Excretion- process of removing metabolic wastes from the body
Metabolic wastes to remove
Carbon dioxide- removed by lungs (respiratory system)
Water- needs to be conserved but still a waste product of cellular respiration; some removed
by skin, lungs, large intestine & urine
Nitrogen- removed by kidneys after being converted to urea by liver; nitrogenous wastes
Feces- removed by digestive system through anus; parts of plants/animals eaten but not
digested; also contains bile & about 10 trillion bacteria (live or dead)
ANATOMY
Kidneys
Main excretory organ of body Organ that produces urine
Located in posterior part of abdominal cavity behind peritoneum (retroperitoneal)
Not fixed in rigid position against abdominal wall but move with diaphragm
Supported by renal arteries/veins & embedded in mass of adipose tissue surrounded by
layer of fibrous tissue
Right is lower than left because of liver
Bean shaped 4-5” long, 2-3” wide, 1-2” thick
About 0.5 lb each
Vessels (renal circulation) & nerves enter/leave through longitudinal fissure (hilum) on
concave side
Covered by fibrous capsule to give a firm smooth covering
Interior
Renal cortex- outer third; granular/reddish brown color; arches over pyramids of
medulla & dips between them forming inward extensions called renal
columns of Bertin; contains most parts of the nephron
Renal medulla- inner 2 thirds; darker in color; consists of renal pyramids (8-15/kidney)
base of pyramid points outward & points directed inward to form papilla
Only contains loop of nephron
Renal pelvis- funnel shaped sac in center of kidney; expanded upper end of ureter;
receives urine from nephron collecting duct; site of kidney stone formation;
made of minor/major calyx
Nephron
About 1 million/kidney
Made of 4 tubules- proximal convoluted, loop of Henle, distal convoluted, collecting
14 mm long & 0.055 mm diameter
Functional/structural unit of kidneys
Bowman’s capsule- upper end of nephron tubule which expands into a saclike
structure with 2 cell layers; blood vessels enter at indentation & form a knot
of capillaries (glomerulus) to fill the cavity; inner wall of Bowman’s capsule
dips between capillaries
Bowman’s capsule + Glomerulus = renal corpuscle
Ureter Pair of narrow tubes leading from kidneys to bladder
Urinary bladder- muscular sac that stores urine with capacity of 470 mL
Urethra- single tube leading from bladder out of body; 5 times longer in males (20 cm to 4 cm)
PHYSIOLOGY
What happens when you eat salty chips & drink lots of liquids? You bloat & get puffy
Why does this temporary retention of water happen?
Increased salt causes sodium imbalance, fluid retained to maintain homeostasis
Functions of Kidneys
Electrolyte balance
Detoxification
Blood pressure regulation
Regulation of blood plasma volume
Concentration of electrolytes & waste products in blood
pH of blood plasma
Ammonia is most common nitrogenous waste of body but it is toxic if it remains in body too long
Body is protected from ammonia poisoning by liver which removes ammonia from blood &
converts it to urea; urea enters bloodstream & is removed by kidneys
Nephron
Functional unit of kidney
~1 million/kidney
Parts Bowman’s capsule- upper end of tubule that expands into a saclike structure with 2
cell layers; glomerulus (blood capillary knot) fills cavity & there is lots of
contact between tubule/capillary
Proximal convoluted tubule
Loops of Henle
Distal convoluted tubule
Collecting duct
In one minute almost a ¼ of blood passes through kidneys
When blood flows into glomerulus
Pressure increases causing blood fluid to filter through capillary wall into Bowman’s capsule
Filtration- process through which materials from blood are forced out of glomerulus & into
Bowman’s capsule
More blood pressure increases the amount of filtrate entering the Bowman’s capsule by blood
pressure
In 24 hours, 180 L of fluid will pass from capillaries but the body loses only 1-1.5 L of fluid/day
in form of urine
Filtrate composition- urea, glucose, ions, amino acids, vitamins
SHOULD NOT CONTAIN- blood cells (RBC or WBC)
Body needs to retain many substances that are removed from the blood by filtration
Reabsorption- process by which materials return to the blood by passing through renal tubule
walls (proximal convoluted tubule) by way of osmosis or active transport
Water re-enters bloodstream by osmosis in proximal convoluted tubule where the membranes
are very permeable to water
Tubule membrane is only slightly permeable to urea so over half of it remains in tubule & is
secreted
Substances such as glucose, amino acids, vitamins, & ions are transported across tubular
membrane into bloodstream by active transport; this reabsorption occurs in the distal
convoluted tubule
Kidneys also play important role in regulating pH of urine
Secretion- process by which some substances such as hydrogen ions pass from blood into filtrate
Fluid/wastes remaining in renal tubule will form urine
Loop of Henle is what maintains higher salt concentration in kidney than in collecting duct
Cells activity transport chloride ions from filtrate into extracellular fluid
This ensures salt concentration of extracellular fluid remains high & promotes reabsorption of
water from collecting duct
Factors affecting amount of urine body makes
Blood supply- more blood means more urine
Ingestion of salt- more salt means less water as body adjusts to salt intake
Ingestion of water- more water means more urine
Urine- Usually amber color
Acidic if eating a mixed diet & alkaline on vegetable diet
Specific gravity of 1.016-1.020
Daily quantity 1.5 L
95% water, 5% metabolic byproducts & unessential chemicals
Nitrogenous wastes of urea/uric acid
Should not contain glucose, proteins, blood cells, bile pigments, hemoglobin
Micturition (urination)
Under control of nervous system
Brought about by stimulation of smooth muscles in walls of ureters, bladder & urethra
Peristalsis process like in digestive system
Bladder capacity- 470 mL but feels full at 250-300 mL
Stretching of bladder wall triggers initiation of micturition
Some Disorders of Excretory System
Glycosuria
Presence of glucose in urine due to either loss of ability of tubules to reabsorb it or excess
glucose in blood
Hematuria
Presence of blood cells in urine
Proteinuria
Presence of excess plasma proteins in urine
This & hematuria indicate kidney is damaged
Kidney stones (calculi)
Form from calcium (calcium oxalate crystals) deposits, proteins, magnesium salts & crystals
of uric acid
May obstruct urinary passage & cause blockage of urine in renal pelvis
Usually visible in Xray
Treatment: surgically removed or broken apart by laser or ultrasound; may also be flushed out
Urinary tract infection
Bacterial or viral infection (E. coli common)
Women more susceptible thatn men
May be asymptomatic but can be detected with bacteria or blood in urine
Urethritis- inflammation of urethral wall
Cystitis- inflammation of bladder lining
Urinary incontinence
Loss of bladder control or uncontrolled urination
Caused by surgery, infections, drugs, constipation, obesity, spinal/nerve damage
Males- can be due to enlargement of prostate
Females- pelvic floor lowers with age & after childbirth which increases pressure
on urethral sphincter
Treatment: behavior changes, dietary changes, pelvic floor exercises, medication, surgery,
collagen injections