Unit 3.7- Homeostasis and the kidney

Define homeostasis

The mechanism by which a constant internal environment is achieved within a living organism

Why is the regulation of the internal environment important?

Regulate temperature and pH- denaturing of enzymes

Water potential of blood- too dilute leads to cells bursting, too concentrated leads to cells shrinking

What does homeostasis prevent?

Large fluctuations around a set point- the body is kept in dynamic equilibrium

What do homeostatic control systems involve?

A negative feedback system

Define negative feedback

When a displacement from a set point results in corrective mechanisms that reduce the displacement

Explain the negative feedback system that controls the glucose conc in the plasma

Too high:

Pancreas detects

Releases insulin

Increases uptake of glucose into cells

Glucose converted into glycogen store

too low:

Pancreas detects

Releases glucagon

Increases conversion of glycogen back into glucose

Glucose released back into blood

Neg feedback system that controls core body temp?

Temp too high:

-thermoregulatory system in brain detects blood temp

-arteries to skin capillaries dilate so more heat is lost from skin

-increased sweating- high latent heat of water so evaporating causes cooling as thermal energy used

Temp too low:

Thermoregulatory centre in brain detects blood temp

Vasoconstriction so less heat lost from skin

Increased shivering- involuntary contraction of muscles under skin-> ATP produced-> heat produced as product

What is positive feedback?

When an effector increases a change

What does oxytocin do?

stimulates uterine contractions at the end of pregnancy

What is excretion?

The removal of waste products of metabolism from the body

What is metabolism?

sum of all chemical reactions in the body

What are the three ways that water is removed from the body?

Urine, exhalation and sweat

What are some excretory organs and what is excreted from them?

Lungs- exhaled air- CO2 and H2O

Kidneys- urine- urea, creatinine, uric acid

Skin- sweat- urea

Liver- faeces- bile pigment

What metabolic process produces:

CO2 and H2O?

Urea, creatinine and uric acid?

Urea?

Bile pigment?

Respiration

Deamination, breakdown of muscle, breakdown of nucleic acid

Deamination

Breakdown of Hb

What are the two functions of the kidney?

1. Excretion

2. Osmoregulation

Define excretion

The removal of waste products of metabolism

What do the two kidneys do?

Filter the blood and remove waste as urine

What is deamination?

The removal of NH2 from excess amino acids

Where does deamination occur?

liver

What happens to the ammonia?

It is converted to urea in liver as ammonia is highly toxic

How is the urea transported?

In the blood

What is osmoregulation?

The control of the water content and solute composition of body fluids such as the blood, tissue fluid and lymph

Where are the kidneys found?

Dorsally in the abdomen

What are the parts of the urinary system?

Vena cava

Aorta

Renal artery

Renal vein

Kidney

Ureter

Bladder

Urethra

Label a kidney

Label the structure of the nephron

Where do each of the kidneys receive its blood supply from?

the renal artery

What happens in the renal vein?

The filtered waste products are excreted by the kidneys as urine

What does urine pass down?

The ureter

What is the bladder?

A muscular sac that stores urine

What is at the base of the bladder?

A sphincter muscle which relaxes during urination so that urine passes out of the body along the urethra

What are the nephrons?

Filtering units

What is the outer cortex?

Where filtration takes place in the nephrons. It has a dense capillary network which receives blood from the renal artery

Where are the renal pyramids?

The medulla

What do the renal funnels collect urine into?

The pelvis

What does the nephron start with?

The Bowman's Capsule in the cortex

What is below the capsule?

The proximal convoluted tubule

What does this lead into?

The loop of henle which runs into the medulla and then back out to the cortex

What does the nephron then form?

The distal convoluted tubule

What does this join to?

A collecting duct which carries the urine through the medulla to the pelvis

What is ultrafiltration?

Fine filtration under very high hydrostatic pressure of small soluble molecules from the blood plasma in the glomerulus to form glomerular filtrate in Bowman's capsule

Why is there high blood pressure in the glomerulus?

1. Diameter of the afferent arteriole entering the glomerulus is greater than the efferent arteriole leaving the glomerulus

2. Blood comes from the renal artery, which is a branch of the aorta, which has blood under high pressure as it has just left the heart

What small molecules are filtered?

Salts

Water

Amino acids

Glucose

Urea

What are too large to be filtered?

Proteins- albumin and globulins

Red blood cells and white blood cells

What is the glomerulus and Bowman's space separated by?

3 layers: the endothelium of the capillary, the basement membrane and the epithelium of Bowman's capsule

What does the endothelium of the capillary have?

fenestrations

What is the basement membrane?

The selective barrier (molecular sieve)

What is the epithelium of Bowman's capsule made up of?

Squamous epithelial cells called podocytes

What do they have?

Pedicels which wrap around a capillary pulling it closer to the basement membrane

What are the gaps between the pedicels called?

Filtration slits

What does the blood that flows from the glomerulus into the efferent arteriole have?

A low water potential as much water has been lost and a high protein concentration remains

Percentage blood filtered=

volume of filtrate produced a minute/ volume of blood entering kidneys per minute x 100

What is selective reabsorption?

The uptake of specific molecules and ions from the glomerular filtrate in the nephron back into the bloodstream

Where does selective reabsorption occur?

proximal convoluted tubule

What is fully reabsorbed?

Amino acids, glucose

What remains in the nephron to be excreted in the urine?

Salts, water and urea

How is the PCT adapted for selective reabsorption?

-Large surface area due to its length

-Large surface area due to microvilli

-Cuboidal epithelial cells have many mitochondria providing ATP for active transport

-Has a close association with the peritubular capillaries of the vasa recta

-Epithelial cells have tight junctions between them. They prevent seepage of reabsorbed materials back into the filtrate

-Epithelial cells have invaginations called basal channels that increase the surface area of the cell membrane at the basement membrane side

How are

salts

water

amino acids

glucose

urea reabsorbed?

Active transport

Osmosis

Co-transport

Co-transport

Diffusion

Describe the process of co-transport

Movement of sodium ions out of the cell by active transport maintains a low conc inside the cell

This maintains a concentration gradient of sodium ions between the lumen and inside the cell

This ensures that sodium ions keeps diffusing into the cell by facilitated diffusion and therefore ensures the continued movement of glucose into the cell by cotransport.

A high concentration of glucose can therefore be maintained within the cell, meaning that it can continue to move out of the cell down a conc grad by fac diff

Define secondary active transport

Uses previously established gradient of Na or hydrogen ions to move other chemicals

What is the glucose threshold and why might it be crossed?

The level of glucose in the blood at which glucose starts to appear in a person's urine.

Transporter proteins become saturated with glucose molecules

A healthy person would never have glucose in their urine, but a person with diabetes might have elevated blood glucose levels that exceed the glucose threshold

At the end of the PCT, what happens?

The filtrate is isotonic with the blood plasma

What takes place following the PCT?

Further water reabsorption of water in the Loop of Henle, DCT and collecting duct

PCT rate of concentration graph

Explain this graph

Glucose and amino acids get selectively reabsorbed across the proximal convoluted tubule into the bloodstream so their concentrations decrease

Identify this

Glomerulus and Bowman's capsule so the cortex of the kidney

And this

Distal convoluted tubule and proximal convoluted tubule

How do they differ in this?

PCT- fuzzy edge due to brush border of microvilli, smaller diameter of lumen

DCT- bigger lumen (wider diameter), less brush border

What is this

Thick walled ascending limb of loop of henle

Thin walled descending limb of loop of henle

Collecting duct- large internal and external diameter

What is the role of the loop of Henle?

To set up a concentration gradient of salts in the tissue fluid of the medulla so that water can flow out of the collecting duct by osmosis should the body become dehydrated

How does the loop of Henle work?

Na+ and Cl- are actively transported out of the ascending limb against a conc grad into the tissue fluid in the medulla

The filtrate becomes less concentrated inside the ascending limb as you go up- higher water potential

Water will move down a water potential gradient by osmosis from the descending limb into the tissue fluid of the medulla, reducing the water potential of the filtrate as you go down the descending limb

Some Na+ and Cl- ions will diffuse into the descending limb, making the filtrate more concentrated

What type of system is this?

A counter current multiplier system

What is the general rule for the loop of henle?

The longer the loop of Henle, the more concentrated the base will be

What happens in the collecting duct?

Water moves down a water potential gradient out of the filtrate ie urine gets more concentrated

Why do diabetics urinate more?

The filtrate in the collecting duct will have a lower water potential than usual, as there will be more glucose in the filtrate, so less water will leave the collecting duct by osmosis down a water potential gradient and so more will be excreted in the urine

Salts and loop of henle??

How does the body gain water?

Drinking, eating

How does the body lose water?

Sweating, excretion, exhalation

What is a negative feedback loop used for?

To restore the normal osmotic concentration in the blood

What is the negative feedback loop under?

Hormonal control

What is the first thing that happens when you're dehydrated?

The osmoreceptor cells detect a fall in the water potential of the blood

Where are osmoreceptors located?

hypothalamus of the brain

What do osmoreceptors do?

Send nervous impulses to the co-ordinator

What is the co-ordinator?

The hypothalamus in the brain

What does the co-ordinator do?

Makes ADH (anti-diuretic hormone) and stores it in the posterior pituitary gland

What else happens to the ADH?

It is released into the bloodstream

What happens because of this?

The effector is effected

What happens?

The ADH binds to receptor proteins on the wall of the collecting duct and DCT of the kidneys

What does this do?

Makes the DCT and CD more permeable to water

What are aquaporins?

Intrinsic membrane proteins which have a pore through which water molecules move

What happens to the aquaporins?

They are added to the cell membranes

What does this allow?

More water to be reabsorbed from the filtrate into the hypertonic tissue fluid within the medulla by osmosis and then into the vasa recta

What will this result in?

An increase in the water potential of the blood back to the set point

What happens then?

This information is fed back to the hypothalamus and less ADH is produced

What will happen if insufficient water can be reabsorbed?

Hypothalamus will trigger thirst

What will happen if the water potential of the blood increases as a result of drinking too much water?

The opposite

What is frequent urination known as?

Diuresis

What does ADH cause?

A small volume of concentrated urine to be produced and for H2O to be reabsorbed into the blood

What does kidney failure result in?

Toxic levels of urea and an inability to remove excess water

What does this lead to?

Bodily fluids increasing in volume and are diluted, which compromises the metabolism

What may kidney failure be due to?

-Diabetes

-High blood pressure (damage to the capillaries of the glomerulus prevents ultrafiltration)

-Auto-immune disease

-Infection

-Crushing injuries