document outlines the urinary system's structure and functions, emphasizing the kidneys as the primary excretory organs responsible for fluid balance and waste removal. It details nephron functionality, including the filtration, reabsorption, and secretion processes, along with the role of hormones in regulating urine concentration and blood pressure. Additionally, it describes urine movement and the micturition reflex involved in the urination process.

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Chapter 18
Urinary System and Fluid
Balance
Lecture Outline
Seeley’s ESSENTIALS OF
ANATOMY & PHYSIOLOGY
Eleventh Edition
Cinnamon VanPutte
Jennifer Regan
Andrew Russo
Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC.

2. Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC.
Urinary System
• The urinary system is the major excretory system of the
body.
• Some organs in other systems also eliminate wastes, but
they are not able to compensate in the case of kidney
failure.
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Urinary System Functions
1. Excretion
2. Regulation of blood volume and pressure
3. Regulation of blood solute concentration
4. Regulation of extracellular fluid pH
5. Regulation of red blood cell synthesis
6. Regulation of Vitamin D synthesis
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Kidney Structures
Renal capsule:
• connective tissue around each kidney
• protects and acts as a barrier
Hilum:
• indentation
• contains renal artery, veins, nerves, ureter
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Kidney Structures
Renal sinus:
• contains renal pelvis, blood vessels, fat
Renal cortex:
• outer portion
Renal medulla:
• inner portion
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Kidney Structures
Renal pyramid:
• Cone shaped structures in the medulla whose bases
project into the cortex
Renal papillae:
• tip of pyramids which drain into calyces
Renal pelvis:
• where calyces join together
• narrows to form ureter
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Ureters and Urinary Bladder
Ureters:
• small tubes that carry urine from renal pelvis of kidney to
bladder
Urinary bladder:
• in pelvic cavity
• stores urine
• can hold a few ml to a maximum of 1000 milliliters
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Urethra
Urethra:
• tube that exits bladder
• carries urine from urinary bladder to outside of body
Internal urethral sphincter
• smooth muscle surrounds urethra at the junction of the
urinary bladder and prevents urine from leaving the
bladder
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Urethra
External urethral sphincter:
• formed of skeletal muscle surrounding the urethra near the
pelvic floor.
• allows a person to voluntarily start or stop the flow of urine
out of the urethra
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Nephron
• The nephron is the functional unit of the kidney.
• Each kidney has over one million nephrons.
• The nephron includes the renal corpuscle, proximal
convoluted tubule, loop of Henle, distal convoluted tubule
and collecting duct
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Renal Corpuscle
The filtration portion of the nephron
Glomerulus:
• A network of capillaries twisted around each other like a
ball of yarn
Bowman’s capsule:
• enlarged end of nephron surrounds glomerulus
• opens into proximal convoluted tubule
• contains podocytes (specialized cells around glomerular
capillaries)
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Renal Corpuscle
• Bowman capsule consists of two layers:
• Outer layer - simple squamous epithelial cells that
become cube-shaped at the beginning of the proximal
convoluted tubule
• Inner layer - cells called podocytes, which wrap around
the glomerular capillaries
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Proximal convoluted tubule:
• where filtrate passes first
• drains filtrate from Bowman capsule
Loop of Henle:
• contains descending and ascending loops
• water and solutes pass through thin walls by diffusion
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Nephron Components

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Nephron Components
Distal convoluted tubule:
• structure between Loop of Henle and collecting duct
Collecting duct:
• empties into calyces
• carry fluid from cortex through medulla
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Types of Nephrons
1. Juxtamedullary nephrons
• renal corpuscles are deep in the cortex near the medulla
• long loops of Henle extend deep into the medulla
• Well adapted for water conservation
• About 15% of nephrons
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Types of Nephrons
2. Cortical nephrons
• Renal corpuscles distributed throughout the cortex (outer
portion)
• Loops of Henle are shorter and closer to the outer edge of
the cortex than juxtamedullary nephrons
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Characteristics of Renal Corpuscle
1. Porous capillaries
- highly permeable due to the presence of pores. Neither
large proteins nor blood cells can fit through them.
2. Porous inner layer of Bowman capsule
- A basement membrane lies between the endothelial cells
of the glomerular capillaries and the podocytes of the
Bowman capsule.
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Characteristics of Renal Corpuscle
3. High pressure
An afferent arteriole supplies blood to the glomerulus for
filtration.
An efferent arteriole transports the filtered blood away from
the glomerulus.
Efferent arteriole has smaller diameter than afferent arteriole
creating a high pressure in the capillaries.
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Filtration Membrane
• The structures in the corpuscle make up the filtration
membrane.
• Consists of capillary endothelium, the basement
membrane, and the podocytes of the Bowman capsule
Filtrate is the fluid filtered from the glomerular capillaries.
Enters the lumen (cavity) inside the Bowman
capsule.
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Flow of Filtrate through Nephron
1. Renal corpuscle
2. Proximal convoluted tubule
3. Descending loop of Henle
4. Ascending loop of Henle
5. Distal convoluted tubule
6. Collecting duct
7. Papillary duct
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Urine Formation
1. Filtration
– occurs in the renal corpuscle, blood plasma leave
glomerulus and enters Bowman space
2. Tubular Reabsorption
– involves removing substances from the filtrate and placing
them back into the blood
3. Secretion
– involves taking substances from the blood at a nephron
area other than the renal corpuscle and putting back into the
nephron tubule
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Urine Formation-Filtration
• Movement of water, ions, small molecules through filtration
membrane into Bowman’s capsule
• 19% of plasma becomes filtrate
• 180 liters of filtrate are produced by the nephrons each
day
• 1% of filtrate (1.8 liters) become urine, the rest is
reabsorbed
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Urine Production-Reabsorption
• 99% of filtrate is reabsorbed and reenters circulation
• The proximal convoluted tubule is the primary site for
reabsorption of solutes and water
• The descending Loop of Henle concentrates filtrate
• Reabsorption of water and solutes from distal convoluted
tubule and collecting duct is controlled by hormones
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Urine Production—Secretion
• Ammonia secretion is passive.
• Secretion of H+, K+, creatinine, histamine, and penicillin is
by active transport.
• These substances are actively transported into the
nephron.
• The secretion of H+ plays an important role in regulating
the body fluid pH.
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Regulation of Urine Concentration and Volume
Renin-angiotensin-aldosterone
- regulates blood pressure; controls fluid & electrolyte
balance
Antidiuretic hormone (ADH)
- helps blood vessels constrict; controls amount of water &
salts in the body
Atrial natriuretic hormone
- acts on kidney to decrease sodium reabsorption
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Urine Movement
Micturition reflex:
• activated by stretch of urinary bladder wall
• action potentials are conducted from bladder to spinal cord
through pelvic nerves
• parasympathetic action potentials cause bladder to
contract
• stretching of bladder stimulates sensory neurons to inform
brain person needs to urinate
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