SBI4U

Ms. Ho-Lau

10.2 Urine Formation in the Nephron

4 Processes

glomerular filtration:

- move water and solutes (except proteins) from blood into nephron

- produces filtrate

tubular reabsorption:

- removes useful substances from filtrate and returns them to the blood

tubular secretion:

- moves additional wastes from blood into filtrate

water reabsorption:

- removes water from filtrate and returns it to the blood

How Urine Forms

1. Glomerular Filtration

moves water and dissolved substances from blood into

the Bowman’s capsule

1. Glomerular Filtration

2 factors contribute to filtration:

1. the permeability of the capillaries of the glomerulus:

the pores in the walls of the capillaries allow water

and dissolved substances to pass through; but

impermeable to proteins and blood cells

2. blood pressure:

the high blood pressure within the glomerulus provides

the force for filtration

1. Glomerular Filtration

1600L to 2000L of blood pass through kidneys each day

produces 180 L of filtrate

This filtrate is identical to blood plasma, minus proteins

and blood cells

Body has to reabsorb water, salts, and nutrients

2. Tubular Reabsorption

proximal tubule and the loop of Henle

reabsorbs ~65% of the filtrate

involves active and passive transport

2. Tubular Reabsorption2. Tubular Reabsorption: Proximal tubule

2. Tubular Reabsorption: Proximal tubule

2. Tubular Reabsorption:

The Loop of Henle

reabsorbs water and ions from filtrate

3 sections

Descending limb

Thin segment of ascending limb

Thick segment of ascending limb

Note: medulla is a very salty environment

2. Tubular Reabsorption: The Loop of Henle

2. Tubular Reabsorption: The Loop of Henle

• Descending limb

permeable to water

• Water diffuses from

filtrate to capillaries

(osmosis)

• [Na+] in filtrate ↑

2. Tubular Reabsorption: The Loop of Henle

• Ascending limb

impermeable to water

but permeable to ions

• Water is not

absorbed

• Na+ diffuses from

filtrate into capillares

2. Tubular Reabsorption: The Loop of Henle

• Na+ are actively transported out

of the filtrate into the capillaries

• replenish medulla’s salty

environment

• At this point, 2/3 of Na+ and

water from the filtrate has been

absorbed

3. Tubular Reabsorption and Secretion:

Distal Tubule

Active reabsorption of Na+

Passive reabsorption of negative ions

Water reabsorbed by osmosis

3. Tubular Reabsorption and Secretion:

Distal Tubule

Active secretion of K+ and H+ from blood into distal tubule to maintain blood pH

Antibiotics and other medications secreted from blood into distal tubule

Secretions are controlled by hormones

4. Water Reabsorption: Collecting duct

Passive reabsorption of water

Hormones can alter the amount of

reabsorption in response to

hydration levels

Urine:

~1% of the volume of the original

filtrate