HOMEOSTASIS AND THE KIDNEY

Question 1

WHAT IS HOMEOSTASIS?

Answer 1

HOMEOSTASIS IS THE MAINTENANCE OF CONSTANT CONDITIONS WITHIN THE BODY IRRESPECTIVE OF EXTERNAL CONDITIONS

Question 2

WHAT ARE THE COMPONENTS OF THE HOMEOSTATIC SYSTEM?

Answer 2

RECEPTOR
CONTROL CENTER
EFFECTOR/CORRECTIVE MECHANISM (including negative and positive feedback system)

Question 3

WHAT IS THE CONTROL SYSTEM?

Answer 3

BRAIN

RECEIVES info from the RECEPTORS (that the monitoring factor has deviated from the set point) and INITIATES the CORRECTIVE MECHANISM in the EFFECTORS

COMMUNICATION between receptors and effectors via the control centre is by hormonal or nervous control

Question 4

WHAT IS THE CORRECTIVE MECHANISM?

Answer 4

The CONTROL SYSTEM receives info from the RECEPTORS (that the monitoring factor has deviated from the set point) and INITIATES the CORRECTIVE MECHANISM in the EFFECTORS

The corrective mechanism implements changes in the body required to cause monitoring factor to return to set point

Question 5

WHAT IS THE NEGATIVE FEEDBACK SYSTEM?

Answer 5

Within the corrective mechanism
Kicks in when the corrective mechanism causes the monitoring factor to return to the set point.
Prevents over-correction of monitoring factor (causing the homeostatic process to occur infinitely)

Question 6

WHAT IS THE POSITIVE FEEDBACK SYSTEM?

Answer 6

A positive feedback loop occurs when the product of a reaction leads to an increase in that reaction (cascade system)

E.g. Blood clotting cascade system (chemicals from wound activates platelets which then release chemicals to activate MORE platelets until wound is scabbed) or pregnant mother going into labour

Question 7

WHAT IS THE PROCESS OF EXCRETION by the kidneys?

Answer 7

Removal of toxic waste products from metabolism from the body via urine

Urine consists of urea, ammonia ( both produced by the breakdown of amino and nucleic acids in the liver) and creatinine (produced by the breakdown of Creatine Phosphate which is used in ATP synthesis in the muscles)

Question 8

WHAT IS THE PROCESS OF ULTRAFILTRATION?

Answer 8

High hydrostatic pressure (pressure that a liquid exerts onto its container which is affected by the force of gravity) which causes the filtration of blood plasma and small molecules below a certain size from the glomerulus into the Bowman’s capsule via the basement membrane.

Substances that ultra-filtrate into the glomerulus form the glomerular filtrate

Question 9

WHAT CAUSES THE HIGH HYDROSTATIC PRESSURE IN THE GLOMERULUS?

Answer 9

Kidneys close to the heart
Afferent arteriole has a wider diameter than the efferent arteriole
Coiling of the capillaries that make up the glomerulus

Question 10

WHAT IS THE COMPOSITION OF THE GLOMERULAR FILTRATE?

Answer 10

GAP-UCAS

Glucose
Amino acids
Proteins (small)
Urea
Creatinine
Ammonia
Salts

Question 11

WHAT IS THE FUNCTIONAL UNIT OF THE KIDNEY?

Answer 11

Nephron

Question 12

WHAT DOES A FUNCTIONING NEPHRON PREVENT?

Answer 12

Basement membrane being damaged under high pressures (can allow larger proteins such as erythrocytes into the Bowman’s Capsule)

Prevents the basement membrane becoming a target for autoimmune disease

Question 13

WHAT ARE THE OPPOSING FORCES TO ULTRAFILTRATION?

Answer 13

Hydrostatic back pressure of the glomerular filtrate

Oncotic pressure caused by the proteins (too big for basement membrane e.g. plasma proteins) in the blood- tries to create an osmotic gradient

The hydrostatic pressure in the glomerulus is so large that the opposing forces are insignificant.

Question 14

What is the process of reabsorption?

Answer 14

Blood products (molecules and ions) required by the body are selectively reabsorbed (facilitated diffusion and active transport) back into the blood via the vasa recta

Question 15

How are glucose, salts and amino acids selectively reabsorbed back into the blood?

Answer 15

Glucose, Salts and Amino acids are selectively reabsorbed by facilitated diffusion.

Active transport is used to ensure ALL of the glucose and the amino acids in the glomerular filtrate is selectively reabsorbed.

Question 16

Why is it called selective reabsorption?

Answer 16

Not all substances present in the glomerular filtrate are reabsorbed back into the blood e.g. urea, creatinine and ammonia.

Question 17

How is water reabsorbed back into the blood?

Answer 17

Due to the selective reabsorption (facilitated diffusion and active transport) of salts, glucose and amino acids back into the blood, the blood now has a more negative water potential compared to the glomerular filtrate.

This allows water in the glomerular filtrate to pass via osmosis back into the blood. (70% of water)

Question 18

How are smaller proteins selectively reabsorbed back into the blood?

Answer 18

Pinocytosis

Question 19

Why does the concentration of urea increase along the length of the nephron?

Answer 19

Urea concentration increases along the length of the PCT due to the reabsorption of water

Up to 50% of urea is reabsorbed by diffusion back into the blood

Question 20

What are the adaptations of the Proximal Convoluted Tubule (PCT) for reabsorption?

Answer 20

Lining of the PCT have cuboidal epithelial cells with microvilli and basal invaginations to create a brush border (increases the SA for reabsorption)

Cuboidal epithelial cells have large concentration of mitochondria (maximise ATP synthesis for Active transport)

Filtrate at the end of the PCT is isotonic to the blood

Cell surface membrane of the cuboidal epithelial cells contain protein carriers (facilitates selective reabsorption)

Question 21

What are the differences between the ascending and descending limb of the Loop of Henle?

Answer 21

The ascending limb is thicker and is impermeable to water

Question 22

Describe the Counter Current Multiplier effect.

Answer 22

> Na+ and Cl- ions move out from the ascending limb (via active transport and facilitated diffusion) and build-up in the interstitial fluid of the medulla creating a negative water potential.
As ions move out of the ascending limb, the glomerular filtrate becomes increasingly dilute becoming hypertonic at the top.
The negative water potential in the interstitial fluid creates an osmotic gradient causing water to move out of the descending limb into the vasa recta.
The osmotic gradient and the permeability of the descending limb causes water to move out of the ascending limb into the vasa recta.
As water moves out of the descending limb, the glomerular filtrate becomes increasing concentrated becoming hypertonic at the bottom.
The concentration of the interstitial fluid does not change- the more ions in the interstitial fluid, the more osmosis occurs therefore cancelling each other out

Question 23

Where are the main sites of water reabsorption?

Answer 23

Proximal Convoluted tubule, Loop of Henle and the Collecting Duct

Question 24

Why is it called the counter-current multiplier effect?

Answer 24

Counter-current= the ascending and descending limb moves in opposite directions
Multiplication= the effect of the osmotic differences between the limbs is significant over the length+depth of the medulla

Question 25

How does having a longer Loop of Henle allow more water to be reabsorbed and retained by the individual?

Answer 25

The longer the Loop of Henle, more ions can pass into the interstitial fluid (via active transport and facilitated diffusion) causing it to have a more negative water potential, hence a steeper osmotic gradient. This allows more water to be reabsorbed into the vasa recta via osmosis instead of being lost in the urine

Question 26

What is the purpose of the Distal Convoluted Tubule?

Answer 26

> Some water is reabsorbed depending on permeability
Controls the ionic composition and pH of the blood
Toxic metabolic waste e.g. creatinine are secreted into the filtrate

Question 27

Describe the process of Osmoregulation.

Answer 27

Homeostatic process that controls the water balance in the body by controlling the volume and concentration of urine under the influence of the Anti-Diuretic Hormone (ADH)

Question 28

How does ADH influence osmoregulation?

Answer 28

ADH varies the permeability of the collecting duct to water which effects the reabsorption of water into the vasa recta

Question 29

Where is ADH produced?

Answer 29

ADH is secreted by the hypothalamus

Question 30

Where is ADH stored?

Answer 30

ADH is stored in the posterior lobe pituitary gland

Question 31

Describe the process of osmoregulation if an individual’s blood is too concentrated.

Answer 31

Osmoreceptors in the Hypothalamus detects the water potential of the blood is more negative
Posterior lobe of the Pituitary gland secretes more ADH into the blood
Walls of the DCT and the collecting duct becomes more permeable to water (aquaporins open)
This causes more water to be reabsorbed back into the blood (via the vasa recta)
Blood becomes more dilute and the water potential returns to the set point
This produces a smaller concentrated volume of urine
As the water potential of the blood reaches the set point, the osmoreceptors reduce the secretion of ADH to prevent over-correction.

Question 32

Describe the process of osmoregulation if an individual’s blood is too dilute.

Answer 32

Osmoreceptors in the Hypothalamus detects the water potential of the blood is less negative
Posterior lobe of the Pituitary gland secretes less ADH into the blood
Walls of the DCT and the collecting duct becomes less permeable to water (aquaporins open)
This causes less water to be reabsorbed back into the blood (via the vasa recta)
Blood becomes more concentrated and the water potential returns to the set point
This produces a larger more dilute volume of urine
As the water potential of the blood reaches the set point, the osmoreceptors increase the secretion of ADH to prevent over-correction.