Urinary system

Question 1

What is aldosterone, vasopressin and angiotensin?

Answer 1

Aldosterone- slow-acting steroid hormone that stimulates sodium reabsorption by kidney tubules.

Vasopressin (ant-diuretic hormone)- rapid-acting peptide produced by the pituitary gland which stimulates water reabsorption.

Angiotensin-converting enzymes- found on endothelial cells.

Question 2

What does an increase in arterial pressure lead the kidneys to do and how do kidneys regulate blood pressure?

Answer 2

An increase in arterial pressure may lead the kidneys to increase urinary loss of sodium and water to decrease plasma volume and eventually decrease blood volume.

The kidneys regulate blood pressure- intra-renal baroreceptors detect changes in stretching with changes in blood volume which stimulates a change in the production of renin.

Question 3

How do kidneys interact with the heart and what happens hours after blood loss?

Answer 3

Cardiac cells produce atrial natriuretic peptide which:
Inhibits sodium reabsorption by kidney tubular cells.
Acts on renal blood vessels to increase filtration rate causing sodium excretion.
Inhibits the action of aldosterone.

Within hours of blood loss:
Compensatory movement of interstitial fluid into the capillaries to increase plasma volume- a redistribution of fluid.
Increase in thirst.
Decrease in salt and water excretion.
Mediated by hormones and kidney function- renin, angiotensin, aldosterone.

Question 4

What is thermoregulation?

Answer 4

The biological process by which an organism maintains its internal body temperature within a narrow, optimal range.

Normal resting core temperature: 36.5-37.5 degrees Celsius, it only occasionally goes outside this limit marginally, beyond 39 degrees, the body may stop functioning properly.
2/3 of all energy liberated from foods is lost as heat.
Heat produced by the average body at rest is 1.25-1.5 kcal per minute.
Heat production during exercise can exceed 15-20 kcal per minute.

Question 5

How can heat be gained and lost during exercise?

Answer 5

Convection (warm air passing by), solar radiation, thermal radiation, and conduction can all increase metabolic heat production during exercise. It’s also affected by clothing, body size, and individual heat tolerance.

Radiation, convection, sweat evaporation, conduction, and respiratory evaporation can cause heat loss during exercise.

Heat loss at rest in ambient conditions:
60% via radiation
12% via convective air currents
3% via conduction (feet to floor)
25% via evaporation (lungs and skin)

During exercise, 80% of heat loss is via evaporation of sweat from skin.

Question 6

What are some physiological responses to body fluid loss?

Answer 6

Blood volume declines.
Osmolarity increases from dehydration.
Increase in change in plasma volume.
Reduction in blood volume (which decreases stroke volume), this increases heart rate to stabilise cardiac output.
Cardiac output decreases once heart rate cannot increase any longer to combat the decrease in stroke volume.

Question 7

What are the functions of the kidneys and what does renal mean?

Answer 7

The kidney can’t restore body water deficit but can help conserve body water and electrolytes during periods of increased loss.

The kidneys:
Remove waste products (urea from break down of protein) and foreign chemicals (drugs) from the blood.
Regulate total body water, salts and acid base balance- kidneys excrete just enough water and salts to maintain homeostasis.
Involved in gluconeogenesis (during fasting, the kidneys produce glucose from amino acids that is released into the blood).
Release hormones that regulate blood pressure (renin) and EPO (increases number of red blood cells).
Produce an active form of vitamin D that influences calcium balance and promotes strong, healthy bones.

Renal- pertaining to the kidneys. The kidneys process the plasma portion of blood by removing substances from it and, in some cases, by adding substances to it.

Question 8

How does urine move through the body and what is a nephron?

Answer 8

We have two kidneys. Water runs through the ureter and into the bladder, where it’s expelled at the urethra. A nephron spans the renal cortex and renal medulla, taking fluid into the renal pelvis where urine is lost via the ureter.

Nephron:
There are 2 types: Cortical and Juxtamedullary- both primarily in renal cortex.
Made up of a network of tubing and blood vessels.
2 components: Renal corpuscle (comprised of Bowman’s capsule- tubing system- and glomerulus) and Tubule (extends from renal corpuscle to collecting duct).

Question 9

How does liquid flow through the nephron and the system of tubes?

Answer 9

Glomerulus->Bowman’s space in Bowman’s capsule->Proximal convoluted tubule->Proximal straight tubule->Descending limb of loop of Henle->Ascending limb of loop of Henle (thin then thick)->Distal convoluted tubule->Cortical collecting duct->Medullary collecting duct->Renal pelvis

Question 10

What is the renal corpuscle?

Answer 10

Forms a filtrate from blood free from cells and proteins. Filtrate leaves corpuscle and enters the tubule. As it flows through the tubule, substances are added/removed. Remaining fluid combines in collecting ducts and exits the kidneys urine.

Anatomy of the renal corpuscle:
The visceral layer of Bowman’s Capsule (Podocytes) surrounds the glomerular capillaries to make up the glomerulus.
Podocyte- large number of extensions that help with filtration. Filtration slits enable the transfer of fluids. Fenestrae are small pores helpful for filtration.

Blood in the glomerular is separated from fluid in Bowman’s space by a filtration barrier consisting of 3 layers:
Single cell lining of the capillary endothelium.
Non-cellular protein-rich layer of basement membrane.
Single cell lining of the Bowman’s capsule.

Question 11

What is dialysis?

Answer 11

A method of filtering blood, separate substances, without the need for the body.
The arterial blood from the patient goes into the dialyzer, where waste products are removed from blood and fresh dialysis fluid is inputted. venous blood then returns to the patient.

Question 12

What are the 2 sets of capillaries in renal circulation?

Answer 12

Glomerular capillaries- specialised for filtering

Peritubular capillaries- supply the nephron/tubules with their own blood supply and they then form the veins where blood leaves the kidney

Question 13

What are juxtamedullary and cortical nephrons?

Answer 13

Juxtamedullary nephrons:
15% of all kidney nephrons.
Renal corpuscle close to cortical-medullary junction.
Henle’s loops plunge deep into the medullar.
Generate osmotic gradient for water reabsorption.

Cortical nephrons:
Represent most of the kidney nephrons.
Corpuscle located in outer cortex.
Henle’s loops do not plunge deep into the medullar.

Question 14

What are juxtaglomerular cells and the macula densa?

Answer 14

Juxtaglomerular cells- secrete renin, influence the formation of angiotensin 2, controls blood pressure (vasoconstriction and sodium/water retention).
Macula densa contain sensors that detect changes in blood composition, they’re part of the ascending loop of Henle.

Question 15

What is the renal control of blood pressure?

Answer 15

There’s a stimuli to renin. The kidneys secrete renin, while the liver produces angiotensinogen (large number of amino acids).
The action of renin on angiotensinogen converts it into angiotensin I (much less amino acids).
Angiotensin converting enzyme converts angiotensin I (inactive) to angiotensin II (active form), which has even less amino acids.
Angiotensin II impacts vasoconstriction of arterioles leading to an increase in pressure. It also impacts the adrenal cortex which releases aldosterone for sodium and water retention, which increases blood pressure.

Question 16

What 3 inputs to juxtaglomerular cells increase renin secretion?

Answer 16

Renal sympathetic nerves

Intrarenal baroreceptors

Macula densa

This would ultimately impact on blood pressure.

Question 17

What substances cannot cross the plasma membrane?

Answer 17

The body has intracellular and extracellular fluid and cells are surrounded by a membrane which is permeable to water, but impermeable to many substances (nonpenetrating solutes- sodium and chloride ions on the outside of the cell membrane, potassium and organic solutes on the inside of the cell membrane).

The osmolarity of extracellular fluid is 285-300 mOsm. The concentration and location of nonpenetrating solutes determines where water goes.

Question 18

What are isotonic, hypotonic and hypertonic solutions?

Answer 18

Isotonic solutions have the same concentration of nonpenetrating solutes as normal extracellular fluid. If cells with an intracellular osmolarity of 300 are placed into a solution of nonpenetrating solutes with an osmolarity of 300, they won’t swell or shrink.

Hypotonic solutions have a lower concentration of nonpenetrating solutes compared to normal extracellular fluid; water rushes into cells to dilute intracellular solutes.

Hypertonic solutions have a higher concentration of nonpenetrating solutes compared to normal extracellular fluid; water rushes out of the cells to dilute extracellular solutes.

Question 19

What is osmotic and volume regulation?

Answer 19

Osmotic regulation- the body recognises there’s too much water and lets it go via urine. Vasopressin secretion decreases and plasma vasopressin decreases to allow an increase in water excretion.

Volume regulation- the body recognises there’s not enough water, so it’s not let go and kept in the body, to pull more water. Vasopressin secretion and plasma vasopressin increases to decrease the excretion of water.

Question 20

What is glomerular filtration, tubular secretion and tubular reabsorption?

Answer 20

Urine formation begins with the filtration of plasma from glomerular capillaries into the Bowman’s space- glomerular filtration, producing glomerular filtrate. The filtrate is cell free and contains all the substance in virtually the same concentration as in plasma- ultra-filtrate.

As plasma moves round the capillary, there’s an opportunity for tubular secretion- the movement of substances form the capillary into the tubule.

Substances can move from the tubule back into the capillary- tubular reabsorption.

Question 21

What forces are involved in glomerular filtration?

Answer 21

Forces impact the movement/flow of substances.

Favouring filtration- P-GC=Glomerular capillary blood pressure. Blood pressure itself forces the plasma out of the capillary and into the tubule.

Opposing filtration- P-BS=Fluid pressure in Bowman’s space. ‘pi’-Gc=Osmotic force due to protein in plasma.

Question 22

What is glomerular filtration rate?

Answer 22

The filtering process is under physiological control (achieved via neural and hormonal input to the afferent and efferent arterioles). Glomerular filtration rate (GFR). If the efferent arteriole constricts/the afferent arteriole dilates, more blood will filter from the capillary into the tubule (increased GFR). If the afferent arteriole constricts/the efferent arteriole dilates, less blood will filter from the capillary into the tubule (decreased GFR).

Question 23

What is sodium and water reabsorption?

Answer 23

The filtered loads are very high (e.g. water), the reabsorption of waste products is incomplete (e.g. urea), the reabsorption of useful components (water and salts) is complete in tubular reabsorption.

We can control how permeable some of the tubules are to sodium and water.

Mechanisms of sodium and water reabsorption:
Sodium reabsorption is an active process occurring in all tubular lumen (except the descending limp of loop of Henle).
Water reabsorption is by osmosis and is dependent on sodium reabsorption.

Question 24

What is the process of coupling of solute and water reabsorption?

Answer 24

It starts with active transport of Na+, it moves from the tubular lumen into the interstitial fluid, facilitated by the Na+/K+ ATPase pump.
As salts are removed from the tubular lumen, local osmolarity decreases. In the interstitial fluid, this increases.
Water concentration is high because Na+ was low, so water moves across the tubular epithelium into the interstitial fluid.
Regardless of the concentration gradient for osmosis, the epithelium must be permeable to water, for this, we need water channel proteins (aquaporins), allowing water to come through, controlled physiologically by vasopressin.

Question 25

What is primary active Na+ reabsorption?

Answer 25

The primary active transport of Na+ out of the cells and into the interstitial fluid is achieved by Na+/K+ -ATPase pumps in the basolateral membrane of the cells. The active transport keeps the intracellular concentration of Na+ low compared to the tubular lumen, so Na+ moves out of the tubular lumen into the tubular epithelial cells. Apical entry of Na+ in the proximal tubule cell occurs by cotransport with various organic molecules or by countertransport with H+. The absorption of NaCl but not water is achieved by the Na-K-2Cl cotransporter (NKCC) in the ascending limb of the loop of Henle. Apical entry of Na+ in the cortical collecting duct occurs primarily by diffusion through Na+ channels.

Question 26

What is the process of water reabsorption to sodium reabsorption?

Answer 26

As Na+ and Cl- are reabsorbed, water can follow passively by osmosis if the apical membrane is permeable water. Na+ is transported from the tubular lumen to the interstitial fluid across the epithelial cells, other solutes (whose reabsorption depends on Na+ transport) contribute to osmosis. The removal of solutes from the tubular lumen decreases the local osmolarity of the tubular fluid adjacent to the cell; the appearance of solute in the interstitial fluid just outside the cell increases local osmolarity. The difference in water concentration between lumen and interstitial fluid causes net diffusion of water from the lumen across the tubular cells’ plasma membranes and tight junctions into the interstitial fluid. Water, Na+ and everything else dissolved in the interstitial fluid move together by bulk flow into peritubular capillaries, completing reabsorption.

Question 27

What is the countercurrent multiplier system?

Answer 27

The opposing flows in the 2 limbs of the Henle loop are called countercurrent flows and the entire loop functions as a countercurrent multiplier system to create a hyperosmotic medullary interstitial fluid. Along the ascending limb, Na+ and Cl- are reabsorbed into the medullary interstitial fluid. The ascending limb is relatively impermeable to water, so little water follows the salt- the interstitial fluid of the medulla becomes hyperosmotic compared to the fluid in the ascending limb. The descending limb doesn’t reabsorb sodium chloride and is highly permeable to water- a net diffusion of water occurs into the more concentrated interstitial fluid until the osmolarities are equal. The loop countercurrent multiplier causes the interstitial fluid of the medulla to become concentrated, this hyperosmolarity draws water out of the collecting ducts and concentrate the urine. The osmolarity difference that exists at each horizontal level is multiplied as the fluid goes deeper into the medulla. The system concentrates the descending loop fluid but then decreases the osmolarity in the ascending loop.

Question 28

What is medullary circulation?

Answer 28

Blood vessels in the medulla (vasa recta) run parallel to the loops of Henle and medullary collecting ducts. As blood flows deeper into the medulla, Na+ and Cl- diffuse into, and water out of- the vessel. After the hairpin, the process becomes reversed, minimising excessive loss of solute from the interstitium by diffusion. At the same time, salt and water being reabsorbed from the loops of Henle and collecting ducts are carried by bulk flow. Due to NaCl and water reabsorption, the amount of blood leaving the medulla is twice the amount that entered. The flow through the medulla is a small portion of total renal blood flow, minimising the washout of the hypertonic interstitium of the medulla.