The Urinary System

Outline• Introduction – Basic Anatomy & Function• Kidneys

– Nephrons• JGA Apparatus• Glomerular Filtration• Tubular Reabsorption• Tubular Secretion

• Regulation of Urine Volume & Concentration• Renal Clearance• Kidney Stones• Ureter• Urinary Bladder• Urethra

Urinary System Organs

Figure 26.1a

Urinary System Organs

Figure 26.1a

Internal Anatomy

Figure 26.3b

Pyelitis & Pyelonephritis• Infection of the renal pelvis and calyces

• Infections that affect the entire kidney are pyelonephritis

Blood and Nerve Supply• Approximately one-fourth (1200 ml) of systemic cardiac

output flows through the kidneys each minute

• Arterial flow into and venous flow out of the kidneys follow similar paths

The Nephron

Figure 26.4b

Renal tubule

Figure 26.4b

Renal tubule

Figure 26.4b

Capillary Beds

Figure 26.5a

Capillary Beds

Figure 26.5a

Vascular Resistance in Microcirculation

Figure 26.6

Juxtaglomerular Apparatus (JGA)

Filtration Membrane

Figure 26.8a

Filtration Membrane

Figure 26.8c

Mechanism of Urine Formation

Glomerular Filtration

• Passive, nonselective process where hydrostatic pressure forces fluids and solutes through the membrane

• The glomerulus is more efficient than other capillary beds

Net Filtration Pressure (NFP)

• The pressure responsible for filtrate formation

Figure 14.08

Glomerular Filtration Rate (GFR)

• The total amount of filtrate formed per minute by the kidneys

• Factors governing filtration rate at the capillary bed

• Normal GFR = 120ml/min

Thought Question

Renal Autoregulation

• Under normal conditions, renal autoregulation maintains a nearly constant glomerular filtration rate

• Autoregulation entails two types of control– Myogenic – Tubuloglomerular feedback

Myogenic Control

• Responds to changes in pressure in the renal blood vessels

• Reflects the tendency of vascular smooth muscle to contract when stretched

Tubuloglomerular Feedback• Senses changes in the JG apparatus

• Directed by the macula densa cells and the JG apparatus

• Cells of the macula densa respond to filtrate flow rate and osmotic signals

• Intrinsic controls cannot handle extremely low systemic blood pressure

Sympathetic Nervous System

• When the SNS is at rest:– Renal blood vessels are maximally dilated

• Under stress:– Norepinephrine is released by the SNS– Epinephrine is released by the adrenal medulla – Afferent arterioles constrict and filtration is

inhibited

Figure 14.20

Renin-Angiotensin Mechanism

• Is triggered when the JG cells release renin

• Renin acts on angiotensinogen to release angiotensin I

• Angiotensin I is converted to angiotensin II by ACE

• Angiotensin II:

• As a result, both systemic and glomerular hydrostatic pressure rise

Figure 14.21

Regulation of GFR

Figure 26.11

Other Factors Affecting Glomerular Filtration

• Prostaglandins (PGE2 and PGI2)

• Nitric oxide

• Adenosine

• Endothelin

Anuria

• Low urine output– May indicate that glomerular blood pressure is

to low to cause filtration

– Usually results from situations where the nephrons cease to function

Thought Question

Tubular Reabsorption• A transepithelial process whereby most tubule contents are

returned to the blood

• Transported substances move through three membranes

• Ca2+, Mg2+, K+, and some Na+ are reabsorbed via paracellular pathways

• Organic Nutrients

• Water and ions

Sodium Reabsorption: Primary Active Transport

• About 80% of the energy used for active transport is used for sodium

• Sodium reabsorption is almost always by active transport

• From there it moves to peritubular capillaries

• Na+ reabsorption provides the energy and the means for reabsorbing most other solutes

Sodium Reabsorption or Excretion

• Controlled by two hormones– Aldosterone – adrenal cortex

(mineralocorticoid)

– Atrial Natriuretic Peptide (ANP) – produced by the heart

Figure 14.22

Figure 14.23

Reabsorption by PCT Cells

Figure 26.12

Figure 14.17

Figure 14.17

Figure 14.29

Figure 14.30

Nonreabsorbed Substances• A transport maximum (Tm):

• When the carriers are saturated, excess of that substance is excreted

• Substances are not reabsorbed if they:

• Urea, creatinine, and uric acid are the most important nonreabsorbed substances

Tubular Secretion• Essentially reabsorption in reverse, where

substances move from peritubular capillaries or tubule cells into filtrate

• Important for:

Regulation of Urine Concentration and Volume

• Osmolality

• Body fluids are measured in milliosmols (mOsm)

• The kidneys keep the solute load of body fluids constant at about 300 mOsm– This is accomplished by the countercurrent

mechanism

Countercurrent Mechanism

• By the time the filtrate reaches the loop of Henle, the amount and flow are reduced by 65% but it is still isosmotic

• The solute concentration in the loop of Henle ranges from 300 mOsm to 1200 mOsm

• Loop of Henle functions as a countercurrent multiplier

• Vasa Recta

Loop of Henle: Countercurrent Multiplier

Figure 26.14

Thought Question

Formation of Dilute Urine• Filtrate is diluted in the ascending loop of Henle

• Dilute urine is created by allowing this filtrate to continue into the renal pelvis

• Sodium and selected ions can be removed by active and passive mechanisms

• Urine osmolality can be as low as 50 mOsm (one-sixth that of plasma)

Formation of Conc Urine• Antidiuretic hormone (ADH, Vasopressin)

inhibits urine output

• In the presence of ADH, 99% of the water in filtrate is reabsorbed

• ADH is the signal to produce concentrated urine

• The kidneys ability to respond depends upon the high medullary osmotic gradient

Loop of Henle: Countercurrent Multiplier

Figure 26.14

Figure 14.24

Diuretics• Chemicals that enhance the urinary output

include:– Any substance not reabsorbed– Substances that exceed the ability of the renal tubules

to reabsorb it

• Osmotic diuretics include:

Renal Clearance• The volume of plasma that is cleared of a

particular substance in a given time

• Renal clearance tests are used to:– Determine the GFR– Detect glomerular damage– Follow the progress of diagnosed renal disease

RC = UV/PRC = renal clearance rateU = concentration (mg/ml) of the substance in urineV = flow rate of urine formation (ml/min)P = concentration of the same substance in plasma

Renal Clearance

Thought Questions

• If renal clearance is less than that of inulin what does that mean?

• If renal clearance is zero what does that mean?

• If renal clearance is greater than that of inulin what does that mean?

Table 14.02

Figure 14.11

Kidney Stones

• Kidney Stones– Calcium, magnesium or uric acids salts that crystalize

in urine

– Large stones blockage increasing pressure excruciating pain

– Predisposing conditions

– Broken up using shock wave lithotripsy

Ureters

Urinary Bladder

Urethra

Figure 26.18a, b

Micturition

Summary• Introduction – Basic Anatomy & Function• Kidneys

– Nephrons• JGA Apparatus• Glomerular Filtration• Tubular Reabsorption• Tubular Secretion

• Regulation of Urine Volume & Concentration• Renal Clearance• Kidney Stones• Ureter• Urinary Bladder• Urethra