Urinary System Notes

Urinary System

Urinary System Functions

• Excretion: The urinary system filters blood to form urine, eliminating waste products.

• Blood Filtration: This process is complex, involving the modification of filtrate through reabsorption of necessary molecules and elimination of waste.

• Blood Volume and Pressure Regulation:

  • The kidneys regulate extracellular fluid by producing either concentrated or dilute urine.

• Solute Concentration Regulation:

  • The system regulates major ion concentrations such as Na^+, K^+, Cl^-, and Ca^{2+}.

• pH Level Regulation:

  • The kidneys secrete H^+ to maintain correct acidity in extracellular fluid.

• Red Blood Cell Synthesis Regulation:

  • The kidneys secrete Erythropoietin, stimulating erythrocyte production.

Anatomy of the Kidneys

• Location: Kidneys are located on either side of the vertebral column on the posterior abdominal cavity wall, spanning from T12 to L3.

• Position: The right kidney sits slightly lower than the left due to the liver's superior position.

• Layers surrounding the kidneys:

  • Renal capsule: A layer of fibrous connective tissue.

  • Adipose tissue layer.

  • Renal fascia: A connective tissue layer.

  • Final adipose tissue layer.

• Hilum:

  • An opening where the renal artery and nerves enter.

  • The renal vein and ureter exit.

• Renal Sinus:

  • A cavity opening after the hilum.

  • Filled with connective and adipose tissue.

• Kidney Structure: Made up of the outer cortex and inner medulla.

• Renal Pyramids:

  • Their bases form the boundary between the cortex and medulla.

• Renal Papillae:

  • The point of the pyramids.

• Minor Calyces:

  • Renal papillae extend into these funnels.

  • There are 8-20 minor calyces.

• Major Calyces:

  • Several minor calyces merge into major calyces.

  • There are 2-3 major calyces.

• Renal Pelvis:

  • Major calyces merge to form the renal pelvis.

• Ureter:

  • The renal pelvis forms a small diameter tube called the ureter, extending to the bladder.

• Renal Columns:

  • Located in between renal pyramids.

Anatomy of a Nephron

• Functional Units: Nephrons are the functional units of the kidneys.

  • Each kidney contains approximately 1.3 million nephrons.

• Components: Consist of four main components:

  • Renal corpuscle: Filters blood.

  • Proximal convoluted tubule (PCT): Returns filtered substances to the blood.

  • Loop of Henle (nephron loop): Conserves water and solutes.

  • Distal convoluted tubule (DCT): Adds additional waste to filtrate.

  • Collecting duct: Connects to several DCTs, carries fluid from cortex to medulla, and empties into the papillary duct.

  • Papillary duct: Empties into the minor calyx.

Flow Through the Kidneys

1. Renal corpuscle

2. Proximal Convoluted Tubule (PCT)

3. Loop of Henle (descending limb then ascending limb)

4. Distal convoluted Tubule (DCT)

5. Collecting Duct

6. Papillary Duct

7. Minor Calyx

8. Major Calyx

9. Renal Pelvis

10. Ureter

11. Urinary Bladder

12. Urethra

Renal Corpuscle

• Components:

  • Glomerulus: A network of capillaries receiving blood from the afferent arteriole and exiting via the efferent arteriole.

  • Bowman’s capsule: A capsule surrounding the glomerulus where fluid is filtered into from the capillaries and then flows to the PCT.

• Filtration Membrane: Allows fluid to flow into the capsule, which is the first major step of urine formation.

Juxtaglomerular Apparatus

• Juxtaglomerular cells: Smooth muscle cells arranged around the afferent arteriole at the entry point to the glomerulus.

• Macula Densa: Specialized cells located in a section of the DCT between the afferent and efferent arterioles.

• Juxtaglomerular apparatus: The contact point of the juxtaglomerular cells and Macula Densa.

• Function: Secretes renin, which aids in the regulation of blood pressure and filtrate formation.

Renal Tubule

• Proximal Convoluted Tubule (PCT)

  • Longer than the DCT.

  • Outer basement membrane with simple cuboidal epithelial cells.

  • Inner surface has many microvilli projections.

• Loop of Henle

  • Thick portions consist of simple cuboidal epithelium cells.

  • Thin portions consist of simple squamous epithelium cells.

• Distal Convoluted Tubule (DCT)

  • Simple cuboidal epithelium cells.

  • Smaller cells than the PCT with fewer microvilli.

• Collecting Duct

  • Simple cuboidal epithelium cells.

  • Larger in diameter compared to the rest of the renal tubule.

Urine Formation

1. Filtration: Non-selectively forcing small molecules and water out of the blood into the Bowman’s capsule where it is called filtrate.

Filtration Membrane

• Capillary endothelial layer: Contains many pores (fenestrations).

• Basement Membrane: Has spaces between fibers.

• Bowman’s Capsule Epithelial layer – Podocyte: Has foot processes creating filtration slits.

Urine Production: Filtration

• Renal Fraction: Percentage of total cardiac output that enters the kidneys.

  • In resting, healthy adults, this is approximately 21%.

• Glomerular Filtration Rate (GFR): The amount of filtrate (plasma) that enters the Bowman’s capsule.

• Renal Blood Flow Rate (per min):

  • Calculated as: Cardiac output (mL/min) x Renal fraction

  • Example: 5600 \, \text{mL} \times 21\% = 1176 \, \text{mL/min}

• Renal Plasma Flow Rate:

  • Calculated as: Renal blood flow rate x Amount of plasma in blood

  • Example: 1176 \, \text{mL/min} \times 55\% = 650 \, \text{mL/min}

• GFR:

  • Calculated as: Renal plasma flow rate x 19% (filtration fraction)

  • Example: 650 \, \text{mL/min} \times 19\% = 125 \, \text{mL/min}

  • In a day, this amounts to approximately 180 L.

Filtration Pressures

• Filtration Pressure: Pressure gradient in the renal corpuscle.

  • Glomerular capillary pressure

    • Outward pressure of blood in the capillaries.

    • Forces solutes and fluid out of capillaries and into Bowman’s capsule.

    • Greater pressure in glomerulus compared to other capillaries.

    • Efferent arteriole is smaller in diameter compared to afferent arteriole and capillaries.

  • Capsular hydrostatic pressure

    • Inward pressure of the filtrate in the Bowman’s capsule pressing back on the capillaries.

  • Blood colloid osmotic pressure

    • Inward pressure resulting from the osmotic force of plasma proteins in the glomerular capillaries.

Intrinsic Mechanisms of Control

• Auto-regulation: Direct regulation of GFR.

  • Myogenic Mechanism

    • Can work in reverse:

      • Increase in afferent arteriole pressure = increase in vessel stretch = constriction = decrease in GFR.

      • Restricts blood flow and lower cap pressure more consistent with efferent arteriole

  • Tubuloglomerular Mechanism

    • Can work in reverse:

      • Increase GFR

      • = increase in flow rate

      • = detection by macula densa cells of DCT

      • = secretion of paracrine hormone from macula densa

      • = increase constriction of afferent arteriole

      • = decreased GFR due to reduced flow = decreased capillary pressure

Release of Vasoactive chemicals inhibited

Tubular Reabsorption

• The process of returning water and solutes back into the blood as the filtrate flows through the renal tubule.

• PCT = Active and selective reabsorption

• Filtrate reduced by 65% at end of PCT

• The key to understanding transport across the basal membrane and apical membrane is understanding the main driving force which is Na^+ concentrations, firstly set by its active transport across the basal membrane.

Countercurrent Multiplier

• Fluid flows in the opposite direction (countercurrent) through two adjacent parallel sections of a nephron loop.

• The descending limb is permeable to water, but not to salt.

• The ascending limb is impermeable to water and pumps out salt.

DCT and Collecting Duct

• Solute and water reabsorption are primarily under hormonal control (e.g. Anti-Diuretic Hormone (ADH))

Tubular Secretion

• The movement of drugs and toxic by-products from the blood into the filtrate.

Ureters and Urinary Bladder

• Ureters

  • Run inferiorly and medially to the bladder and enter on the posterolateral surface

  • Peristaltic contractions move urine through the ureters

• Urinary Bladder

  • Hollow, muscular container, reservoir for urine

  • Max 1L, discomfort = 500 mL

  • Urinary bladder is able to distend

    • Large folds inside (similar to stomach)

    • Cells are transitional cells which stretch

    • Outer smooth muscle is able to stretch

    • Smooth muscle and elastic connective tissue prevent urine from exiting

    • Contraction of the smooth muscle assists with forcing the urine out

  • External urinary sphincter – skeletal muscle which controls the urine flow through the urethra

• Micturition

  • Elimination of urine from the urinary bladder.

    • Stimuli

      • Stretch of the bladder wall

      • Stretch receptors produce action potentials

    • Response

      • Parasympathetic stimulation = contraction of the bladder’s smooth muscle

      • Decreased somatic stimuli = relaxation of the external urinary sphincter

      • Higher centers of the brain can inhibit or stimulate the reflex

Regulation of Volume and Concentration

• Recall that the DCT and collecting duct are regulated by hormonal mechanisms depending on the conditions of the body

  • If water needs to be retained = water is reabsorbed and results in urine which is concentrated and of a small volume

  • If water needs to be lost = then the dilute filtrate can pass through the DCT and collecting duct with no change in concentration = large volume which is dilute

• Mechanisms which work together and assist with this are the

  • Renin-angiotensin-aldosterone hormone mechanism

  • Antidiuretic hormone mechanism

• Water Balance

  • Dehydration

  • Volume = Decreases

  • Osmolality = Increases

  • Why?

Anti-Diuretic Hormone (ADH)

• Also known as Vasopressin

• Sensitive to changes in blood osmolality and BP

  • Increase in blood osmolality = increase in ADH secretion

    • Response

      • Increased water reabsorption at the kidneys = decrease in urine output = decrease in blood osmolality

  • Decrease in blood pressure (and blood volume) = increased ADH secretion

    • Response

      • Increased water reabsorption = decrease in urine output = increase in blood pressure and volume

ADH action

• If we were so overhydrated we had no ADH…

  • Osmolality of extracellular fluids :small volume of dilute urine increase

  • ADH release from posterior pituitary : drinking large amount of water volume no change

  • Number of aquaporins (H20 channels) in collecting duct : saline ingestion of isotonic osmolarity no change

  • H2O reabsorption from collecting duct + : ingestion of hypertonic saline normal volume and osmolarity

• If we were so dehydrated we had maximal ADH…

  • Increase Incomplete compensation from descending limb of nephron loop

  • Osmolality of extracellular fluids + : eating salt without drinking water decrease

  • ADH release from posterior pituitary number of aquaporins (H20 channels) in collecting duct :replacement of sweat loss with plain water small volume of concentrated urine increase

  • H2O reabsorption from collecting duct + : hemorrhage for dehydration decrease

Renin-Angiotensin-Aldosterone Mechanism/System (RAAS)

• Renin sensitive to changes in blood pressure

  1. Renin is secreted in response to:

     • Reduced afferent arteriole stretch (reduced blood pressure)

     • Low Na^+ levels detected by the Macula Densa cells in the DCT

  2. Renin converts angiotensinogen (produced in the liver) to angiotensin 1

  3. Angiotensin 1 is converted to Angiotensin 2

     • Response (target tissue = adrenal cortex) = vasoconstriction = increase in blood pressure Increases Aldosterone and ADH secretion

  4. Aldosterone Increases the rate of Na^+ reabsorption

How does the RAAS maintain GFR?

• Angiotensin 2 = constriction of afferent and efferent arteriole = less renal blood flow = increasing systemic blood pressure

  • However to maintain GFR

    • Angiotensin has a greater effect on efferent arteriole

    • Therefore more blood remains in Glomerulus = GFR remains the same

How does the RAAS increase blood pressure?

• Stimulates release of Aldosterone

  • Increases sodium reabsorption in kidneys

  • Water follows sodium = increase in blood volume = increase BP

• Stimulates release of ADH

  • Increases H2O reabsorption = increases blood volume = increase BP

• Directly stimulates sodium reabsorption in kidneys

• Vasoconstriction of arteries/arterioles = increased blood pressure

• Atrial Natriuretic Peptide

  • Increased blood volume = increased atrial stretch

  • Atrial Myocardial cells stretch and release Atrial Natriuretic Peptide

Regulation of Blood pH

• pH is a measure of the H ions in the body

• The function of enzymes is greatly affected by the concentration of hydrogen ions (H^+).

  • Inverse relationship

    • Increase in H^+ = Decrease in pH = acidosis (acid)

    • Decrease in H^+ = Increase in pH = alkalosis (base)

  • Concentration is controlled by acids which release H^+ and bases which remove H^+

Buffer Systems

• The H^+ concentration is regulated by:

  • Chemical buffers (seconds)

  • Respiratory systems (1-3 minutes)

  • Renal Mechanisms (hours to a day)

• Carbonic Acid/Bicarbonate Buffer System (chemical buffer)

  • Important for the buffering of by products of intense exercise such as Lactic acid

• Respiratory Regulation

  • Influenced by the carbonic/bicarbonate buffer system

Renal Buffer

• Kidney tubules directly increase or decrease the rate at which H^+ are reabsorbed or secreted