Urinary Study Guide

List the functions of the kidney.

• Filter the blood and excrete the toxic metabolic wastes

• Regulate blood volume, pressure, and osmolarity by regulating water output

• Secretes erythropoietin

• Help regulate calcium homeostasis and bone metabolism by helping synthesize calcitriol

• Clears hormones and drugs

• Detoxify free radicals

• Supports blood glucose levels (in extreme starvation)

What is urea?

A by-product of protein catabolism.

List the path that fluids take to become urine.

Renal corpuscle:

1) Glomerulus

2) Glomerular capsule

Renal Tubule:

3) Nephron loop (descending limb)

4) Nephron loop (ascending limb)

5) Distal convoluted tubule

6) Collecting duct

Drainage System:

7) Minor calyx

8) Major calyx

9) Renal pelvis

10) Ureter

Storage Structure:

11) Urinary bladder

Elimination structure:

12) Urethra

• 1-6 = Urine formation, 7-12 = Urine excretion

Describe the flow of blood through the kidney.

1) Abdominal aorta

2) Renal artery

3) Segmental artery

4) Interlobar artery

5) Arcuate artery

6) Cortical radiate artery

Blood vessels associated w/ nephrons

7) Afferent arteriole

8) Glomerulus

9) Efferent arteriole

10) Peritubular capillaries/vasa recta

11) Cortical radiate vein

12) Arcuate vein

13) Interlobar vein

14) Renal vein

15) Inferior vena cava

Define a nephron and describe its two principal components. Describe the two types of nephrons.

Nephron: One of approx. 1 million blood-filtering, urine producing units in each kidney. Functional unit of the kidney

• Renal corpuscle: Filters the blood plasma

• Renal tubule: Converts filtrate to urine

Two types of nephrons:

• Cortical nephron: 85% of nephrons, in renal cortex

• Juxtamedullary nephron: The rest of the nephrons, glomerulus is lower, nephron loops dip into medulla

Define the three stages of fluid as blood plasma is converted to urine.

• Glomerular filtrate: Similar to blood plasma but without proteins

• Tubular fluid: Fluid from proximal convoluted tubule to distal convoluted tubule

• Urine: Collecting duct on because it undergoes little alteration after this point

Briefly describe the four steps of urine conversion.

• Glomerular filtration: The beginning of urine formation, the glomerular capillaries take in water and other substances to filter into the glomerular capsule

• Tubular reabsorption: Keeps the good substances back in the blood.

• Tubular secretion: Removes the bad and useless stuff in the urine.

• Water conservation: Concentrating urine, done when necessary

Describe glomerular filtration and how it differs from capillary filtration.

Glomerular filtration is similar to plasma filtration, their difference is that plasma filtration in the blood is made up of proteins which glomerular filtration has none of. 

• The beginning of urine formation, the glomerular capillaries take in water and other substances to filter into the glomerular capsules

• The substance would have to pass through a filtration membrane made up of 3 layers

  • Fenestrated endothelium of the capillary: Their small openings act as a filter, however does not let in red blood cells

  • Basement membrane: Made up of proteoglycan gel that some molecules small enough can pass through but most of them would get stuck in the process. Even if the molecules are small enough it may not go through because it is negatively charged. 

  • Filtration slits: Made up of podocytes with projections that resembles tentacles which will swaddle the capillaries.

• Glomerular filtration has a higher blood hydrostatic pressure compared to capillaries because their afferent arterioles are much bigger. The glomerular filtration has a smaller efferent arteriole which causes the space to become more crowded. Glomerular inward force is 32 mm while capillaries are 18 mm

• Proteins are needed for the osmotic pressure to pull, glomerular has no proteins which results in one type of osmotic pressure that pulls fluid into the blood. Its pressure is much higher compared to the capillaries for rapid filtration.

Describe what happens in tubular reabsorption vs tubular secretion.

• Tubular Reabsorption: Happens int he proximal convoluted tubule; Selects what comes across

  • Sodium ions are reabsorbed via active transport → Negatively charged chloride are attracted to positively charged sodium ions → As the concentration of ion increases in plasma, osmotic pressure increases → Water moves from the proximal tubule to the capillary by osmosis

• Tubular Secretion: Contributes to acid-base balances by secreting hydrogen bicarbonate ions, extracts nitrogenous wastes from the blood, clears drugs and contaminants from blood

Uptake by Peritubular Capillaries

• Accumulation of reabsorbed fluid creates high fluid pressure that drives water into capillaries

• Narrowness of efferent arteriole lowers the blood hydrostatic pressure from 60 → 80 mm Hg so theres less resistance to reabsorption

• Water is filtered out in the glomerulus but proteins remain in blood so there is increased osmotic pressure, strongly favoring reabsorption

What is the transport maximum? What is the significance if an ion exceeds its transport maximum?

Transport maximum: Is the limited amount of fluid can be reabsorbed, this point is reached when the transporters are holding too much fluid.  

• Substances that don't get picked up by transported will automatically go to your urine because there are no transporters that can carry it. 

Why is it important to maintain homeostasis in your GFR?

• If too high, fluid flows through the renal tubules too rapidly for them to reabsorb the usual amount of water and solutes

  • Urine output rises → Dehydration or electrolyte depletion

• If too low, fluid flows sluggishly through tubules, reabsorbs waste that should be eliminated in the urine.

Describe in detail the three mechanisms that control GFR.

Renal Autoregulation: Ability for nephrons to adjust their own blood flow and GFR without external control

• Myogenic mechanism: Arterioles constrict when arterial blood pressure rises and stretches them and relaxes when arterial blood pressure drops

• Tubuloglomerular feedback: Glomerulus receives feedback on status of downstream fluid and adjusts filtration.

Sympathetic Control: Activates the sympathetic nervous system to control kidney blood vessels. If afferent arterioles narrow GFR will decrease, if the arterioles widen the GFR increases. If efferent arterioles narrows the GFR increases, if the arterioles widen GFR decreases.

Hormonal Control: Is controlled by renin and angiotensin. A decrease in blood pressure is detected by baroreceptors in the arteries to activate the granular cells to secrete renin. Renin mixed with Angiotensinogen to create angiotensin I. The more Angiotensin I there it the lungs will create ACE to break it down and convert it to Angiotensin II which works as:

• Vasocontrictor systemically and increase blood pressure.

• Narrows the efferent arterioles to increase GFR. It will lower the pressure in the peritubular capillaries to keep water in the body. 

• Stimulates the adrenal cortex to create aldosterone and activate the distal convoluted tubule and collecting duct to reabsorb sodium.

• Activates the hypothalamus to tell the posterior pituitary gland to secrete ADH, helps with water retention and feelings of thirst. 

Draw a nephron and label the PCT, nephron loop, DCT and collecting duct and describe what happens in each during sodium reabsorption.

Describe what effect the four major hormones have on water balance and how they are regulated.

1) Aldosterone: Retains salt by storing it in the distal convoluted tubule. Low concentrations of sodium/high concentration of potassium will activate adrenal gland to secrete Aldosterone and encourage the distal convoluted tubule to reabsorb sodium and expel potassium. More sodium goes back to the blood and water will follow, and potassium will go to the urine. 

2) Parathyroid hormone: Prevents phosphate from being absorbed in the kidneys as it stops calcium from being released into the blood. Allows for more calcium to enter the distal convoluted tubule and nephron loop, water will follow and increase fluid volume.  

3) ADH: Will be secretes if there is too much water, too much ions in the blood, or not enough blood volume. The hypothalamus and posterior pituitary gland release ADH for the kidneys to retain more water and amplify the feeling of thirst. More water being stored means less urine output. 

4) Natriuretic peptides: Responds to high blood pressure by widening afferent arterioles while narrowing the efferent arterioles and allow more filtration rate. To lower fluid volume it disables renin, aldosterone, ADH, and sodium from being secreted.

Describe the countercurrent multiplier. Be sure to include the locations.

Countercurrent multiplier: The ability to take in salt and transport it to the medullary tissue, this depends on the dilution of fluids that are traveling opposite of it. 

1) The proximal convoluted tubule secretes more salt

2) Traveling towards the medulla tissue the osmolarity of extracellular fluid increases, and more water osmolysis out of the descending limb.

3) Less water in the descending limb makes it much concentrated with salt.

4) The ascending limb will kick out salt back into the medulla if there is too much in the 

5) More salt leaving the ascending limb causes the extracellular fluid in the medulla to be more concentrated with salt.

Describe countercurrent exchange. Be sure to include the location.

• The blood flow in the adjacent parallel capillaries are traveling in opposite ways

• As the blood flows down towards the medulla the vessels will bring in salts from the tissue into the blood, water will come into the tissue as the blood goes up the cortex, the salt will be left in the medulla and water continues to travel with the blood. 

What is the micturition reflex? How can we control it?