Urinary System (Lecture) Exam 3

Components of the Urinary System

• Kidneys

• Ureters

• Urinary bladder

• Urethra

Homeostatic Kidney Functions

• Regulation of blood ionic composition

• Regulation of blood pH

• Regulation of blood volume and blood pressure

• Regulation of Maintenance of blood osmolarity

• Production of hormones

• Excretion of metabolic wastes and foreign substances (drugs/toxins)

• Regulation of blood glucose level

Osmolarity

Total # of dissolved particles per liter of solution

Protection and Support of the Kidneys

• Retroperitoneal

• Protective Coverings

  • Renal Capsule- innermost

  • Adipose Tissue

  • Renal Fascia- outermost

Cortex

Outer region of Kidney

Medulla

Inner region of Kidney

Renal Pelvis

A funnel-shaped, basin-like structure located at the central, medial part of the kidney

Renal Sinus

A fat-filled, hollow cavity located in the very center of the kidney

Nephron

Structural & Functional unit of the Kidney

Blood Supply of the Kidneys

• Although kidneys constitute <0.5% of total body mass, they receive 20-25% of resting cardiac output

• ~1100mL of blood flows through kidneys each minute

Nerve Supply of the Kidneys

• Renal nerves primarily carry sympathetic outflow

• They regulate blood flow through the kidneys

Blood Flow in the Kidney

Aorta → Renal Artery → Segmental Artery → Interlobar Artery → Arcuate Artery → Cortical Radiate Artery → Afferent Arteriole → Glomerulus → Efferent Arteriole → Peritubular capillaries/vasa recta → Cortical Radiate Vein → Arcuate Vein → Interlobar Vein → Renal Vein → Inferior Vena Cava

Renal Corpuscle

• Glomerulus

• Glomerular (Bowman’s) capsule

Renal Tubule

• Proximal convoluted tubule (PCT)

• Nephron loop/Loop of Henle

• Distal convoluted tubule (DCT)

Two Types of Nephrons

• Cortical Nephrons (~85%)

• Juxtamedullary Nephrons (~15%)

Cortical Nephrons

Essential for filtering blood, reabsorbing water and electrolytes, and producing the majority of urine

Juxtamedullary Nephrons

Concentrate urine, regulate water balance, and create a hyperosmotic environment

Glomerular (Bowman’s) Capsule

Filtrate is collected between the visceral and parietal layers

Juxtaglomerular Apparatus (JGA)

• The ascending loop contacts the afferent arteriole at the macula densa

  • Detect changes in Na+ and Cl- concentration

  • Signal juxtaglomerular cells to release renin

• The wall of the afferent arteriole contains smooth muscle cells called juxtaglomerular cells (aka Granular cells)

  • Regulates blood pressure in the kidney in conjunction with the ANS

The Filtration Membrane

• Endothelium of fenestrated capillary

• Basement membrane of capillary

• Filtration slits of visceral layer

Glomerular Filtration

Blood plasma and dissolved substances get filtered into the glomerular capsule

Tubular Reabsorption

All along the renal tubule & collecting duct, water, ions, and other substances get reabsorbed from eh renal tubule lumen into the peritubular capillaries and ultimately into the blood

Tubular Secretion

All along the renal tubule & collecting duct, substances such as wastes, drugs, and excess ions get secreted from the peritubular capillaries into the renal tubule. These substances will make their way into the urine.

Excretion of a solute

(Glomerular filtration + secretion) - reabsorption

Filtration

• The hydrostatic pressure & colloid osmotic pressure on either side of the capillary membrane of the glomerulus influence GFR

• The capacity is enhanced by:

  • Thin, porous membrane

  • Increased surface area

  • High glomerular BP

Glomerular Filtration

• Driven by blood pressure (blood hydrostatic pressure)

• Opposed by capsular hydrostatic pressure & blood colloid osmotic pressure

• Water & small molecules move out of the glomerulus

• ~150-180L of fluid filtered into the glomerular capsule daily

Blood hydrostatic pressure

55mmHg

Blood colloid osmotic pressure

30mmHg

Capsular hydrostatic pressure

15mmHg

Net outward pressure

10mmHg

Net Filtration Pressure (NFP)

The sum of osmotic and hydrostatic pressures

Glomerular Filtration Rate (GFR)

• The volume of filtrate formed by both kidneys each minute

• Remains relatively constant when MAP is between 80-180mmHg

Average Adult Male (GFR)

~125mL/min

Average Adult Female (GFR)

~105mL/min

GFR is controlled by

• Renal Autoregulation

• Neural Regulation

• Hormonal Regulation

High GFR

Useful substances are lost

Low GFR

Waste products may not be removed sufficiently

Regulation of Glomerular Filtration Rate

• Renal Autoregulation: Intrinsic Controls

• Neural & Hormonal Control: Extrinsic Controls

Renal Autoregulation: Intrinsic Controls

• Intrinsic ability of kidney to maintain constant blood pressure and GFR

• Functions by 2 mechanisms:

  • Myogenic response

  • Tubuloglomerualr feedback mechanism

Neural & Hormonal Control: Extrinsic Controls

• Process external to kidneys

• Involve physiologic process to change GFR

• Decreases with extensive sympathetic stimulation

• Two Hormones contribute to regulating GFR

  • Angiotensin II

  • Atrial natriuretic peptide (ANP)

Angiotensin II

Constrict afferent and efferent arterioles → Decreases GFR

Atrial Natriuretic Peptide (ANP)

Relax mesangial cells → Increase capillary surface area → increase GFR

Renal Autoregualtion

• Myogenic mechanism

• Tubuloglomerular feedback

Myogenic Mechanism

Increased BP → stretches afferent arteriole → afferent arteriole constricts → restores GFR

Tubuloglomerular Feedback

Macula densa on DCT monitors tubular fluid & signals juxtaglomerular cells (smooth muscle, surrounds afferent arteriole) to constrict afferent arteriole to decrease GFR

Transport Maximum (T_m)

Maximum rate of reabsorption/secretion of a substance by the renal tubules

Renal Threshold

The plasma concentrated at which the transport mechanism saturates, and the substances begins to appear in the urine.

Water Reabsorption

• Obligatory water reabsorption (necessary) -90%

• Facultative water reabsorption (optional)- 10%

Obligatory water reabsorption (necessary) -90%

• Water is absorbed along with solutes

• PCT & descending loop of Henle

Facultative water reabsorption (optional)- 10%

Regulated by ADH (DCT, collecting duct)

Reabsorption and Secretion in the PCT

• Diffusion

• Primary Active Transport

• Secondary Active Transport

Primary Active Transport in the PCT

Uses ATP (e.g. Na+ & K+ pumps)

Secondary Active Transport

• Driven by ions’ electrochemical gradient

• Symporters move substances in the same direction

• Antiporters move substances in opposite directions

Reabsorption Routes

• Paracellualr reabsorption

• Transcellular reabsorption

Paracellualr Reabsorption

Passive fluid leakage between cells

Transcellualr Reabsorption

Directly through the tubule cells