5.2.6 The Mammalian Kidney: Function

Question 1

How is proximal convoluted tubule adapted?

Answer 1

• Lots of microvilli, large SA
• Lots of mitochondrion, energy provided for active transport
• Lots of cotransport proteins (for each type of molecule)

Question 2

What is the functional unit of the Kidney?

Answer 2

The Nephron

Question 3

How is urine produced? (2 step process)

Answer 3

• Ultrafiltration
• Selective Reabsorption

Question 4

How does ultrafiltration occur

Answer 4

• Renal artery -> Afferent Arteriole -> Nephron -> (Glomerulus and bowman’s capsule)
• Afferent arteriole has a larger diameter than efferent arteriole (capillaries leaving glomerulus)
• So there is a higher BP in glomerulus than that of the bowman’s capsule
• High BP causes smaller molecules to be forced out of the capillaries in the glomerulus into the bowman’s capsule, forming glomerular filtrate
• Proteins are stopped by the basement membrane
• So the solute concentration in blood plasma in glomerular capillaries is higher than that of the bowman’s capsule
• So water potential in the blood plasma in the glomerular capillaries is lower than in the bowman’s capsule
• So water moves from bowman’s capsule to glomerulus
• However pressure gradient outweighs solute gradient
• RBCs ,WBCs and platelets stay in blood (too large to pass through holes in capillaries)
• Basement membrane filters out proteins
• Main substances that pass out are amino acids, water, glucose, urea and inorganic ions

Question 5

Structure of membranes between bowman’s capsule and glomerulus

Answer 5

Endothelium of capillary
- First layer
- Lots of small holes (fenestrations)

Basement membrane
- Collagen fibres and glycoproteins
- Form mesh preventing passage of molecules with greater than 69000 Mr

Epithelium of Bowman’s capsule
- Podocytes (finger-like projections)
- Ensures ther are gaps between cells so fluid from blood in glomerulus can move throuhg to the lumen of the bowmans capsule

Question 6

What are the main components of glomerular filtrate?

Answer 6

Amino acids, water, glucose, urea and inorganic ions (mainly Na+, K+ and Cl-)

Question 7

What molecules are reabsorbed at the proximal convoluted tubule?

Answer 7

• ALL Glucose (actively transported)
• Amino acids (actively transported)
• Vitamins
• Inorganic ions
• Urea
• The movement of these solutes out of the tubule into the blood increases the solute concentration of the blood so water moves out of tubule and into blood by osmosis

Question 8

Selective Reabsorption Process

Answer 8

• Na+ actively transported out of epithelial cells (in PCT wall) into the blood plasma
• Lower concentration of Na+ in epithelial space
• Na+ diffuses in (down concentration gradient) from lumen of PCT
• Passing through cotransporter proteins which transports a solute with it, (glucose, amino acid, etc)
• Once inside epithelium, solutes diffuse down concentration gradient into blood plasma from epithelial cells into the blood
• Selective reabsorption has occured

Question 9

Textbook notes on Descending Limb

Answer 9

• Na+ and Cl- diffuse into descending limb
• Water diffuses out by osmosis

Question 10

Textbook notes on Ascending Limb

Answer 10

• Na+ and Cl- actively transported out of ascending limb
• Decreases water potential of fluid in medulla
• Causing H2O from descending limb to diffuse out into the medulla by osmosis