The Excretory System: Urine Formation

… eww

The Excretory System

• Mammals must excrete many different wastes in order to remove toxic substance from the body

• The wastes form as a result of the metabolic processes of the body and degeneration of cells (ie. urea, ammonia, CO2, lactic acid, etc.)

• Much of the excretory system flows through the kidneys

Kidneys

• Part of excretory system, controlled by nervous and endocrine system

• Weighing ~0.5 kg each, kidneys may hold as much as 25% of the body’s total blood volume at any given time

• Involved in the production of urine

KidneysStabilize internal environment by:• regulating water content (osmoregulation)• eliminating poisonous by-products of biological reactions• regulating ion concentration

Mechanisms of Regulation:• diffusion/osmosis• forced filtration• active transport

Nephron

• Each kidney contains over 1 million slender tubules called nephrons

• The functional units of the kidney

Nephron Structure

Glomerulus:• High-pressure capillary

bed• The site of blood filtration

Bowman’s Capsule:• Funnel-like structure• ‘Beginning’ of the nephron

Each nephron has its own independent blood supply (renal arterioles and veins)

Nephron Structure

Components not entering capsule:

• Blood proteins• Blood Cells• Platelets

Nephron Structure

Proximal Tubule:• Re-absorption of

molecules important to the blood

Loop of Henle:• Concentration of

urine (solutes to be excreted)

Nephron Structure

Distal Tubule:• Final re-absorption

and secretion of molecules

Collecting Duct• Collection of urine

from multiple nephrons

• Joins with the ureters; ultimately the bladder

Urine Formation

Accomplished by 3 functions of the nephron:

Forced filtration of blood from the afferent arteriole into the glomerulus, and through Bowman’s capsule under high pressure

• proteins, blood cells and platelets are too large to pass through & stay in the blood

• sodium chloride, glucose, amino acids, ions enter the nephron

Urine FormationRe-absorption of most fluids and solutes from the

filtrate back into the blood• for every 120 mL of fluids filtered, only 1 mL becomes

urine• many solutes are removed from the nephron into the

capillary beds surrounding the convoluted tubules• passive transport moves Na+, Cl- and HCO3

• active transport moves glucose and amino acids• an osmotic gradient is created by the solutes in the

capillary bed, drawing water out of the nephron (in all tubules but the ascending Loop of Henle)

Urine Formation

Secretion moves wastes from blood into the distal tubule of the nephron

• Urea, uric acid, ammonia, excess ions, minerals, drugs, etc. moved by active transport

How it all works:

How it all works:

YouTube Link:

http://www.youtube.com/watch?v=vNvZaGcLzEo

Lab Exercise

Comparing Solutes in the Plasma, Nephron & Urine

• Activity: 7.5.1 - p.363