Chapter 26: The Urinary System

Question 1

List the function of the kidneys (8)

Answer 1

1. Regulation of blood ionic composition
2. Regulation of blood pH - release H+ and conserve HCO3- to regulate pH
3. Regulation of blood volume - occurs by conserving/eliminating water
4. Regulation of blood pressure - release renin which increases BP
5. Maintenance of blood osmolarity - regulates loss of water and solutes
6. Production of hormones - produce calcitriol and erythropoietin
7. Regulation of blood glucose level - can use glutamine in gluconeogenesis
8. Excretion of wastes and foreign substances - forms urine

Question 2

Describe the appearance and location of kidneys

Answer 2

Pairs of reddish, kidney bean-shaped organs located just above the east between the peritoneum and posterior wall of the abdomen

Since their position is posterior to the peritoneum of the abdominal cavity, the organs are said to be retroperitoneal

The right kidney is slightly lower than the left due to the liver

Question 3

Define the renal hilum

Answer 3

External anatomic feature

indentation near the centre of the concave border of kidney through which the ureter emerges from the kidney along with blood vessels, lymphatic vessels, and nerves

Question 4

Describe the 3 layers of tissue that surround each kidney

Answer 4

1. Renal capsule (deep) - smooth, transparent sheet of dense irregular connective tissue that is continuous with the outer coat of the ureter; barrier against trauma and helps maintain shape
2. Adipose capsule (middle) - mass of fatty tissue surrounding the renal capsule protects from trauma and holds it firmly in place in the abdominal cavity
3. Renal fascia (superficial) - thin layer of dense irregular connective tissue the anchors the kidney to the surrounding structures and abdominal wall

Question 5

Describe the two internal distinct anatomical regions of the kidney: the renal cortex and renal medulla

Answer 5

Renal cortex - superficial smooth light red region that is divided into an outer cortical zone and an inner juxtamedullary zone

Renal Medulla - deep darker reddish-brown inner region that consists of several cone-shaped renal pyramids - their apex (narrow inner region), called renal papilla, points toward the renal hilum

Question 6

Define renal comulms

Answer 6

Portions of the renal cortex that extend between renal pyramids

Question 7

Define renal lobe

Answer 7

Consists of a renal pyramid, some of the renal column on either side of the renal pyramid, and the renal cortex at the base of the renal pyramid

Question 8

Define parenchyma

Answer 8

The functional portion of the kidney that constitutes the renal cortex and renal pyramids of the renal medulla

Within parenchyma are functional unites called nephrons

Question 9

Describe the following components of the kidneys: papillary ducts, minor and major calyces, renal pelvis, and renal sinus

Answer 9

Papillary ducts collect filtrate (filtered fluid) formed by the nephrons and they extent through the renal paella of the pyramids and drain into the minor and major calyces

A minor calyx receives urine from the papillary function of one renal papilla and delivers it to a major calyx - at this stage it is urine as no further reabsorption can occur

Urine trains into a single large cavity called the renal pelvis and then out through the ureter to the urinary bladder

The hilum expands into a cavity within the kidney called the renal sinus, which contains parts of the renal pelvis, the calyces, and branches of the renal blood vessels and nerves

Question 10

Describe the blood and nerve supply of the kidneys

Answer 10

Renal artery -> segmental arteries -> interlobular arteries -> arcuate arteries -> cortical radiate arteries -> branch to form afferent arteries -> glomerus (ball-shaped capillary network) -> reunite to form efferent arteries -> divide into peritubular capillaries -> vasa recta (large loop-shaped capillaries) -> reunite to form cortical radiate veins -> arcuate veins -> interlobular veins -> blood leaves kidney though a single renal vein that exits at the renal houma and caries venous blood to inferior vena cava

Question 11

Describe the renal corpuscle and renal tubule of a nephron

Answer 11

Renal corpsucle - where blood plasma is filtered; it has 2 components:

• Glomerulus (capillary network)
• glomerular capsule or Bowman’s capsule, a double walled epithelial cup that surrounds the glomerular capillaries

Renal tubule - where filtered fluid passes; has 3 main sections:

• Proximal convoluted tubule (PCT) - attached to glomerular capsule and coiled
• Nerphon loop (loop of henle) - extends into renal medulla and returns to cortex
• Distal convoluted tubule (DCT) - further away from capsule and coiled

Question 12

Define the collecting duct (CD)

Answer 12

A single duct that distal convoluted tubules of several nephrons empty into

These then unite and converge into several hundred papillary ducts, which drain into the minor calyces

Question 13

Differentiate between the descending and ascending limb of the nephron loop

Answer 13

Descending - begins in the renal cortex and extends downward into the renal medulla

Ascending - the loop makes a turn from the medulla and returns to the renal cortex where it terminates at the distal convoluted tubule

Question 14

Differentiate between cortical and juxtamedullary nephrons

Answer 14

Cortical (80-85%) - renal corpuscles lie in the outer portion of the cortex and they have short nephron loops that penetrate only into the outer region of the renal medulla

juxtamedullary (15-20%) - renal corpuscles lie deep in the cortex, close to the medulla, and their have a long nephron loop that extends into the deepest region of the medulla

Question 15

What are the two portions of the ascending limb of the nephron loop of juxtamedullary nephrons

Answer 15

1. Thin ascending limb - thinner epithelium

2. Thick ascending limb

Question 16

Describe the histological features of the glomerular capsule

Answer 16

Consists of visceral and parietal layers:

• visceral layer consists of modified simple squamous epithelial cells called podocytes that have foot like projections that wrap around the single layer of endothelial cells of the glomerular capillaries
• parietal layer consists of simple squamous epithelium and forms the outer wall of the capsule

Fluid filtered from the capillaries enters the capsular space between the two layers, which is the lumen of the urinary tube

Question 17

Describe the histological features of renal tubule and collecting duct

Answer 17

In the proximal consulted tubule (PCT) the cells are simple cuboidal epithelial cells with prominent brush borders of microvilli and in most of the distal convoluted tubule (DCT) the cells are simple cuboidal epithelial cells

In each nephrons the columnar tubule cells are crowded and thus referred to as the macula densa. Alongside the densa the walls of the afferent arteries contain modified smooth muscle fibers called juxtaglomerular cells (JG), together constituting the juxtaglomerular apparatus (JGA)

In the last part of the DCT are principal cells (receptors for both antidieuratic hormone and aldosterone) and a small number of intercalate cells that play a role in homeostasis of blood pH

Question 18

Identify the 3 basic functions performed by the nephrons and collecting ducts and indicate where each occurs

Answer 18

1. Glomerular filtration - In the glomerulus, blood plasma and dissolved
   substances (smaller than most proteins) get filtered into the glomerular capsule
2. Tubular reabsorption - All along the renal tubule and collecting duct, water,
   ions, and other substances get reabsorbed from the renal tubule lumen into the peritubular capillaries and ultimately into the blood
3. Tubular reabsorption - All along the renal tubule and collecting duct, substances such as wastes, drugs, and excess ions get secreted from the peritubular capillaries into the renal tubule. These substances ultimately make their way into the urine

Question 19

Define glomerular filtrate and filtrate fraction

Answer 19

glomerular filtrate - the fluid that enters the capsular space

filtrate fraction - the fraction of blood plasma in the afferent arterioles of the kidneys that becomes glomerular filtrate

Question 20

Describe the filtration membrane

Answer 20

A leaky barrier that consists of the glomerular capillaries and the podocytes that completely encircles the capillaries

Substances filtered from the blood cross 3 filtration barriers:
1. FENESTRATION (PORE) OF GLOMERULAR ENDOTHELIAL CELL: prevents filtration of blood cells but allows all components of blood plasma to pass through

2. BASAL LAMINA OF GLOMERULUS: prevents filtration of larger proteins
3. SLIT MEMBRANE BETWEEN PEDICELS: prevents filtration of medium-sized proteins

Question 21

Describe the principle of filtration

Answer 21

The use of pressure to force fluids and solutes through a membrane is the same in glomerular capillaries as in blood capillaries elsewhere in the body

However, the volume of fluid filtered by the renal corpuscle is much larger than in other blood capillaries of the body for 3 reasons:

1. Glomerular capillaries present a large surface area for filtration because they are long and extensive
2. The filtration membrane is thin and porous
3. Glomerular capillary blood pressure is high

Question 22

Discuss the pressures that promote and oppose glomerular filtration

Answer 22

1. Glomerular blood hydrostatic pressure (GBHP) - promotes filtration by forcing water and solutes in blood plasma through the filtration membrane
2. Capsular hydrostatic pressure (CHP) - opposes filtration and represents a ‘back pressure’
3. Blood colloid osmotic pressure (BCOP) - opposes filtration and is due to the presence of proteins

Question 23

Define net filtration pressure (NFP)

Answer 23

Total pressure that promotes filtration and is determined as follows:

NFP = GBHP - CHP - BCOP

Question 24

Define glomerular filtration rate (GFR) and the three mechanisms that regulate it

Answer 24

GFR - the amount of filtrate formed in all renal corpuscles of both kidneys each minute

1. RENAL AUTOREGULATION OF GFR - kidneys are capable of maintaining control renal flow flow despite changes in BP, a process called renal auto regulation that consists of 2 mechanisms:
   - Myogenic mechanism: Stretched smooth muscle fibers contract, thereby narrowing lumen of afferent arterioles to decrease GFR

• Tubuloglomerular feedback: Decreased release of nitric oxide (NO) by juxtaglomerular apparatus causes constriction of afferent arterioles to decrease GFR
  2. NEURAL REGULATION OF GFR - Constriction of afferent arterioles through activation of a1 receptors and increased release of renin tp decrease GFR

3. HORMONAL REGULATION OF GFR - two hormones contribute to the regulation:
   - Angiotensin II: Decreased blood volume or blood pressure stimulates production of angiotensin II which causes contraction of afferent and efferent arterioles which decreases GFR

• Atrial natriuretic peptide (ANP): Stretching of atria of heart stimulates secretion of ANP. By causing relaxation of the glomerular mesangial cells, ANP increases the capillary surface area available for filtration increasing GFR

Question 25

Differentiate between paraceullar and transceullar reabsorption

Answer 25

In paracellular reabsorption, water and solutes in tubular fluid return to the bloodstream by moving between tubule cells

In transcellular reabsorption, solutes and water in tubular fluid return to the bloodstream by passing through a tubule cell

Question 26

Differentiate between primary active and secondary active transport

Answer 26

Primary - the dingy derived from hydrolysis of ATP is used to pump a substance accross a membrane (ex. Na-K pump)

Secondary - energy stored in an ions electrochemical gradient, rather than hydrolysis of ATP, drives another substance across a membrane (symporters and anti porters)

Question 27

Define transport maximum

Answer 27

Each type of transporter (symporter and antipodes) has an upper limit on how fast it can work and is measured in mg/min

Question 28

What is the difference between obligatory and facultative water reabsorption?

Answer 28

Obligatory - water reabsorbed with solutes in tubular fluid because the water is ‘obliged’ to follow the solutes when reabsorbed; occurs in the proximal convoluted tubule and the descending limb of the nephron loop because these segments are permeable to water

Facultative - reabsorption of the final 10% that is regulated by ADH and occurs mainly in the collecting ducts