The Excretory System

• Regulates volume and composition of body fluids

  • removes wastes (filtration)

  • returns essential substances for reuse (collection)

Wastes

• substances produces in excess of the body’s needs

• dangerous to health if accumulated

  • Water, CO2, Ions (Na+, Cl-, H+, etc.)

Excretion: separating and eliminating wastes from body fluids

• respiratory system

  • Gases (Ex. CO2)

• skin

  • perspiration (Ex. Water, salts, some urea)

• digestive system

  • cellular chemicals (Ex. water, salts, lipids, etc.)

• excretory/urinary system

  • most metabolic wastes

Excretory/Urinary system

• metabolic wastes, suspended or dissolved in blood

• filtered and removed by excretory/urinary system via urine

  • 95% water + nitrogen + inorganic salts

  • nitrogenous wastes → metabolism of amino acids and nucleotides → urea as main product

• Pooping is not excretion

Organs of Excretory System

• Kidney

  • 2 fist sized organs

  • On each side of body → near lower back

  • Protected by fat layer + some muscles + rib cage

  • can live with only one kidney → living organ donation possible

• Ureters

  • ~28cm

  • Muscular tubes

  • Connect Kidneys to urinary bladder

  • Moves urine by peristalsis

• Urinary Bladder

  • Muscular

  • stores urine (~600mL)

  • Urination → relaxation of TWO muscular sphincters

    • urinary bladder fills with urine

    • stretch receptors → nerve impulses to brain

    • nerve impulses from spinal cord → urinary bladder muscles contact + BOTH sphincters relax → urination

  • Inner sphincter → involuntary

  • Secondary sphincter → voluntary

• Urethra

  • ~20cm males → merges with vas deferens from the testes to make a singular tract out of the body

  • ~4cm females → separate opening for urinary and reproductive tract

Kidneys

• Homeostasis

  • producing urine to rid body of dissolved wastes

  • keeping pH constant

  • keep salt/water balance of blood

• Nephrons

  • Functional unit of the kidney → filtering substances from blood + transform into urine

  • Microscopic → over one million nephrons in each kidney

  • Several nephrons enter one collecting duct

  • Structure → filter + tube + duct

Nephron

• Filter → Bowman’s Capsule

  • Renal Artery enters capsule → capillary network (glomerulus)

  • Glomerulus walls = filter

    • Impermeable: proteins, red blood cells, large molecules → stay in blood

    • Permeable: water, small molecules, ions, urea → enters nephron → filtrate

• Tube → A tubule

  • Three sections

    • Proximal Tubule

    • Loop of Henle

    • Distal Tubule

  • Reabsorbs useful chemicals (Ex. Glucose, ions, etc.) → return to blood via active transport

• Duct: Collecting Duct

  • Reabsorbs water from filtrate → return to blood via renal vein

  • Remaining substance = urine

    • chemically different from initial filtrate

Urine Formation:

1) Glomerular Filtration

• Water + solutes from blood into nephron → return to blood via renal vein

• remaining substance = urine

  • chemically different than initial filtrate

• TWO FACTORS influencing filtration efficiency

  • glomerulus permeability → pores in the glomerulus capillary walls ONLY allow small molecules through

    • proteins, blood cells, large molecules stay in blood

  • blood pressure → increased pressure in glomerulus compared with other capillaries → forced filtration

2) Tubular Reabsorption

• Returns useful substances from filtrate back to blood

• Proximal Tubule

  • Large surface area for reabsorption → glucose, ions, water, etc.

  • Cells → many mitochondria → energy (ATP) for Active Transport

    • Positive Ions → Na+, K+

    • Glucose, Amino Acids and other solutes

  • Negative ions (Ex. Cl-) → attracted to positive ions → passively follow

  • Water → passive movement down concentration gradient established by ion transport (osmosis)

• Descending Loop of Henle

  • Renal medulla → “salty” environment

  • cells of descending loop: permeable to water + not very permeable to ions

    • water → osmosis → from filtrate into capillaries

    • Filtrate Na+ concentration increases

• Bottom of loop

  • maximum Na+ concentration

  • Impermeable to water + slightly permeable to solutes

    • solutes diffuse into capillary and then blood

• Ascending Loop of Henle

  • Thick walled → impermeable to water

  • Active transport of Na+ out of filtrate

    • Keeps medulla salty

    • makes filtrate less concentrated than tissues and blood

  • 2/3 of Na+ and Water has not been reabsorbed

• Distal Tubule

  • Active transport of Na+ ions from filtrate into capillaries → Cl- passively follows

    • Establish concentration gradient

  • water follows by osmosis into capillaries

3) Tubular Secretion

• Additional wastes from blood into filtrates

• Distal Tubule

  • K+ and H+ are actively transported into distal tubule from blood stream/capillaries

  • H+ ion secretion → optimal pH of blood

4) Water Reabsorption

• Return water from filtrate into blood

• Collecting Duct

  • Medulla → Salty

  • Water passively reabsorbed → osmosis

  • Depends on blood plasma concentration

    • dehydrated → concentrated blood plasma → permeability of distal tubule and collecting duct to water is increased → more water reabsorption

• Filtrate is now urine with only 1% of its original volume

Regulating Water Reabsorption

• Amount of water reabsorption depends on

  • blood volume

  • concentration of blood plasma

• osmotic pressure: force generated by water as it moves my osmosis

• increase in blood plasma concentration → increase in osmotic pressure

• Blood plasma concentration too high (Ex. Dehydration) → high osmotic pressure

  • stimulates osmoreceptors in hypothalamus → pituitary gland → antidiuretic hormone (ADH) released

  • ADH travels to kidney

    • increase permeability of distal tubule + collecting duct

    • increase water reabsorption

    • decrease water excretion in urine

• Blood plasma concentration too dilute → low osmotic pressure

  • osmoreceptors stops ADH release

    • decrease permeability of distal tubule + collecting duct

    • decrease water reabsorption

    • increase water excretion in urine

• Aldosterone

  • Hormone maintaining Na+ balance

  • Decrease Na in blood → aldosterone released

    • stimulates Na+ reabsorption in distal tubule + collecting duct

    • Cl- follows passively

    • water follows by osmosis

    • Aldosterone → retains BOTH salt and water

    • increase K+ in blood → aldosterone released

      • Stimulates K+ secretion into distal tubule + collecting duct