Histology of the Kidney and Urinary Tract

Question 1

What is the excretory function of the kidney?

Answer 1

• As the blood passes through the kidney, an ultrafiltrate is produced
• Excess water and ions, some drugs, toxins and metabolic breakdown products (urea, creatinine) are excreted in the urine

Question 2

What is the homeostatic function of the kidney?

Answer 2

• **Regulating and maintaining extracellular fluid volume and composition: **
  
  • selective secretion and re-absorption
    
    • water, ions, (e.g. Na+, K+, H+, Ca/P04 ) and other compounds
• Maintenance of acid-base balance
  
  • generation of bicarbonate
  • selective secretion of H+ ions

Question 3

What is the endocrine function of the kidney?

Answer 3

• Monitoring the O2 carrying capacity of the blood via erythropoietin
• Regulating blood pressure through the renin-angiotensin system

Question 4

Describe the following:

• Cortex:
• Medulla:
• Kidney lobe:

Answer 4

• Cortex:
  
  • granular in appearance and homogeneous in consistency
  • Medullary Rays: linear arrays of tubules extending into the cortex
• Medulla:
  
  • striated appearance and consists of 6-18 Renal Pyramids
  • Renal Papilla: apex or tip of a renal pyramid
• Kidney lobe:
  
  • ​macroscopic subdivision consisting of a renal pyramid and its surrounding cortex

Question 5

Describe the following:

• Lobule:
• Caspule:

Answer 5

• Lobule:
  
  • microscopic subdivision consisting of a medullary ray and the cortical tissue (primarily nephrons) on either side
  • tubules of these nephrons connect with the collecting ducts within the medullary rays
• Caspule:
  
  • consists mainly of fibrous connective tissue and surrounds the kidney
    
    • parenchyma is not subdivided by septa

Question 6

The total blood volume of the body passes through the kidneys every ….

Answer 6

4-5 minutes

• Note: kidney receives 20-25% of cardiac output

Question 7

How much fluid is extracted from the blood each minute?

How much is reabsorbed and how much is excreted?

Answer 7

125 ml of fluid is extracted from the blood each minute as filtrate [180 L/day]

• 124 ml is reabsorbed in the kidney tubules
• 1 ml is excreted as urine

Question 8

Arterial Supply to the kidney:

Answer 8

Renal A ⇒ Lobar A ⇒ Interlobar A ⇒ Arcuate A ⇒ Interlobular A ⇒ Afferent Arteriole

Question 9

What is the microvasculature of the kidney?

Answer 9

Afferent arteriole ⇒ glomerulus ⇒ efferent arteriole

1. Tubular Plexus
   
   • supplies tubules of the cortical nephrons
2. Vasa Recta long capillary loops
   
   • supplying tubules of juxtamedullary nephrons

Question 10

What are the components of the nephron? What is the function of the nephron?

Answer 10

1. Components:
   
   • Renal Corpuscle
   • Renal Tubule
2. Function: a Filter and a Fluid Modifier (Recycle/Secrete)
   
   • kidney produces an ultrafiltrate of the blood
   • recycles many components that are in the filtrate
   • other compounds are added to the filtrate as it goes through the tubular system

Question 11

What is the difference between the developmental and functional viewpoints of the nephron?

Answer 11

1. **Developmental Viewpoint ⇒ **nephron consists of:
   
   • Renal Corpuscle, Proximal Tubule, Loop of Henle, Distal Tubule [collecting ducts not included]
2. **Functional Viewpoint **⇒ nephron consists of:
   
   • Renal Corpuscle, Proximal Tubule, Loop of Henle, Distal Tubule & Collecting Duct
   • Whole structure = Uriniferous Tubule

Question 12

What are the components of the renal corpuscle?

Answer 12

1. Glomerulus
2. Visceral Layer of the Renal Capsule (Bowman’s)
3. Parietal Layer of the Renal Capsule
4. Mesangium

Question 13

What can be seen in the renal corpuscle at the EM level?

Answer 13

• spherical, double-layered sac (Renal Capsule) that surrounds a network of capillaries (Glomerulus)
• Vascular Pole where the arterioles enter and exit
• Urinary Pole that is continuous with the proximal convoluted tubule

Question 14

Renal corpuscles are found only in the ______ _____.

Answer 14

Renal corpuscles are found only in the kidney cortex

Question 15

What is the glomerulus? What supplies and drains it?

Answer 15

• a network of capillary loops supplied and drained by an arteriole
• Afferent (supplying) Arteriole is larger in diameter than Efferent (draining) Arteriole
  
  • Size difference creates a pressure differential that drives glomerular filtration

Question 16

Where are podocytes located?

Answer 16

visceral layer of Bowman’s capsule

Question 17

Describe the composition of Bowman’s Capsule:

Answer 17

• a double-layered epithelial sac surrounding the glomerulus
• Parietal Layer (outer) is a simple squamous epithelium
• Visceral Layer (inner) a simple epithelium composed of cells called Podocytes
• Urinary Space: space between the two epithelial layers
  
  • continuous with the proximal tubule
  • glomerular filtrate enters this space

Question 18

What makes up the glomerular filtration barrier?

Answer 18

• Capillary Endothelium
  
  • discontinuous, containing numerous 70-100 nm pores
  • pores are freely permeable to water and solutes ≤ 6-8 kD
  • moderately permeable to molecules 8-16kD
  • luminal surface has a negative charge because it is coated with a glycocalyx
• Basement Membrane
  
  • primary barrier that prevents protein from entering the glomerular filtrate

Question 19

What gives podocytes their name?

Answer 19

Pedicles: 1° and 2° foot processes

Question 20

The space between pedicles is called the _________ ____.

Answer 20

The space between pedicles is called the Filtration Slit.

Question 21

What bridges the Filtration Slit? What is the major protein?

Answer 21

Filtration Slit is bridged by an electron dense Filtration Slit Diaphragm, a modified adherens junction consisting of the protein Nephrin

Question 22

What is the function of the glomerular mesangium?

Answer 22

• physical support
• regulation of glomerular blood flow
• turnover of glomerular basement membrane

Question 23

What is the role of specialized pericyte/smooth muscle cells?

Answer 23

Found in the glomerular mesangium:

• contain receptors for atrial neuretic peptide (ANP) and angiotensin II
• secrete endothelin, cytokines and prostaglandins

Question 24

• Where does the convoluted portion of the proximal tubule begin?
• What is the function and histology of this portion?

Answer 24

• begins at the urinary pole and located in cortex
• Function and Histology:
  
  • substantial reabsorption
  • cuboidal/columnar cells with granular cytoplasm and basal nuclei
  • apical brush border w/glycocalyx obscures lumen
  • lysosomes and apical vesicles
  • numerous mitochondria at base of cell provide energy for transport
  • complex lateral interdigitations between epithelial cells make lateral cell membranes indistinguishable

Question 25

* What is another name for the **straight portion of the proximal tubule**? * What kind of cells make up this portion?

Answer 25

**Thick Descending Limb of Henle** * cuboidal epithelium

Question 26

* What are the parts of the **loop of Henle**? * Where is it located?

Answer 26

* **4 parts:** 1. **straight portion of the proximal tubule** (thick descending limb) 2. **thin descending** limbs 3. **thin ascending** limbs 4. **straight portion of the distal tubule** (thick ascending limb) * located in the **medulla**

Question 27

What determines the **length** of the **loop of Henle**?

Answer 27

Length is determined by the _location of its renal corpuscle with respect to the corticomedullary junction_ * **Cortical Nephrons** * external to the juxtamedullary zone * _short loops_ * **only a Descending Thin Limb** * **Juxtamedullary Nephrons** * _long looped_ * **Ascending and Descending Thin Limbs**

Question 28

What are the major differences between the thick and thin loops?

Answer 28

* _thick portions_ of the loop are lined with _cuboidal epithelium_ * _thin segments_ are lined with _simple squamous epithelium_ * cell membranes in the _ascending thin limb_ between epithelial cells are _interdigitated_, resulting in _water_ _impermeability_

Question 29

What is the **function** and **histology** of the **straight portion of the distal tubule**?

Answer 29

**Thick Ascending Limb** * **Histology:** * lined with _cuboidal epithelium_ * _scant microvilli_, _efficient tight junctions_ * Lateral & basal membane _interdigitations_ * _abundant mitochondria_ * **Function:** * **Impermeable to water** * _Na+, Cl-, and K+ reabsorbed_ * glucose, amino acids, proteins reabsorbed through facilitated transport * _H+ ions secreted_

Question 30

What is the **function** and **histology** of the **convoluted** **distal tubule**?

Answer 30

**Early Distal Tubule** * **Histology:** * lined with _cuboidal epithelium_ * _scant microvilli_ * fewer basal interdigitations * fewer mitochondria * **Function:** * Na+ (**Aldosterone responsive**), Cl-, K+, HCO3 reabsorbed * K+, urate, H+ ions, NH₃ secreted

Question 31

* What is the **histology** of the **collecting tubules**? * What does the portion repsond to?

Answer 31

* epithelium contains **principal cells** (cuboidal) * _transition segment_ between the nephron and the collecting duct * **Antidiuretic Hormone (ADH) dependent segment** * Na+ is reaborbed and K+ is secreted

Question 32

**Describe Renal Tubular Disease:** * Causes * Pathophysiology * Results in ....

Answer 32

* **Caused by** toxins, drugs, infections, metabolic disturbances, ischemia * **Affects reabsorptive and secretory functions** resulting in either _polyuria or oligo/anuria_ * **Renal failure** may develop due to _accumulation of toxic substances_ * **Acidosis** results because of _failure of H ion excretion_

Question 33

* How are **principal cells** composed? * How do the **principal cells** respond in the **presence of ADH**? * What happens if there is an **absence of ADH**?

Answer 33

* Principal Cells: * **one primary cilium** (flow sensor) * ADH sensitive **AQP-2 water channels** * **In the presence of ADH** * **​**_urea and water diffuse out_ of the collecting duct and into the renal interstitium * **increases urine tonicity** * **In the absence of ADH** * **​water is excreted** from the collecting ducts * leading to **Polyuria** and **hypotonic urine** * Diabetes Insipidus

Question 34

With regards to principal cells, polycystic kidney disease results from what?

Answer 34

defects in **Polycystin 1 & 2** * proteins that mediate the function of the primary cilium

Question 35

Where is the renal interstitium? What are the components?

Answer 35

* interstitial (stromal) tissue is found in the **renal cortex & medulla** * stroma is finer in cortex * interstitium components: 1. interstitial connective tissue 2. interstitial cells (fibroblasts) in cortex & medulla

Question 36

* What is the role of the tubular-interstitium-vascular interaction? * What are the components? * Describe the countercurrent multiplier and counter current exchanger:

Answer 36

* **provides a mechanism for modifying and concentrating urine** * Components: 1. Collecting ducts 2. Loops of Henle 3. Vasa Recta * **Countercurrent Multiplier** * urine concentration * **Countercurrent Exchanger** * protects ion gradient

Question 37

**JG Apparatus Components:**

Answer 37

1. **Renin producing (JG) cells** * Specialized smooth muscle cells in the wall of the afferent arteriole 2. **Extraglomerular mesangial (lactis) cells** * Connected to JG cells via _gap junctions_ 3. **Macula Densa** * columnar cells of the distal convoluted tubule * **detects Na+ and Cl- concentration for JG cells** resulting in _alterations of the filtration rate_ and _auto-regulation of blood volume_

Question 38

How can JG aparatus components affect systemic blood pressure and blood volume?

Answer 38

JG apparatus components can **increase** systemic blood pressure (BP) & blood volume (BV) through the **angiotensin system** Renin release ⇒ angiotensin conversion ⇒ ↑ in aldosterone secretion ⇒ ↑ Na and water reabsorption

Question 39

* Describe the role of erythropoiten: * What stimulates its production?

Answer 39

* **↑ mitosis of red blood cell precursors** * **↑ release of red cells from marrow** * Probably produced by cortical interstitial cells * Transported to bone marrow * **Production stimulated by:** * high altitude * hemorrhage * impaired pulmonary function

Question 40

* What is kidney failure? * Acute kidney injury vs. End-stage renal disease:

Answer 40

* **Kidney Failure** * _Inability of the kidney to remove accumulated metabolites from blood_ * Acute kidney injury * Clinical Picture - _oligouria_ \<400ml/day, _unexpected weight gain_ or _edema_, _increased toxins in blood_ * Prognosis depends on cause, severity, treatment, age * End-stage renal disease * Irreversible injury ⇒ end-stage renal disease ⇒ uremia + hematuria * Glomerular injury, autosomal dominant polycystic disease, others

Question 41

What are the **layers of the ureter**?

Answer 41

* **Mucosa** * _transitional (uro)epithelium_ * _lamina propria_ contains abundant _elastic tissue_ * **Muscularis** * _smooth muscle_ * in ureter -2 layers in the upper 2/3 of the ureter; 3 layers lower 1/3 of the ureter * **Adventitia** * _fibrous connective tissue_

Question 42

How is the urinary bladder composed?

Answer 42

* **Transitional epithelium** * 3 layers of smooth muscle

Question 43

What is the difference between the male and female urethra:

Answer 43

1. **Male** * 15-20 cm; 3 parts (prostatic, membranous, penile) * Transitional – pseudostratified sq. * Shared urinary & reproductive systems 2. **Female** * 3-5 cm * Transitional – pseudostratified sq. * Urinary system only

Question 44

What are some common clinical problems leading to obstruction of the excretory passages?

Answer 44

1. **Benign Prostatic Hypertropyhy** * also known as nodular hyperplasia * Older males \>45 years * Can cause urethral obstruction 2. **Renal Calculi** (kidney stones) * Common in USA (7-21/1000), men, sedintary individual * Hereditary disposition * Hypercalcemia, pH change, supersaturation of ions enhance stone formation 3. **Bladder Cancer** * Associated with smoking * Majority in US involve the uroepithelium