the structural basis of kidney function

Question 1

4 key functions of the kidneys and where they take place

Answer 1

Functions of the Kidney The kidney’s main function is the production of urine but in doing this, it has these actions:
§ Filtration of blood plasma – Glomerulus.
§ Selective reabsorption of contents – Proximal convoluted tubule.
§ Retention of unwanted contents in urine – Loop of Henle.
§ Concentration selection or urine – Distal convoluted tubule.

Question 2

Mechanism of Urine Production

5 steps

Answer 2

Mechanism of Urine Production

1. Filtration:
   a. The glomerulus filters the blood that passes through it.
   b. Filtrate ONLY contains molecules that weigh less than 50,000 Da.
2. Reabsorption:
   a. The proximal convoluted tubule reabsorbs material.
   b. Material includes: ions, glucose, amino acids, small proteins and water.
3. Creation of hyper-osmotic extracellular fluid:
   a. The Loop of Henle and vasa recta create this.
   b. The hyperosmolarity is created by the countercurrent mechanism.
4. Adjustment of ion content in urine:
   a. The distal convoluted tubule and collecting ducts do this.
   b. These structures control the levels of Na+, K+, H+ and NH4+ excreted.

Question 3

The Nephron – The Renal Corpuscle

Components:

Answer 3

The Nephron – The Renal Corpuscle (and associated visible Juxta-Glomerular Apparatus):
Components:
Bowmen’s capsule containing the
glomulerus (a network of modified capillaries), podocytes and
mesangial cells associated with glomerulus.

Question 4

The Nephron – The Renal Corpuscle

Blood supply

Answer 4

§ Blood supplied at the vascular pole of the corpuscle by the AFFERENT which leaves via the EFFERENT.
§ The glomerular capillaries are at a HIGH pressure.

Question 5

renal corpuscle

filtration barrier consists of

Answer 5

Filtration Barrier:
§ Fenestrae exist in the capillary endothelium.
§ A specialised basal lamina exists.
§ There are filtration slits between the foot processes of the podocytes which allows passage of ions and molecules ONLY less than 50,000 Da.

Question 6

Drainage of filtrate in the renal corpuscle

Answer 6

Drainage of Filtrate:

at urinary pole of corpuscle drains to proximal convoluted tubule

Question 7

Function of the proximal convoluted tubule

Answer 7

Function - Reabsorbs 70% of glomerular filtrate.
§ Na+ movement is by basolateral Na+ pumps INTO the extracellular space (from duct).
o Water and anions then follow the Na+.
o Glucose is re-absorbed by Na+/Glucose co-transporter.
o Amino acids re-absorbed by Na+/amino-acid co-transporter.
§ Protein uptake by endocytosis.

Question 8

Structural features of the proximal convoluted tubule

Answer 8

Structure:
§ Cuboidal epithelium, sealed by tight junctions, brush border at apical membrane.
§ Interdigitations of basolateral membrane.
§ Contains aquaporin proteins for water diffusion and PROMINENT mitochondria.

Question 9

describe the countercurrent mechanism in the loop of henle

Answer 9

The Nephron – Loop of Henle – The Countercurrent Mechanism:
Descending Thin Tubule – Absorbs water into the extracellular space:
§ Maintains a passive osmotic equilibrium – aquaporins present.
§ Made up of simple squamous epithelium.

Ascending Thick Limb:
§ Na+ and Cl- actively pumped out of tubular fluid – prominent mitochondria.
§ Membranes LACK aquaporins so have a LOW permeability to water.
o This results in a hypo-osmotic tubular fluid and a hyper-osmotic extracellular fluid.
§ Comprised of cuboidal epithelium with few microvilli.

Vasa Recta – The network of capillaries surrounding the LoH:
§ The blood is in rapid equilibrium with extracellular fluid.
§ The loop structure stabilises the hyper-osmotic concentration of Na+.

Question 10

Distal Convoluted Tubule – COMES BACK TO THE

and what does it do?

Answer 10

The Nephron – Distal Convoluted Tubule – COMES BACK TO THE GLOMERULUS:
§ Adjusts the ion content – Na, K, H and NH4.
o Controlled by aldosterone.
§ Made of cuboidal epithelium, few microvilli, but lots of mitochondria
§ Loops back round to the glomerulus to form the juxta-glomerular apparatus (has macula densa cells).

Question 11

Medullary collecting duct
-controls
-passes through
rate of reabsorption depends on

Answer 11

Medullary Collecting Duct:
§ Completes ion adjustment and CONTROLS the urine osmolarity.
o The duct passes through the hyper-osmotic medulla and so water passes out of the tube into the extracellular fluid to concentrate the urine.
o The rate of reabsorption depends upon aquaporin 2 production into the apical membrane.
§ ADH/Vasopressin control.
o Basolateral aquaporin 3 are not under any control.
§ Again, simple cuboidal epithelium.
§ Has a smooth muscle wall for peristalsis (with 2 layers).
§ Little active pumping so few mitochondria but lots of secretory activity organelles.
§ Drains into minor calyx of papilla of medullary pyramid and major calyces and pelvis have urinary epithelium

Question 12

juxtaglomerular apparatus

Answer 12

-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