Chapter 25 - Urinary System and Renal Physiology

prefix for kidneys

nephro-

sufix for kidneys

-uria

amount of fluid filtered from blood by kidneys every single day

200 liters

Function of the kidney

maintaining the composition of the body’s extracellular fluids by filtering the blood

1. Regulate total body water volume and concentration of solutes in water

2. Regulate concentration of ions in ECF

3. Acid-base balance

4. Remove toxins, metabolic wastes, & other foreign substances

5. Hormone production-EPO and renin

Location of kidneys

• back

• size of bar of soap

• Each kidney lies between the parietal peritoneum and dorsal body wall

Kidney is what type of organ?

Retroperitoneal organ

• no viseral peritoneum, just against bodywall

Medial portion of kidney is __

Concave

what enters renal hilum

• ureters

• renal blood vessels

• lymphatics

Where is adrenal gland located

sits immediately superior to each kidney

Supporting external structures of the kidneys

1. Renal fascia

2. Perirenal fat capsule

3. Fibrous capsule

Renal fascia

• Supporting external structure of the kidneys

• dense connective tissue

• Function: anchors kidneys to surrounding structures

Perirenal fat capsule

• Supporting external structure of the kidneys

• fat mass surrounding kidneys

• Function: Cushions kidneys from physical trauma

  • still kidneys easily damaged

Fibrous capsule

• Supporting external structure of the kidneys

• thin, transparent capsule

• Function: prevents disease from spreading to kidneys from other parts of body

Major internal regions of kidneys:

1. Renal Cortex

2. Renal Medulla

3. Renal Pelvis

Renal Cortex

• a major internal region of kidneys

• outter most region

• provides area for glomerular capillaries and blood vessel passage, EPO, & renin produced here

Renal Medulla

• a major internal region of kidneys

• Contain several renal pyramids → packed with capillaries & urine-collecting tubules

• Function: allows for some water reabsorption, electrolyte balance, disposal of waste and H+ ions

Renal Pelvis

• a major internal region of kidneys

• Open space (largest) in center of each kidney

• Pelvis branches to form major calyces (calyx)

  • Major calyces lead into minor calyces at tip of each renal pyramid

• Function of calyces and pelvis: urine collection from renal medulla

Pelvis branches to form

major calyces (calyx)

Major calyces lead into __ at tip of each renal pyramid

minor calyces

Function of calyces and pelvis

urine collection from renal medulla

what delivers blood to kidneys?

Renal arteries deliver to kidneys → divide into smaller blood vessels to serve major regions of kidney

Renal arteries split into

Largest to smallest

1. Segmental arteries (5)

2. Interlobar arteries

3. Arcuate arteries

4. Cortical radiate arteries

Segmental arteries

• there are 5

• the largest of the smaller blood vessels branching off the renal arteries

• ensure blood reaches all branches

Interlobar arteries

• the 2nd largest of the smaller blood vessels branching off the renal arteries

• travel between renal pyramids

Arcuate Arteries

• the 2nd smallest of the smaller blood vessels branching off the renal arteries

• arc over bases of pyramids

Cortical radiate arteries

• the smallest of the smaller blood vessels branching off the renal arteries

• supply renal cortex

Veins of kidney

• smallest to largest

1. Cortical radiate veins

2. Arcuate veins

3. Interlobar veins

4. Renal veins

Nerve Supply to the Kidneys

Renal plexus → autonomic nerve fibers & ganglia

Sympathetic vasomotor fibers regulate blood supply to each kidney

Function: Adjusts diameter of renal arterioles to adjust blood flow to glomeruli

Renal plexus

autonomic nerve fibers & ganglia of kidneys

_ regulate blood supply to each kidney

Sympathetic vasomotor fibers

nephron

• is the functional unit of the kidney

• Function: Responsible for forming filtrate and eventually urine in the kidneys

General structure of nephron

Each nephron contains a renal corpuscle and renal tubule

renal corpuscle

• structure in the nephron

  • Located entirely within renal cortex

• filters blood to form the filtrate

renal tubule

• structure in the nephron

• reabsorbs some substances from the filtrate & secretes other substances into the filtrate

Subdivisions of renal corpuscle

1. glomerulus

2. Glomerular capsule

Glomerulus

• Subdivision of renal corpuscle

• cluster of capillaries

• Blood enters glomerulus via afferent arteriole, leave via efferent arteriole

• Capillaries are very porous → some fluid & substances in blood easily filtered out of capillary

• Fluid is called filtrate

filtrate

• some fluid in blood easily filtered out of capillary

• raw material used to produce urine

Blood enters glomerulus via

afferent arteriole

Blood leaves glomerulus via

efferent arteriole

Glomerular capsule

• Subdivision of renal corpuscle

• double-layered structure that completely surrounds glomerular capillaries

• Inner layer has podocytes with foot processes

Inner layer of glomerular capsule has

• podocytes with foot processes

Renal Tubules & Collecting Duct

Begins in renal cortex, extends into renal medulla, then returns to renal cortex

Subdivisions of renal tubules

1. Proximal convoluted tubule (PCT)

2. Nephron Loop

   1. descending limb

   2. ascending limb

3. Distal convoluted tubule (DCT)

Proximal convoluted tubule (PCT)

• leads immediately off from glomerulus

• Located in renal cortex

• Large cuboidal epithelial cells with dense microvilli

Nephron Loop

• formerly Loop of Henle

• Travel between renal cortex and renal medulla

• Composed of

  • Descending limb

  • Ascending limb

• Function: allows the kidneys to vary the concentration of urine according to how much water is reabsorbed at nephron loop

Descending limb Nephron Loop

• Leads off from PCT

• High permeability to H2O, impermeable to solutes

Ascending limb Nephron Loop

• Continuous with DCT

• High permeability to solutes, impermeable to H2O

Distal Convoluted Tubule (DCT)

• Located in cortex, composed of small cuboidal epithelia

• Smaller diameter than PCT, contain no microvilli

Collecting Ducts

• Each collecting duct receives filtrate from tubules of multiple nephrons

  • Collecting ducts fuse together, dump urine into minor calyces

Important cell types in collecting ducts

1. Principal cells → maintain Na+ balance in body

2. Intercalated cells → help maintain acid-base balance

Types of Nephrons

1. Cortical Nephrons

2. Juxtamedullary Nephrons

• have the same structures (previously described), BUT there are slight modifications to those structures

Cortical Nephrons

• Located almost entirely in the cortex

  • Small portion of nephron loop found in renal medulla

Juxtamedullary Nephrons

Nephron loops deeply invade renal medulla

Capillary Beds of Nephrons

1. Glomerulus Capillaries

2. Peritubular Capillaries

3. Vasa Recta

Glomerulus capillaries

Maintains high pressure to increase filtrate production

Peritubular Capillaries

• Low pressure capillaries arising from efferent arteriole

• Cling to proximal & distal tubules of cortical nephrons

  • Function: Reabsorb water & solutes from tubule cells

• Empty into cortical radiate veins → filtered blood returns to circulation

Vasa Recta

• Found only on juxtamedullary nephrons

  • Run parallel to long nephron loop

• Help form concentrated urine

Juxtaglomerular complex

• portion of nephron where portion of ascending limb lies against afferent & efferent arterioles

• Overall function: Regulate blood pressure & filtration rate of the glomerulus

cellular modifications at Juxtaglomerular complex

1. Macula densa

2. Granular cells (Juxtaglomerular cells)

3. Extraglomerular mesangial cells

Macula densa

• 1 of the cellular modifications at Juxtaglomerular complex

• chemoreceptor cells

• Function: Monitor NaCl content of filtrate entering distal convoluted tubule

Granular cells

• aka: Juxtaglomerular cells

• 1 of the cellular modifications at Juxtaglomerular complex

• specialized smooth muscle cells cells

• Found in arteriolar walls of afferent arteriole

• Can sense blood pressure in afferent arteriole

• Also stimulated by macula densa cells

The granules in granular cells _

• secrete renin

• Renin mostly affects the efferent arteriole!

• Low NaCl concentration = increased renin release

• Low pressure in arteriole = increased renin release

Extraglomerular mesangial cells

• 1 of the cellular modifications at Juxtaglomerular complex

• Packed between tubule and arterioles

• function: provide structural support and regulate blood flow within the glomerulus.

Diuresis

Urine formation

Urine Formation Steps

1. Glomerular Filtration

2. Reabsorption

3. Secretion

Glomerular filtration (Urine Formation Step)

production of a cell and protein-free filtrate that serves as the raw material for urine

How does Glomerular filtration work?

• Pressure forces fluid out of glomerular capillary & into glomerular capsule

• The filtration membrane allows passage of water, small solutes into glomerular capsule

  • aided by foot processes of podocytes

what do foot processes of podocytes in glomerular filtration

• foot processes create filtration slits

• Slits prevent passage of macromolecules/large-sized materials into filtrate

Filtration Pressures

• pressures that force fluid into or out of glomerulus

Outward Filtration Pressure

• force fluid out of glomerulus

• promotes filtrate formation

• outward pressure here is always HIGH

• composed of: Hydrostatic pressure in glomerular capillaries (HP_gc)

Hydrostatic pressure in glomerular capillaries (HP_gc)

• blood pressure of the glomerular capillaries that forces fluid out of glomerulus and into the space of the glomerular capsule

Inward Filtration Pressure

• oppose filtrate formation

• composed of

  1. Hydrostatic pressure in capsular space (HP_cs)

  2. Colloid osmotic pressure in glomerular capillaries (OP_gc)

Is the net filtration pos or neg?

• positive

• means forming more filtrate

Hydrostatic pressure in capsular space (HPcs)

• part of the inward filtration pressure

• pressure exerted by filtrate that is already in the glomerular capsule

Colloid osmotic pressure in glomerular capillaries (OPgc)

• part of the inward filtration pressure

• proteins that are still in capillaries will “pull” water back in

Which is smaller, Hydrostatic pressure in capsular space (HP_cs) or Colloid osmotic pressure in glomerular capillaries (OP_gc)?

HP_cs is smaller than OP_gc, usually at 15mmHg

• OP_gc has more influence on the inward pressure

Glomerular Filtration Rate

• the total volume of filtrate formed per minute for all nephrons in the kidneys

• highly regulated

• ideal is 125mL/min for men

What part of the glomerular capillary does filtration occur

Filtration occurs along the entire length of a glomerular capillary

Factors affecting GFR

1. Net Filtration Pressure (NFP)

2. Surface area of capillaries

   • increased area increases rate

3. Filtration membrane permeability

Regulation of GFR

• Tightly regulated for two reasons:

  1. Kidneys need constant GFR to make filtrate and maintain extracellular homeostasis

  2. Regulating GFR regulates blood pressure in entire body

  • Ex: decreasing GFR will decrease urine output

    • bc blood volume

Primary variable controlled to regulation GFR

• HP_gc

  • When HP_gc increases → NFP & GFR also increase

  • When HP_gc decreases → NFP & GFR decrease

Control of HP_gc can be

• intrinsic (renal)

• extrinsic (CNS)

Renal autoregulation of GFR

• intrinsic

• kidneys adjust resistance to blood flow

• Intrinsic controls can maintain GFR for blood pressures ranging 80-180 mm Hg

  • Myogenic mechanism

  • Tubuloglomerular Feedback Mechanism

Myogenic mechanism

• Renal autoregulation of GFR

• Rising systemic blood pressure stretches afferent arteriole

  • In response to stretch, the afferent arteriole's smooth muscle cells contract, causing the arteriole to constrict

• Blood flow into glomerulus restricted to maintain GFR at desirable rate

Tubuloglomerular Feedback Mechanism

• Renal autoregulation of GFR

• controlled by macula densa

  • Remember: macula densa monitor NaCl concentrations

• Increasing GFR = decrease in reabsorption rate

  • Response: Macula densa cause vasoconstriction of afferent arteriole → decreases blood flow into glomerulus → decreases GFR

Neural Mechanisms of GFR regulation

• Extrinsic

• The sympathetic nervous system will override renal autoregulation

  • happens during stressful situations, exercise, or when blood pressure drops significantly

  • fast

• Norepinephrine released by sympathetic system in response to low blood pressure

  • Vascular smooth muscle contracts

  • afferent arterioles contract

Hormonal Mechanisms of GFR regulation

• Extrinsic

• Renin-Angiotensin-Aldosterone mechanism → overall effect is to increase BP

• Granular cells of JGC stimulated to release renin

Activation of granular cells to release renin can involve

1. Stimulation by sympathetic nervous system

2. Activated macula densa cells

   • Macula densa sense low NaCl concentrations due to decreased GFR

3. Reduced stretch of arteriole walls

   • Decreased stretch of arteriole walls = low blood flow/pressure in afferent arteriole

Reabsorption (Urine Formation Step)

• selectively moving substances from the filtrate back into the blood

  • 99% of filtrate is reabsorbed by the body

• Substances can either move in between kidney tubule cells (paracellular) or through kidney tubule cells (transcellular)

• includes rebsorption of

  • Na+

  • Nutrients and ions

  • Water

paracellular (in terms of reabsorption)

Substances move in between kidney tubule cells

Transcellular (in terms of reabsorption)

Substances move through kidney tubule cells

What type of process is reabsorption of Na+

transcellular, active process

What type of process is Reabsorption of nutrients & ions

• Can be transcellular or paracellular

• Nutrients are co-transported with Na+ via transcellular route

• Ions can be transcellular or paracellular

What type of process is Reabsorption of water

• Passive

• Some H20 absorbed via paracellular route

• Transmembrane protein aquaporin allows water to cross plasma membrane of tubule cell

  • PCT has many aquaporins → water is always absorbed here

  • Collecting ducts have no aquaporins until antidiuretic hormone (ADH) is present

• Number 4 in pic

aquaporin

• transmembrane protein

• allows water to cross plasma membrane of tubule cell

• in PCT

• in Collecting duct only when antidiuretic hormone is present

  • Collecting ducts have no aquaporins until antidiuretic hormone (ADH) is present

Any pathway using a transport protein has a

• transport maximum (T_m)

  • based on the number of binding sites

  • The more transport proteins for a specific molecule, the higher the amount absorbed

  • What happens when all the transport proteins for a particular substance are bound?

    • in diabetes mellitus this leads of glucosuria

Reabsorption in the proximal convoluted tubule (PCT)

• Contain villi and microvilli

• All glucose, amino acids, most nutrients reabsorbed here

• Most water and Na+ also reabsorbed here (~65%)

• Most electrolytes reabsorbed here

• Uric acid and urea also reabsorbed here

Reabsorption in the nephron loop

• Water reabsorption is not coupled to solute reabsorption here

• Water can leave the descending limb, but not the ascending limb

• Solutes can leave the ascending limb, but not the descending limb

• Importance: the difference in permeability between the ascending limb and descending limb allows the nephron to form dilute or concentrated urine

Reabsorption in the DCT & Collecting Duct

• Most water and solutes have already been reabsorbed by PCT and nephron loop

• Hormonally controlled

  1. Antidiuretic hormone (ADH)

  2. Aldosterone

  3. Atrial natriuretic peptide (ANP)

  4. Parathyroid hormone (PTH)

Antidiuretic hormone (ADH)

• inhibits urine formation by increasing water reabsorption to blood

• If water is returned to blood → it will not be put in urine

• Aquaporins inserted into collecting ducts

  • Amount of ADH is directly proportional to number of aquaporins inserted