Excretory System

Tony Serino, Ph.D.

Assistant Professor of BiologyMisericordia Univ.

Excretory System

• Remove wastes from internal environment• Wastes: water, heat, salts, urea, etc.• Excretory organs include: Lungs, Skin, Liver, GI

tract, and Kidneys• Urinary system account for bulk of excretion

Fluid Input & Output

Urinary System

Ureter Histolgy

Mucosa

Muscularis

Adventitia

-about 25 cm long, retroperitoneal, moves urine by peristalsis; volume of urine moved is called a jet (1-5 jets/min)-ureters enter the bladder wall obliquely, allowing them to remain closed except during peristalsis

Urinary Bladder(Remanent of Allantois)

Urinary Bladder Histology

Mucosa

Submucosa

Muscularis

(Serosa)

(Detrusor Muscle)

Urinary Bladder Filling

• Highly distensible• 10-600ml normally• Capable of 2-3X that

volume• Under normal

conditions, the pressure does not significantly increase until at least 300 ml volume is reached

Urethra

Urethra Histology

-epithelium changes from transitional to stratified squamous along its length-large numbers of mucous glands present

Bladder (Storage) Reflex

Voluntary control

• As urine accumulates, the bladder wall thins and rugae disappear

• Innervation (sympathetic) to the sphincter muscles (particularly the internal sphincter) keeps the bladder closed and depresses bladder contraction

Micturition Reflex (Voiding)• Urine volume increases, and

the smooth muscle increases pressure in bladder

• Stretch receptors in detrusor muscle, increase parasympathetic activity in the splanchnic nerve cause increase bladder contraction and internal sphincter relaxation

• Voluntary relaxation of external sphincter by a decrease in firing of the pudendal nerve

Kidney Location (x.s.)(Retroperitoneal)

Human Kidney

Hilus

Cortex vs. Medulla

Capsule

Anatomy of Kidney

Major and Minor Calyx

Arterial Supply

Venous Drainage

Renal Circulation

Renal Cortex Blood Flow

Glomerulus

BP in Renal Vessels

Nephron (two types)

Epithelium of Nephron

Urine Formation Overveiw

a. Pressure Filtration

b. Reabsorption

c. Secretion

d. Reabsorption of water

d

Glomerulus (SEM)

GlomerulusBowman’s Capsule

Podocytes

Filtration in GlomerulusCapillary Lumen Endothelium

Fenestration

Basement Mem.Pedicels

Slit pores

Glomerular Filtrate

Glomerular Filtration• A pressure filtration produced by the BP, fenestrated

capillaries of glomerulus, and the podocytes creates the glomerular filtrate

• Slit size allows filtration of any substance smaller than a protein

• Blood proteins create an osmotic gradient to prevent complete loss of water in blood,

• Pressure in Bowman’s capsule also works against filtration

• Volume of filtrate produced per minute is the Glomerular Filtration Rate (GFR)

• Average GFR = 120-125 ml/min

Forces controlling Glomerular Filtration

PCT and DCT (H&E stain)

Tubular Reabsorption• 75-85% of glomerular filtrate reabsorbed in PCT

• Some of the reabsorption is by passive diffusion– Example: Na+

• Much of the reabsorption is active, most linked to the transport of Na+; known as co-transport

• The amount of transporter proteins is limited; so most actively transported substances have a maximum tubular transport rate (Tm)

Reabsorption

Loop of Henle and CD

• Provides mechanism where water can be conserved; capable of producing a low volume, concentrated urine

• Loop of Henle acts as a counter-current multiplier to maintain a high salt concentration in medulla

• CD has variable water permeability and must pass through the medulla

• Allows for the passive absorption of water

Counter-current Multiplier• Descending is permeable to

water but not salt; loss of water concentrates urine in tube

• Ascending is permeable to NaCl but not water; Salt now higher in tube than interstitium; first passively diffuses out then near top is actively transported out

• Results in a self-perpetuating mechanism; maintaining a high salt concentration in center of kidney

Vasa Recta• Supply long loops of

Henle• Provide mechanism to

prevent accumulation of water in interstitial space

• Passive diffusion allows the blood to equilibrate with osmotic gradient in extracellular space

Counter-current Exchange

Tubular Secretion

• PCT and DCT both actively involved in secretion (active transport of substances from the blood to the urine)

• Both ducts play important roles in controlling amount of H+/HCO3

- lost in urine and therefore blood pH

• DCT actively controls Na+ reabsorption upon stimulation by aldosterone (controls final 2% of Na+ in urine)

Summary

Re-absorption

Loses water

Loses NaCl Selective Secretion & Re-absorption

Water Re-absorptionwith ADH present

Dehydration & Water Intoxication

Thirst

ADH release Reabsorption of Water in CD

Hypertonic, low volume urine

Juxtaglomerular Apparatus

Renin-Angiotensin-AldosteroneBP

Decreased Stretch in JG cells

Renin Release

Angiotensinogen Angiotensin I

Angiotensin II

Converting Enzyme arteriolar constriction

Aldosterone Release Na+ reabsorption

BP

water retention and BV

Decreased Na in Urine in DCT

stimulation of Macula Densa

Declining BP Regulation

Stimulates thirst

Increase BP Regulation (ANP)

Acid/Base Transport