Excretion and the Urinary System

Excretion Overview

• Every day through metabolism the body produces damaging waste products
• CO2 is excreted through the respiratory system
• The digestive system eliminates indigestible material
• The kidneys continuously clear the blood of undesirable substances

Urinary System Facts

• The human body contains ~ 5 liters of blood that circulates continuously
• Normal fluid intake is ~ 2-3 liters daily
• 1200 liters of blood circulate through the kidneys daily which is the entire blood plasma volume, 60X per day
• The kidneys extract 180 liters of filtrate from the blood, which is ~7.5 l/hr
• If all filtrate extracted was excreted the body would be totally dehydrated within an hour
• Most of the filtrate is recovered; only about 1.5 liters of urine is produced daily

Kidney functions

• Filters blood allowing toxins, metabolic wastes, and excess ions to be excreted in the urine
• Regulates volume and chemical makeup of the blood
• Maintains the proper balance between water, salts, acids and bases
• Involved in gluconeogenesis during extended fasted
• Produces rennin to help regulate blood pressure and erthropoietin to stimulate red blood cell production
• Activates vitamin D

Other organs of the Urinary System

• Urinary Bladder: provides a temporary storage reservoir for urine produced by the kidneys
• Ureters: transports urine from the kidneys to the bladder
• Urethra: transports urine from the bladder to outside of the body

Urinary System Structures

• Kidneys
• Ureter
• Urinary bladder
• Urethra
• Renal arteries
• Renal veins

Kidney Details

• Bean shaped
• Approximately the size of a fist
• Retroperitoneal meaning it's located behind the abdominal cavity lining
• Located between 12th thoracic vertebra-3rd lumbar vertebra
• Partially protected by floating ribs (false ribs)
• The right kidney is slightly lower than the left

Kidney Blood Supply

• Blood enters the kidney through the renal artery
• Blood leaves through the renal vein
• The adrenal gland is NOT part of the urinary system

Kidney Structure

• Kidney treatment happens within the cortex and the medulla that ultimately produces urine
• Urine drains into the minor calyces, then major calyces, then renal pelvis then ureter

Nephron

• The nephron is the functional unit of the kidney
• Parts of the nephron: Glomerulus which is a capillary network, Bowman’s capsule, Proximal convoluted tubule, Loop of Henle, Distal convoluted tubule and the Collecting duct

Blood Supply to Nephrons

• Vasculature on top of tubules - arterioles on either side of glomerular capillaries
• Afferent artery - glomerular capillaries does filtration, note not all blood is filtered
• Efferent artery runs via peritubular capillaries, which wrap around the epithelium portion of nephron proximal/distal tubules
• 99% reabsorption happens in proximal tubules into peritubular capillaries into vasculature
• Vasa Recta - capillaries close to juxtamedullary nephrons only
• Vasa Recta are specialized peritubular capillaries that are hairpins close to thin/thick ascending and thin descending limbs of loop of Henle

Nephron Capillary Beds

• Each nephron has two capillary beds: Glomerulus and Peritubular capillaries
• The glomerulus is fed by an afferent arteriole and drained by an efferent arteriole
• Blood pressure in the glomerulus is high because arterioles possess high resistance
• Afferent arterioles have a larger inner diameter than efferent arterioles
• Fluids and solutes are forced out of the blood along the entire length of the glomerulus

Peritubular Capillary Beds and Vasa Recta

• These are low pressure capillaries that are adapted for absorption
• They contain porous capillaries
• Arise from efferent arterioles
• Capillary bed surrounds renal tubules
• Empty into the renal venous system
• The vasa recta capillaries of the juxtamedullary nephrons have similar properties but are long, straight arterioles

Processes in urine formation

• Blood composition and urine formation involve three processes: Glomerular Filtration, Tubular Reabsorption and Secretion

Processes for Urine Production

• Filtration: Blood pressure forces water and dissolved solutes across a membrane; Filtration depends on solute size
• Reabsorption: Removal of water and solute molecules from filtrate after entering renal tubules; Occurs by diffusion, active processes and osmosis; Water and solutes re-enter circulation at peritubular capillaries
• Secretion: Transport of solutes across tubular epithelium into filtrate; Water, sodium, and potassium may re-enter filtrate at Loop of Henle and collection ducts

Glomerulus Details

• Glomerulus: a group of renal capillaries, connecting afferent and efferent arterioles
• Afferent arterioles bring blood to the glomerulus, efferent arterioles take blood away, to the peritubular capillaries
• The kidney has the only portal system where capillaries are fed and drained by arterioles
• Unlike veins, arterioles contain smooth muscle, which can contract, affecting blood pressure in the area

Bowman’s Capsule Details

• Bowman’s capsule encircles a glomerulus
• The capsule has 2 layers of epithelium, separated by Bowman’s space
• The inner layer of epithelium is in intimate contact with glomerular capillaries
• The visceral layer of Bowman’s capsule or filtration membrane, surrounds glomerular fenestrated capillaries
• The specialized cells called podocytes have many extensions call pedicels
• Pedicels of adjoining podocytes interdigitate, forming filtration slits
• The surface of the pedicels is negatively charged

The Filter Membrane

• Filter membrane lies between the blood and the interior of the glomerular capsule
• It is composed of: the porous fenestrated endothelium of the glomerular capillaries, the basement membrane and Podocytes, which forms the visceral membrane of the glomerular capsule

Glomerular capillaries Details

• Glomerular capillaries are fenestrated, or contain openings, with a pore diameter of ~70nm
• Fenestrated capillaries are about 100X more permeable than continuous body capillaries; The capillary bed provides a very large surface area for filtration
• Size of the fenestrations prevents blood cells, platelets and most proteins from crossing endothelium; Most water and solutes DO cross
• The negative charge on pedicel membranes plays a part in blockage of small proteins
• Albumin= 7.0nm; IgG= 11.0nm, kidney filtrate contains all plasma components except protein. Note that some extremely small proteins will slip through the membrane

Pressure Filtration

• Pressure filtration forces fluids and dissolved substances through a membrane
• In the case of the kidney, fluid is forced through the capillary fenestrations passing through slits between pedicels, into Bowman's space
• Degree of filtration occurring in Bowman’s capsule depends on the net filtration pressure, which is determined by three forces
• Pressure within the glomerulus is defined as glomerular blood hydrostatic pressure
• Pressure in Bowman’s capsule is capsular hydrostatic pressure
• Osmotic pressure within the glomerulus is blood colloid osmotic pressure

Glomerulus Blood

• Blood enters through an afferent arteriole
• Blood exits the glomerulus via an efferent arteriole
• The arterioles are encircled by smooth muscle and can regulate pressure within the glomerulus

Calculating Filtration Pressure

• Glomerular blood hydrostatic pressure is 60mm
• Blood colloid osmotic pressure is 32mm
• Capsular hydrostatic pressure is 18mm
• Net filtration pressure is glomerular blood hydrostatic pressure -( blood colloid osmotic pressure + capsular hydrostatic pressure)

Glomerular Pressure Variation

• Glomerular blood hydrostatic pressure varies with constriction of afferent and efferent arterioles
• Constricting the afferent arteriole: Renal plasma flow decreases and glomerular filtration rate decreases
• Dilating the afferent arteriole: Renal plasma flow increases and glomerular filtration rate increases
• Constricting the efferent arteriole: Renal plasma flow decreases but glomerular filtration rate increases
• Dilating the efferent arteriole: Renal plasma flow increases but glomerular filtration rate decreases

Glomerulus Function

• Afferent arteriole constriction: glomerular pressure decreases, decreasing filtration
• Efferent arteriole constriction: pressure in the glomerulus increases, increasing filtration
• If the GFR is too high: Needed substances cannot be reabsorbed quickly enough and are lost in the urine
• If the GFR is too low: Everything is reabsorbed, including wastes that are normally disposed of