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Urinary System
Chapter 18
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2. Learning Objectives Lesson 18.1: Urinary System
Identify the major organs of the urinary system and give the generalized function of each.
Name the parts of a nephron and describe the role each component plays in the formation of urine.
Explain how the kidneys act as vital organs in maintaining homeostasis.
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3. Learning Objectives Lesson 18.1: Urinary System (Cont.)
Explain the importance of filtration, reabsorption, and secretion in urine formation.
Discuss the mechanisms that control urine volume, including the normal amount and composition of urine.
Explain the process of urine elimination.
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Kidneys
Location of the kidneys
Under back muscles, behind parietal peritoneum, just above waistline
Right kidney usually a little lower than left (Figure 18-1)
Gross structure: Internal anatomy (Figure 18-2)
Renal cortex
Outer layer of kidney substance
Renal medulla
Inner portion of kidney
Renal pyramids
Triangular divisions of medulla
Both kidneys are protected a bit by the lower rib cage.
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Kidneys (Cont.)
Gross structure: Internal anatomy
Renal papilla (pl. papillae)
Narrow, innermost end of pyramid
Renal pelvis
Expansion of upper end of ureter; lies inside kidney
Calyx (pl. calyces)
Division of renal pelvis
Once the peritoneum has been cut/opened, the potential for spread of infection throughout the entire abdominal cavity increases.
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Urinary System
Figure 18-1, A, shows the anterior view of urinary organs.
Figure 18-1, B, shows surface markings of the kidneys, eleventh and twelfth ribs, spinous processes of L1 to L4, and lower edge of pleura viewed from behind.
Figure 18-1, C, shows horizontal (transverse) section of abdomen, which shows the retroperitoneal position of the kidneys.
Figure 18-1, D, shows x-ray film of urinary organs.
B From Abrahams P, Marks SC, Hutchings RT: McMinn’s color atlas of human anatomy, ed 5, Edinburgh, 2003, Mosby.
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7. Internal Structure of the Kidney
Figure 18-2, A, shows an artist’s rendering of a coronal section of the kidney.
Figure 18-2, B, is a photomicrograph of a coronal section of a preserved human kidney.
From Abrahams PH, Spratt JD, Loukas M, van Schoor AN: McMinn & Abrahams’ clinical atlas of human anatomy, ed 7, Edinburgh, 2013, Elsevier Ltd.
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Kidneys (Cont.)
Microscopic structure
Nephrons are microscopic units of kidneys
Consist of renal corpuscle and renal tubule (Figure 18-3)
Renal corpuscle (Figure 18-4)
Bowman capsule
Cup-shaped top of a nephron
Glomerulus
Network of blood capillaries tucked into the Bowman capsule
More than a million microscopic units called nephrons make up each kidney’s interior.
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Kidneys (Cont.)
Microscopic structure
Renal tubule
Proximal convoluted tubule (PCT)
First segment of a renal tubule
Nephron loop (Henle loop)
Extension of proximal tubule; consists of straight descending limb, hairpin turn, and straight ascending limb
Distal convoluted tubule (DCT)
Part of tubal distal to ascending limb of nephron loop
Collecting duct (CD)
Straight part of a renal tubule
The shape of a nephron is unique, unmistakable, and admirably suited to its function of producing urine.
It looks like a tiny funnel with a very long stem.
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Location of Nephrons
Figure 18-3: Magnified wedge cut from a renal pyramid shows an example of a cortical nephron and a juxtamedullary nephron.
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11. Renal Corpuscle
Figure 18-4, A, is a schematic drawing that shows the relationship of glomerulus to Bowman capsule (together called the renal corpuscle) and adjacent structures.
Figure 18-4, B, is a scanning electron micrograph showing several glomeruli and their associated blood vessels.
Courtesy Andrew P Evan, University of Indiana.
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Kidneys (Cont.)
Overview of kidney function
Excretes toxins and nitrogenous wastes
Regulates levels of many chemicals in blood
Maintains water balance
Helps regulate blood pressure via secretion of renin
Secretes erythropoietin (EPO)
Homeostasis cannot be maintained if the kidneys fail and the condition is not soon corrected.
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Formation of Urine
Occurs by a series of three processes that take place in successive parts of nephron (Figure 18-6 and Table 18-1)
Filtration
Goes on continually in renal corpuscles
Glomerular blood pressure causes water and dissolved substances to filter out of glomeruli into Bowman capsule
Normal glomerular filtration rate 125 mL per minute
The kidneys have 2 million (or more) nephrons.
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Formation of Urine
Figure 18-6: Diagram shows examples of the steps in urine formation in successive parts of a nephron: filtration, reabsorption, and secretion.
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15. Functions of Parts of Nephron in Urine Formation
Table 18-1 shows the functions of parts of the nephron in urine formation.
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16. Formation of Urine (Cont.)
Reabsorption
Movement of substances out of renal tubules into blood in peritubular capillaries
Water, nutrients, and ions are reabsorbed
Water is reabsorbed by osmosis from proximal tubules
Secretion
Movement of substances into urine in the distal and collecting tubules from blood in peritubular capillaries
Hydrogen ions, potassium ions, and certain drugs are secreted by active transport
Ammonia is secreted by diffusion
The kidneys’ nephrons form urine by way of a combination of these three blood-balancing processes: filtration, reabsorption, and secretion.
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17. Control of Urine Volume
Antidiuretic hormone (ADH)
Secreted from the posterior pituitary gland
Decreases the amount of urine by collecting ducts permeable to water
Aldosterone
Secreted by the adrenal cortex
Stimulates the tubules to reabsorb sodium at a faster rate
Atrial natriuretic hormone (ANH)
Secreted from the heart’s atrial wall
Stimulates kidney tubules to secrete more sodium and thus lose more water
The body has ways to control the amount and composition of the urine that it excretes – mainly by controlling the amount of water and dissolved substances that are reabsorbed by the kidney tubules.
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18. Control of Urine Volume (Cont.)
Abnormalities of urine volume
Anuria: Absence of urine
Oliguria: Scanty amount of urine
Polyuria: Unusually large amount of urine
Because a change in urine volume/output is a significant indicator of many diseases, measurement of both fluid intake and output is necessary.
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Elimination of Urine
Ureters (Figure 18-10)
Structure
Narrow, long tubes with expanded upper end (renal pelvis) located inside kidney and lined with mucous membrane
Function
Drain urine from renal pelvis to urinary bladder
The renal pelvis is the basinlike upper end of the ureter located inside of the kidney.
Episodes of renal colic (pain caused by the passage of kidney stone) is painful for a couple of reasons, such as tearing or stretching of the urinary lining and cramping muscles.
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Ureter Cross Section
Figure shows the cross section of a ureter.
From Telser A, Young J, Baldwin K: Elsevier’s integrated histology, Philadelphia, 2008, Mosby.
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21. Elimination of Urine (Cont.)
Urinary bladder (Figure 18-11)
Structure
Elastic muscular organ, capable of great expansion
Lined with mucous membrane arranged in rugae, as is stomach mucosa
Functions
Storage of urine before voiding
Voiding
When the bladder is filled, its inner surface may stretch until it is smooth.
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22. Structure and Location of the Urinary Bladder
Figure 18-11, A, shows a frontal view of a fully distended male bladder dissected to show the interior.
Figure 18-11, B, shows a sagittal section of the female urinary system (left) and the male urinary system (right) showing relationship of the bladder to other anatomical structures.
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23. Elimination of Urine (Cont.)
Urethra
Structure
Narrow tube from urinary bladder to exterior
Lined with mucous membrane
Opening of urethra to the exterior called urinary meatus
Functions
Passage of urine from bladder to exterior of the body
Passage of male reproductive fluid (semen) from the body
Cystitis
Bladder infection
Urethra is a narrow tube; only about 4 cm long in a woman and about 8 cm long in a man.
In the man, a urethra has two functions: (1) terminal portion of urinary tract and (2) passageway for movement of semen.
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24. Elimination of Urine (Cont.)
Micturition
Passage of urine from body (also called urination or voiding)
Regulatory sphincters
Internal urethral sphincter (involuntary)
External urethral sphincter (voluntary)
Bladder wall permits storage of urine with little increase in pressure
Two sphincters act as valves that guard the pathway leading from the bladder.
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25. Elimination of Urine (Cont.)
Micturition
Emptying reflex
Initiated by stretch reflex in bladder wall
Bladder wall contracts
Internal sphincter relaxes
External sphincter relaxes, and urination occurs
Muscular wall of the bladder permits this organ to accommodate a considerable volume of urine with very little increase in pressure until a volume of 300 mL to 400 mL is reached.
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26. Elimination of Urine (Cont.)
Micturition
Urinary retention
Urine produced but not voided
Urinary suppression
No urine produced but bladder is normal
Urinary incontinence
Urine is voided involuntarily; a common bladder control problem in elderly people
May be caused by spinal injury or stroke
Retention of urine may cause cystitis
Micturition is a complex body function; it requires control and integration of both voluntary and involuntary nervous system components.
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Urinalysis
Physical, chemical, and microscopic examination of urine
Sign of a disease
Changes in normal characteristics of urine
Appearance of abnormal urine characteristics
Like blood, urine is a fluid that reveals much about the function of the body.
Urinalysis may help determine the presence and nature of a pathologic condition.
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Questions?
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