Ch 26 Kidney and Excretory System Notes

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Chapter 26

Urinary System

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26.1 Functions of the Urinary System • Filtering of blood: involves three processesfiltration, reabsorption, secretion. • Regulation of – Blood volume – Concentration of blood solutes: Na+, Cl-, K+, Ca2+, HPO4-2 – pH of extracellular fluid: secrete H+ – Blood cell synthesis

• Synthesis of vitamin D 26-3

26.2 Kidney Anatomy and Histology Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Kidney Ureter Urinary bladder Urethra

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Location and External Anatomy of Kidneys • Location – Lie behind peritoneum (retroperitoneal) on posterior abdominal wall on either side of vertebral column – Lumbar vertebrae and rib cage partially protect – Right kidney slightly lower than left

• External Anatomy – Renal capsule: fibrous connective tissue. Surrounds each kidney – Perirenal fat • Engulfs renal capsule and acts as cushioning

– Renal fascia: thin layer loose connective tissue • Anchors kidneys and surrounding adipose to abdominal wall

– Hilum • Renal artery and nerves enter and renal vein and ureter exit kidneys • Opens into renal sinus (cavity filled with fat and loose connective tissue) 26-5

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Liver Spleen Adrenal glands

Renal artery Renal vein

Tenth rib

Left kidney Right kidney Inferior vena cava

Abdominal aorta Ureters Common iliac vein Common iliac artery

Urinary bladder

Urethra

(a)

Body wall

Anterior view

Anterior

Parietal peritoneum Renal vein Peritoneal cavity Renal artery

Liver Inferior vena cava

Renal fascia

Abdominal aorta Adipose tissue

Psoas major muscle

Renal capsule

Vertebra Back muscle (b)

Kidney Posterior Inferior view

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Internal Anatomy of Kidneys • Cortex: outer area Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Renal capsule Cortex Medulla Artery and vein in the renal sinus

Segmental artery

Renal sinus (space)

Hilum (indentation)

Renal pyramid

Renal artery Renal vein

Renal papilla Minor calyx

Renal pelvis

Major calyx Renal column Medullary rays (a)

Ureter

– Renal columns: part of cortical tissue that extends into medulla

• Medulla: inner area; surrounds renal sinus – Renal pyramids: cone-shaped. Base is boundary between cortex and medulla. Apex of pyramid is renal papilla, points toward sinus.

• Calyces – Minor: papillae extend into funnel of minor calyx – Major: converge to form pelvis

• Pelvis: enlarged chamber formed by major calyces • Ureter: exits at the hilum; connects to urinary bladder

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The Nephron • Functional and histological unit of the kidney • Parts of the nephron: Bowman’s capsule, proximal tubule, loop of Henle (nephronic loop), distal tubule • Urine continues from the nephron to collecting ducts, papillary ducts, minor calyses, major calyses, and the renal pelvis • Collecting ducts, parts of the loops of Henle, and papillary ducts are in the renal medulla

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glomerulus Bowman capsule

Renal corpuscle (cut)

Proximal convoluted tubule Nephron Loop of Henle Distal convoluted tubule Proximal convoluted tubule Distal convoluted tubule

Renal corpuscle

Blood supply Juxtamedullary nephrons have loops of Henle that extend deep into the medulla.

Cortical nephrons have loops of Henle that do not extend deep into the medulla.

Cortex

Thick segment ascending limb Loop of Henle

Thin segment ascending limb Renal pyramid of the medulla

Thin segment descending limb

Collecting ducts

Papillary duct

Renal papilla

To a minor calyx

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Renal Corpuscle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

• Bowman’s capsule: cup-shaped structure at the beginning of the nephron • Glomerulus: network of capillaries. Blood enters through afferent arteriole, exits through efferent arteriole.

Renal corpuscle

Bowman capsule Glomerulus

Proximal convoluted tubule

Afferent arteriole

Distal convoluted tubule

Efferent arteriole

(a) The renal corpuscle consists of the Bowman capsule and the glomerulus. The Bowman capsule is the enlarged end of a nephron, which is indented to form a double-walled chamber. The Bowman capsule surrounds the glomerulus, which is a network of capillaries. Blood flows from the afferent arteriole into the glomerulus and leaves the glomerulus through the efferent arteriole.

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Histology of the Nephron Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Renal corpuscle

Bowman capsule Glomerulus

(a) Juxtamedullary nephron

Proximal convoluted tubule Distal convoluted tubule

• Proximal tubule: simple cuboidal epithelium with many microvilli • Loops of Henle

Nucleus Mitochondrion Microvilli

Basement membrane

(d) Distal convoluted tubule. The cells have sparse microvilli and numerous mitochondria, and they actively reabsorb Na+, K+, and Cl–. Ascending limb, loop of Henle

Collecting duct

Invagination

• Distal tubule: shorter than proximal tubule.

Descending limb, loop of Henle

Mitochondrion

Basement membrane Tight junction

Nucleus

(b) Proximal convoluted tubule. The luminal surface of the epithelial cells is lined with numerous microvilli. The basal surface of each cell rests on a basement membrane, and each cell is bound to the adjacent cells by tight junctions. The basal margin of each epithelial cell has deep invaginations, and numerous mitochondria are adjacent to the basal membrane. Active reabsorption and secretion are major functions.

Microvilli

• Collecting ducts: form where many distal tubules come together. Larger in diameter,. Form medullary rays and lead to papillary ducts

Mitochondrion Nucleus Basement membrane

(e) Collecting duct. The cells have some microvilli and numerous mitochondria, and they actively reabsorb Na+, K+, and Cl–. Papillaryduct

Mitochondrion Microvilli

Nucleus Basement membrane

(c) Descending limb of the loop of Henle. The thin segment of the descending limb is composed of simple squamous epithelial cells that have microvilli and contain a relatively small number of mitochondria. Water easily diffuses from the thin segment into the interstitial fluid.

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Arteries and Veins of the Kidneys Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Arterial supply: 1. Renal arteries branch from abdominal aorta

5. Interlobular artery

4. Arcuate artery 3. Interlobar artery 2. Segmental artery

11. Interlobular vein

12. Arcuate vein

13. Interlobar vein

1. Renal artery 14. Renal vein Medulla Cortex

Ureter

Renal pyramid Renal column

(a)

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Arteries and Veins of the Kidneys •

6. 7. 8. 9.

The part of the circulation involved with urine formation Afferent arterioles supply blood to glomerulus Glomerulus Efferent arterioles exit the renal corpuscle Vasa recta: capillaries that course into medulla along with loops of Henle, then back toward cortex (peritubular capillaries)

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8. Efferent arteriole

Proximal convoluted tubule

Distal convoluted tubule

7. Glomerulus 6. Afferent arteriole

Bowman capsule

9. Peritubular capillaries (blood flows to the vasa recta or directly to the interlobular veins)

5. Interlobular artery Arcuate artery 11. Interlobular vein Arcuate vein

Ascending limb, loop of Henle Descending limb, loop of Henle 10. Vasa recta

Collecting duct

(b)

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Arteries and Veins of the Kidneys Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

• Venous drainage 11. Renal veins

5. Interlobular artery

4. Arcuate artery 3. Interlobar artery 2. Segmental artery

11. Interlobular vein

12. Arcuate vein 13. Interlobar vein

1. Renal artery 14. Renal vein Medulla Cortex

Ureter

(a)

Renal pyramid Renal column

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26.3 Urine Production Nephrons considered functional units of the kidney: smallest structural component capable of producing urine Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Urine formation results from the following three processes: 1 Filtration

2

Tubular reabsorption

3 Tubular secretion

Filtration (blue arrow) is the movement of materials across the filtration membrane into the Bowman capsule to form filtrate. Solutes are reabsorbed (purple arrow) across the wall of the nephron into the interstitial fluid by transport processes, such as active transport and cotransport. Water is reabsorbed (orange arrow) across the wall of the nephron by osmosis. Water and solutes pass from the interstitial fluid into the peritubular capillaries.

Peritubular capillaries Interstitial fluid 2 3 1 Filtrate

Rest of the nephron Bowman capsule Glomerular capillaries

To interlobular veins Urine

Renal corpuscle

Efferent arteriole Afferent arteriole

Solutes are secreted (green arrow) across the wall of the nephron into the filtrate.

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Filtration • Movement of fluid, derived from blood flowing through the glomerulus, across filtration membrane • Filtrate: water, small molecules, ions that can pass through membrane • Pressure difference forces filtrate across filtration membrane • Glomerular filtration rate (GFR): amount of filtrate produced each minute. 180 L/day • Average urine production/day: 1-2 L. Most of filtrate must be reabsorbed

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Tubular Reabsorption: Overview • Tubular reabsorption: occurs as filtrate flows through the lumens of proximal tubule, loop of Henle, distal tubule, and collecting ducts • Results because of – – – – –

Diffusion Facilitated diffusion Active transport Symport Osmosis

• Substances transported to interstitial fluid and reabsorbed into peritubular capillaries: inorganic salts, organic molecules, 99% of filtrate volume. These substances return to general circulation through venous 26-16 system

Urine Production • In Proximal convoluted tubules – Na+ and other substances removed – Water follows passively – Filtrate volume reduced

• In descending limb of loop of Henle – Water exits passively, solute enters – Filtrate volume reduced 15%

• In ascending limb of loop of Henle – Na+, Cl-, K+ transported out of filtrate – Water remains

• In distal convoluted tubules and collecting ducts – Water movement out regulated by ADH • If absent, water not reabsorbed and dilute urine produced • If ADH present, water moves out, concentrated urine produced 26-17

Urine Concentration Mechanism • When large volume of water consumed – Eliminate excess without losing large amounts of electrolytes – Response is that kidneys produce large volume of dilute urine

• When drinking water not available – Kidneys produce small volume of concentrated urine – Removes waste and prevents rapid dehydration

• Mechanisms that create urine of variable concentration – Maintenance of high concentration of solutes in medulla – Countercurrent functions of loops of Henle – Control of permeability of distal nephron to water 26-18

Urea • Responsible for large part of high osmolality in medulla • Descending limbs of loops of Henle permeable to urea; urea diffuses into interstitial fluid • Ascending limbs and distal tubules impermeable to urea • Collecting ducts permeable to urea; some diffuses out into interstitial fluid • Urea flows in a cycle maintaining high urea concentration in medulla

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Filtrate flow

Descending limb, loop of Henle

Ascending limb, loop of Henle Collecting duct

Urea

Thin segment Urea is excreted in the urine. Urea contributes to the interstitial fluid solute concentration and reenters the thin segments of the loop of Henle.

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Urine Movement • Hydrostatic pressure forces urine through nephron • Peristalsis moves urine through ureters from region of renal pelvis to urinary bladder. Occur from once every few seconds to once every 2-3 minutes – Parasympathetic stimulation: increase frequency – Sympathetic stimulation: decrease frequency

• Ureters enter bladder obliquely through trigone. Pressure in bladder compresses ureter and prevents backflow 26-20

Anatomy and Histology of Ureters and Bladder • Ureters: bring urine from renal pelvis to urinary bladder. • Urinary bladder: hollow muscular container. In pelvic cavity posterior to symphysis pubis.

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Transitional epithelium

(a)

Kidney

Ureter

Connective tissue (lamina propria) Smooth muscle layer Connective tissue (adventitia) (b)

Parietal peritoneum Urinary bladder Opening of ureter Trigone Opening of urethra Location of the external urethral sphincter

Transitional epithelium Connective tissue (lamina propria) Smooth muscle layer (detrusor muscle) Connective tissue (adventitia) (c)

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Anatomy and Histology of Urethra • Male: extends from the inferior part of the urinary bladder through the penis • Female: shorter; opens into vestibule anterior to vaginal opening • Internal urinary sphincter: in males, elastic connective tissue and smooth muscle keep semen from entering urinary bladder during ejaculation • External urinary sphincter: skeletal muscle surrounds urethra as it extends through pelvic floor. Acts as a valve

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Micturition Reflex Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cerebrum 1 Urine in the urinary bladder stretches the bladder wall. 2 Action potentials produced by stretch receptors are carried along pelvic nerves (green line) to the sacral region of the spinal cord. 3 Action potentials are carried by parasympathetic nerves (red line) to contract the smooth muscles of the urinary bladder. 4 Ascending pathways carry an increased frequency of action potentials up the spinal cord to the pons and cerebrum when the urinary bladder becomes stretched. This increases the conscious urge to urinate. 5 Descending pathways carry action potentials to the sacral region of the spinal cord to tonically inhibit the micturition reflex, preventing automatic urination when the bladder is full. Descending pathways facilitate the reflex when stretch of the urinary bladder produces the conscious urge to urinate. This reinforces the micturition reflex. 6 The brain voluntarily controls the external urethral sphincter through somatic motor nerves (purple), causing the sphincter to relax or constrict.

Pons

4

5

Ascending pathways

Descending path ways

2 Sacral region of spinal cord

Pelvic nerves

Parasympathetic nerves 3

Ureter

Somatic motor nerves

1

6 Urinary bladder

External urethral sphincter

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26.7 Effects of Aging • Gradual decrease in size of kidneys, but only onethird of one kidney necessary for homeostasis • Amount of blood flowing through gradually decreases • Number of glomeruli decrease and ability to secrete and reabsorb decreases • Ability to concentrate urine declines and kidney becomes less responsive to ADH and aldosterone • Reduced ability to participate in vitamin D synthesis contributing to Ca2+ deficiency, osteoporosis, and bone fractures 26-25