URINE FORMATION IN THE NEPHRON9.2

Formation of Urine 3 main steps: -

Filtration, -Reabsorption, -Secretion

1. Filtration Dissolved solutes pass

through walls of glomerulus into Bowman’s capsule

Plasma proteins, platelets, and blood cells are too large to pass

2. Reabsorption 1 mL urine formed / 120 mL

fluid filtered the rest is reabsorbed

Occurs by both active & passive transport

Occurs until the threshold level of a substance is reached the rest is excreted

Solutes actively transported out of nephron create an osmotic gradient

3. Secretion Movement of

wastes from blood into nephron

Nitrogen-containing wastes

Histamine H+ Drugs (ie: penicillin)

Cells loaded with mitochondria line distal tubule aid in active transport

4 PROCESSES IN THE FORMATION OF URINE Glomerular Filtration

Moves water and solutes from blood plasma into nephron

Creates filtrate Tubular Reabsorption

Removes useful substances and returns them to blood (eg. sodium)

Tubular Secretion Moves additional wastes and excess substances

from the blood into the filtrate Water Reabsorption

Removes water from filtrate and returns it to blood

Glomerulus

GLOMERULAR FILTRATION Factors contributing to filtration:

Permeability of capillaries Glomerular capillaries have many pores in their walls Prevent blood cells and proteins from passing through

– most other substances can pass Blood Pressure

4X greater in glomerulus than in other capillary beds in the body

Provides force for filtration 180 L / day of filtrate

Very similar to plasma at this point

Proximal Tubule

TUBULAR REABSORBTION: PROXIMAL TUBULE 65% of filtrate is reabsorbed in the proximal

tubule Involves active and passive transport Cells of proximal tubule are loaded with

mitochondria to aid in transport Focuses on transporting Na+, glucose

Negative ions follow Water follows by osmosis

Loop of Henle

TUBULAR REABSORPTION: PROXIMAL TUBULE (CONTINUED)

Loop of Henle Function: absorb water and ions Descending loop very permeable to water,

slightly permeable to ions Medulla is salty drives movement of water by

osmosis Concentration of Na+ inside Loop of Henle increases

Ascending loop becomes more permeable to solutes Thin-walled section: Na+ move along concentration

gradients, passing into blood vessels Thick-walled section: Na+ is now moved out of filtrate

by active transport Helps to maintain salty environment of medulla

Distal Tubule

TUBULAR REABSORPTION: DISTAL TUBULE Reabsorption of ions decreases the

concentration of the filtrate, causing water to be reabsorbed by osmosis

Potassium and hydrogen ions are actively secreted into the distal tubule from the bloodstream

Other substances (eg. Drugs like penicillin) are secreted into the distal tubule

Collecting Duct

REABSORPTION FROM THE COLLECTING DUCT Passive reabsorption of water in collecting

duct High concentration of ions outside the collecting

duct, in the medulla If blood plasma is too concentrated, permeability

to water in the duct will increase Filtrate is ~4X more concentrated by the

time it exits the duct Filtrate is now called urine

****TABLE 9.2, Pg 315 – excellent study tool

Summary