The Renal System

Question 1

What are the functions of the kidney?

Answer 1

• regulation of water balance
• regulation of pH and inorganic ion balance (sodium, potassium, calcium)
• excretion of metabolic and nitrogenous waste products (urea from proteins, uric acid from nucleic acids, creatinine from muscle creatine and hb breakdown products)
• hormone secretion (erythropoietin, renin)
• removal of toxic chemicals
• regulation of blood pressure (renin-angiotensin system)
• control of formation of RBC via erythropoietin
• vitamin D activation and calcium balance
• gluconeogenesis (formation of glucose from amino acids and other precursors)

Question 2

Describe the position and relations of the kidney

Answer 2

• the kidney is situated upon the posterior wall of the abdominal cavity, one on either side of the vertebral column just below the rib cage
• the right kidney is slightly lower than the left owing to its relationship to the liver
• each kidney is about 10cm long, 6.5cm wide and about 3cm thick and weighs around 100g

Question 3

Describe how blood flows through the kidneys

Answer 3

• the kidneys receive about 25% of the total cardiac output
• the renal blood supply arises from the aorta via the renal artery
• it returns to the inferior vena cava via the renal vein

Question 4

Describe the endocrine activity of the kidney

Answer 4

• the kidney secretes two hormones (renin and erythropoietin)
• renin is produced in the afferent arteriole and is secreted when the blood supply to the kidneys is reduced and in response to low sodium levels
  
  • -> it acts on angiotensinogen which is present in the blood to form angiotensin
  • -> this raises blood pressure and encourages sodium reabsorption
• erythropoietin is produced in response to low oxygen levels
  
  • -> it stimulates an increase in the production of RBC from the bone marrow

Question 5

Describe the two different types of nephron

Answer 5

• cortical –> 85-90% are cortical nephrons, they have short loops of Henle and are involved in the control of plasma volume during normal conditions
• juxtomedullary –> They have longer loops of Henle extending into the renal medulla, they facilitate increased water retention when availability of water is restricted

Question 6

Describe the process of filtration (production of urine)

Answer 6

• blood flows into the glomerulus via the afferent arteriole which is at a higher pressure then the blood that leaves the glomerulus through the efferent arteriole
- the difference in diameters ensure that the blood in the glomerulus is kept a high pressure
• the high pressure forces liquid and small molecules in the blood out of the capillary and into the renal capsule
• the liquid and small molecules pass through 3 layers to get into the bowman’s capsule and enter the nephron tubules
- the endothelium of capillaries –> contains gaps in which blood passes through as well as the substances dissolved in it
- the basement membrane –> fine mesh of collagen fibres and glycoproteins that do not allow large molecules like proteins to pass through
- epithelium of bowman’s capsule –> contains finger like projections called podocytes that fluid from the glomerulus can pass through into the bowman’s capsule

Question 7

Describe the process of selective reabsorption (production of urine)

Answer 7

• the sodium potassium pumps remove sodium ions from the cells lining the PCT - this reduces the concentration of sodium ions in the cell cytoplasm
• sodium ions are then transported into the cell along with amino acids or glucose via facilitated diffusion
• as the glucose and amino acid concentrations rise inside the cell they are able to diffuse out of the opposite side of the cell into the tissue fluid, they may also be actively removed
• from the tissue fluid these substances then diffuse into the blood and are carried away
• the readsorption of salts, glucose or amino acids reduces the water potential of the cell and increases the water potential in the tubule fluid so water will enter the cells and be reabsorbed into the blood by osmosis
• larger molecules such as small proteins that may have entered the tubule will be reabsorbed by endocytosis

Question 8

How are the cells lining the PCT specialised for selective reabsorption?

Answer 8

• microvilli - increase the surface area for reabsorption
• co transporter proteins - transports glucose or amino acids
• sodium potassium pumps - pump sodium ions out of the cell and potassium ions into the cell actively against concentration gradient
• many mitochondria - provides energy from ATP for reabsorption process

Question 9

What happens if the water level in the blood is high? (A person is hydrated)

Answer 9

• the water potential of the blood is monitored by osmoreceptors in the hypothalamus of the brain
• when the water potential is high, they do not shrink, and stimulate neurosecretory cells in the hypothalamus
- they produce and release anti diurectictic hormone which flows down the axon to the posterior pituitary gland where it is stored until needed
- when the neurosecretory cells are stimulated they send action potentials down their axons which causes the release of ADH
• however less impulses are sent to the neurosecretory cell so less ADH is released
• so less ADH travels in the blood to the kidney and binds to glycoproteins (receptors) in the membrane of the cells in the wall of the collecting duct
- this causes less vesicles containing aquaporins (water channels) to move and fuse with the cell membrane
• this causes the cell membrane to become less permeable to water so less water is reabsorbed by osmosis from the collecting duct into the capillary
• this means that a larger volume of dilute urine is produced which decreases the water potential of the blood

Question 10

What happens when water levels in the blood are low? (A person is dehydrated)

Answer 10

• the water potential of the blood is monitored by osmoreceptors in the hypothalamus of the brain
• when the water potential is very low, they shrink, and stimulate neurosecretory cells in the hypothalamus
- they produce and release anti douse tic hormone which flows down the axon to the posterior pituitary gland where it is stored until needed
- when the neurosecretory cells are stimulated they send action potentials down their axons which causes the release of ADH
• ADH travels in the blood to the kidney and binds to glycoproteins (receptors) in the membrane of the cells in the wall of the collecting duct
- this causes vesicles containing aquaporins (water channels) to move and fuse with the cell membrane
• this causes the cell membrane to become more permeable to water so more water is reabsorbed by osmosis from the collecting duct into the capillary
• this means that a smaller volume of concentrated urine is produced which increases the water potential of the blood

Question 11

Describe the function of the ureters

Answer 11

• the ureters transport urine from the kidneys to the bladder by waves of peristalsis
• about every 30 seconds a peristaltic contraction begins at the renal pelvis and sweeps along the ureter forcing urine towards the urinary bladder

Question 12

Describe the structure of the ureters

Answer 12

• each ureter is about 0.3cm in diameter and 25-30cm long, running from the renal helium to the posterior wall of the bladder
• each ureter is lined with smooth muscle and transitional epithelium, the lumen has a star shaped cross section
• the ureters lie upon the posterior abdominal wall outside the peritoneal cavity, entering the bladder at an oblique angle one at a each side of the base of the trigone
• as urine accumulates in the bladder the ureters are compressed, forming a valve which prevents urinary reflux

Question 13

Describe the structure and function of the bladder

Answer 13

• the bladder is a distensible hollow organ also composed of smooth muscle which acts as a reservoir for urine
• it is intermittently emptied under conscious control
• stretch receptor within the muscle and trigone provide the signals that the bladder is full
• the normal capacity of the bladder is approx 700-800ml however the natural desire to void urine becomes conscious when the level of urine reaches approx 300ml

Question 14

What happens if there is inflammation present in the trigone region of the bladder?

Answer 14

• inflammation in the trigone region caused by infection and or trauma often results in a frequent and urgent desire to void urine but on voiding only small amounts of urine are passed

Question 15

Describe the structure and function of the urethra

Answer 15

• urine is voided via the urethra
• the female urethra is only 4cm in length compared to around 20cm length in males which predisposes women to UTI’s
• there are small mucus-secreting glands in the urethra that help to protect the epithelium from the corrosive urine
• the upper internal sphincter at the exit from the bladder is composed of smooth muscle and is under autonomic control
• the external sphincter is composed of skeletal muscle and is under voluntary control

Question 16

Describe the process of micturition

Answer 16

• the process of micturition is a coordinated response that is due to the contraction of the muscular wall of the bladder, reflect relaxation of the internal sphincter or the urethra an voluntary relaxation of the external sphincter
• as the bladder fills with urine, stretch receptors in the wall of the urinary bladder are stimulated which then relay parasympathetic sensory nerve impulses to the brain generating awareness of fluid in the bladder
• the urge to micturate can be voluntarily resisted and postponed until a suitable time
• this is due to the conscious descending inhibition of the reflex bladder contraction and relaxation of the external sphincter
• if the urge to micturate is not voluntarily resisted then the bladder will empty of urine by the muscle wall contracting, the internal sphincter opening by the action of Bell’s muscles and voluntary relaxation of the external sphincter
• this is assisted by an increase in pressure in the pelvic cavity as the diaphragm is lowered and the abdominal muscles contract

Question 17

What happens to the bladder during pregnancy?

Answer 17

• as the bladder lies below the uterus its capacity is compromised by the growing uterus in early pregnancy
• later on, once the pregnant uterus has become an abdominal organ the pressure on the bladder is relieved
• finally at the end of pregnancy, bladder capacity is again compromised as the presenting part of the fetus engages, occupying space within the true pelvis cavity and thus restricting the space available to the bladder

Question 18

How do pregnancy hormones affect the renal system?

Answer 18

• under the influence of progesterone, bladder capacity increases to about 1000ml by late pregnancy and the walls of the ureters relax, which allows them to dilate, bend or kink
• if this occurs then it can result in a slowing down of urinary flow, causing women to be more at risk from infection

Question 19

What happens to the amount of glomerular filtrate produce during pregnancy?

Answer 19

• during pregnancy large amounts of urine are produced due to an increase in glomerular filtrate as this helps to eliminate the additional wastes created by maternal and fetal metabolism

Question 20

What happens to the kidneys during pregnancy?

Answer 20

• kidneys enlarge by about 1cm in length and by about 30% in volume due to an increase in renal blood flow and vascular volume

Question 21

How can hypertensive disorders in pregnancy disrupt renal function?

Answer 21

• the detectable presence of protein within the urine may indicate the larger molecules than normal are being forced into the bowmans capsule
• this is caused by the increased blood pressure resulting in abnormal ultrafiltration