Functions of the Urinary System
Kidneys dispose of waste products in urine |
Kidneys’ regulatory functions include: |
Nitrogenous wastes |
Production of renin to maintain blood pressure |
Toxins |
Production of erythropoietin to stimulate red blood cell production |
Drugs |
Conversion of vitamin D to its active form |
Excess ions |
The urinary system consists of kidneys, ureters, urinary bladder, and urethra.
Kidneys
The kidneys lie against the posterior abdominal wall, underneath the 12th rib.
They are retroperitoneal (posterior to the parietal peritoneum).
• The ribs help protect the kidneys, as does a heavy cushion of fat encasing each
organ.
• Structures (such as blood vessels, the ureters, and nerves) enter and leave the
kidney through a slit called the hilum—located in a concave notch on the medial
side.
Interior of the Kidney
A tough, fibrous capsule surrounds each kidney. The interior consists of the renal cortex and renal medulla; the renal cortex forms the outer region and the renal medulla forms the inner region. |
Rrenal columns extened from the renal cortex,and divide the interior into cone-shaped sections, renal pyramids. The base of each pyramid faces outward toward the cortex; the point of the pyramid, renal papilla,faces the hilum. The renal papilla extends into a cup called a minor calyx. |
The calyx collectsV urine leaving the papilla. Two or three minor calyces join together to form a major calyx The major calyces converge to form the renal pelvis, which receives urine from the major calyces. The renal pelvis continues as the ureter, which channels urine to the urinary bladder |
Blood Flow Through Kidneys
Aorta → Renal Artery → Afferent Arterioles → Glomerulus → Efferent Arterioles → Pelvic Capillaries → Renal Vein → Inferior Venal Cava → Right Atrium |
Kidney Protection
Three protective layers enclose the kidney |
Fibrous capsule encloses each kidney |
Perirenal fat capsule surrounds the kidney and cushions against blows |
Renal fascia is the most superficial layer that anchors the kidney and adrenal gland to surrounding structures |
Renal Tubule
Arising from Bowman’s capsule is the proximal convoluted tubule. |
The renal tubule straightens out and dips into the medulla before turning sharply and returning to the cortex. |
The collecting duct receives drainage from the distal convoluted tubules of several different nephrons. |
Thousands of microvilli that allow absorption line the inside of the proximal convoluted tubule |
his entire segment—which consists of a descending limb and an ascending limb—is called the loop of Henle |
The collecting duct passes through the medulla and merges with other collecting ducts before draining into a minor calyx. |
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After returning to the cortex, the ascending limb coils again, forming the distal convoluted tubule. |
Renal Circulation – Blood Supply
One-quarter of the total blood supply of the body passes through the kidneys each minute |
Renal artery provides each kidney with arterial blood supply |
Renal artery divides into segmental arteries → interlobar arteries → arcuate arteries → cortical radiate arteries |
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As it enters the kidney, it divides into smaller and smaller arteries. The arteries pass through the renal columns and into the renal cortex. |
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Blood leaves the kidney through the renal vein, which empties into the inferior vena cava. |
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Nephrons—the filtration units of the kidney—primarily lie in the kidney’s outer region (the cortex). Loops from the nephron dip into the inner region (the medulla). |
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Filtrate from Glomerulus to Urine Excretion
1. Glomerulus --->
2. Bowman's capsule --->
3. Proximal Convoluted Tubule --->
4. Loop of Henle --->
5. Distal Convoluted Tubule --->
6. Collecting Duct --->
7. Renal Pelvic (minor and major calyxes) --->
8. Ureters --->
9. Urinary Bladder --->
10. Urethra |
Renal Tubule
Extends from
glomerular capsule and
ends when it empties
into the collecting duct
▪ Subdivisions
• Proximal convoluted tubule
(PCT)
• Nephron loop (loop of Henle)
• Distal convoluted tubule
(DCT)
Renal Corpuscle
Bowman’s capsule (or glomerular capsule) consists of two layers of epithelial cells surrounding the glomerulus in an open-ended covering. ( |
Fluid filters out of the glomerulus and collects in the space between the two layers of Bowman’s capsule. |
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From there, it flows into the renal tubule on the other side of the capsule. |
The renal corpuscle consists of a glomerulus and Bowman’s capsule.
Renal Corpuscle
The renal corpuscle is the beginning of the nephron. (The two main components of
a nephron are a renal corpuscle [which filters blood plasma] and a renal tubule
[where urine is formed])
Nephron
Nephrons are the filtration units of the kidney |
Afferent Arterioles - arise from smaller arteries in the cortex; each afferent arteriole supplies blood to one nephron |
Blood leaves the glomerulus through an efferent arteriole |
Nephrons need a constant flow of blood. (More than 20% of the blood pumped by the heart each minute goes to the kidneys.) |
Earise from smaller arteries in the cortex; each afferent arteriole supplies blood to one nephron, glomerulus |
The efferent arteriole leads to a network of capillaries around the renal tubules, peritubular capillaries |
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These capillaries pick up water and solutes reabsorbed by the renal tubules. |
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Blood flows from the peritubular capillaries into larger and larger veins that eventually feed into the renal vein |
▪ Structural and functional units of the kidneys
▪ Each kidney contains over a million nephrons
▪ Each nephron consists of two main
structures
• Renal corpuscle
• Renal tubule
Renal Circulation – Venous
Venous blood flow |
Cortical radiate veins → arcuate veins → interlobar veins → renal vein |
Renal vein returns blood to the inferior vena cava |
Renal Circulation – Blood Supply
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Renin-Angiotensin-Aldosterone System
Urine Formation
Glomerular filtration |
Tubular reabsorption |
Tubular secretion |
Blood flows into the glomerulus through the afferent arteriole and leaves through the efferent arterioles |
After filtrate leaves the glomerulus, it enters the renal tubules, where additional chemicals are removed from the filtrate and returned to the blood |
Chemicals are added to the filtrate |
The walls of glomerular capillaries are dotted with pores; water and small solutes (such as electrolytes, glucose, amino acids, vitamins, and nitrogenous wastes) filter out of the blood and into Bowman’s capsule. |
Sodium, water, glucose, amino acids, chloride, potassium, and bicarbonate moves by active transport out of the proximal convoluted tubule and into the bloodstream of the peritubular capillaries |
Wastes such as ammonia (NH3 ) and uric acid, as well as drugs (such as aspirin and penicillin), are secreted out of the blood and into the tubules. |
. Blood cells and most plasma proteins are too large to pass through the pores. The fluid that filters into Bowman’s capsule flows into the renal tubules. |
About half of the nitrogenous waste urea is also reabsorbed. |
Tubular secretion of hydrogen ions also occurs, helping to regulate the body’s pH. |
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Sodium and chloride are actively pumped out of the ascending limb of the loop of Henle into interstitial fluid. |
Water diffuses out of the descending limb of the loop of Henle, further concentrating the filtrate |
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The distal convoluted tubule and collecting ducts reabsorb variable amounts of water and salts. |
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The distal convoluted tubule and collecting ducts reabsorb variable amounts of water and salts. |
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Specialized cells play a role in acid-base balance, reabsorbing potassium and secreting hydrogen into the tubule. |
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Several different hormones 23 Tubular Reabsorption and Secretion View animation on “Urine formation” Copyright ©2020 F.A. Davis Company Chapter 19: Urinary System 23 help regulate reabsorption by the cells in the distal convoluted tubule. |
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The collecting duct reabsorbs water and concentrates the filtrate, resulting in urine. |
Urine formation involves three processes:
1. Glomerular filtration
2. Tubular reabsorption
3. Tubular secretion
Basic Stages of Urine Formation
1. Glomerular Filtration:
Creates a plasma-like filtrate of the blood.
2. Tubular Reabsorption:
Removes useful solutes from the tubular
fluid, returning them to the blood
3. Tubular Secretion:
Removes additional waste from the blood,
adds them to the tubular fluid
4. Water Conservation:
Removes water from the urine and returns
it to the blood; concentrates waste
Urine Composition
Color – the color is generally yellow but varies in degree and clarity |
Odor – freshly voided urine should have no particular order unless affected by certain ingredients. Urine that has sat smells of ammonia |
pH – pH is generally acidic but can range from 4.5-8.0 |
Volume – 0.75L – 2L/24 hours |
Specific Gravity – 1.003 – 1.032 |
Composition of Urine
Consists of 95% water; 5% dissolved substances |
Reveals information about health of kidneys and other organs |
The dissolved substances include nitrogenous wastes—such as urea, uric acid, ammonia, and creatinine—as well as other solutes, such as sodium, potassium, and sulfates. |
Solutes in Urine
Solutes normally found in urine |
Solutes NOT normally found in urine |
Sodium and potassium ions |
Glucose |
Urea, uric acid, creatinine |
Blood proteins |
Ammonia |
Red blood cells |
Bicarbonate ions |
Hemoglobin |
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WBCs (pus) |
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Bile |
Hormones That Affect the Urinary System
• Aldosterone, ANP, and PTH all act on the renal tubules; ADH acts primarily on the
collecting duct.
• The adrenal cortex secretes aldosterone when blood levels of Na+ decline or
concentration of K+
increases.
• The atria of the heart secretes ANP when blood pressure rises; this inhibits the
secretion of aldosterone and ADH.
• The posterior pituitary gland secretes ADH.
• The parathyroid glands secrete PTH in response to low calcium levels.
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Urinary System
The ureters and urethra serve as passageways for conducting urine away from the
kidneys and out of the body while the bladder stores urine until it can be
eliminated.
The ureters connect the renal pelvis of each kidney with the bladder
Ureters
Attaching the kidney to the urinary bladder |
Peristalsis aids gravity in urine transport |
Continuous with the renal pelvis |
Enter the posterior aspect of the urinary bladder |
Run behind the peritoneum |
Urinary Bladder
Smooth, collapsible, muscular sac situated posterior to the pubic symphysis |
Trigone—triangular region of the urinary bladder base based on three openings |
Mucous transitional epithelium lines the bladder. When the bladder relaxes, this layer forms folds, rugae. As urine fills the bladder, the rugae flatten and the epithelium thins, allowing the bladder to expand. |
Two openings from the ureters (ureteral orifices) |
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One opening to the urethra (internal urethral orifice) |
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At the point where the urethra leaves the bladder is a ring of smooth muscle that forms the internal urethral sphincter. This sphincter contracts involuntarily to retain urine in the bladder. |
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External urinary sphincter exists where the urethra passes through the pelvic floor; this sphincter consists of skeletal muscle and is, therefore, under voluntary control. |
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The urethra is a small tube that conveys urine away from the bladder and out of the body; it opens to the outside of the body at the external urinary meatus. |
▪ In males, the prostate
surrounds the neck of the
urinary bladder
Urinary Bladder
Smooth, collapsible, muscular sac situated posterior to the pubic symphysis |
Trigone—triangular region of the urinary bladder base based on three openings |
Mucous transitional epithelium lines the bladder. When the bladder relaxes, this layer forms folds, rugae. As urine fills the bladder, the rugae flatten and the epithelium thins, allowing the bladder to expand. |
Two openings from the ureters (ureteral orifices) |
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One opening to the urethra (internal urethral orifice) |
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At the point where the urethra leaves the bladder is a ring of smooth muscle that forms the internal urethral sphincter. This sphincter contracts involuntarily to retain urine in the bladder. |
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External urinary sphincter exists where the urethra passes through the pelvic floor; this sphincter consists of skeletal muscle and is, therefore, under voluntary control. |
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The urethra is a small tube that conveys urine away from the bladder and out of the body; it opens to the outside of the body at the external urinary meatus. |
▪ In males, the prostate
surrounds the neck of the
urinary bladder
Urethra
Thin-walled tube that carries urine from the urinary bladder to the outside of the body by peristalsis
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