Structure & Function of the Urinary System

Functions

• Regulation of extracellular fluid volume & blood pressure

  • works with CVS to ensure tissues get enough O2 and BP within normal values

• Regulation of plasma osmolarity

• Maintenance of ion balance

  • in response to diet intake, urinary loss helps to maintain proper levels of Na+, K+, Ca2+ ions

• Homeostatic regulation of pH

  • remove or conserve either H+ or HCO3- (bicarbonate ions) as needed

• Excretion of waste

  • removes metabolic wastes dissolved in plasma e.g. uric acid and creatine

• Secretion of hormones and enzymes

  • erythropoietin (RBC production), renin (sodium balance and BP homeostasis) & vit D conversion to control Ca2+ balance

Structure of nephron

the nephron is the functional unit of the kidney

Renal corpuscle filters blood plasma:

• Glomerulus capillary network

• Glomerular (Bowman’s) capsule double walled up surrounding glomerulus

Renal tubule filtered fluid passes into:

• proximal convoluted tubule

• descending, loop of henle (nephron loop) and ascending

• Distal convoluted tubule

Distal convoluted tubule of several nephrons empty into single collecting duct

Renal exchange processes

• Urinary excretion of substance depends on its filtration, reabsorption and secretion.

Glomerular filtration

Filtration barriers:

• Glomerular capillary endothelium

• Basal lamina

• Epithelium of Bowman’s capsule- podocytes

Forces that influence glomerular filtration:

• Hydrostatic/blood pressure (55 mmHg)

  • pressure of flowing blood in glomerular capillaries

  • favors movement of filtrate into Bowman’s capsule

• Colloid osmotic pressure (30 mmHg)

  • plasma proteins entering capsule create a gradient that favors movement back into capillaries

• Hydrostatic fluid pressure (15 mmHg)

  • in fluid up in enclosed Bowman’s capsule

  • creates a gradient that favors movement back into capillaries

Glomerular filtration rate (GFR)

• volume of fluid that filters into Bowman’s capsule per unit time

• average GFP 125mL/min or 180L/day

• Factors influencing GFP

  • Net filtration pressure

  • Filtration coefficient

    • surface area of glomerular capillaries

    • permeability between capillary & Bowman’s capsule

• 20% of plasma volume that pass through glomerulus is filtered

• <1% of filtered fluid is excreted

  

• Autoregulation of glomerular filtration rate takes place over a wide range of blood pressures.

Regulation of glomerular filtration rate

Myogenic response

• Intrinsic ability of vascular smooth muscle to respond to pressure changes

• ↑afferent arteriole resistance →↓GFR

• ↑efferent arteriole resistance → ↑GFR

Tubuloglomerular feedback

• Paracrine control through loop of Henle

Hormones and autonomic neurons

• By changing resistance in arterioles

• By altering the filtration coefficient

• Angiotensin II - vasoconstrictor

• Prostaglandins - vasodilators

Reabsorption

• movement of filtered solutes and water from lumen of tubule back into plasma

• takes places in proximal tubule and distal segment of nephrons

Principles governing tubular reabsorption of solutes & water:

• active transport to create concentration of electrochemical gradient

• water osmotically follow solutes

• Transepithelial transport (passing through cells)

  • substances cross both apical and basolateral membrane

• Paracellular pathway (passing around cells)

  • substances pass through junction between two adjacent cells

Principles governing the tubular reabsorption of solutes:

Some solutes and water move into and then out of epithelial cells (transcellular or epithelial transport); other solutes move through junctions between epithelial cells (the paracellular pathway). Membrane transporters are not shown in this illustration.

Secretion

Transfer of molecules from extracellular fluid into lumen of nephron:

• dependent on membrane transport proteins to move organic compounds

• active process move substrates against concentration gradient & use secondary active transport to move into lumen

• secretion of K+ and H+ is important in homeostatic regulation

• enables nephron to enhance excretion of substance

  • adds to substances collected during filtration, making excretion more effective

Fluid & Electrolyte Homeostasis

Water balance in the body

• Water makes up 50-60% of total body weight

• main entry of water is through food & drink

• most lost water in urine

• homeostasis maintains water balance unless there is pathology or an abnormal ingestion of water

Kidneys in water balance

• Kidneys cannot replenish lost water; only preserve or get rid of excess amounts

• volume loss replaced from the environment

• renal filtration will stop if there is a major loss causing extremely low blood pressure and blood volume

Urine concentration

• Osmolarity of urine measure of how much water is excreted by kidneys

• osmolarity changes as filtrate flows through nephron

• Diuresis - removal of excess water in urine

  • diuretics: drugs that promote urine excretion

• Kidney controls urine concentration

  • varying amounts of water and Na reabsorbed in distal nephron (distal tubule & collecting duct)

Osmolarity changes through nephron

Countercurrent multiplier system

Exchange is enhanced by active transport of solutes

• Two components:

  • loops of Henle that leave the cortex, dip down into the more concentrated environment of the medulla, then ascend into the cortex again.

  • peritubular capillaries - vasa recta, also forming hairpin loops.

Water reabsorption

• Distal tubule and collecting duct cells alter permeability to water

• process involves adding or removing water pores (aquaporins) in apical membrane

• depends on secretion of vasopressin/antidiuretic hormone (ADH)

Control of vasopressin secretion

3 stimuli controlling vasopressin secretion:

• Plasma osmolarity > 280mOsM

  • higher the osmolarity, more vasopressin released by posterior pituitary

  • osmoreceptors in hypothalamus detect changes in osmolarity

• Blood pressure

• Blood volume

Renin-Angiotensis-Aldosterone System (RAAS)

• Angiotensis II (ANG II) is the usual signal controlling aldosterone release from the adrenal cortex

• The RAS pathway begins when afferent arterioles secrete renin

• Renin converts inactive angiotensinogen, into angiotensis I (ANG I)

• ANG I converted into ANG II by angiotensis-converting enzyme (ACE)

• ANG II → adrenal gland → synthesis and release of aldosterone

• Aldosterone reabsorb Na+ at collecting duct

ACE2 & SARS-CoV-2 (COVID-19 Virus)

• ACE2 is present in many cell types and tissues including the lungs, heart, blood vessels, kidneys, liver and gastrointestinal tract.

• ACE2 is highly abundant on type 2 penumocytes in alveoli

• When the SARS-CoV-2 virus binds to ACE2, it prevents ACE2 from performing its normal function to regulate ANG II signalling

• ANG II increases blood pressure and inflammation, death of cells in alveoli