Urinary System (15 & 16)

LO1: Explain the function of the Urinary System

LO2: Describe the macroanatomy of the Urinary System

LO3: Describe the microanatomy of the Urinary System

LO4: Identify diseases of the Urinary System

LO5: Explain glomerular filtration

LO6: Describe tubular reabsorption

LO7: Describe tubular secretion

LO8: Explain the process of excretion from the kidney

Function of the Urinary System

Renal System

  • Regulation of plasma ionic composition: By increasing or decreasing the excretion of specific ions in the urine (Na+, K+, Ca2+, Mg2+, Cl-, HCO3 2-and H2PO4 -, H+, and HPO4)

  • Regulation of plasma volume and blood pressure by controlling the rate at which water is excreted in the urine affecting total blood volume and pressure

  • Regulation of plasma osmolarity

  • Regulation of plasma hydrogen ion concentration (pH)

  • Removal of metabolic waste products and foreign substances from the plasma

Kidney’s

  • Function as endocrine hormones and secrete

    • Erythropoietin (which stimulates erythrocyte production by the bone marrow)

    • Renin (which is necessary for the production of angiotensin II, to regulate salt and water balance for long-term control of blood pressure)

  • The activation of vitamin D3 to regulate blood calcium and phosphate levels

  • Gluconeogenesis

Describe the macroanatomy of the Urinary System

The Kidney’s

Blood Supply of the Kidney’s

Describe the microanatomy of the Urinary System

Structure of a Nephron

Types of Nephrons

  • Cortical nephrons

    • 85%

    • Renal corpuscles in the outer part of the cortex.

    • Short loop of Henle extending into outer medulla.

  • Juxtamedullary nephrons

    • 15%

    • Renal corpuscle in the proximity of the base of the renal pyramid.

    • Long loops of Henle, extending deep into the inner region of the pyramids (important for urine concentration mechanism).

Identify diseases of the Urinary System

Chronic Kidney Disease

  • Gradual loss of kidney function over months or years

  • Caused by diabetes mellitus, hypertension, glomerulonephritis or polycystic kidney disease

  • Leads to: decreased glomerular filtration rate, impaired electrolyte and acid-base balance, overactive RAA, hypertension

Diuretics

  • Medications that increase urine output by altering renal tubular reabsorption

  • Different types

    • Loop diuretics inhibit Na+ reabsorption

    • Thiazides act on distal convoluted tubule ―

    • Potassium-sparing diuretics affect collecting ducts

  • Reduce blood volume

  • Can affect electrolyte balance

Glomerular Filtration

Golmerular Filtration

  • The bulk flow of protein-free plasma from the glomerular capillaries into Bowman’s capsule.

  • The glomerular filtrate must cross three barriers to enter Bowman’s capsule

    • the capillary endothelial cell layer,

    • the surrounding epithelial cell layer

    • the basement membrane that is sandwiched between them

  • Water and small solutes cross the filter freely while cells and proteins do not

  • Filtration at the renal corpuscle is driven by Starling forces (hydrostatic and osmotic pressure gradients) existing across the walls of glomerular capillaries.

  • The filtrate resembles plasma in composition, except that it lacks most of the proteins found in plasma.

Pressure

  • Glomerular capillary hydrostatic pressure (PGC) favours filtration and is equal to the blood pressure in the glomerular capillaries (60 mm Hg)

  • Bowman’s capsule hydrostatic pressure (PBC) opposes filtration and is typically about 15 mm Hg.

  • Glomerular osmotic pressure (πPC)

    • Created by the presence of non-permeating solutes.

    • Opposes filtration because proteins in the plasma draw filtrate back into the glomerulus.

  • Bowman’s capsule osmotic pressure (πBC) is where the osmotic pressure in Bowman’s capsule favours filtration.

  • Fluid Pressure is a measurement of the force per unit area on an object in the fluid or on the wall of the vessel

  • Oncotic Pressure is a form of osmotic pressure induced by the proteins, notably albumin, in a blood vessel's plasma (blood/liquid) that causes a pull on fluid back into the capillary.

Tubular Reabsorption & Secretion

Excretion from the Kidneys

  • Excretion by the kidneys entails the elimination of solute and water from the body in the form of urine.

  • The rate at which a substance is excreted in the urine has a direct bearing on the volume and composition of the plasma.

  • Simple rule: material that enters the lumen of the renal tubules is excreted unless it is reabsorbed

  • Amount Excreted = amount filtered – amount reabsorbed + amount secreted

  • The rate at which a solute is excreted in the urine (expressed in moles/min) depends on:

    • The amount filtered

    • The rate at which the solute is secreted

    • The rate at which solute is reabsorbed.

  • If the amount of solute excreted per minute is less than the amount filtered, then the solute was reabsorbed in the renal tubules.

  • If the amount of solute excreted per minute is greater than the amount filtered, then the solute was secreted in the renal tubules.