THE URINARY SYSTEM
Systems Involved in Excretion
(Elimination of metabolic waste)
• Urinary system
Urine
– _____
• Respiratory system
Carbon dioxide
– ____________
• Digestive system
– _____
Feces
• Integumentary system
Sweat
– _____
Functions of the Urinary System
•
•
•
•
Excretion of metabolic wastes
Regulation of fluid and electrolyte balance
Regulation of blood pH
Regulation of blood pressure by the enzyme
renin
• Production of Erythropoietin hormone that
stimulate RBC formation
• Activation of vitamin D to Calcitriol that
regulate blood calcium levels
Anatomy of the urinary system
• Kidneys filter blood and
produce urine
• Ureters transport urine to
urinary bladder
Kidney----• Urinary bladder stores
Ureter---------urine until eliminated
• Urethra is tube for
elimination of urine
Urinary bladder---------Urethra----------------------
• Renal capsule
sheaths kidney
• Renal cortex is
outer region
• Renal medulla is
inner region with
8-18 renal
pyramids
• Renal papilla is
apex or tip of
renal pyramid
• Renal columns
are cortical tissue
between
pyramids
Anatomy of the Kidney
Renal capsule-----
Renal papilla-----Renal cortex----Renal pyramid-------Renal column---------------
Anatomy of the Kidney
• Minor calyx
receives urine
from renal papilla
---------------Minor calyx • Major calyx
collects urine from
Major calyx--------two or more minor
Renal pelvis-------------calyces
• Renal pelvis
collects urine from
all major calyces
---Ureter
• Ureter carries
urine from renal
pelvis to bladder
THE NEPHRON
• Microscopic filtering units of kidneys
composed of renal tubules and a blood supply
• Called functional units – make most of urine
• About 1 million per kidney
• Mostly in renal cortex with some tubules
extending into renal pyramids of medulla
THE NEPHRON
• Structure
– Renal corpuscle
• Loops of capillaries called
glomerulus
• Double layered Bowman’s
capsule surrounds glomerulus
– Renal tubules
• Proximal convoluted tubule
(PCT) connects to Bowman’s
capsule
• Loop of Henle with
descending and ascending
limbs
• Distal convoluted tubule
(DCT) is last part of nephron
• DCT of several nephrons
connect to a collecting duct
(CD)
NEPHRON ANATOMY
Bowman’s capsule---------------- -------------Glomerulus
PCT---------------------------------------DCT
Descending limb of Henle----------------
-----------Ascending limb of Henle
---------CD
Renal Corpuscle Anatomy
---Bowman’s capsule
Afferent arteriole---------DCT-----------------
-------Glomerulus
-------PCT
Efferent arteriole-----
--------Capsular space
Physiology of the Urinary System
•Four processes required for urine formation
1. Filtration from the glomerulus blood into
Bowman's capsule
2. Tubular Reabsorption from renal tubules to blood
3. Tubular Secretion from blood into the renal
tubules
4. Excretion of the final product (urine)
1
3
2
4. Urine
FILTERATION
• High glomerular blood hydrostatic pressure
of about 55 mmHg forces water and solutes
through the filtration membrane
– Formed by glomerular capillaries and inner
layer of Bowman’s capsule
FILTERATION
• Water and many different solutes pass from
glomerulus blood into the Bowman’s capsule
• Resulting fluid in capsular space called filtrate
• Filtrate contains
– Wastes such as urea and uric acid that must be
eliminated in the urine
– Useful (vital) substances such as water, organic
nutrients and electrolytes that must be kept
FILTERATION
• Amount of blood flow through kidneys in
one minute = renal blood flow (RBF)
– About 1200 mL/minute
• Amount of filtrate formed in one minute =
glomerular filtration rate (GFR)
– Between 105 and 125 mL/minute
• So around 10% of RBF becomes filtrate
TUBULAR REABSORPTION
• Most of filtrate moves from the nephron tubules
back into the blood
– Water, organic nutrients and electrolytes are kept
• Most reabsorption in proximal convoluted tubule
Blood of peritubular capillaries
TUBULAR REABSORPTION
• Water reabsorbed by Osmosis
• Solutes reabsorbed by:- Diffusion
- Active transport
- Co-transport
- Pinocytosis
TUBULAR SECRETION
• Some solutes move from the blood of the
peritubular capillaries into the filtrate
– Potassium, ammonium, hydrogen and
bicarbonate ions are secreted into filtrate
• Helps regulate acid-base balance and
electrolyte concentrations
Blood of peritubular capillaries
WATER REABSORPTION
•
•
•
•
Each day, between 150-180 L of filtrate
Each day, 1-2 L of urine
Each day, about 148 – 178 L reabsorbed
Amounts of H2O reabsorbed
–
–
–
–
–
65% in proximal convoluted tubules
15% in descending limbs of Henle
None in ascending limb of Henle
10% in distal convoluted tubules
10% in collecting ducts
WATER REABSORPTION
• Each day we lose about as much water as
we gain
• If loss exceeds gain, become dehydrated
• If gain exceeds loss, become over-hydrated
• Controlled by reabsorption of varying
amounts of water
Types of Water Reabsorption
• Obligatory water reabsorption
– In PCT and descending LOH
– More constant
• Facultative water reabsorption
– In DCT and CD
– Variable, depending on degree of hydration
– This reabsorption is controlled by ADH
Setting Stage for Water Reabsorption
•
•
•
•
65% of water is reabsorbed in the PCT
15% is reabsorbed in descending LOH
No water reabsorbed by ascending LOH
Sodium chloride is actively reabsorbed by
ascending LOH
• Filtrate becomes about 4X as concentrated at
bottom of LOH
Steps in Concentration of Urine
1. As water loss exceeds gain, hypothalamus
stimulate secretion of Antidiuretic hormone
(ADH) from the posterior pituitary
2. ADH makes the cells of the DCT and CD
more permeable to water
3. More water leaves the DCT and CD and
enters concentrated interstitial fluid and
blood at the bottom of the LOH
Steps in Concentration of Urine
4. Concentrated interstitial fluid and blood
take more water out of the filtrate and
concentrate the urine
5. Aldosterone from adrenal cortex increases
salt reabsorption from tubules and thus
helps to concentrate urine
6. Results in less, dark & concentrated urine
Concentration of Urine-ADH Present
ADH
DCT and CD more
permeable to water
and more water
reabsorbed
Steps in Dilution of Urine
1. As water gain exceeds loss, hypothalamus
stops secreting Antidiuretic hormone (ADH)
2. DCT and CD become less permeable to
water and less water is reabsorbed
3. Adrenal cortex decreases aldosterone
secretion, thus less salt is reabsorbed from
tubules and this helps to dilute urine
4. More water stays in filtrate and urine
resulting in more, light and dilute urine
Dilution of Urine
No ADH, so DCT and
CD less permeable to
water and less water
reabsorbed
Urine: the final product
• Urine composition
– 95% H2O
– 5% solutes (solids)
• Electrolytes
• Nitrogenous wastes such as urea, creatinine and uric acid
protein metabolism
– Urea from __________________
– Creatinine from ________________
muscle metabolism
– Uric acid from ____________________
nucleic acid metabolism
Urine: the final product
• Characteristics of Urine
– Color:- Yellowish to water clear
– Turbidity:- Clear without turbidity
– Specific gravity:- 1.00 to 1.03
– pH:- Acidic
Clinical Terms
• Nephritis:- Inflammation of the nephrons
• Renal failure:- inability to produce urine
• Hemodialysis:- blood dialysis to remove
waste metabolites when there is renal
failure
Clinical Terms
• Cystitis:- inflammation of urinary bladder
• Urethritis:- inflammation of the urethra
• Calculi:- formation of kidney stones