Ch 26: Urinary System

What is the order of blood flow in the kidney?

1. Renal artery 2. Segmental arteries 3. Interlobar arteries 4. Arcuate arteries 5. Cortical radiate (interlobular) arteries 6. Afferent arterioles 7. Glomerulus 8. Cortical radiate veins 9. Arcuate veins 10. Interlobar veins 11. Renal vein

Functions of the kidney

1. maintenance of electrolyte balance in the body 2. maintenance of acid-base balance in the blood 3. maintenance of plasma osmolarity

In a person with normal blood pressure, what is the average glomerular filtration rate?

125 mL/min

Calculate the net filtration pressure if capillary hydrostatic pressure is 60 mm Hg, capillary osmotic pressure is 25 mm Hg, and capsular hydrostatic pressure is 10 mm Hg.

25 mm Hg 60 - (25 + 10) = 25 mm Hg The two pressure that oppose filtration must be subtracted from the force favoring filtration.

The average pH of normal urine is about __________.

6.0

The secretion of ADH is directly stimulated by

A change in body fluid osmolarity

Describe a pyelogram.

A pyelogram is an image of the urinary system. A pyelogram is obtained by using an X-ray. A pyelogram uses a radiopaque dye that is administered intravenously.

List three age-related changes in the urinary system.

A reduction in the GFR as a result of fewer glomeruli. A 30-40 percent decrease in the number of functional nephrons from age 25-85. A decrease in muscle tone of the external urethral spincter which can lead to incontinence. Reduced sensitivity to ADH Problems with urinary retention in males due to an enlarged prostate. Increase in the formation of kidney stones.

Following a protein-rich meal, which of the following conditions may be noticed?

Aminoaciduria

What is an ion that is secreted?

Ammonia

What stimulates the secretion of aldosterone?

Angiotensin II

Angiotensin I is converted to angiotensin II by _____________.

Angiotensin converting enzyme (ACE), which is produced in the capillaries of the lungs

During filtration, which of the following acts as a physical barrier?

Basal lamina, podocytes, and capillary endothelium

To regulate pH, what is secreted into the fluid in the collecting system?

Bicarbonate and Hydrogen

Which is the pressure that opposes the formation of filtrate by drawing water *out* of the filtrate and back into the *plasma*?

Blood colloid osmotic pressure (BCOP)

ADH stimulates the reabsorption of water in __________.

Both the distal convoluted tubule and the collecting ducts.

Glucose is reabsorbed ______

By secondary active transport Through transmembrane proteins Into peritubular capillaries By facilitated diffusion

What are the forces opposing filtration at the glomerulus?

Capsular hydrostatic pressure and blood osmotic pressure

What is the function of the renal pelvis?

Collects newly formed urine

What is the last step before urine is released?

Conscious relaxation of the external urinary sphincter occurs

Which of the following statements about the regulation of the glomerular filtration rate (GFR) is false? Constriction of the afferent arteriole increases the GFR. Renin is released when the GFR is low. Constriction of the efferent arterioles increases the GFR. Constriction of the afferent arteriole increases the GFR.

Constriction of the afferent arteriole increases the GFR.

Autoregulation controls the glomerular filtration rate (GFR) by __________.

Constriction of the afferent arterioles Dilation of the afferent arteriole Constriction of the efferent arteriole

What type of nephron is 85% of all nephrons?

Cortical nephrons

Which of the following statements is NOT true regarding cortical nephrons? Cortical nephrons account for approximately 15 percent of all nephrons. The efferent arterioles of cortical nephrons deliver blood to a network of peritubular nephrons. Cortical nephrons are located almost entirely within the superficial cortex of the kidney. Cortical nephrons have a relatively short nephron loop.

Cortical nephrons account for approximately 15 percent of all nephrons.

What type of transport occurs when two chemical substances cross the membrane in the same direction, bound to the same carrier, in the process of reabsorption?

Cotransport

If the osmotic pressure in the glomerular capillaries increased from 28 mm Hg to 35 mm Hg, would net filtration increase or decrease?

Decrease (bc osmotic pressure opposes filtration.) Increase in osmotic pressure would decrease filtration.

How does the constriction of afferent arterioles affect glomerular filtration rate (GFR)?

Decrease GFR Now the diameter is more close to the efferent arteriole diameter.

What is the stimulus that leads to the release of renin by the juxtaglomerular complex (JGC)?

Decrease in blood pressure Obstruction of blood flow to the kidneys Decrease in blood volume

What ion changes stimulate aldosterone?

Decreased plasma sodium levels and Increased plasma potassium levels.

How does the dilation of efferent arterioles affect GFR?

Decreases GFR Now the diameter's of the afferent and efferent arterioles are more similar.

Secretion of ADH would _____

Decreases urine output Increases urine concentration

How does the addition of aldosterone effect urine?

Decreases urine volume

How does aldosterone affect urine volume?

Decreases volume

What effect does constricting the efferent arteriole have on the filtration process?

Decreasing the volume of blood leaving the glomerular capillaries Increasing glomerular filtration rate Increasing the blood pressure in the glomerular capillaries

What is the triple-layered muscle responsible for urinary bladder contractions?

Detrusor

What is the primary site of the secretion of potassium and hydrogen ions into the tubular fluid?

Distal convoluted tubule

Which portion of the nephron is under ADH and aldosterone stimulation?

Distal convoluted tubule

What parts of the kidney make up the juxtaglomerular complex (JGC)?

Distal convoluted tubule and afferent arteriole

In which of the following parts of the nephron does aldosterone control the reabsorption of sodium ions?

Distal convoluted tubule and collecting duct

What type of vessels take black OUT of the glomerulus?

Efferent vessels

The ability to control the micturition reflex depends on the control of the _________.

External urethral sphincter

Which mechanism is important in the reabsorption of glucose and amino acids when their concentrations in the tubular fluid are relatively high?

Facilitated transport

In which of the basic processes involved in urine formation does blood pressure force water and solutes across the wall of the glomerular capillaries?

Filtration

What are the three stages of urine formation?

Filtration Reabsorption Secretion

What are the three parts of the filtration membrane in the kidney?

Foot processes of podocytes Basement membrane Fenestrated endothelium

What is meant by the term nephrolithiasis?

Formation of kidney stones

What are the blood vessels found in the renal corpuscle that produce filtrate?

Glomerular capillaries

Where is filtrate formed?

Glomerular capsule

why does most reabsorption occur in the PCT?

Glomerular filtrate still contains some products that should not be excreted, so it must be reabsorbed into the peritubular capillaries (ex: glucose, water, amino acids, NaCl)

What is the outward pressure forcing water and solute molecules across the glomerular wall?

Glomerular hydrostatic pressure

Which of the following is the term for the blood pressure in the capillaries of the renal corpuscle?

Glomerular hydrostatic pressure

The reabsorption of water and solutes ____

Is passive Depends on concentration gradients Is into peritubular capillaries

Which of the following parts of the nephron has two limbs with thick segments and thin segments?

Nephron loop

Which part of the nephron is primarily responsible for establishing a concentration gradient in the interstitial fluid in the medulla of the kidney?

Nephron loop

What is/are the primary site(s) in the nephron where the loss of water, sodium, and potassium ions is regulated?

Nephron loop and collecting duct

Where does reabsorption occur?

PCT (a lot) Loop of Henle (a lot) DCT (a little) Collecting duct (a little)

Where are nephrons located in the kidney?

Partly in the renal cortex and partly in the renal medulla

What are the cells that surround the glomerular capillaries and limit filtration?

Podocytes

Where is ADH released from?

Posterior pituitary gland

The male urethra is divided into three segments. Which of the following is the correct order that urine passes through the urethra?

Prostatic, membranous, spongy

Filtrate produced in the renal corpuscle flows directly into the

Proximal convoluted tubule

Glucose reabsorption occurs in the ____

Proximal convoluted tubule

In which of the following areas of the nephron are most nutrients reabsorbed?

Proximal convoluted tubule

Sixty to seventy percent of the water in the filtrate is reabsorbed in the __________.

Proximal convoluted tubule

What is the primary site of nutrient reabsorption in the nephron?

Proximal convoluted tubule

What gets reabsorbed in the distal convoluted tubule? What gets secreted in the DCT?

Re-absorbed: calcium, sodium, others. Secreted: Hydrogen ions, potassium ions, and ammonia ions.

What is the process that can return useful solutes like electrolytes and glucose to the blood?

Reabsorption

What are three functions of the proximal convoluted tubules?

Reabsorption of organic nutrients like glucose and amino acids Active secretion of hydrogen ions Active reabsorption of ions like sodium, potassium, and magnesium

What is the primary function of the nephron?

Regulating the concentration of water and other substances in the body. They filter the blood, reabsorb what the body needs, and excrete the rest as urine.

Regulation of blood volume is an important function of the urinary system. Which of the following is NOT involved with the regulation of blood volume? Production of renin Production of erythropoietin Regulation of glucose levels Regulation of water levels

Regulation of glucose levels

Which of the following brings oxygenated blood into the kidney?

Renal artery

What structure does the glomerulus and glomerular capsule (Bowman's capsult) form?

Renal corpuscle

Where does the kidney filter the blood?

Renal corpuscle

Blood enters each kidney through the renal arteries. What is the order in which blood travels from the renal artery to the glomeruli?

Segmental arteries, interlobar arteries, arcuate arteries, cortical radiate arteries, afferent arterioles, glomerulus

When the solute gradient in the interstitial space is increased, urine _____

Volume decreased Concentration increased

How does reabsorption affect blood volume?

Water not re-absorbed in excreted in urine. Urinating lowers blood volume. Decreased water reabsorption --> increased water in urine --> decreased blood volume

Water reabsorption in the PCT vs DCT?

Water reabsorption is always obligatory in the proximal convoluted tubule but not in the distal convoluted tubule.

Which of the following is not accurate concerning urinary reflexes? The pontine storage center inhibits urination by decreasing parasympathetic activity and increasing somatic motor nerve activity at the external urethral sphincter. When urine is being stored, afferent impulses from stretch receptors in the urinary bladder stimulate sympathetic outflow causes the detrusor muscle to contract and the internal urethral sphincters to relax. The voiding reflex begins when afferent impulses from stretch receptors in the bladder stimulate interneurons that relay sensations to the pontine micturition center. During voiding, sacral spinal reflexes casue the detrusor muscle to contract and the internal urethral sphincter to relax.

When urine is being stored, afferent impulses from stretch receptors in the urinary bladder stimulate sympathetic outflow causes the detrusor muscle to contract and the internal urethral sphincters to relax.

Aldosterone stimulates ion pumps along the distal convoluted tubule DCT and the collecting duct causing __________.

a reduction in the number of sodium ions lost in the urine

Dilation of the afferent arteriole and constriction of the efferent arteriole when blood flow and glomerular pressure decline cause __________.

an elevation of glomerular blood pressure to normal levels

During reabsorption of water in the proximal convoluted tubule, what causes water to diffuse from the lumen into the interstitial space?

an increase in the osmolarity of the interstitium Submit

What is the primary regulatory mechanism that maintains glomerular filtration rate (GFR)?

autoregulation (self-regulation of constant blood flow)

The pressure that represents the resistance to flow along the nephron and conducting system is the __________.

capsular hydrostatic pressure (CHP)

What is hypokalemia?

deficiency of potassium in the bloodstream

Smooth muscle in the wall of the urinary bladder is the

detrusor muscle

What are the most important substances to be reapsorbed?

glucose and amino acids.

When antidiuretic hormone levels rise, the distal convoluted tubule becomes __________ and __________.

more permeable to water; water reabsorption increases.

Blood supply to the proximal and distal convoluted tubules of the nephron is provided by the ____

peritubular capillaries

In a nephron, the long tubular passageway through which the filtrate passes includes the __________.

proximal and distal convoluted tubules and the nephron loop

The filtration of plasma that generates approximately 180 liters/day of filtrate occurs in the __________.

renal corpuscle

Two main parts of a nephron

renal corpuscle and renal tubule

What is the most superficial region of the kidney?

renal cortex

The plasma concentration at which a specific substance or ion begins to appear in the urine is called the ________________.

renal threshold

Which artery of the kidney does not have a vein with the same name?

segmental artery

The active transport of which ion out of proximal convoluted tubule cells causes the reabsorption of both water and solutes?

sodium

Colloid osmotic pressure is the osmotic pressure resulting from ___________.

suspended proteins in the blood

The decreased intracellular concentration of sodium in tubular cells during active transport is caused by which of the following mechanisms?

the sodium-potassium ATPase pump in the basolateral membrane Submit

What gets secreted into the PCT?

urea, uric acid, creatinine, ammonium ions, drugs.

How do the kidneys assist the liver?

1. Kidneys assist the liver in detoxifying poisons 2. Generate amino acids for metabolic use during starvation by deaminating amino acids.

How does aldosterone affect ions? so

* Increases sodium reabsorption*by increasing the number of sodium-potassium in the luminal membrane of the proximal convoluted tubule.

What results in an increased glomerular filtration rate?

- Decreasing the efferent arteriole diameter - Increasing the afferent arteriole diameter

What results in a decrease in glomerular capillary pressure?

- Increasing the efferent arteriole diameter - Decreasing the afferent arteriole diameter

The driving forces for filtration in the nephron:

-Hydrostatic pressure gradients -Osmotic pressure gradients

What occurs in the filtration stage of urine production?

1. Blood flows into the glomerulus from the afferent arteriole. 2. Filterable blood components (water, waste) will move towards the center of the glomerulus. The accumulation of components are called glomerular filtrate. 3. Non-filterable components exit via the efferent arteriole.

What is glomerular filtration?

1. Blood from the afferent vessel enters the glomerulus. 2. Impure plasma enters the Bowman's capsule through the filtration slits in between the podocytes of the capsule's basement membrane. RBCs and plasma proteins are too large to fit through the slits (aka glomerular filtration barrier) 3. The rest of the blood in the glomerulus exits through the efferent arterioles.

What triggers renin release by the granular cells in the afferent arterioles?

1. Drop in BP and blood volume 2. Decreased sodium in DCT (bc more time for reabsorption earlier) 3. CNS innervation.

Urinary system flow

1. Glomerular capsule (browman's capsule) 2. Proximal convoluted tubule 3. Descending limb of loop of Henle 4. Ascending limb of loop of Henle 5. Distal convoluted tubule 6. Collecting duct 7. Papillary duct 8. Minor calyx 9. Major calyx 10. Renal pelvis 11. Ureter 12. Urinary bladder 13. Urethra

Structures that form the filtration membrane in the nephron:

1. Glomerulus 2. Glomerular capsule

In the event of a severe increase in systemic blood pressure, what mechanism would increase GFR?

ANP and BNP ANP and BNP are released from the heart when an increase in blood pressure or volume stretches the walls of the heart. Both ANP and BNP increase GFR, which increases urine production and reduces blood volume and pressure. ANP increases glomerular filtration rate. ANP increases glomerular filtration rate (GFR) through its direct vasodilata- tion effects on the afferent arterioles, which allows more blood to enter the glomerulus for ultrafiltration. ANP also reverses afferent arteriolar vasoconstriction induced by norepinephrine

Nephron: secretion vs. absorption

Absorption = substances in nephron ---> blood Secretion = blood ---> collecting duct

What is the primary function of the juxtaglomerular complex?

Adjusts glomerular-filtration rate by affecting blood pressure and volume

Where is aldosterone produced?

Adrenal cortex Located directly above the kidneys.

The myogenic mechanism of renal autoregulation primarily involves smooth muscle in which blood vessels?

Afferent arterioles Under normal physiological conditions, this works to maintain GFR despite moderate changes in systemic blood pressure.

What type of vessels take blood INTO the glomerulus?

Afferent vessels

How does aldosterone aid in sodium reabsorption?

Aldosterone increases the number of passive sodium channels in the luminal membrane of the distal tubule and collecting duct, thus aiding sodium reabsorption.

What structures are located in the outer renal cortex?

Glomerulus Convoluted tubules

Which of the following is NOT a waste product normally found in urine?

Glucose

When the glucose transport maximum is reached,

Glucose is excreted in the urine and not all of the glucose is reabsorbed

Name three results that can be found from an urinalysis?

High glucose in the urine (glucosuria) can indicate diabetes. High white blood cells in urine (pyuria) can indicate a renal infection. High red blood cells in urine (hematuria) can indicate vascular damage to the urinary system.

Where does the ureter penetrate the kidney?

Hilum

What is the primary driving force (pressure) that produces glomerular filtration?

Hydrostatic pressure of blood (blood pressure)

Dangerously low glomerular fitration rate (GFR), which can lead to acute kidney failure, can be caused by all the of the following except ______________. Dehydration Shock Hemorrhage Hypertension

Hypertension

Which of the following is the result of prolonged aldosterone stimulation?

Hypokalemia

ADH is produced by the ___

Hypothalamus

What fluid enters the glomerular capsule?

Impure plasma (plasma without plasma proteins)

How does the dilation of afferent arterioles affect GFR?

Increase GFR Now there is an even greater difference in diameter compared to the efferent arteriole.

When there is a severe rise in systemic BP, does our body increase GFR or decrease GFR?

Increase, so urine production can increase, which will lower blood volume and BP.

What is the effect of aldosterone along the DCT, the collecting tubule, and the collecting duct?

Increased conservation of sodium ions and water

What is the effect of taking a diuretic?

Increased water loss in the urine

How does the constriction of efferent arterioles affect GFR?

Increases GFR. Now there's an even greater difference in diameter compared to the afferent arteriole.

Which other body system excretes the same wastes, like urea, that the urinary system excretes?

Integumentary system

What is glomerular blood pressure?

It is the driving force for glomerular filtration. It is caused by the difference in diameters of the afferent vessel (thick) and the efferent vessel (thin). These diameters can be adjusted, so glomerular BP can be adjusted if needed.

Which organ in the urinary system produces urine?

Kidney

What role do the kidneys play for RBC's ?

Kidney releases EPO (erythropoietin), which is necessary for RBC's production and maturation in the red bone marrow.

What are the cells found between adjacent glomerular capillaries that can change the diameter of the capillaries and adjust blood flow?

Mesangial cells

What term means urination?

Micturition

What occurs in the reabsorption stage of urine production?

Molecules and ions will be re-absorbed into the circulatory system. The fluid passes through the nephron's tubules, loop, and collecting duct. Ions and water are removed as the fluid osmolarity (ion concentration) changes.

What is the name for the regulation of the glomerular filtration rate, where the afferent arterioles adjust their diameter in response to blood pressure changes?

Myogenic mechanism

What is the formula for net filtration pressure?

NFP = NHP - BCOP

Which of the following transporters in the luminal membrane results in secretion?

Na+ - H+ countertransport

What is an example of a countertransporter found in the renal tubules?

Na+ / H+

What gets reabsorbed in the PCT?

NaCl (70%), water (70%), glucose (>95%), amino acids (>95%)

Which hormone produced by the heart increases the glomerular filtration rate (GFR)?

Natriuretic peptides

What structure performs the primary function of the kidneys?

Nephron

In which of the following areas of the nephron would countercurrent multiplication occur?

Nephron loop

Where does filtration of the blood occur inside the kidneys?

Renal cortex

Which of the following is a dense, fibrous layer that anchors the kidney to surrounding structures?

Renal fascia

In what part of the kidney are the renal pyramids located?

Renal medulla

Where are the renal columns located?

Renal medulla

What is the expanded, funnel-shaped structure in the kidney that leads to the ureter?

Renal pelvis

What structures are located in the inner renal medulla?

Renal pyramids Loop of Henle

What enzyme is released by the juxtaglomerular complex to regulate GFR?

Renin Renin catalyzes the conversion of angiotensinogen to angiotensin I, a critical first step in the hormonal regulation of GFR during a low pressure event.

How does renin raise BP and blood volume back to normal?

Renin converts angiotensinogen (released from the liver) --> angiotensin I which gets converted to angiotensin II. Angiotensin II is a vasoconstrictor, so it increases BP. Angiotensin II also constricts the efferent arterioles, which raises glomerular BP and increases glomerular filtration rate, which then increases sodium in the DCT. Angiotensin II also stimulates aldosterone release from the adrenal glands. Aldosterone triggers the DCT to increase sodium reabsorption. Water follows Na+, so this increases blood volume, thus increasing BP. Angiotensin II also triggers ADH release from posterior pituitary glands. Triggers more reabsorption of water by the DCT and collecting duct. This increases blood volume and pressure.

Which of the following occurs first when there is a decrease in the glomerular filtration rate?

Renin is released from cells of the juxtaglomerular complex.

Which of the following GFR-regulating mechanisms is initiated by cells of the juxtaglomerular complex?

Renin-angiotensin system Renin is secreted by cells in the juxtaglomerular complex in response to a decrease in glomerular BP. --> angiotensin II circulation in the blood. * Activation of the renin-angiotensin system increases GFR *

What are three systems that excrete wastes from the body?

Respiratory Digestive Integumentary Urinary

One of the functions of the urinary system is to help in the regulation of pH. How does the nephron correct an acidosis condition in the blood?

Secreting H+ and reabsorbing HCO3-

Which process in the formation of urine removes chemicals from the blood and puts them into the urine and is not driven by blood pressure?

Secretion

What occurs in the secretion stage of urine production?

Secretion occurs in the collecting duct. Some substances (ex: H+ and creatinine) are removed from the blood through the peritubular capillary network into the collecting duct.

What are three functions of the collecting ducts?

Sodium ion reabsorption by aldosterone-sensitive ion pumps that exchange sodium ions for potassium ions Reabsorption of urea through diffusion Bicarbonate ion reabsorption in exchange for chloride ions

Describe the purpose and normal results of a BUN (blood urea nitrogen) test.

Some nitrogenous wastes are expected in the urine, but high levels should not remain in the blood. The test measure the ability of the kidneys to excrete nitrogenous wastes like urea, uric acid, and creatinine. The test measures the amount of urea in the blood.

Sympathetic innervation into the kidney is responsible for __________.

Stimulation of renin release Regulation of glomerular blood flow and pressure Direct stimulation of water and sodium ion reabsorption

How do sympathetic fibers play a role in the autonomic mechanism for regulating GFR?

Sympathetic fibers override local controls to decrease the GFR. Sympathetic activity causes *vasoconstriction* of *afferent* arterioles to *decrease GFR* when there is an acute fall in blood pressure.

Where does secretion occur?

The PCT - a little (urea, uric acid, creatinine) The DCT - a lot (H+ ions, K ions, ammonium ions)

What are three characteristics of carrier-mediated transport processes?

The distribution of carrier proteins can vary in different regions of the cell surface The membrane of a single tubular cell contains many types of carrier proteins A specific substrate bind to a carrier protein that facilitates movement across the membrane

Which part of the kidney functions as an endocrine organ to produce erythropoietin and the enzyme, renin?

The juxtaglomerular complex

What causes urinary incontinence?

The loss of muscle tone in the urinary sphincter muscles.

With ADH added but in the absence of aldosterone,

Urine volume decreased Potassium concentration increased

How does the loop of Henle concentrate urine?

The renal medulla has a higher concentration of salts due to reabsorption by the loop of Henle. The descending limb of the loop of Henle pushes water out to be reabsorbed. The ascending loop of Henle is impermeable to water (no water can move out).

Interstitial fluid is located in _____

The spaces surrounding kidney tubule

If ADH is not added to collecting ducts,

The urine volume will increase

Define glomerular filtration rate

The volume of filtrate created by the kidneys per minute.

Glucose is transported _____

Through the basolateral membrane by facilitated diffusion

The ureters are lined by what kind of epithelium, which allows them to be stretched when undergoing peristalsis and moving urine?

Transitional

When referring to reabsorption of molecules in the kidney, Tm is the abbreviation for __________.

Transport maximum, the number of available carrier proteins in the renal tubules

The area in the urinary bladder that is bounded by the openings of the ureters and the entrance to the urethra is the __________.

Trigone

What is the most abundant nitrogenous waste that the kidneys remove from the blood?

Urea

Which of the following organic wastes is generated by the catabolism of amino acids?

Urea

Name three waste products

Urea Creatinine Uric acid

Contraction of the muscular bladder forces urine out of the body through the __________.

Urethra

The amount of filtrate the kidneys produce each minute is the __________.

glomerular filtration rate

What is the passive force that creates filtrate?

glomerular hydrostatic pressure (GHP)

When plasma glucose concentrations are higher than the renal threshold, glucose concentrations in the filtrate exceed the transport maximum (Tm), and __________.

glucose appears in the urine (glucosuria)

What gets reabsorbed in the loop of Henle?

important salts (sodium, calcium, potassium, magnesium)

A decrease in glomerular hydrostatic pressure from 50 mmHg to 40 mmHg _____________.

is devastating to kidney function since at 40 mmHg, kidney filtration stops because the net filtration pressure falls to 0 mmHg

The primary cause of incontinence in the elderly is due to ______

loss of muscle tone in the urethral sphincters

The group of tall cells in the distal convoluted tubule that serve as either chemoreceptors or baroreceptors are the __________ cells.

macula densa