Organisms Respond to their Internal and External Environments: Homeostasis - Control of Blood Water Potential, Ultrafiltration & Selective Reabsorption

Question 1

Define the term osmoregulation

Answer 1

• Control of the water potential of the blood by the kidney

Question 2

What are the four main steps by which water is reabsorbed by the kidney?

Answer 2

• Ultrafiltration
• Selective reabsorption
• Loop of Henle
• Distal convoluted tubule and collecting tube

Question 3

What happens during ultrafiltration?

Answer 3

• Blood enters kidney via renal artery
• Afferent arterioles takes blood into glomeruli
• Efferent arterioles takes blood away from glomeruli
• Efferent arteriole is narrower, so blood in glomeruli is under high hydrostatic pressure
• High pressure forces small molecules in blood out of capillary and into Bowman’s capsule, through three layers
• Capillary endothelium, basement membrane and epithelium of Bowman’s capsule
• Substances that enter Bowman’s capsule are known as glomerular filtrate

Question 4

Which molecules enter Bowman’s capsule during ultrafiltration?

Answer 4

• Water
• Glucose
• Amino acids
• Ions
• Urea

Question 5

Which molecules do not enter Bowman’s capsule during ultrafiltration?

Answer 5

• Proteins

- Blood cells

Question 6

Why can’t proteins and blood cells enter Bowman’s capsule during ultrafiltration?

Answer 6

• These molecules are too large to pass through the basement membrane

Question 7

How do small molecules pass through the endothelium of the capillary during ultrafiltration?

Answer 7

• Cells have gaps between them called fenestrations

Question 8

What is the basement membrane made of?

Answer 8

• Continuous sheet made of protein

Question 9

What cells make up the epithelium of Bowman’s capsule?

Answer 9

• Podocytes

Question 10

Where does the glomerular filtrate go?

Answer 10

• Glomerular filtrate passes out of Bowman’s capsule and into proximal convoluted tube (PCT)
• PCT is main site of re-absorption

Question 11

What happens during selective reabsorption?

Answer 11

• Useful substances (glucose, amino acids, ions and water) are reabsorbed
• While glomerular filtrate flows along PCT, loops of Henle and along DCT
• Useful substances are reabsorbed into the blood by active transport and facilitated diffusion
• This lowers water potential of blood, more water enters blood by osmosis

Question 12

How is the PCT adapted for reabsorption of useful substances back into the blood?

Answer 12

• Epithelium of wall of PCT has microvilli

- Provides large surface area

Question 13

What happens to the remaining filtrate after selective reabsorption?

Answer 13

• Passed along PCT, loop of Henle, DCT and collecting duct
• While reabsorbing water
• Filtrate remains is urine, which passes along ureter and bladder

Question 14

What substances are usually found in urine?

Answer 14

• Water
• Dissolved salts
• Urea
• Hormones
• Excess vitamins

Question 15

What substances are usually not found in urine?

Answer 15

• Proteins
• Blood cells
• Glucose

Question 16

Why are proteins and blood cells not usually found in urine?

Answer 16

• Too big to be filtered out of blood and into basement membrane of nephron tubules

Question 17

Why is glucose not usually found in urine?

Answer 17

• Glucose is reabsorbed back into the blood by active transport and facilitated diffusion
• During selective reabsorption