Urinary System

Question 1

Urinary System

Answer 1

This functions to maintain the volume and composition of body fluids within normal limits (osmoregulation) and for removing the body of waste products of the cellular metabolism (aka excretory system). It is closely associated with the reproductive system by location (aka urogenital/urinogenital system) and the major part is the kidneys (aka renal system).

Question 2

Homeostatic Function of Urinary System

Answer 2

Homeostasis is the maintenance of all body systems in a normal range. This is involved in water and electrolyte balance, the removal of nitrogenous wastes and toxins and maintaining normal pH (excreting H+, reabsorbing HCO3-, maintains at 7.4).

Question 3

Osmoregulation

Answer 3

All animals balance the gain and loss of water and dissolved substances e.g. Na+. Water is gained through food, drink and as a product of cellular respiration (metabolism). Water is lost through urination, defecation and evaporation (sweating and breathing).

Question 4

Osmosis

Answer 4

Water will move through semi-permeable membranes. It will move from a place of lower solute concentration through a semi-permeable membrane to a higher solute concentration area.

Question 5

Adaptations to Prevent Dehydration

Answer 5

All terrestrial vertebrates have an outer skin of water-resistant cells. Embryos of these organisms develop in fluid filled amniotic sac surrounding as protective membranes (in eggs for reptiles in wombs for mammals). There are behavioural changes such as drinking water and seeking shade that also assist.

Question 6

The Kidneys

Answer 6

These organs play a major role in coserving water and regulating the osmotic pressure of blood. When fluid intake is high the kidneys excrete dilute urine excreting water while keeping salts (electrolytes). When fluid intake is low the kidneys conserve water by forming concentrated urine (excreting salts and keeping water). We can concentrate urine 4x more than blood.

Question 7

Nitrogenous Waste Removal

Answer 7

When proteins are broken down they become amino acids and nucleic acids are broken down into nitrogenous bases however a biproduct of both reactions is amino groups (NH2).

Question 8

Ammonia

Answer 8

NH3 formed from broken down proteins and nucleic acids amino groups is too toxic to be stored in the body and doesn’t diffuse readily into the air however is highly soluble in water and diffuses rapidly across the cell membrane. IF an animal is surrounded by water NH3 can readily diffuse out of cells. It must be transported and excreted in large volumes of very dilute solutions to be effective. This is the method of nitrogenous waste removal used by most aquatic animals.

Question 9

Urea

Answer 9

CONH2NH2 is the product formed in mammals, amphibians, sharks and some bony fish when the amino groups of broken down proteins and nucleic acids remain. This is highly soluble in water and 100,000x less toxic than ammonia (NH3). This means it can be stored in the body in a concentrated solution. This means water is required in order to dispose of it.

Question 10

Uric Acid

Answer 10

This is the product formed in birds, insects, reptiles and land snails when amino groups are left over from protein and nucleic acid breakdown. This is a more complex molecule than urea and ammonia making it relatively nontoxic however largely insoluble in water. This means excretion of this substance minimises water loss however requires more energy which must be balanced out in the body.

Question 11

Urinary System Organs

Answer 11

Kidneys are 2 structures (left is lower down due to displacement from the liver) which produce urine by filtering the blood. Ureters which transport urine from the kidneys to the bladder. The bladder is used to store urine. The urethra passes urine from the bladder to the outside of the body. The left/right renal arteries and left/right renal veins are what provide a blood supply to both kidneys. The adrenal glands are a pair of glands which sit above both kidneys and release hormones.

Question 12

Gross Anatomy of Kidneys

Answer 12

These organs are surrounded by a fibrous capsule. There is a blood supply (renal artery/vein) which enters the kidney and divides into many smaller arteries a nd veins. These are made up of an outer renal cortex which filers the blood and an inner renal medulla which concentrates the filtrate. There are many renal pyramids in between which there are many renal columns (cortical tissue dips down into the medulla). The pyramids produce urine which drains into minor calyx from a buildup at the renal papilla (ends of the renal pyramid). The minor calyx’s empty into the major calyx which further empties into the renal pelvis and finally leaves tis organs through the ureters.

Question 13

Renal Blood Vessels

Answer 13

Each kidney is supplied by renal arteries branching off the aorta and blood leaves the kidneys via the renal vein and drains into the inferior vena cava. Despite comprising <1% of body weight they receive 20-25% of cardiac output. The human blood volume is 5L however 1100-2000L of fluid pass through the capillaries in kidneys every day where they extract 180L of filtrate in this time. If all the filtrate was excreted as urine we would lose vital nutrients and dehydrate. Kidneys refine the filtrate, concentrating the urea and returning most of the water and solutes to the blood and leaves typically 1.5L of urine.

Question 14

Ureters

Answer 14

Urine enters this structure from the renal pelvis of the kidneys. This stretches causing a contraction of the muscle wall in a peristaltic wave which helps to transport the urine down to the bladder. Urine enters the bladder from below which is why the muscle contractions are required for its transport. These have a mucosa made up of transitional epithelium which is folded in a relaxed state, a lamina propria is present, muscularis is 2-3 layers of smooth muscles. This tube lacks a true submucosa which assists in propelling urine to the bladders (via peristalsis).

Question 15

Bladder

Answer 15

This is a muscular sac located on the floor of the pelvic cavity with a capacity of 500mL with a maximum of 800mL. The muscularis of this organ has 3 layers of smooth muscle. The internal structure of this organ shows rugae (folds/ridges) which is common in older men with prostate issues. The mucosa of this structure is lined with transitional epithelium meaning it is highly distensible (stretchy) and has a lamina propria. As this structure fills it expands superiorly, rugae flatten and epithelium thins from 5-6 layers to 2-3 layers which also gives the cells a squamous appearance.

Question 16

Urethra

Answer 16

This conveys urine out of the body which is short in females and longer in males. In males there is an internal and external sphincter which is a thickening of the smooth muscle of the bladder. The internal sphincter is involuntarily controlled and compresses this structure and retain urine in the bladder. The external sphincter is present in men and women which is makeup of skeletal muscles of the pelvic floor allowing voluntary control over this structure. This is similar to the way in which anal sphincters are designed to allow for voluntary control at the very end of the waste removal process.

Question 17

Neural Control of Micturition

Answer 17

This is also known as urinating. As the bladder fills stretch receptors in the bladder wall send signals to the sacral spinal cord which sends back contraction signals through motor neurons to contract muscle of the bladder and relax the internal urethral sphincter. This process leads to the emptying of the bladder. The stimulation from stretch receptors is also sent to the brain which we can sense in order to make voluntary decisions to decide whether it is appropriate at the time to urinate by relaxing our external urethral sphincters.

Question 18

Kidney Anatomy

Answer 18

The outer part is the cortex and the inner part is the medulla. The basic functional unit is called a nephron. The nephrons extract filtrate from the blood and refine the filtrate into a smaller amount of urine. These are found in the renal pyramids with many of these structures per kidney.

Question 19

Basic Stages of Urine Formation

Answer 19

1. Glomerular filtration - this creates a plasma like filtrate of blood.
2. Tubular reabsorption - this removes useful solutes from the filtrate back into the blood.
3. Tubular secretion - this adds additional wastes from the blood into the tubule to add to the filtrate.
4. Water conservation - this removes water from the urine and returns it to blood (concentrates wastes).

Question 20

Nephron

Answer 20

Each kidney has 1.2 million of these. Each of these structures is supplied by an arteriole from the renal artery which forms a glomerulus (twisted up capillary) which sits in the Bowman’s capsule which makes up a structure called the renal corpuscle (Bowman’s capsule _ glomerulus). Blood will then leave this area and enter the renal tubule where the filtrate is converted into urine. This tubule consists of the proximal convoluted tubules (PCT), loop of Henle and distal convoluted tubule (DCT). The final filtrate will then enter a collecting duct which is supplied by several of these structures. These can be juxtamedullary (just above the medulla) or cortical (in the cortex).

Question 21

Flow of Glomerular Filtrate

Answer 21

glomerular capsule -> PCT -> nephron loop -> DCT -> collecting duct -> papillary duct -> minor calyx -> major calyx -> renal pelvis -> ureter -> urinary bladder -> urethra.

Question 22

Renal Circulation

Answer 22

The renal artery divides many times to form arterioles that enter the glomerulus of each nephron which is the afferent arteriole. In the glomerulus the arteriole becomes thinner and more twisted up until it finally gains structure again and leaves the glomerulus which is the efferent arteriole. This efferent arteriole then forms a capillary network (fenestrated capillary) around the PCT and DCT called the peritubular capillaries and around the loop of Henle called the vasa recta.

Question 23

Renal Corpuscle

Answer 23

This is made up of the Bowman’s capsule and glomerulus. The Bowman’s capsule has a parietal (outer) - simple squamous epithelium, visceral (inner) - podocytes (wrap around the capillaries of the glomerulus) and capsular space (between inner and outer layers). There is a vascular pole where the afferent arteriole enters and the efferent arteriole leaves. There is a urinary pole on the opposite side of the corpuscle to the vascular pole where the renal tubule begins. There is a difference between the efferent and afferent arterioles with the latter being wider creating hydrostatic pressure in the glomerulus which helps to drive water and compounds through the filtration membrane to form the filtrate.

Question 24

Glomerular Filtration

Answer 24

This is when a glomerular filtrate is formed. The glomerulus acts as a mechanical filter which blood is pushed through by the increased pressure in the glomerulus due to the volume of the afferent arteriole being larger than that of the efferent one and also forces the filtrate of the plasma into the capsular space. Many solute molecules small enough to pass through the filtration slits are carried through by water in which they are dissolved however RBCs and plasma proteins remain in capillaries (too large).

Question 25

Filtration Membrane

Answer 25

This is used to create the glomerular filtrate and consists of 3 barriers which are the epithelium (endothelium) of capillaries, basement membrane and the filtration slits between the podocyte cell extensions (pedicels). Water, electrolytes, glucose, amino acids, fatty acids, vitamins, urea, uric acid, creatinine etc. pass through this barrier. Blood cells, plasma proteins, large anions, protein bound minerals and hormones and most molecules larger than 8nm in diameter don’t pass through the barrier. Damage to this barrier can allow proteins into urine causing proteinuria or allow blood cells into urine causing hematuria.

Question 26

Glomerular Filtration Rate (GFR)

Answer 26

This is around 150L/day in females and 180L/day in males. This is 30-35x the volume of blood in the body. 99% of filtrate is reabsorbed to form 1-2L of urine per day. If this rate is too high fluid flows through the renal tubules too rapidly to reabsorb enough water and solutes, urine output rises and dehydration and/or electrolyte depletion can occur. It this rate is too low then wastes are reabsorbed which can cause Azotemia (high levels of nitrogen containing compounds in the blood). In order to control this rate nephron can adjust their blood flow which enables a stable rate despite changes is systemic arterial blood flow. In sympathetic nervous responses and the release of adrenaline causes the constriction of afferent arterioles therefore reducing this rate and redirecting flood to the heart, brain and skeletal muscles.

Question 27

Reabsorption & Secretion

Answer 27

In the reabsorption process useful solutes from the filtrate are taken and returned back to the blood. In secretion additional wastes from the blood are taken and added to the filtrate.

Question 28

Nephron Tubule

Answer 28

The PCT (in cortex) has simple cuboidal epithelium with microvilli (contains brush border) and many mitochondria and is involved with absorption. The loop of Henle (in medulla) has a descending and ascending limb. It has a thin segment with simple squamous epithelium which is very permeable to water. It has thick segments with simple cuboidal epithelium with many mitochondria which is involved in the active transport of salts. The DCT (in cortex) contains simple cuboidal epithelium without microvilli (no brush border). The collecting and papillary ducts (medulla) contain simple cuboidal epithelium.

Question 29

Proximal Convoluted Tubule (PCT)

Answer 29

The filtrate passes first into this structure. This is located in the cortex and is very long. IT is surrounded by peritubular capillaries, lines with simple cuboidal epithelium with microvilli for absorption and has abundant mitochondria for ATP for active transport. It is involved with 65% of the reabsorption of the glomerular filtrate returning water and solutes back to blood. It is also involved in tubular secretion removing some substances from blood and secreting then back into the tubule.

Question 30

Tubular Reabsorption

Answer 30

There are 2 routes by which this can occur which involve a transcellular or paracellular route. In transcellular routes there is an active and passive transport of electrolytes and water will passively follow the active transport of solutes (osmosis). In the paracellular route there are leaky junctions between epithelial cells which allows water to pass between cells where the water will carry dissolved solutes along as well (solvent drag).

Question 31

Renal Threshold

Answer 31

The upper limit of the amount of a substance that can be reabsorbed which is also known as ‘transport maximum’.

Question 32

Tubular Secretion

Answer 32

This removes additional wastes from the blood and adds them to the filtrate. It enables the kidneys to increase the urine concentration of materials to be excreted. This is a process of waste removal e.g. urea, uric acid, bile acids, ammonia, drugs, morphine, penicillin etc. This process is also involved in acid-base balance with regulating H+ and HCO3- ions to regulate blood pH.

Question 33

Water Conservation

Answer 33

The nephron removes water from the filtrate and returns it to the blood and concentrates wastes. This occurs in the loop of Henle and collecting duct with water being returned via the vasa recta. This process is done due to the concentration gradient from the cortex the the medulla. In the descending limb H2O is reclaimed due to the low concentration of the filtrate (water leaves) whereas in the ascending limb salts are reclaimed as the filtrate becomes more concentrated (salts e.g. NaCl leaves).

Question 34

Countercurrent Multiplier of Nephron Loop

Answer 34

1. More salt is continually added by the PCT.
2. The higher the osmolarity of the ECF, the more water leaves the descending limb by osmosis.
3. The more water that leaves the descending limb the salter the fluid is that remains in the tubule.
4. The salter the fluid in the ascending limb the more salt the tubule pumps into the ECF.
5. The more slat is pumped out of the ascending limb the saltier the ECF is in the renal medulla.

Question 35

Vasa Recta

Answer 35

These capillaries enter the medulla adjacent to the ascending limb of the loop of Henle and leaves the medulla adjacent to the descending limb. Blood flows in the opposite direction to the flow of the filtrate. As blood flows downward into the medulla the water diffuses out of the capillaries and salt diffuses into them. As blood flows back up to the cortex salt diffuses out of the capillaries and water diffuses into them. This maintains the concentration gradient between the cortex and medulla.

Question 36

Collecting Duct

Answer 36

This structure is partially in the medulla with this section being permeable to water so it reabsorbs water (back into vasa recta) and concentrates the urine 4x. Antidiuretic hormone (ADH) from he posterior pituitary increases bloods osmotic pressure or decreases blood volume. increased ADH increases water permeability of the collecting duct and DCT which increases the concentration of urine. Decreased ADH caused by overhydration increases the volume of dilute urine meaning it is less concentrated.

Question 37

Urea

Answer 37

IT glomerular filtration constantly adds this to the filtrate. The thick segments of the ascending limb and DCT are impermeable to this meaning more of this in the filtrate of the DCT. The part of the collecting duct in the medulla is permeable to this so it leaks out increasing the concentration gradient in the interstitial fluid of the medulla. Some of this enters the descending limb of the loop and the lower portion of the ascending limb to travel through the loop again to the collecting duct. This constant recycling of this substance maintains the high osmolarity of it in the deep medulla.

Question 38

Renal Medulla

Answer 38

The varying ability of different mammals to form a concentrated urine correlates closely with the lengths of the loops of Henle which can increase the concentration gradient of the medulla. This increase leads to higher water reabsorption and therefore more concentrated urine. For aquatic mammals loops of Henle are short with beavers concentrating urine only to 2x blood plasma. For humans there is a relatively longer loop of Henle which allows concentration of urine 4x blood plasma. In desert mammals there are very long loops of Henle with camels having urine 8x blood plasma in concentration and Australian hopping mice having 22x blood plasma.