Ch. 17: Physiology of the Kidneys

What does renal mean?

Pertaining to the kidneys

What is the main function of the renal system?

Regulation of extracellular fluid environment in the human body through urine formation

Name four other functions the renal system has:

1. Regulates blood volume
2. Eliminates organic waste products of metabolism
3. Regulates balance of electrolytes
4. With the respiratory system, maintains acid-base balance/pH of plasma

Name the structures within the renal system:

- Kidneys (2)
- Ureters (2)
- Urinary Bladder
- Urethra

Kidney functions

- formation of urine
- water and electrolyte balance
- secretion of toxins and drugs into urine
- gluconeogenesis

What is gluconeogensis?

Synthesis of glucose from AAs during prolonged fasting (also occurs in liver)

Ureters function

transfer of urine to bladder

Urinary Bladder function

storage and micturition

Urethra function

micturition

In the Female Renal System, what do the kidneys do?

Form a filtrate of the blood that's modified by reabsorption and secretion

Urine destined for ________ moves from the kidneys along the ______ to the _______.

excretion; ureters; bladder

*remember KIDNEYS -----> URETERS -----> BLADDER

Where is the urine excreted?

Through the urethra

What is the outer layer of the kidney called?

Renal Cortex

What is the renal cortex the site of?

Glomerular filtration and convoluted tubules

What is the inner layer of the kidney called?

Renal Medulla

What is the renal medulla the site of?

- Loops of Henle
- Drainage of collecting ducts into the renal pelvis and ureter

What is Micturition?

Contractions of smooth muscle in ureter wall (another word for urination)

What does micturition cause?

Urine to move from ureter to bladder

Bladder walls are made up of what type of muscle?

Smooth muscle (detrusor muscle)

Contraction of detrusor muscles produces what?

Micturition

At the base of the bladder is the:

Internal urethral sphincter

Below the internal urethral sphincter is the:

external urethral sphincter

What does the external urethral sphincter do?

- Surrounds the urethra
- Contractions can prevent urination

What two things determine the contraction/relaxation of these muscles?

1. Neuronal input due to stretching of the bladder when it fills
2. Voluntary decision making

How does blood enter and exit the the kidney?

- Enters through the renal artery
- Exits through the renal vein

What is the Nephron?

The functional unit of the kidneys, consists of a renal corpuscle and tubule

What two things make up the renal corpuscle?

Glomerular capsule + Glomerulus

About how many nephrons are there in a kidney?

> 1,000,000

What does the branching of the renal artery lead to?

Afferent Arterioles

What do the afferent arterioles do?

Bring blood to the glomeruli

*Blood from renal artery ----> afferent arterioles ----> glomeruli*

How much percent of plasma filters out of the glomerulus? And where does it filter out to?

20%; and into surrounding glomerular capsule & then into a tubule

Name the order of what occurs after the filtration of plasma

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

What does the remaining 80% blood do?

- Exits renal corpuscle through efferent arteriole to the peritubular capillaries
-The blood then drains into veins that exit kidney as the renal vein

Name the order of what occurs to the 80% of blood

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

What does the glomerular/bowman's capsule do?

- surrounds glomerulus

In the nephron, fluid filters from the ______ into the _____.

glomerulus; glomerular capsule

What does the proximal convoluted tubule do?

- Filtrates from glomerulus, enters its lumen
- Reabsorption of salt, water into peritubular capillaries that surround tubule
- Secretion of substances into filtrate

What does the collecting duct do?

- Distal convoluted tubule empties into it
- Duct drains into renal pelvis and then ureters

Glomerula Filtration:

Appx 20% of plasma filters out of glomerulus (capillaries) into the glomerular capsule

How is the plasma filtered?

Filters through large pores in glomerular capillaries called fenestrae

Around _____ L of glomerular filtrate is produced each day, but urine ______ is only around _____ L per day.

180; excretion; 1-2

How much percent of the filtrate is excreted as urine?

1%

How much percent of the filtrate is reabsorbed to the vascular system? And why?

99%; to maintain blood volume and pressure

What is reabsorption?

Return of filtrate from tubules to peritubular capillaries by osmosis

Urine volume varies on what?

On fluid needs of body, so volume of fluid that's reabsorbed also varies

Where is salt and water in filtrate reabsorbed?

Proximal tubule or descending limb of loop henle

What does filtration refer to?

Movement of fluid/solutes from the glomerulus into the capsule and then into the tubules

*Glomerulus ---> Capsule ----> Tubules

What does reabsorption refer to?

Movement of materials from the tubules into peritubular capillaries

Which electrolyte is actively transported out of filtrate, and which one follows?

Na+ and Cl- follows passively by electrical attention

Due to _______, _______ follows salt into the peritubular capillaries

osmosis; water

If water wants to be reabsorbed into the bloodstream by osmosis what needs to happen to the ISF?

The ISF that surrounds the tubule must be hypertonic

Why is the fluid hypertonic?

B/c of the Countercurrent Multiplier System

What allows limbs to interact to create high osmotic pressure in the ISF?

Countercurrent flow in ascending and descending limbs in nephrons and close proximity of limbs

What occurs in the ASCENDING limb of the loop of Henle?

- Na+ is actively PUMPED into ISF
- Cl- follows Na+ bc of electrical attraction
- Not permeable to water so fluid in ascending limb becomes diluted
- NaCl accumulates in the ISF here, which increases the osmolarity of ISF

The countercurrent multiplier system does which of the following?
A) pumps CI- out of the tubule
B) secretes NaCl into the urine
C) allows water to diffuse out of the ascending limb
D) pumps Na+ out of the tubule
E absorbs water into the tubule

D) pumps Na+ out of the tubule

What occurs in the 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 the ascending limb, where Na+ is actively pumped out and Cl- follows, creating diluted tubular fluid and more concentrated ISF

What is the 3 steps within the Countercurrent Multiplier System in Tubule?

1) Extrusion of NaCl from ascending limb makes ISF more concentrated. Na+ is pumped out, Cl- follows due to electrical attraction
2) In descending limb, water diffuses out by osmosis and enteres 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 b/c of the lack of permeability to water

Fluid that enters the collecting duct is _____. What does this conclude?

hypotonic; ISF surrounding collecting duct is hypertonic

What is the collecting duct permeable to and not permeable to? What does this cause water to do?

Permeable to water but not salt so water diffuses out of duct into hypertonic ISF by osmosis

What causes ADH secretion to increase?

Plasma osmolarity increasing

Why must both plasma osmolarity and ADH secretion increase?

To increase permeability of collecting duct to water

What is the action of osmoreceptors in ADH secretion?

Senses changes in water intake that alters plasma osmolarity

Release of _____ is altered to affect water _______ in the kidneys. What does this affect?

- ADH; Reabsorption
- It affects the volume of urine excretion to maintain blood volume

What is renal plasma clearance?

Volume of plasma that's "cleared" of a substance by kidneys per unit time

How are substances from the plasma removed?

By filtration from glomerulus, or secretion into filtrate

What is secretion?

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

During Renal Plasma Clearance what occurs to the filtered glucose and AAs?

They're completely reabsorbed in the proximal tubule by active transport

What happens when the concentration of glucose exceeds the capacity of the transporters?

The excess glucose is exerted in the urine = glucosuria

When there is excess glucose in the urine, this is called glucosuria.
A)True
B) False

A)True

When is filtered Na+ and K+ reabsorbed?

In early part of nephron

What do the concentrations of Na+ and K+ in the urine excreted depend on?

Physiological needs/homeostasis and are adjusted late in the nephron

What do decrease plasma [Na+] activate?

Renin-angiotensin-aldoesterone system

Name some results in secretion of aldosterone (adrenal cortex)

- Stimulates Na+ reabsorption to increase plasma
- Causes passive reabsorption of Cl-
- Water follows by osmosis to increase blood volume

What is the main function of ADH in regards to homeostasis of Plasma Na+

Regulates water reabsorption to regulate urine volume and blood volume

What does the Renin-angiotensin-aldoesterone system do?

Stimulates secretion of aldosterone when Na+ intake is low

What does aldosterone do?

Stimulates Na+ reabsorption in the cortical collecting ducts

How is Potassium filtered?

From the glomerulus, some is reabsorbed

What does aldosterone do when [K+] is high?

Stimulates potassium secretion

What's the equation for Renal Control of Acid/Base Balance?

CO2 + H20 -----> H2CO3 -------> H+ + HCO3-

How do kidneys regulate blood pH?

By excreting H+ in the urine and by reabsorbing bicarbonate

Why is urine slightly acidic?

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

What occurs in acidosis?

- Increase plasma [H+] and more in filtrate
- Bicarbonate is synthesized in proximal tubule cells to compensate
- Bicarbonate enters blood

What occurs in alkalosis?

- Decreased plasma [H+] and less in filtrate
- Less bicarbonate is reabsorbed to compensate