Lecture: Urinary System

Question 1

Le Milieu Intérieur

Answer 1

Internal environment
Coined by Claude Bernard

Question 2

Major functions of the urinary system

Answer 2

-Excretion of toxins and metabolic by-products
-Homeostasis of major ions
-Homeostasis of blood volume and osmolarity
-Acid-base balance
-Stimulation of erythrocyte production
-Synthesis of vitamin D

Question 3

What are the functional compartments of the kidney?

Answer 3

The medulla (renal pyramid) and the cortex

Question 4

Renal papilla

Answer 4

Where filtrate is collected in channels at the end of the renal medulla

Question 5

Minor calyx

Answer 5

Located at the tip of the renal papilla. Each pyramid corresponds to one minor calyx. They join to form the major calyx

Question 6

Renal column of Bertin

Answer 6

portion between the pyramids

Question 7

Hilum of the kidney

Answer 7

Middle of the kidney where the major renal artery enters and renal vein leaves

Question 8

What is the functional unit of the kidney? What is it composed of?

Answer 8

The nephron is the kidney functional unit. It is composed of a glomerulus and a kidney tubule

Question 9

Juxtamedullary nephron

Answer 9

• Very long loop of Henle
• Important for concentrating urine

Question 10

Cortical nephron

Answer 10

Located higher up in cortex

Question 11

What are the subdivisions of the kidney tubule?

Answer 11

Bowman’s capsule, Proximal convoluted, Thick descending, Thin descending and ascending, Thick ascending, Distal convoluted, Collecting duct

Question 12

Bowman’s capsule

Answer 12

receptacle where the network of capillaries of the glomerulus is found.
Like a glove that completely covers the network of capillaries.

Bowman’s capsule is composed of a visceral and a parietal layer
- Visceral layer is composed of epithelial cells known as podocytes
- Parietal layer is composed of squamous epithelial cells (continuous with proximal convoluted)

Question 13

Proximal convoluted tubule

Answer 13

• Exclusively found in the cortex
• Similar morphology to the thick descending limb
• Permeable to: glucose, ions, water, and amino acids
• Reabsorbs the molecules and sends them back to into the circulatory system

The proximal tubule plays a key role in the reabsorption of key nutrients (e.g. glucose; amino-acids, vitamins) and ions. Its brush border (microvilli) increases the surface and absorption area. Cells contain many mitochondria to sustain active transport. The proximal convoluted tubule is very long and is responsible for much of the reabsorption.

Question 14

Thick descending limb

Answer 14

• Crosses the junction between medulla and cortex
• Part of the Loop of Henle
• Similar morphology to the proximal convoluted tubule

Question 15

Thin ascending limb

Answer 15

• Thin ascending limb is NOT permeable to water
• No aquaporin channels
• It has specialized transporters to take out Na+ and Cl-

Question 16

Thick ascending limb

Answer 16

• Crosses the junction between medulla and cortex
• Similar morphology to the distal convoluted tubule
• Part of the Loop of Henle

Contains a large number mitochondria and Na+/K+ ATPases and regulate ion transport (i.e. Na+, Cl-, Ca++)

Morphological feature
* No brush border
Properties
* Impermeable to water
* Permeable to Na+/K+

Question 17

Distal convoluted tubule

Answer 17

• Only in the cortex
• Similar morphology to the thick ascending limb
• Permeable to Ca2+, Na+, and Cl- which flow out

Contains a large number mitochondria and Na+/K+ ATPases and regulate ion transport (i.e. Na+, Cl-, Ca++)

Morphological feature
* No brush border
Properties
* Impermeable to water
* Permeable to Na+/K+

Question 18

Collecting duct

Answer 18

• Has specialized channels, some of which are permeable to water, others are permeable to ions
• At this level, transport is regulated by hormones

Question 19

Why is the tubule very long and convoluted?

Answer 19

High surface area in order to maximize exchange

Question 20

ADH/Vasopressin

Answer 20

Binds to receptor to increase # of aquaporin channels in collecting duct so that H2O flows out

From Google:
Water balance: ADH helps the kidneys reabsorb water, reducing the amount of urine produced. ADH levels are usually higher at night to prevent urination while sleeping.

Question 21

Peritubular capillaries

Answer 21

In cortical nephron: capillaries around the convoluted tubules and the Loop of Henle

In juxtamedullary nephron: capillaries only around the convoluted tubules. Going further down, it becomes vasa recta instead.

Question 22

Vasa recta

Answer 22

“straight vessel”
capillaries around the Loop on Henle found ONLY in juxtamedullary nephrons

Question 23

How does the osmolarity of the medulla play a role in urine concentration?

Answer 23

The concentration of the medulla goes from 300 mOs (close to blood osmolarity) to 1200 mOs. This high osmolarity allows water to flow out and follow its concentration gradient

Question 24

Thin descending limb

Answer 24

• Part of the Loop of Henle
• Thin descending limb is permeable to water which flows out following its concentration gradient

Morphological features
- Extremely thin
- Few basal or luminal specializations (no microvilli)
- Not many active processes
Functional properties
- Permeable to water
- Little permeability to Na+
- Allows selective reabsorption of H2O

Question 25

Principal cells

Answer 25

Cells of the collecting duct that can re-absorb water and Na+ via the action of hormones (ADH and aldosterone respectively), contributing to control blood volume and blood pressure. Cells have only few and short microvilli Lecture Notes: The majority cell termed principal cell responsible for ADH-dependent water resorption through aquaporin receptors

Question 26

Intercalated A and B cells

Answer 26

Cells in collecting duct that modulate acid/base balance via H+ and HCO3 exchange. Cells are very rich in mitochondria for intense ion transport. B cells: specialized to transport bicarbonate ions A cells: H+ exchange From Lecture Notes The minority intercalated cell responsible for acid-base adjustment

Question 27

Renal corpuscle

Answer 27

Glomerulus + Bowman’s capsule

Question 28

Glomerulus

Answer 28

Big network of capillaries Has fenestrated endothelium

Question 29

Vascular pole

Answer 29

The side of the glomerulus where the afferent and efferent arteries are contained

Question 30

Urinary pole

Answer 30

The side of the glomerulus where the ultra-filtrate collects

Question 31

Podocytes

Answer 31

Epithelial cells that make up the visceral layer of the Bowman's capsule

Question 32

Pedicels

Answer 32

Finger-like structures formed from podocyte processes. Pedicels cover the surface of capillaries. Pedicels from two podocytes interdigitate to form filtration slits. This is done to contribute to the filtration of blood because the space (filtration slits) limit the movement of large molecules from the blood vessels to the urinary space. Podocyte prevents large molecules from leaving the capillaries and reaching the urinary space.

Question 33

Which structures make up the filtration barrier?

Answer 33

The filtration barrier involves: - endothelial pores - filtration slits - glomerular basement membrane (GBM) - endothelial glycocalyx - glycocalyx of podocytes

Question 34

Glomerular basement membrane (GBM)

Answer 34

The GBM is a fusion of the basal lamina of the endothelial cell and the podocyte

Question 35

The filtration barrier is based on which two criteria?

Answer 35

The filtration barrier is based on the size and charge of a molecule. Several molecules of the filtration barrier are negatively charged: - Sulfated proteoglycans - Heparan The charge barrier is due to negatively charged molecules that cover the podocyte and endothelial cells. Negatively charged molecules are less likely to cross the barrier

Question 36

Mesangial cells

Answer 36

3rd cell in glomerulus between the capillary and the podocyte - Have a phagocytic activity that contributes to remove cellular debris - Are specialized contractile pericytes that participate in the structural integrity of glomerular capillaries and contribute to modulate filtration - Release growth factors that maintain the integrity of the capillary network - Produce the glomerular matrix (or mesangial matrix, which is abnormally expanded in diabetic nephropathy) Functions as resident macrophage, smooth muscle and fibroblast for the glomerulus

Question 37

What are the three components of the juxtaglomerular apparatus (JGA)?

Answer 37

1. Macula densa 2. Juxtaglomerular cells 3. Extraglomerular mesangial cells From Lecture Notes: The point of close approximation of the returning distal tubule to its originating glomerulus is the location of structures collectively referred to as the juxtaglomerular apparatus. These structures are important in transmitting signals about the efficiency of filtration. The juxtaglomerular apparatus consists of three structures; one epithelial, one a cell of CT, and one a modified smooth muscle cell.

Question 38

Macula densa

Answer 38

The macula densa is a specialized region of the distal convoluted tubule near the vascular pole. (The distal convoluted tubule always circles back next to the Bowman's capsule) Macula densa cells detect Na+ levels in the lumen of the distal tubule and signal juxtaglomerular cells to release renin when Na+ is low (indicates a low filtration rate) Cells of the macula densa also contribute to increase prostaglandins levels via the activity of COX-1 and COX-2 enzymes (also in tubules and medullary interstitial fibroblasts) From Lecture Notes: a specialization of about 25 (in humans) of the distal tubule epithelial that are closest to its glomerulus of origin. The macula densa senses salt balance in the filtrate at its apical surface & communicates with the below cell types via paracrine signaling.

Question 39

Juxtaglomerular cells

Answer 39

Juxtaglomerular cells are specialized smooth muscle cells in the wall of the afferent (or efferent) arteriole. Juxtaglomerular cells release renin in response to a decrease in Na+. Renin increases the production of angiotensin II, which increase the reabsorption of Na+ and blood volume. Have granules that contain renin. From Lecture Notes: The last and first few smooth muscle cells in the wall of the afferent and efferent arterioles as they transition to the glomerular capillary. These cells are positioned to control the glomerular filtration rate both directly by changing the diameter of their associated vessel, and indirectly by the secretion of renin, which affects the renin-angiotensin-aldosterone system that controls systemic blood volume and vascular resistance.

Question 40

Extraglomerular mesangial cells

Answer 40

Extraglomerular mesangial cells are specialized mesangial cells that release signaling molecules contributing to regulate the glomerular filtration rate (and other functions!) From Lecture Notes: The nearby cells in the connective tissue compartment that form a cap over the vascular pole of the glomerulus, and thus sit generally between the other two structures. These cells have numerous poorly understood functions including being a resident macrophage, compartmentalizing the glomerulus, and affecting glomerular perfusion in a manner similar to capillary pericytes.

Question 41

What role does the juxtaglomerular apparatus play?

Answer 41

The juxtaglomerular apparatus plays a key role in the modulation of glomerular filtration rate by modulating the caliber of AA and EA The juxtaglomerular apparatus plays a key role in modulating blood volume and osmolarity by activating the renin-angiotensin system and Na+ transport

Question 42

What is the glomerular filtration rate (GFR)?

Answer 42

The rate at which blood is filtered smaller AA diameter = decreased GFR smaller EA diameter = increased GFR

Question 43

Prostaglandins

Answer 43

Acts on afferent arterioles Can increase GFR by increasing the dilation of afferent arterioles Kidney PGs are vasodilatory and can increase renal blood flow and glomerular filtration rate (GFR) by dilating afferent arterioles

Question 44

Angiotensin II

Answer 44

Acts on efferent arterioles Can increase GFR by decreasing the dilation of efferent arterioles

Question 45

Erythropoietin

Answer 45

Erythropoietin is released by specialized fibroblasts in response to a decrease in oxygen tension in the renal matrix. The number of erythropoietin-producing cells increase during hypoxia and at high altitude.

Question 46

Peritubular fibroblasts

Answer 46

Peritubular fibroblasts can differentiate into myofibroblasts in stress conditions and this can lead to kidney fibrosis. CT will start occupying more space at the detriment of the normal function of the kidney which is to filter blood.

Question 47

Renal sinus

Answer 47

Between the lobes, and continuous with the hilum is a connective tissue space termed the renal sinus, within which are the first branches of the major vessels, and the distal branches of the ureter, the major calyces (calyx s.) and minor calyces.