Kideny And Excretion

Question 1

What are the major functions of the kidney?

Answer 1

Maintain homeostasis of body fluid volume, regulating blood pressure
Maintain homeostasis of plasma solute composition, help control plasma pH
Excreting waste products such as urea, uric acid, ammonia, and phosphate

Question 2

Kidney Structure

Answer 2

Two kidneys- each a fist-sized organ made up of outer cortex, inner medulla
Functional unit of the kidney is the nephron: contains about ~1M nephrons
Blood enters nephron, flows into capillary bed called glomerulus
Bowman’s capsule and glomerulus make up renal corpuscle
Filtrate from bowman’s capsule moves to proximal tubule, into the loop of Henle (medulla), into the distal convoluted tubule in the cortex, and out to the collecting duct

Question 3

Where does urine created in the kidney flow to in order to exit the body?

Answer 3

Urine created by kidney, emptied into renal pelvis, then ureter, then bladder, then drained to external environment by urethra

Question 4

How does the bladder keep the urethra closed?

Answer 4

Through the action of the urinary sphincter muscles to avoid leaking urine
During urination, urinary sphincter muscles relax and a muscle in the bladder contracts to allow the release of urine

Question 5

Nephron

Answer 5

Functional unit of the kidney
Each human kidney contains ~1M nephrons
Functions to filter out necessary blood proteins and cells, and concentrate remaining waste productions for excretion

Question 6

How does blood enter a nephron?

Answer 6

Flows into a capillary bed called glomerulus
Bowman’s capsule and glomerulus make up renal corpuscle
Hydrostatic pressure forces some plasma through fenestrations of glomerular endothelium and into Bowman’s capsule
Fenestrations screen out blood cells and large proteins
Fluid entering bowman’s capsule is called filtrate or primary urine

Question 7

How do the blood vessels control the hydrostatic pressure at the glomerulus?

Answer 7

Diameter of afferent (to glomerulus) and efferent (away from glomerulus) arterioles can constrict or dilate to control blood flow and thus hydrostatic pressure
Constriction of afferent arterioles decreases blood flow and hydrostatic pressure -> less filtration
Constriction of efferent arterioles decreases exit of blood, increasing hydrostatic pressure and increasing filtration

Question 8

Proximal Tubule

Answer 8

Filtrate moves from Bowman’s capsule to proximal tubule
Location of secretion and most reabsorption takes place
Drugs, toxins, uric acid, bile pigments, and other solutes are secreted into filtrate by cells of proximal tubule
Hydrogen ions are secreted through antiport system driven by sodium concentration gradient
Reabsorption occurs via passive or active transport
Secondary active transport proteins in apical membranes carry glucose, most proteins, and other solutes back into blood. When saturated, additional solutes reach urine
Simple or facilitated diffusion: water (permeable tight junctions)

Question 9

Transport maximum

Answer 9

Concentration of a solute that saturates its transport proteins
Once solute reaches transport maximum, any more solute cannot get across protein

Question 10

What is the result of the proximal tubule in terms of filtration?

Answer 10

Removal of valuable nutrients; glucose, protein, solutes
Amount of filtrate in nephron is reduced, solute composition altered, overall concentration of solutes in filtrate is unchanged

Question 11

Loop of Henle

Answer 11

Functions to increase the solute concentration, and therefore osmotic pressure of the medulla
Solute concentration of the filtrate leaving loop of Henle is decreased
Initial descending and final ascending segments of loop of Henle differ in permeability to solutes and water
Descending: low permeability to salt and high permeability to water
Ascending: high permeability to solutes, impermeable to water

Question 12

Descending Loop of Henle

Answer 12

Where filtrate enters the medulla and Loop of Henle
Low permeability to salt and high permeability to water
Water passively diffuses out of loop of Henle and into medulla
Filtrate osmolarity in segment of nephron increases

Question 13

Ascending Loop of Henle

Answer 13

Where filtrate leaves the medulla in the loop of henle
Has high permeability to solutes and active transport mechanisms for solutes near end, highly impermeable to water
Solutes pass out first by passive diffusion, and then through active pumps into medulla
Increases solute concentration in medulla, while reducing it in the filtrate
Capillary bed called vasa recta surrounds loop of Henle and helps maintain medulla concentration

Question 14

What types of metabolites are generally excreted vs. reabsorbed in kidneys?

Answer 14

Water soluble metabolites are generally excreted, along with excess sodium and other unneeded ions
Water, glucose, and most ions are reabsorbed back into the blood to maintain homeostasis

Question 15

How do the kidneys work with the lungs and heart?

Answer 15

Maintain blood plasma pH through a balance of acidic carbon dioxide and alkaline bicarbonate
Critically important for maintaining proper blood pressure in cardiovascular system

Question 16

Countercurrent flow in loop of Henle

Answer 16

As filtrate moves through descending and ascending loops, flows in opposite directions in parallel to tubules that are very close together and separated only by a small amount of tissue and fluid
Vasa recta has counter-current flow in opposite direction: blood in vasa recta descends next to ascending loop, and ascends next to descending loop

Question 17

Single Effect

Answer 17

Process by which active transport of solute by pumps in wall of thick ascending loop creates concentration gradient
Result is that solute is more highly concentrated outside the tubule (medulla) than inside the tubule (filtrate)
Water cannot offset effect due to impermeability to water
Creates moderate osmotic gradient between filtrate and interstitial fluid (200 mOsm conc. Diff)

Question 18

What is the net result of the single effect?

Answer 18

Solutes exit the ascending loop of Henle and water exits the descending loop of Henle
Dilutes the filtrate in ascending limb, while concentrating the medulla
Concentrates filtrate in descending limb of the loop of Henle

Question 19

Counter-current multiplier mechanism

Answer 19

Applies single effect (creates static gradient) to dynamic system, where fluid is continually moving through loop of Henle
Process is generally in homeostasis in body

Question 20

Explain the concentration gradients in the loop of Henle

Answer 20

Due to the water flowing out of the filtrate in the descending loop of Henle, combined with the solutes flowing out in the ascending loop of Henle, solutes are concentrated towards the hairpin turn of the loop, creating a osmolarity gradient that is much higher the lower into the medulla the loop dips, and which rises the as the filtrate ascends up in the loop towards the cortex.

Question 21

How does the vasa rect not diffuse the concentrations in the medulla?

Answer 21

The hairpin loop structure of the vasa recta, combined with particularly slow blood flow through vessels, allows blood circulation to occur without major disruption to medulla’s concentration gradient
Water diffuses out of vasa recta in concentrated areas of medulla and solutes are absorbed into blood, so bottom of vasa recta is as concentrated as medulla
In ascending area, free flow of solutes out of vasa recta and water flow into vasa recta causing blood to be only slightly more concentrated than surrounding medulla
Looped vasa recta delivers oxygen and nutrients and removes waste products without disturbing conc. Gradient

Question 22

Distal Tubule

Answer 22

Filtrate moves out of medulla and into renal cortex entering the distal tubule
Distal tubule reabsorbs Na+ and Ca2+, while secreting K+, H+, and HCO3-
Aldosterone acts on distal tubule to increase number of sodium and potassium membrane transport proteins in their membranes, causing blood pressure to increase
Net effect of distal tubule is to lower filtrate osmolarity
ADH causes cells to become more permeable to water in collecting tubule, water flows out of tubule back into body, concentrating filtrate

Question 23

Collecting Duct

Answer 23

Distal tubule empties into collecting duct
Carries filtrate into highly osmotic medulla
Impermeable to water usually, but sensitive to ADH which can allow water to enter medulla via passive diffusion down concentration gradient
Osmotic pressure created by densely concentrated solutes in medulla cause substantial portion of free water in filtrate of collecting duct to diffuse back into body, allowing formation of concentrated urine

Question 24

Renal Pyramids

Answer 24

Collecting ducts line up side by side to form this structure

Lead to renal calyx, which empties into renal pelvis

Question 25

Juxtaglomerular Apparatus

Answer 25

Monitors filtrate pressure in the distal tubule
Specialized cells secrete enzyme renin when filtrate pressure is too low
Renin-angiotensin-aldosterone system (RAAS) regulates blood pressure and fluid balance
Renin, initiates cascade that produces angiotensin I from angiotensinogen (zymogen), traveling to lung and converted to angiotensin II, causing blood vessels to constrict, raising blood pressure
Angiotensin II also causes adrenal cortex to secrete aldosterone
Aldosterone acts on distal tubule, stimulating formation of membrane proteins that absorb sodium and secrete potassium to increases blood pressure (water follows sodium absorption, increasing blood volume)