Week 6 - The Urinary System

Question 1

How would you introduce the kidneys in the setting of a lab exam (structure/function/location)?

Answer 1

• Structure: paired, bean-shaped structures that have a reddish-brown colour. Surrounding each kidney is a fibrous capsule, perinephric fat and renal fascia that play a supportive role.
• Location: kidneys extend approximately from T12 to L3, that lie obliquely in the paravertebral gutters, retroperitoneally.
  The right kidney sits lower than the left. Rib 12 overlies the upper pole of the right kidney, whilst rib 11 overlies the upper pole of the left kidney.
• Function:
• • urine formation = regulates levels of fluid and electrolytes, and removes metabolic wastes, drugs and toxins
• • there are also endocrine and metabolic functions = including blood pressure regulation via the production of renin

Question 2

What are the external gross structures of the kidney?

Answer 2

Each kidney has an upper and lower pole. Its lateral border is convex, whilst its medial border is concave and has a vertical cleft where the renal hilum is located. This extends into an internal cavity, known as the renal sinus.

Question 3

Identify and describe the gross internal structures of the kidney

Answer 3

• Cortex lies beneath the capsule
• Medulla = arranged as pyramids that have a papilla at its apex and a base; their striped appearance is due to parallel bundles of urine-collecting tubules. Renal columns separate pyramids, and are inward extensions of cortical tissue. A kidney lobe includes the pyramid and surrounding cortical tissue.
• Renal pelvis = collects urine draining from the minor calyx to the major calyx, and is a funnel-shaped expansion that is continuous with the ureter leaving the hilum

Question 4

Define the term nephron. How many nephrons are contained in the kidney?

Answer 4

The nephron is the basic functional unit of the kidneys. There are approximately 1 million nephrons in each kidney.

Question 5

Describe the key components of a nephron i.e. what structures are in the (i) renal corpuscle and (ii) renal tubule?

Answer 5

Renal corpuscle:

• glomerulus = knot of capillaries that receive blood from an afferent arteriole, and drained by an efferent arteriole
• Bowman’s capsule = a cup-shaped hollow structure that surrounds the glomerulus, continuous with the renal tube

Renal tubule:

• proximal convoluted tubule = receives filtrate from Bowman’s capsule
• loop of henle
• distal convoluted tubule

Question 6

What features of the glomerulus and glomerular capsule allow for the easy formation of filtrate?

Answer 6

• glomerulus have fenestrations (capillary pores) allow for easy formation of filtrate
• Bowman’s capsule has podoctyes, which are foot processes, that contain filtration slits to pick up filtrate

Question 7

Differentiate a cortical nephron from a juxtamedullary nephron. i.e. where is each type located? What type is most abundant in the kidney? Which type is associated with vasa recta/peritubular capillaries?

Answer 7

Cortical nephron:

• location: predominantly in the cortex
• abundance: 85%
• associated with: peritubular capillaries, that cling to adjacent renal tubules

Juxtamedullary nephron:

• location: close to cortex-medullar junction
• abundance: 15%
• associated with: vasa recta, long bundles of vessels

Question 8

Define the term juxtaglomerular apparatus (complex)

Answer 8

a region where the most distal portion of the loop of Henle lies against the afferent (and sometimes efferent) arteriole of its glomerulus

Question 9

List the functions of the specialised cells of the juxtaglomerular apparatus

Answer 9

• Macula densa cells = monitor the NaCl content of filtrate entering the distal convoluted tubule
• Juxtaglomerular cells = contains granules of renin
• Extraglomerular mesangial cell = help to regular MD and JG cells

Question 10

Describe the normal composition of urine

Answer 10

95% water

5 % solutes (normal solutes = urea, Na+, K+, phosphate, sulphate, creatinine, uric acid)

Question 11

What urinary constituents are considered “abnormal”?

Answer 11

blood proteins
WBCs (pus)
bile pigments
glucose
ketones

Question 12

List and describe the three major processes involved in urine formation

Answer 12

• GLOMERULAR FILTRATION
• • occurs in the renal corpuscle
• • produces a cell-free and protein-free filtrate
• TUBULAR REABSORPTION
• • selective reabsorption of substances that the body needs to keep
• • 99% of water and salt; all glucose and amino acids
• • anything that is not reabsorbed becomes urine
• TUBULAR SECRETION
• • selective addition of substances from blood into the filtrate

Question 13

What forms the filtration membrane?

Answer 13

capillary with fenestrations

membrane:

• capillary endothelium
• basement membrane
• foot processes of podocyte of glomerular capsule

glomerular

Question 14

Define the term glomerular filtration rate (GFR)

Answer 14

the volume of filtrate formed each minute by the kidneys

Question 15

Discuss three factors that can affect GFR

Answer 15

NET FILTRATION PRESSURE (key determinant is the hydrostatic pressure of the glomerular capillaries)
TOTAL SURFACE AREA AVAILABLE (can be impacted by disease status)
FILTRATION MEMBRANE PERMEABILITY (can be impacted by disease status)

Question 16

List some consequences of an inadequate GFR

Answer 16

excess fluids
solutes
wastes

Question 17

The body possesses intrinsic and extrinsic mechanisms that maintain GFR
Summarise the intrinsic controls of GFR i.e. the myogenic mechanism, tubuloglomerular mechanism and the role of prostaglandin E2.

Answer 17

Autoregulation: maintenance of GFR due to mechanisms local to the kidney

Myogenic mechanism: vascular smooth muscle of arteriole contracts when placed on stretch, and relaxes when not.
High systemic BP: afferent arterioles constrict to limit NFP and prevent glomerular damage
Low systemic BP: afferent arterioles dilate

Tubuloglomerular mechanism: initiated by macula densa cells (chemoreceptors sensitive to NaCl concentration of filtrate)
High GFR = not enough time for reabsorption of NaCl from filtrate. MD cells detect high NaCl and release adenosine (induces vasoconstriction of afferent arterioles)

Prostaglandin E2: produced by renal cells (tubules and medulla) and has a paracrine function (locally-acting hormone)
Maintains GFR by inducing vasodilation of afferent arterioles.
Consequence = excess of NSAIDs lead to inhibition of PGE2 = decreased GFR

Question 18

Outline the two extrinsic controls of GFR.

Answer 18

• hormonal mechanism = activation of RAAS occurs when systemic BP (and consequently, GFR) is low
  1. juxtaglomerular cells release renin
  2. renin catalyses a cascade that increases systemic BP, renal perfusion and GFR
• neural controls = sympathetic nervous system acts upon kidneys when systemic BP is low (baroreceptor reflex)
  1. renal sympathetic nerves activate B1-adrenergic receptors on juxtaglomerular cells
  2. renin release catalyses a cascade that increases systemic BP, renal perfusion and GFR

Question 19

In the renal tubule and collecting duct, indicate the regions at which specific nutrients are reabsorbed

Answer 19

Proximal convoluted tube -
AT = Na+, HCO3-, AAx, glucose
PT = H2O, Cl-, K+, Ca2+, urea

Loop of Henle -
Descending...
AT = none
PT = H2O
Ascending...
AT = Na+, Cl-, K+
PT = H2O, Ca2+

Distal convoluted tube -
AT = Na+
PT = H2O, Ca2+

Collecting duct -
AT = Na+
PT = H2O

AT = active transport
PT = passive transport

Question 20

In the renal tubule and collecting duct, indicate the regions at which specific nutrients are secreted

Answer 20

Proximal convoluted tube -
some drugs, poisons and H+

Loop of Henle -
urea

Distal convoluted tube -
K+ and some drugs

Collecting duct -
K+

Question 21

How would you introduce the ureters and bladder in the setting of a lab exam (structure/function/location)?

Answer 21

Structure: paired, narrow tubes with a thick muscular wall

Function: conveys urine from the kidneys to the bladder

Location: upper half is in the abdomen, whilst the lower half is in the pelvis. The abdominal part extends from the kidney to the bifurcation of the common iliac artery.

Question 22

Name the type of epithelium that is located in the renal pelvis, ureters and bladder. What special properties does this epithelium have?

Answer 22

Transitional epithelium - able to maintain integrity when distended (stretched)

Question 23

What name is given to the smooth muscle of the bladder wall? Which nerves provide motor supply to this muscle?

Answer 23

detrusor muscle

supplied by pelvic splanchnic nerve (S2-S4) (PNS)

Question 24

Compare and contrast the male urethra to the female urethra

Answer 24

Different:
Male urethra - serves both urinary and reproductive function
~20cm long
passes through the prostate and entire corpus spongiosum

Female urethra - serves a urinary function only
~4cm long
most of urethra is embedded within the anterior vaginal wall

Question 25

Consider how the urethra might be affected in the following disease states: (i) prostatic enlargement (ii) lower urinary tract infection (iii) cauda equina syndrome

Answer 25

i. prostatic enlargement can cause acute kidney injury due to urinary tract obstruction.
ii. LUTI may develop into urethra and ??
iii. cauda equina syndrome affects nerve supply to bladder. This can cause backflow of filtrate to urethra

Question 26

What are the three events of micturition?

Answer 26

Contraction of detrusor muscle
Relaxation of IUS
Relaxation of EUS