5.10 Renal System

Question 1

Give some key functions of the renal system

Answer 1

-regulating blood volume
-regulating blood pressure
-regulating blood ionic concentration
-blood pH
-blood osmolarity
-blood glucose concentration
-excretion of waste
-production of hormones such as calcitriol and erythropoietin

Question 2

What is the functional unit of the kidney

Answer 2

The nephron

Question 3

What makes up the renal corpuscle

Answer 3

The glomerulus and the bowmans capsule

Question 4

Describe the glomerulus

Answer 4

A tangled capillary network that is surrounded by the bowman’s capsule

Question 5

Describe how blood is supplied to and from the glomerulus

Answer 5

The afferent arteriole brings blood into the renal corpuscle and the efferent arteriole takes blood out of the renal corpuscle. The vasa recta are a group of blood vessels (forming the peritubular capillaries) that branch of from these arterioles

Question 6

Describe the bowman’s capsule

Answer 6

A cup shaped, double walled structure that surrounds the glomerulus. It is made of 2 simple squamous epithelium layers. The inner visceral layer wraps around the endothelial cells of the glomerular capillaries. The outer parietal layer forms the outer wall of the bowman’s capsule. The space between these 2 layers is the capsular space. The proximal convoluted tubule then branches of from the bowman’s capsule.

Question 7

Where does ultrafiltration of the blood take place

Answer 7

Through the filtration membrane in the renal corpuscle

Question 8

How much blood is filtered through the renal corpuscle every day

Answer 8

180L of blood passes through the filtration membrane every day

Question 9

Describe the 3 components of the filtration membrane that blood has to pass through.

Answer 9

-the endothelial cell of the glomerulus, which has small fenestration pores in it for the filtrate to pass through
-the basal lamina, which has a dense protein matrix filtering out larger molecules
-the visceral layer of the bowmans capsule with podocyte cells with projections and filtrations slits in between the projections

Question 10

What is needed for the blood to pass through the filtration membrane in the bowmans capsule.

Answer 10

The filtration is a passive process, so it relies on high blood pressure in the glomerulus to force the filtrate through to the PCT. This high blood pressure is due to the afferent arteriole having a wider lumen diameter than the efferent arteriole. This force is known as the net filtration pressure.

Question 11

How is the net filtration pressure calculated

Answer 11

The NFP is the net hydrostatic pressure minus the net osmotic pressure. This is calculated as the the glomerular blood hydrostatic pressure minus the capsular hydrostatic pressure and the blood colloid osmotic pressure.

Question 12

What is the glomerular filtration rate

Answer 12

The volume of fluid filtered from the glomerular capillaries into the bowmans capsule per unit of time. The average values are between 105-125 ml/min.

Question 13

How is the glomerular filtration rate controlled and affected

Answer 13

The glomerular filtration rate is controlled by constriction/dilation of the afferent and efferent arterioles. The GFR can be affected by changes in glomerular blood pressure, the osmotic pressure of the plasma, blood flow to the glomeruli and the permeability in the renal corpuscle

Question 14

What are the 2 types of regulation mechanisms of the GFR

Answer 14

Intrinsic (autoregulation) regulation

Extrinsic regulation

Question 15

Describe intrinsic (autoregulation) regulation mechanisms of the GFR

Answer 15

There are myogenic mechanisms where smooth muscle in the arteriole contracts in response to vessel stretch from increased blood pressure

There is also a tubuloglomerular feedback mechanism where macula densa cells monitor the filtrate flow. With an increased filtrate flow rate, the juxtaglomerular cells (which make up the juxtaglomerular apparatus) in the afferent arterioles will constrict and reduce blood flow into the glomerulus.

Question 16

Describe the extrinsic regulation mechanisms of the GFR

Answer 16

In times of severe haemorrhage or dehydration, sympathetic stimulation will constrict the afferent arteriole to reduce blood flow into the glomerulus

In response to low blood pressure, juxtaglomerular cells will secrete renin causing vasoconstriction to increase blood pressure

Question 17

Briefly describe the layout of the renal tubule

Answer 17

The renal tubule is comprised of the proximal convoluted tubule, the loop of henle and the distal convoluted tubule. The PCT and DCT are just in the renal cortex, but the loop of henle extends between the renal cortex and renal medulla. The loop of henle is made up of the descending limb, the thin ascending limb and the thick ascending limb.

Question 18

Describe the structure of the collecting duct.

Answer 18

The collecting duct receives the filtrate from the DCT of multiple nephrons so is not part of an individual renal tubule. It is comprised of the cortical collecting duct, outer medullary collecting duct and inner medullary collecting duct.

Question 19

What proportions of substances are reabsorbed in the PCT

Answer 19

67% of Na, Cl and K
All of the glucose and amino acids

Question 20

Describe how sodium is reabsorbed into the tubular cells from the first half of the PCT

Answer 20

-there is a Na/H antiporter
-there are Na symporters with bicarbonate, glucose, amino acids and others

Question 21

Describe how sodium is reabsorbed into the tubular cells from the second half of the PCT

Answer 21

In the second half of the PCT, Na is reabsorbed with Cl. There are both transcellular and paracellular routes to this.
Transcellular - Na enters via antiporters such as an Na/H

Paracellular - the Cl moves between the cells along its concentration gradient, this creates an electrical gradient which facilitates the movement of the Na through the paracellular pathways

Question 22

In the PCT, how does Na leave the tubular cells into the blood stream

Answer 22

via Na/K - ATPase pumps

Question 23

What types of substances are secreted by the PCT

Answer 23

Organic cations/anions, end products of metabolism, drugs and toxic chemicals. Many of these can not be filtered as they are bound to plasma proteins

Question 24

How are substances secreted into the PCT

Answer 24

Via specific transporter proteins, either organic anion transporters (OATs) or organic cation transporters (OCTs). All the substances excreted in the PCT uses these transporters

Question 25

How can improper secretion in the PCT lead to toxicity build up

Answer 25

Organic ions compete with the drugs and waste products for use of the specific transporters (OATs and OCTs)

Question 26

Give two additional functions of the PCT

Answer 26

-gluconeogenesis
-converting forms of vitamin D

Question 27

What does reabsorption in the loop of henle generate

Answer 27

A countercurrent multiplier which establishes osmotic gradients so that the collecting ducts can variably reabsorb water to create dilute/concentrated urine according to the ADH levels. This is done by the reabsorption of water and ions in the loop of henle.

Question 28

Describe the permeabilities of the descending limb of the loop of henle

Answer 28

It is permeable to water but impermeable to solutes like NaCl

Question 29

Describe the permeabilities of the thin ascending loop of henle

Answer 29

It is impermeable to water but permeable to solutes like NaCl

Question 30

Describe how sodium is reabsorbed into the thick ascending loop of henle

Answer 30

Via transcellular - a 1Na/1K/2Cl (NKCC2) symporter moves Na across the apical membrane, and paracellular pathways. Na/K - ATPase pumps then move the Na into the bloodstream.

Question 31

Where is the kidney tubules is it controlled how much water will be lost with urine

Answer 31

The DCT and collecting duct

Question 32

What are the roles of principal cells in the late DCT and collecting duct

Answer 32

They reabsorb NaCl and
secrete K. AQP2 aquaporins are inserted in the apical membrane and AQP 3 and 4 aquaporins are inserted in the basal membrane.

Question 33

What are the roles of intercalated cells in the late DCT and collecting duct

Answer 33

They secrete H+ or bicarbonate and they reabsorb K+

Question 34

What is changes in water balance manifested by

Answer 34

Changes in blood plasma osmolarity. A major determinant of this plasma osmolarity is the Na+ concentration

Question 35

Give examples of factors that can alter water reabsorption in water balance

Answer 35

Hormones, nerves and starlings forces

Question 36

Describe the action of antidiuretic hormone

Answer 36

There are osmoreceptors in the hypothalamus and their feedback stimulates the release of ADH from the posterior pituitary gland. This ADH then stimulates the insertion of aquaporins, so more water is reabsorbed into the collecting duct. This concentrates the urine. When ADH levels are low, a hypoosmotic (dilute) urine must reabsorb solute but not water, this occurs in the ascending limb of the loop. When ADH levels are high, a hyperosmotic (concentrated urine) must reabsorb water without reabsorbing the solute, this occurs via countercurrent mutiplication

Question 37

The balance of what ion is closely tied to extracellular fluid (including plasma ) volume

Answer 37

Na

Question 38

Describe sensors that detect a change in Na balance and thus extracellular fluid volume

Answer 38

-baroreceptors in the cardiac atria, right ventricles, large pulmonary vessels, aortic arch, carotid sinus and arterioles of the kidneys - alter sympathetic outflow and ADH secretion

-juxtaglomerular apparatus of the kidneys - alters renin secretion

-hepatic sensors and Na sensors in the CNS - alters the sympathetic outflow