Advanced Human Physiology Exam 3 Study Guide

A comprehensive set of flashcards covering advanced human physiology topics related to the urinary and digestive systems, created for exam preparation.

Functions of the urinary system

1. Maintain water balance in body + osmolarity of body fluids 2. Regulate quantity/conc. of extracellular fluid ions 3. Maintain plasma volume 4. Maintain pH (acid-base balance) by controlling elimination of acids + bases in urine 5. Excrete foreign compounds/waste 6. Produce erythropoietin (to produce new RBCs) 7. Produce renin for salt conservation 8. Convert vitamin D to active form

What do kidneys do?

Maintain homeostasis by filtering blood, regulate plasma constituent concentration, eliminate metabolic waste.

Anatomical order of urinary system

1. Kidney 2. Ureter 3. Bladder 4. Urethra

Kidney

Double organ, bean-shaped, located in lower abdominal cavity (back), and produces urine.

Ureter

Collects urine from the kidneys + transports it to the bladder.

Bladder

Temporarily stores urine.

Urethra

Excretes urine from the body.

Nephrons produce __________ of urine

Selective quantity of urine depending on concentration needs.

What collects drops of urine?

The renal pelvis.

Renal cortex

Outer layer of the kidney (darker on microscope slide).

Renal medulla

Inner layer containing renal pyramids.

Nephron

Working/functional unit of kidneys that processes urine while filtering + regulating blood before it returns to the heart.

What is urine a result of?

Filtration.

What components does a nephron consist of?

Vascular + tubular components.

What does the tubular component consist of?

• Bowman's capsule - Proximal tubule - Loop of Henle - Distal tubule - Collecting duct.

Bowman's capsule

Collects glomerular filtrate.

Proximal tubule

Uncontrolled reabsorption + secretion.

Loop of Henle

Establishes osmotic gradient in renal medulla (important for kidney to produce urine with varying concentration).

Distal tubule + collecting duct

Controlled reabsorption of Na+ and H2O.

What is the fluid that leaves the collecting duct?

Urine.

What does the vascular component of nephrons consist of?

• Afferent arteriole - Glomerulus - Efferent arteriole - Peritubular capillaries.

Afferent arteriole

Carries blood to glomerulus.

Glomerulus

Tuft of capillaries that filters plasma into the tubular component.

Efferent arteriole

Carries blood away from glomerulus.

Peritubular capillaries

Supply renal tissue with blood.

Vasa recta

• In juxtamedullary nephrons - Capillaries that wrap around the loop of Henle.

Renal corpuscle

Glomerular capillaries + Bowman's capsule.

Cortical nephrons

• 80% - Short/no loop of Henle - Mostly located in cortex - No contribution to hypertonic medullary interstitium.

Juxtamedullary nephrons

• 20% - Long loop of Henle - Located in cortex + medulla - Generate gradient in the medulla (important for water reabsorption).

Glomerular filtration

Uncontrolled filtration of protein-free plasma from the glomerulus.

During glomerular filtration, what is filtration based on?

Size.

Tubular reabsorption

Movement of filtered substances from tubular lumen into peritubular capillaries (nephron to blood).

Tubular secretion

Movement of non-filtered substances from capillaries into tubular lumen (blood to nephron).

How much of plasma is filtered with each stop at the kidneys?

20%.

Glomerular filtration steps

1. Blood enters by afferent arteriole + passes through glomerular capillaries. 2. Plasma proteins + large cells continue trip to efferent arteriole. 3. Water, ions, + small molecules are filtered/collected in Bowman's capsule + move in the nephron as filtrate.

Filtrate

Waste product due to filtration of blood through 3 layers of the capillary wall.

3 layers of capillary walls

1. Capillary endothelium (large, fenestrated pores) 2. Basement membrane 3. Bowman's epithelium (podocytes).

Are podocyte cells selective barriers?

No, they are non-selective - anything that is small enough to fit in the space between the podocytes is allowed to pass through.

After fluid accumulates in Bowman's capsule, where does it move next?

Proximal convoluted tubule.

Glomerular capillary blood pressure (Pgc)

Hydrostatic pressure caused by pressure of blood into capillary walls (pressure exerted on glomerular capillaries).

Plasma-colloid osmotic pressure (πp)

Opposes filtration, determined by plasma proteins.

Bowman's capsule hydrostatic pressure (Pbc)

Opposes filtration, determined by pressure accumulation in Bowman's capsule.

Why does πbs not usually exist?

If Pgc > Pbc + πp = filtration into Bowman's capsule.

What is πp due to?

• Due to movement of water into glomerular capillaries due to plasmosmotic pressure inside glomerular capillaries (increased concentration of plasma proteins inside glomerular capillaries).

Are there plasma proteins inside of the Bowman's capsule?

No, so water is forced to move into glomerular capillaries.

GFR

Glomerular filtration rate.

What is the typical GFR?

$125 ext{ mL/min}$ or $180 ext{ L/day}$.

How many times is blood filtered through the kidneys per day?

36 times.

What can GFR be altered by?

1. Protein concentration in blood (affects osmotic pressure) 2. Hydration (if you drink a lot/not enough water, it affects systemic bp) 3. Urinary tract obstructions 4. Mean arterial bp (main).

Even if there are changes in MAP, what does the kidney want to do?

Maintain constant GFR.

If MAP increases, what is the resulting GFR and why?

• Increased GFR - due to increased glomerular capillary bp.

Local control on GFR

• Autoregulation through juxtaglomerular apparatus (JGA) - maintains GFR within a range when the kidney detects an increased/decreased GFR.

Within the JGA, where are granular cells?

Afferent arteriole.

Within the JGA, where are macula densa cells?

Distal convoluted tubule.

How are the granular + macula densa cells connected?

Intimately connected.

Steps in JGA locally regulating GFR

1. Macula densa cells sense too much filtration (high GFR) + communicate that to granular cells. 2. Granular cells constrict afferent arteriole. 3. Blood flow is decreased + so is GFR.

How is GFR systemically controlled?

• Sympathetic input - innervates granular cells to constrict afferent arteriole - causes renin secretion.

If MAP increases, what do afferent arterioles do to compensate?

• AA automatically constrict to decrease GFR.

If MAP decreases, what do afferent arterioles do to compensate?

• AA automatically dilate to increase GFR (and increase urine output).

What happens when there is a urinary tract blockage?

• Excess fluid in Bowman's space that cannot leave - increased pressure in Bowman's space - decreased net filtration pressure - decreased GFR.

If the amount of a substance that is filtered is less than the amount that is secreted, then:

The substance was secreted.

What happens to Pgc and GFR when you constrict AA?

• Decrease blood coming into glomerular capillaries - decrease Pgc - decrease GFR.

What would be input to constrict AA?

Diarrhea, decreased bp (want to hold onto fluid).

What happens to Pgc and GFR when you constrict EA?

• Decreased blood leaving glomerular capillaries - increased Pgc - increased GFR.

What happens to Pgc and GFR when you dilate EA?

• More blood leaving glomerular capillaries - decreased Pgc - decreased GFR.

What happens to Pgc and GFR when you dilate AA?

• More blood into glomerulus - increased Pgc - increased GFR.

What is tubular reabsorption important for?

Brings important molecules that were filtered back into the blood (water, Na+, glucose).

How much of filtrate volume is reabsorbed?

99% (almost complete reabsorption).

Possible routes of reabsorption

1. Luminal membrane → basolateral membrane → renal interstitial fluid → peritubular capillaries → blood 2. Tight junctions → renal interstitial fluid → peritubular capillaries → blood.

Luminal membrane

Faces the tubular lumen containing filtrate.

Basolateral membrane

Faces the interstitial fluid.

Substances needing protein carriers have a __________ transport maximum

Transport maximum.

Transport maximum

Max amount of movement across the plasma membrane for a specific molecule (max amount that can be reabsorbed).

What happens when a transporter becomes fully saturated?

Body cannot filter any more of it back to the blood, so the substance is excreted.

When sugar is less than 300 mg/mL, what is the relationship between filtration + reabsorption?

Linear (everything that is filtered is reabsorbed).

When sugar is more than 300 mg/mL, what happens?

Sugar is excreted because all of the transporters are full.

How was diabetes discovered?

Diabetics had glucose in their urine.

What is the most abundant cation in filtrate?

Na+.

Is Na+ reabsorption active or passive?

Active.

Where does Na+ reabsorption occur?

All tubules, except descending loop of Henle.

How is Na+ reabsorption controlled in the distal convoluted tubule and collecting duct?

Hormonal by aldosterone (and renin).

How is water reabsorbed?

Osmosis through aquaporins.

When are aquaporins not present?

ADH is absent.

Where is H2O reabsorption hormonally controlled? What is it controlled by?

• Distal convoluted tubule + collecting duct - ADH.

In the proximal tubule + ascending loop of Henle, what type of reabsorption occurs?

Na+ because the transporters are present.

In the proximal tubule + ascending loop of Henle, are H2O and Na+ coupled?

Yes.

In the distal convoluted tubule + collecting duct, what type of regulation occurs?

Hormonal regulation; last change for filtrate to be changed before urine is excreted from body.

In the distal tubule + collecting duct, are H2O and Na+ coupled?

No.

What percent of Na+ is reabsorbed in the proximal tubule?

65%.

What percent of Na+ is reabsorbed in the loop of Henle?

25%.

What percent of Na+ is reabsorbed in the distal tubule + collecting duct?

10%.

In the proximal tubule, how is Na+ reabsorbed?

1. ATPase Na+/K+ pump pumps Na+ out of the cell + K+ into the cell (low concentration of Na+ and high concentration of K+ inside). 2. Cotransport: Na+ moves with its concentration gradient (from lumen into the cell) to move glucose into cell (against its gradient). 3. Countertransport: Na+ moves with its concentration gradient (from lumen into the cell) to move H+ into tubular lumen (against its gradient). 4. Once Na+ gets into cells, Na+/K+ pump pumps Na+ from cell to interstitial fluid into the blood.

What determines Na+ reabsorption in the proximal tubule?

Presence of transporters.

How is H2O reabsorbed in the proximal tubule?

Passively; follows Na+.

What is the pathway for H2O to be reabsorbed in the proximal tubule?

Paracellular, not regulated; follows Na+ by osmosis.

What is the pathway for H2O to be reabsorbed in the distal tubule + collecting duct?

Transcellular; requires aquaporins, regulated by vasopressin, requires energy.

How is Na+ reabsorbed in the distal tubule + collecting duct?

1. Na+/K+ ATPase pump establishes gradient (low concentration of Na+ and high concentration of K+ inside the cell). 2. Simple diffusion moves Na+ from tubular lumen into the cell and K+ from inside the cell into the tubular lumen. 3. Na+ inside the cell is pumped into the IF, then back into blood. 4. K+ joins the filtrate + is excreted.

How does aldosterone regulate Na+ reabsorption in the distal tubule + collecting duct?

Aldosterone builds Na+ channels + Na+/K+ pumps to increase Na+ reabsorption.

When would you need to save Na+?

Low bp, diarrhea.

Proximal tubule Na+ reabsorption review

Na+ reabsorption high + constant - moves by cotransport + countertransport across luminal membrane - moves by Na+/K+ pumps across basolateral membrane.