Renal

Question 1

list the functions of the kidney

Answer 1

Kidneys are essentially excretory organs - rid the body of waste products of metabolism
Also other important functions:
1. Water balance
2. Ionic balance (Na+ and K +)
3. Acid-base balance (H +)
4. Blood pressure regulation (Renin-Angiotensin System)
5. Calcium regulation (Vit D)
6. Erythrocyte production (Erythropoietin)

Question 2

describe the macroscopic structure of the kidney

Answer 2

3 distinct regions: renal cortex, renal medulla, renal pelvis

renal cortex is outermost part, forms continuous smooth outer portion with a number of projections that extend down between pyramids

renal medulla consists of approx 8-12 renal pyramids, a cone shaped section of the kidney

renal pelvis forms the expanded upper portion of the ureter which is funnel shaped

Question 3

describe microscopic structure of the kidney/nephrons

Answer 3

nephrons form functional units of kidney
nephron consists of glomerulus and a renal tubule
approx 1 million nephrons per kidney
nephrons filter blood, perform selective reabsorption
is part of the homeostatic mechanism of the body- helps regulate amount of water, salts, glucose, aura, and other minerals in the body
nephron is a filtration system responsible for reabsorption of water and salts

Question 4

what is the function of the bowman’s capsule/glomerular capsule?

Answer 4

contains first part of nephron, is part of filtration system
when blood enters the kidneys it enters the bowmans capsule which separates the blood into 2 components- a filtrated blood component and a filtrate which is moved through the nephron

Question 5

what is the function of the proximal convoluted tubule (PCT)

Answer 5

filtrate drains into PCT from bowman’s capsule, where reabsorption of salt water and glucose now occurs. at the same time certain substances including uric acid and drug metabolites are actively transferred from blood capillaries into the tubule for excretion

Question 6

what is the function of the loop of henle

Answer 6

divided into descending and ascending loops- ascending loop is much thicker than the descending loop
main function of the loop of henle is to generate a concentration gradient which creates a region of high concentration of sodium in medulla of the kidney

Question 7

what is the function of the descending limb of the loop of henle

Answer 7

descending portion is highly permeable to water and has low permeability for ions and urea, required urine is concentrated in this part of the nephron, is relatively impermeable to solute but permeable to water so that water moves out by osmosis and fluid in the tubule becomes hypertonic

Question 8

what is the function of the ascending limb of the loop of henle

Answer 8

thin section of the ascending loop of henle is virtually impermeable to water but permeable to solute, especially Na and Cl, thus these ions move down concentartion gradient. urea which was absorbed into medullary interstitial from collecting duct diffuses into ascending limb. this keeps urea within the interstitium of the medulla where it also has a role in containing urine

thick section of ascending loop of henle and early distal tubule are virtually impermeable to water . sodium and chlorine are actively transported out of the tubule, making the tubular fluid very hypotonic

Question 9

describe function of the distal convoluted tubule

Answer 9

is an important site for

• active secretions of ions and acids
• plays a part in the regulation of calcium ions by excreting calcium ions in response to calcitonin hormone
• selective reabsorption of water
• arginine vasopresssin receptor 2 proteins located in DCT
• plays a role in regulating pH by absorbing bicarbonate and secreting protons (H+) into filtrate

Question 10

how does ADH effect concentration of urine is the DCT and collecting ducts

Answer 10

if it is present the DCT and collecting duct become permeable to water. as the collecting duct passes through the medulla with a high solute concentration the interstitium, the water moves out of the lumen of the duct and concentrated urine is formed

Question 11

describe the function of the collecting ducts/papillary ducts/renal pelvis

Answer 11

several collecting ducts converge and drain into a larger system called the papillary ducts which in turn empty into the minor cortex. from here filtrate, now called urine, drains into renal pelvis. it is the final stage where sodium and water are reabsorbed

Question 12

describe perfusion of kidneys

Answer 12

the kidney filters at least 20-25% of blood during resting cardiac output- approx 1200ml of blood flows through the kidney each minute
each kidney receives t’sown blood supply directly from the aorta via the renal artery, which is divided into anterior and posterior renal arteries
each nephron receives one afferent arteriole which further subdivides into the glomerulus (“tuft” of capillaries). the glomerular capillaries reunite and leave the Bowman’s capsule as efferent arterioles, which unite to form the accurate veins and finally into interlobular veins
blood leaves the kidneys through the renal vein, which then flows into the the interior vena cava

Question 13

summarise composition of the glomerular filtrate

Answer 13

being an ultra filtrate of plasma, the glomerular filtrate will have an essential similar ionic concentration as plasma as the ions can pass through easily) i.e. Na+ K+ Cl+ urea
As these ions mostly determine the osmotic pressure of blood the filtrate has an osmotic pressure essentially similar to plasma. formed elements (RBCs WBCs platelets) are not normally found , nor are large proteins e.g. albumin

Question 14

explain the reabsorbative processes at work in the proximal convoluted tubule

Answer 14

the proximal tubule is the major reabsorbative segment of the nephron and accounts for nearly 2/3 of all filtered water, sodium and chlorine. the proximal tubule is the segment where the majority of critical organic solutes such as glucose and amino acids are reabsorbed. this segment also plays an important role in acid base balance as it is involved in bicarbonate reabsorption and secretion of organic acids

large reabsorpatative capacity of the proximal tube relies on low intracellular sodium concentration caused by action of the sodium potassium pump - the low intracellular sodium conc is used to actuate reabsorption of sodium, other solutes and water

Question 15

explain concept of renal plasma clearance

Answer 15

clearance is the volume of plasma from which a substance is completely removed by the kidney in a given amount of time

Question 16

how is plasma clearance estimated

Answer 16

quantity of urine x (concentration of substance in urine/concentration of substance in plasma)

Question 17

What is countercurrent multiplication?

Answer 17

Countercurrent multiplication in the kidneys is the process of using energy to generate an osmotic gradient that enables you to reabsorb water from the tubular fluid and produce concentrated urine. This mechanism prevents you from producing litres and litres of dilute urine every day, and is the reason why you don’t need to be continually drinking in order to stay hydrated.

Question 18

The three segments of the loops of Henle have different characteristics that enable countercurrent multiplication- describe each (descending, thin ascending, thick ascending)

Answer 18

The thin descending limb is passively permeable to both water and small solutes such as sodium chloride and urea. As active reabsorption of solutes from the ascending limb of the loop of Henle increases the concentration of solutes within the interstitial space (space between cells), water and solutes move down their concentration gradients until their concentrations within the descending tubule and the interstitial space have equilibrated. As such, water moves out of the tubular fluid and solutes to move in. This means, the tubular fluid becomes steadily more concentrated or hyperosmotic (compared to blood) as it travels down the thin descending limb of the tubule.

The thin ascending limb is passively permeable to small solutes, but impermeable to water, which means water cannot escape from this part of the loop. As a result, solutes move out of the tubular fluid, but water is retained and the tubular fluid becomes steadily more dilute or hyposmotic as it moves up the ascending limb of the tubule.

The thick ascending limb actively reabsorbs sodium, potassium and chloride. this segment is also impermeable to water, which again means that water cannot escape from this part of the loop. This segment is sometimes called the “diluting segment”.

Question 19

Countercurrent multiplication moves sodium chloride from the tubular fluid into the interstitial space deep within the kidneys. Although in reality it is a continual process, the way the countercurrent multiplication process builds up an osmotic gradient in the interstitial fluid can be thought of in two steps- describe each

Answer 19

The single effect. The single effect is driven by active transport of sodium chloride out of the tubular fluid in the thick ascending limb into the interstitial fluid, which becomes hyperosmotic. As a result, water moves passively down its concentration gradient out of the tubular fluid in the descending limb into the interstitial space, until it reaches equilibrium.
Fluid flow. As urine is continually being produced, new tubular fluid enters the descending limb, which pushes the fluid at higher osmolarity down the tube and an osmotic gradient begins to develop.
As the fluid continues to move through the loop of Henle, these two steps are repeated over and over, causing the osmotic gradient to steadily multiply until it reaches a steady state. The length of the loop of Henle determines the size of the gradient - the longer the loop, the greater the osmotic gradient.

Question 20

Explain how ADH secretion is regulated

Answer 20

regulated by baroreceptors and osmoreceptors

low pressure baroreceptors barorecpetors in the large veins and atria detect stretch of these structures due to increased intravascular volume. decreased plasma volume leads to secretion of adh

osmoreceptors are receptors in the hypothalamus that detect changes in tissue osmolality - increased osmolality leads to secretion of adh. adh triggers appearance of H2O channels in collecting duct. due to higher osmotic pressure in the medullary interstitium water moves out f the duct

Question 21

Describe the regulatory functions of the kidneys

Answer 21

1. water balance- vasopressin
2. sodium and potassium balance- aldosterone and atrial natriuretic peptide act on the DCT to exact fine control over sodium ion excretion
3. acid-base balance, involves excretion of hydrogen ions, partly liked to regulation of sodium ions

Question 22

Show how the renin-angiotensin system influences salt and water balance

Answer 22

sodium is the principle extracellular cation while potassium is the main intracellular cation. it is important to regulate the output of these ions, as input varies. those ions are mostly reabsorbed at the PCT (around 70% of Na+ and 100% of K+), the role of DCT being to regulate reabsorption of Na+ and secretion of K+ under the influence of aldosterone. aldosterone secretion is controlled via the renin-angiotensin system. part of the DCT has aldosterone sensitive sodium/potassium pump, promoting sodium reabsorption and potassium secretion. aldosterone may act by increasing DCT inward promation to sodium, forcing the pump to work harder to pump out Na+

Question 23

explain how aldosterone secretion is regulated

Answer 23

Aldosterone is part of a group of linked hormones, which form the renin-angiotensin-aldosterone system. Activation of this system occurs when there is decrease in blood flow to the kidneys following loss of blood volume or a drop in blood pressure (eg due to a haemorrhage) or decrease in plasma sodium concentration. Renin is an enzyme that leads to a series of chemical reactions resulting in the production of angiotensin II, which in turn stimulate aldosterone release. Aldosterone causes an increase in salt and water reabsorption into the bloodstream from the kidney thereby increasing the blood volume, restoring salt levels and blood pressure. Once salt levels and blood pressure are corrected and the body becomes rehydrated, the level of renin in the bloodstream falls and therefore the amount of aldosterone in the blood also falls, meaning more water is excreted in the urine. The renin-angiotensin-aldosterone system is an example of a negative feedback system.

Question 24

describe effect of natriuretic peptides

Answer 24

is a hormone involved in tubular secretion and reabsorption, is a powerful vasodilator and is a protein produced by the myocytes in the atria of the heart i response to increased blood pressure. amp stimulates the kidneys to excrete sodium and water from the renal tubules thus decreasing blood volume which in turn lowers blood pressure

Question 25

what are the 2 roles of the kidney in acid-base balance

Answer 25

1. reabsorb bicarbonate from urine

2. excrete hydrogen ions into urine

Question 26

what happens in the proximal tubule in terms of acid-base balance

Answer 26

there is reabsorption of bicarbonate ions which is filtered at glomerulus
there is production of ammonium

Question 27

explain how bladder stores and eliminates urine

Answer 27

the bladder is a hollow smooth muscular sack which stores urine. when the bladder is empty the inner section of the bladder forms folds but as the bladder expands without a significant rise in the internal pressure of the bladder. when the volume of urine in the bladder reaches about 300ml, stretch receptors in the bladder walls stimulate –> excite sensory fibres which relay info to sacral area of spine–> info integrated in spine &relayed to 2 different sets of neurons:

• parasympathetic motor neurons excite and contract muscles in bladder –> pressure within bladder increases and internal sphincter opens
• somatic motor neurons supplying the external sphincter via pudendal nerve are inhibited –> external sphincter opens and urine flows out

Question 28

what are consequences of renal failure

Answer 28

when kidneys are not working properly waste products and fluids can build up to dangerous levels
renal failure may occur with an serious illness or operation. if blood supply to kidneys is reduced considerably kidneys may be permanently damaged if this lasts long enough.

Question 29

how can renal failure be managed

Answer 29

no cure but treatments to manage symptoms and stop it getting worse:

• lifestyle changes
• medication to control associated problems such as hugh blood pressure/high cholesterol
• dialysis in advance CKD
• kidney transplant in advanced CKD