Structural Basis of Kidney Function

Question 1

Answer 1

Question 2

What is found in the kidney cortex and medulla?

Answer 2

Different sections of nephrons are located in different parts of the kidney: Thecortex contains the renal corpuscle, proximal, and distal convoluted tubules. The medulla and medullary rays contain the loops of Henle and collecting ducts.

Question 3

MEchanism of urine production in the kidney?

How is blood filtered in the kidney?

Where are materials reabsorbed in the kidney and name some of those materials?

What mechanism creates an hyper-osmotic extracellular fluid and state what part of a nephron helps with this?

Answer 3

Question 4

What does the renal corpuscle compose of?

The glomerulus consists of ……………… with associated ………………. (visceral epithelial cells); these “wrap” around the capillaries, with long processes/feet that leave slits/gaps between them = fenestrations

What is the blood supply of the renal corpuscle?

Decribe the filtration of the renal corpuscle?

Answer 4

Renal corpuscle

 Structure composed of the glomerulus + Bowman’s capsule

 The glomerulus consists of capillaries with associated podocytes (visceral epithelial cells); these “wrap” around the capillaries, with long processes/feet that leave slits/gaps between them = fenestrations

 Blood supply: from afferent to efferent arteriole, the blood supply enters at the vascular pole of the corpuscle

 Filtration barrier: the fenestrae between the processes of the podocytes have a specialised basal lamina which allows the passage of ions and molecules <~50,000m weight to pass from the blood (i.e. first step in producing urine)

 The filtrate then drains into the proximal convoluted tubule at the urinary pole of the corpuscle

Afferent arteriole is greater then the efferent arteriole- builds up pressure

The endothelium of the renal corpuscle is fenestrated

The basement membrane is also modified to act as a filter.

Question 5

How does the proximal convuluted tubule cells preform selective reabsorption?

Answer 5

Selective Reabsorption

 Material that needs to be retained in the blood is then reabsorbed in the proximal convoluted tubule (70% reabsorbed)

 This includes ions, glucose, amino acids, small proteins, water etc

 Methods of uptake:

o Na+ uptake by basolateral Na+ pump. Water and anions then follow the Na+ (along osmotic + electrochemical gradient)

o Glucose uptake is via Na=/glucose co-transporter

o Amino acids by Na+/amino acid co-transporter

o Protein uptake by endocytosis

Question 6

List 5 structural features of the kidney?

Answer 6

 Structural features:

o large diameter lumen (larger than distal convoluted tubule)

o cuboidal epithelium sealed with tight junctions (act as paracellular seals)

o brush border at apical surface +basolateral interdigitations - increase the membrane surface area

o aquaporins (membrane protein channel carriers) – mediate transcellular water diffusion

o prominent mintochondria (reflect high energy requirements

Question 7

Explain the Creation of hyper-osmotic (concentrated) ECF?

Answer 7

Achieved by loop of Henle (both descending + ascending limb) + vasa recta (blood vessels supplying loop) by a countercurrent mechanism

 Decending limb:

o Thin, simple squamous epithelium

o Aquaporins present on apical membrane allow passive H2O reabsorption to continue until a passive osmotic equilibrium is established

 Ascending limb:

o Thick, cuboidal epithelium with few microvilli + prominent mitochondria (required for the active pumping of ions)

o Na+ + Cl- are actively pumped out of the tubular fluid into the ECF. However there are very waterimpermeant tight junctions between the epithelial cells, which also lack aquaporins, therefore H2O not reabsorbed further

o This results in a HYPER-osmotic ECF (with a corresponding HYPO-osmotic tubular fluid)

 Vasa recta

o Loop structure stabilises the hyperosmotic ECF by forming a rapid equilibrium

Question 8

Adjustment of ion content of urine

What part of the nephron is responsible for the above?

What is the type of epithelium is found there?

What does this part of the nephron do? Name 2 things

Answer 8

Adjustment of ion content of urine

Acheieved by distal convoluted tubule (also known as cortical collecting duct)

 Cuboidal epithelium, with few microvilli + numerous large mitochondria, complex lateral membrane interdigitations with Na+ pumps

 Adjusts Na+,K+, H+ and NH4+ (urea) – under control by aldosterone

 Re-equibirilates luminal fluid + ECF (under control by vasopressin)

 Also specialisation at macula densa, which forms part of the juxtaglomerular apparatus (this regulates the function of the nephron vie renin-angiotensin system – found at vascular pole of renal corpuscle)

Question 9

Where does the Final adjustment of urine concentration occur?

What is the type of epithelium present in the tissue that does the above role in the nephron?

Why does it have few mitachondria?

What does the rate of final adjustment of urine content depend on?

What is it controlled by?

What does the basolteral membrane have and what is its function?

Urine then drains into ………… …………. at apex of medullary pyramid

then to ………… …………. then to ………… …………. –> ureter

Answer 9

Final adjustment of urine concentration

Occurs in medullary collecting duct – involves the movement of water down osmotic gradient into extracellular fluid; completes ion adjustment + controls urine osmolarity

 Simple cuboidal epithlium with single cilium per cell and no interdigitations on cell boundaries

o Little active pumping therefore fewer mitochondria required

o However contain organelles associated with secretory activity e.g. golgi

 Rate due to aquaporin-2 in apical membrane

o Content is varied by exo/endocytosis mechanism

o Controlled by vasopressin (ADH; antidiuretic hormone)

 Basolateral membrane has aquaporin-3 for transcellular water movement, but this is not under vasopressin control

 Urine then drains into minor calyx at apex of medullary pyramid

then to major calyx then to renal pelvis –> ureter

o Both calyces + renal pelvis have specialised urinary epithelium

o Urinary epithelium – a specialised epithelium also known as urothelium; properties are resistance to urine, ability to stretch

 Cells appear squamous or cuboidal depending on degree of stretch

 The luminal cells are also highly specialised for low permeability

Question 10

Answer 10

Juxtaglomerular apparatus

Endocrine specialisation

Secretes renin to control blood pressure via angiotensin

Senses stretch in arteriole wall and [Cl-] in tubule

Cellular components are

• macula densa of distal convoluted tubule
• juxtaglomerular cells of afferent arteriole