Three - Urinary System

Main Functions and Anatomy - Kidneys

Location: Superior lumbar region against posterior abdominal wall

Functions:

• Regulation of fluid and electrolyte balance

• Excretion of metabolic waste and foreign substances

• Regulation of pH (acid-base balance)

• Maintenance of blood pressure - through blood volume and renin production

• Regulation of erythropoiesis (red blood cell production hormone)

• Metabolic function

  • Activating vitamin D

  • Production of new glucose when starving

Gross Anatomy:

Three regions

• Renal cortex

  • Outer region containing cortical nephrons

• Renal medulla

  • Cine shaped tissue called renal pyramids

• Renal pelvis

  • Funnel shape continuous with ureter

  • Drains papillae into minor then major calyces

Main Functions and Anatomy - The Nephron

Consists of:

1. Renal Corpuscle: tight collection of capillaries (glomerulus)
   Specialisation for filtration

2. Real Tubule: proximal tubule PT, nephron loop, distal tubule DT
   Reabsorption and secretiom
   
   

Classes:

• Urine forming units of the kidneys

• Around 1 million per kidney

• Divided into two types

1. Cortical nephron

   1. Found in renal cortex

   2. 85% of all nephrons

2. Juxtamedullary nephrons

   1. Nephron loop descend deep into renal medulla

   2. 15 % of all nephrons

   3. Important to form concentrated urine

Renal Blood Supply

Nephron Blood Supply

1. Glomerular capillaries - specialised for filtration

2. Peritubular capillaries - readily absorb solutes and water from tubule cells

3. Vasa recta - important for forming concentrated urine

They also provide oxygen and nutrients to surrounding cells

Urine Production

Three Major Processes

1. Glomerular Filtration

   • Passive, filtrate = plasma - proteins, RBCs

2. Tubular Reabsorption

   • Passive or active transport, what's needed is reabsorbed

3. Tubular Secretion

   • Active transport, adds to filtrate

Urine Production = filtration + secretion - reabsorption

Glomerular Filtration Membrane (GFM)

Consists of:

• Glomerular capillaries

• Basement membrane

• podocytes

Filtration of solutes from blood is determined by size and charge;

• Small molecules filter freely

• Large molecules and blocked (remain in blood)

• Negative charged molecules are repelled (remain in blood)

Allows free passage of water and most solutes (ions, glucose, amino acids, small proteins, vitamins, nitrogenous wastes)

Glomerular Filtration

A non-selective process where blood pressure forces fluid through GFM into glomerular capsule = positive net filtration pressure, hydrostatic pressure drives filtration, does not consume metabolic energy.

Forces determining filtration pressure:

1. Glomerular hydrostatic pressure (GHP)

2. Glomerular colloid osmotic pressure (GCOP)

3. Capsular hydrostatic pressure (CHP)

Net Filtration Pressure = GHP - (GCOP + CHP)

• 50mm Hg - (30mm Hg + 10mm Hg) = 10mm Hg

Glomerular Filtration Rate (GFR)

Total volume of filtrate formed by renal nephrons each minute. Determines how quickly blood is cleansed of metabolic waste. Influences the kidneys’ ability to maintain homeostasis.

GFR closely regulated in order to;

• Remove nitrogenous wastes from the blood 

• Remove any excess solutes and water from the blood 

• Ensure the body tissues have a constant blood volume and pressure

GFR needs to be relatively constant;

• Maintains production of filtrate and therefore adequate reabsorption and secretion

• Maintains extracellular fluid homeostasis

Factors Determining GFR

• Net filtration pressure (NFP)

• Total surface area (SA) available for filtration

• Filtration membrane permeability (PERM)

Glomerular Filtration Rate (GFR) = Net Filtration Pressure (NFP) x Surface Area (SA) x Filtration Membrane Permeability (PERM)

Tubular Resorption

Selective process that reclaims most of filtrate formed during glomerular filtration.Tubular Secretion

• Proximal tubule - site of most reabsorption

• Nephron loop - reabsorption of water and solutes, not coupled

• Distal tubule and collecting duct - reabsorption regulated by hormones

Tubular Secretion

Selective process moving substances from blood into filtrate, occurs along the length of tubule. Important for:

• Removal of unwanted substance such as drugs and metabolites that have been passively reabsorbed

• Elimination of excess K+

• Maintenance of blood pH through removal of H+, organic acids or bases

Intrinsic Regulation of GFR

Renal autoregulation;

• Protects glomerulus from damage at high blood pressures

• Ensures kidneys still receive sufficient blood flow to allow necessary filtration and removal of wastes

• Operates as long as mean arterial pressure is maintained at 80-180mmHg

Myogenic Mechanism

Tubuloglomerular Feedback Mechanism

Relies on inherent properties of arteriole walls Composed of vascular smooth muscle Increased BP = stretched vascular smooth muscle = constricted afferent arteriole in response = lower GHP = higher GFR Decreased BP allows dilation of afferent arterioles = higher GHP = lower GFR Both responses serve to maintain normal NFP (Net filtration pressure)  and GFR

Relies on interactions between nephron tubule and glomerulus Response to changes in solute concentration in filtrate within tubule Flow dependant GFR increased = less time for reabsorption = concentration of NaCl in filtrate remains high = macula densa cells respond to cause constriction of different arteriole = lower GHP = higher GFR = slower flow of filtrate = more time for processing = more reabsorption of NaCl

Renin-Angiotensin-Aldosterone-System

1. Blood pressure decreases = GFR decreases

2. Juxtaglomerular cells release Renin

3. Renin causes angiotensinogen = angiotensin-I

4. ACE causes angiotensin-I = angiotensin-II

5. Promotes vasoconstriction of efferent arterioles

6. GRF

Formation and Passage of Filtrate Summary

1. Blood enters glomerular through afferent arteriole

2. Filtered by filtration membrane

3. Filtrate enters glomerular capsule

4. Filtrate enters proximal tubule (65% reabsorbed)

5. Filtrate enters nephron loop

6. Filtrate enters distal tubule

7. Filtrate enters collecting duct

8. Passes through papillae then enters minor→major calyces→renal pelvis

9. Drainers into ureter→bladder→urethra

Regulation of Blood Pressure

• Short term neural and hormonal controls - by altering resistance and CO

• Long term renal regulation - by altering blood volume

Long-Term Renal Regulation of Blood Pressure

1. Direct renal mechanism

Alters blood volume independently of hormones

• Increased BP or blood volume causes elimination of more urine - lower BP

• Decreased BP or blood volume, kidneys conserve water - higher BP 

2. Indirect renal mechanism

The renin-angiotensin-aldosterone system (RAAS)

• Lower arterial blood pressure = release of renin

• Renin commences a cascade of events catalyzing angiotensin - angiotensin I - angiotensin II

Functions of Angiotensin II

Renin released by granular cells of the kidneys, renin is released when:

• Lower arterial pressure

• Lower body NaCl content

• Lower stretch of afferent arteriole
  
  All aim to increase EFC volume and therefore blood pressure

Aldosterone vs Antidiuretic Hormone (ADH)

Aldosterone	Antidiuretic Hormone (ADH)
Released in response to; Angiotensin II  Higher plasma K+  Acts at the DT and CD to; Increase Na+ reabsorption Increase water reabsorption Increase K+ secretion Increase ECF volume Decrease urine output	Inhibits diuresis (urine production)  Released when solute concentration of blood increases (increased osmolarity)  Or when stimulated by angiotensin II DT and CD become more permeable to water (insertion of more aquaporins)  Results in increased water reabsorption - THEREFORE decreased urine output  ADH secretion inhibited by diuretic medications and alcohol

Blood Pressure and Urine Output