4.2 - Excretion in humans
● ***Excretion: removal of metabolic waste from organism
● Metabolic reactions are chemical reactions which take place inside cells
● Many metabolic reactions produce waste products not required by cells
● ***Excretion ≠ egestion (expulsion of faeces - food that has never been
absorbed into body cells)
● Metabolic wastes in human body
○ Carbon dioxide & water: from aerobic respiration of sugars
○ Urea (尿素): produced by liver, from breakdown of amino acids (comes
from breakdown of excess proteins that are no longer required in body)
○ Other excess minerals from diet, e.g.: sodium, medical drugs (antibiotics)
● Why there is need of excretion
○ Waste material might reach toxic concentrations
○ Osmotic (滲透的) effects of increased concentration of body fluids
○ ***Loss of space in storing waste products (i.e.: water)
● Main routes of excretion
○ Lungs - excretion of carbon dioxide and water\vapour (through
ventilation)
○ Skin - excretion of water and excess mineral ions (through sweat)
○ Kidneys - excretion of water, urea, mineral ions etc. (through urination)
Kidneys as excretory organs (the urinary system)
● ⅓ of aortic blood goes to both kidneys
● Remove waste materials, e.g.: urea from blood
● Keep water content of bloodstream constant (osmoregulation)
● Urine produced carried in ureters to bladder, then to urethra
● Bladder is muscular sac, can expand to store urine produced and hold for hours
● When sphincter muscles are relaxed at the exit of bladder, urine is forced down
urethra, bladder empties
1
, ●
How kidneys remove urea from blood
● Nephron (腎單位)
○ About 1 million nephrons in each kidney {microscopic}
■ ***In the end, all nephrons join together to the ureter
○ Renal artery branches into smaller arteries called arterioles, each of which
leads to knot/ball of capillaries called glomerulus (腎小球)
○ Glomerular capillaries sit inside cup-shaped Bowman’s capsule (腎小囊),
which is double-walled structure leading into kidney tubule
○
2
● ***Excretion: removal of metabolic waste from organism
● Metabolic reactions are chemical reactions which take place inside cells
● Many metabolic reactions produce waste products not required by cells
● ***Excretion ≠ egestion (expulsion of faeces - food that has never been
absorbed into body cells)
● Metabolic wastes in human body
○ Carbon dioxide & water: from aerobic respiration of sugars
○ Urea (尿素): produced by liver, from breakdown of amino acids (comes
from breakdown of excess proteins that are no longer required in body)
○ Other excess minerals from diet, e.g.: sodium, medical drugs (antibiotics)
● Why there is need of excretion
○ Waste material might reach toxic concentrations
○ Osmotic (滲透的) effects of increased concentration of body fluids
○ ***Loss of space in storing waste products (i.e.: water)
● Main routes of excretion
○ Lungs - excretion of carbon dioxide and water\vapour (through
ventilation)
○ Skin - excretion of water and excess mineral ions (through sweat)
○ Kidneys - excretion of water, urea, mineral ions etc. (through urination)
Kidneys as excretory organs (the urinary system)
● ⅓ of aortic blood goes to both kidneys
● Remove waste materials, e.g.: urea from blood
● Keep water content of bloodstream constant (osmoregulation)
● Urine produced carried in ureters to bladder, then to urethra
● Bladder is muscular sac, can expand to store urine produced and hold for hours
● When sphincter muscles are relaxed at the exit of bladder, urine is forced down
urethra, bladder empties
1
, ●
How kidneys remove urea from blood
● Nephron (腎單位)
○ About 1 million nephrons in each kidney {microscopic}
■ ***In the end, all nephrons join together to the ureter
○ Renal artery branches into smaller arteries called arterioles, each of which
leads to knot/ball of capillaries called glomerulus (腎小球)
○ Glomerular capillaries sit inside cup-shaped Bowman’s capsule (腎小囊),
which is double-walled structure leading into kidney tubule
○
2
