ch. 17: physiology of the kidneys

renal

pertaining to the kidneys

main function of renal system is...

regulation of extracellular fluid (ECF) environment in the human body through urine formation

via regulation of ECF, the renal system...

regulates blood volume

eliminates organic waste products of metabolism: urea (protein breakdown), uric acid (nucleic acids), creatinine (muscle creatine), end products of hemoglobin breakdown

regulates balance of electrolytes (Na+, K+, HCO3-, other ions)

with respiratory system, maintains acid-base balance/pH of plasma

kidneys (2)

formation of urine

water and electrolyte balance

secretion of toxins and drugs into urine

gluconeogenesis

gluconeogenesis

synthesis of glucose from AAs during prolonged faster (also occurs in liver)

ureters (2)

transfer of urine to bladder

urinary bladder

storage and micturition (urination) via the urethra

urethra

flow of urine from bladder to outside (micturition)

the paired kidneys form a...

filtrate of the blood that is modified by reabsorption and secretion

urine destined for excretion moves from the...

kidneys along the ureters to the bladder. it is then excreted through the urethra

the outer layer of the kidney is the...

renal cortex; it is the site of glomerular filtration as well as the convoluted tubules

the inner layer of the kidney is the...

renal medulla; this is the location of the longer loops of Henle, and the drainage of the collecting ducts into the renal pelvis and ureter

micturition

contractions of smooth muscle in ureter wall cause urine to move from ureter to bladder

bladder walls are...

smooth muscle (detrusor muscle)

what produces micturition

contraction of detrusor

internal urethral sphincter (smooth muscle) is at...

base of bladder

external urethra sphincter (skeletal muscle) is...

below internal urethral sphincter and surrounds the urethra

its contraction can prevent urination

contraction and relaxation of these muscles is determined by...

neuronal input, due to stretching of the bladder when it fills

voluntary decision making

blood enters the kidney via the...

renal artery and exits via the renal vein

in the kidney, there is...

extensive branching and capillary networks including the glomeruli

nephron

functional unit of the kidneys, consisting of a renal corpuscle (glomerular capsule + glomerulus) and tubule

nephrons per kidney

over one million

blood enters the kidney through

renal artery

branching of renal artery leads to

afferent arterioles, which bring blood to the glomeruli

glomeruli

a glomerulus is a capillary network in renal corpuscle

blood from renal artery -> afferent arterioles -> glomeruli

twenty percent of plasma from glomerulus...

filters out of glomerulus into glomerular capsule and then moves into tubule

blood from renal artery -> afferent arterioles -> glomerulus

filtrate from glomerulus -> glomerular capsule -> tubule -> collecting duct -> renal pelvis -> ureters

blood remaining in glomerulus (80% of blood) exits...

renal corpuscle through efferent arteriole to the peritubular capillaries

this blood drains into veins that exit the kidney as the renal vein

80% of the blood in glomerulus -> efferent arteriole -> peritubular capillaries -> renal vein

glomerular capsule or bowman's capsule

-surrounds the glomerulus

-fluid filters out of glomerulus into capsule

proximal convoluted tubule

filtrate from glomerulus enters its lumen

reabsorption of salt, water, etc. into peritubular capillaries that surround tubule

secretion of substances into filtrate

collecting duct

distal convoluted tubule empties into it

duct drains into renal pelvis and then into ureters

glomerular filtration

filters through large pores in glomerular capillaries called fenestrae

filtrate (or ultrafiltrate) is cell-free and mostly protein-free; otherwise similar to plasma

reabsorption of salt and water

around 180 L of glomerular filtrate produced each day, but urine excretion is only around 1-2 L per day

1% of the glomerular filtrate is excreted as...

urine and 99% of filtrate returns to vascular system (reabsorbed) to maintain blood volume and pressure

reabsorption

return of filtrate from tubules to peritubular capillaries, via osmosis

urine volume varies depending on...

fluid needs of body (e.g. to maintain blood volume and pressure), so volume of fluid reabsorbed varies

most salt and water in filtrate are...

reabsorbed in the proximal tubule. some reabsorbed in descending limb of loop of Henle

filtration refers to the...

movement of fluid and solutes from the flomerulus into the capsule and then into the tubules

reabsorption refers to the...

movement of materials from the tubules into the peritubular capillaries

i.e. back into general circulation

Na+ is actively transported out of...

filtrate and Cl- follows passively by electrical attraction

due to osmosis, water follows the salt into the peritubular capillaries

countercurrent multiplier system

for water to be reabsorbed into bloodstream by osmosis, ISF surrounding tubule must by hypertonic -> causing water to move out of tubule

fluid is hypertonic due to

countercurrent multiplier system

countercurrent flow (opposite direction flow) in...

ascending and descending limbs in nephrons and close proximity of limbs allow them to interact to create high osmotic pressure in ISF

countercurrent multiplier system in ascending limp of the loop of Henle

Na+ is actively pumped into ISF

Cl- follows Na+ because of electrical attraction

not permeable to water so fluid in the ascending limb becomes diluted

NaCl accumulates in the ISF here, increasing the osmolarity of ISF

countercurrent multiplier system in descending limb of the loop of Henle

permeable to water but not salt

ISF is hypertonic compared to filtrate here so water leaves descending limb by osmosis -> ISF -> capillaries

hypertonic fluid then enters ascending limb, where Na+ is actively pumped and Cl- follows, creating diluted tubular fluid and more concentrated ISF

countercurrent multiplier system summary

1. extrusion of NaCl from ascending limb makes ISF more concentrated here. Na+ is pumped out and Cl- follows due to electrical attraction

2. in descending limb, water diffuses out via osmosis (and enters capillaries). this increases osmolarity of tubular fluid and decreases its volume as the fluid descends

3. fluid at the bend of the loop has a high osmolarity, 1200 mOsm. the saltiness of the ISF is multiplied here because of the lack of permeability to water

changes in water intake alter...

plasma osmolarity, which is sensed by hypothalamic osmoreceptors

release of ADH is altered to...

affect water reabsorption in the kidneys. this affects the volume of urine excreted, to maintain blood volume

renal plasma clearance

volume of plasma that is cleared of a substance by kidneys per unit time (i.e. substance is removed from plasma)

substances are removed from plasma via...

filtration from glomerulus or secretion into filtrate

secretion

the movement of substances from the peritubular capillaries into the tubular fluid, for excretion in urine

reabsorption of a substance reduces it

renal plasma clearance

filtered glucose and AAs are completely reabsorbed in...

proximal tubule via active transport

when the concentration of glucose exceeds the capacity of the transporter, i.e. the transport maximum...

the excess glucose is excreted in the urine

glucosuria

the excess glucose is excreted in the urine

glucosuria occurs when...

plasma glucose concentration is too high, 180-200 mg/dl

e.g. in diabetes mellitus

much of the filtered Na+ and K+ is...

reabsorbed in early part of nephron

concentrations of Na+ and K+ in the urine excreted...

depend on physiological needs/homeostasis, and are adjusted late in nephron

decreased plasma [Na+] activates...

renin-angiotensis-aldosterone system -> secretion of aldosterone (adrenal cortex)

decreased plasma [Na+] leads to...

stimulates Na+ reabsorption, to increase [Na+]

causes passive reabsorption of Cl-

water follows via osmosis to increase blood volume

also stimulates K+ secretion into filtrate when plasma [K+] is high

ADH regulates...

water reabsorption to regulate urine volume and blood volume

renin-angiotensin-aldosterone system stimulates

secretion of aldosterone when Na+ intake is low

aldosterone stimulates Na+...

reabsorption in the cortical collecting ducts

potassium is filtrated from...

the glomerulus

some is reabsorbed in proximal convoluted tubule

aldosterone stimulates potassium secretion in the collecting duct when...

plasma [K+] is high

kidneys regulate blood pH by...

excreting H+ in the urine, and by reabsorbing bicarbonate

CO2 + H2O <-> H2CO3 <-> H+ + HCO3-

kidneys alter....

plasma [bicarbonate] to regulate [H+]

urine is slightly acidic because...

almost all of the filtered bicarbonate is reabsorbed in tubules and H+ is excreted

in acidosis (pH , 7.35)...

there is increased plasma [H+] and more H} in filtrate

bicarbonate is synthesized in the proximal tubule cells to compensate and then bicarbonate enters the blood

in alkalosis (pH > 7.45)...

there is decreased plasma [H+] and less H+ in filtrate, so less bicarbonate is reabsorbed to compensate