1 - Homeostasis

CCEA A2 1 Biology Homeostasis and the Kidney

What are the two forms of coordination in multicellular animals?

1. Nervous - rapid response between specific parts

2. Endocrine - Slower and less specific

What is Homeostasis?

The maintenance of constant or steady state conditions within the body.

What are the stages of a self-regulating system?

• Set point - the desired level at which the system operates moderated by receptors

• Receptors - this detects any deviation from the set point and informs the controller

• Controller - coordinates information from various sources and sends instructions to an appropriate effector

• Effector - that brings about the necessary change needed to return the system to the set point. This returns normally creates a feedback loop.

• Feedback loop - informs the receptor of the changes to the system brought about by the effector

whats a corrective mechanism?

Brings about the changes required to return the factor to its normal levels/set point if the receptors show a departure from normal levels/set point for the factor being controlled.

What occurs when there’s a deviation from the set point that causes changes that result in an even greater deviation from the normal?

Positive Feedback

example of a corrective mechanism

If mammals overheat the corrective mechanism can include sweating and the vasodilation of capillaries in skin

What is involved in the corrective mechanism?

A negative feedback system which occurs as the return of the factor being controlled to its set point causing the corrective measures to be turned off preventing over-correction as reduces corrective action as set point approaches

Why is homeostasis important?

• provides optimum conditions for Enzyme reactions in terms of temp and pH as they are sensitive to this

• Water potential - changes to this in blood and tissue fluid may cause cells to shrink or expand, and then the cells will not operate normally. avoids osmotic problems in cells and body fluids

• Biochemical reactions are in dynamic equilibrium- changes can alter the equilibrium

• Maintain a constant internal environment independent of external - they will have a wider geographical range and therefore have a greater chance of finding food and shelter

What environmental features affect the functioning of the cell?

1. Temperature - Too low=slow reactions Too high=denature protein

2. Amount of Water - Lack=reaction slow/stop Too much=cells swell/burst

3. Amount of glucose - Too little=reduced respiration Too much=water drawn out

What is excretion?

The removal of toxic waste from metabolism e.g. Urea/Nitrogenous gas/ammonia, CO2 and bile to prevent them from becoming toxic

The Kidney

An organ which removes toxic waste products of metabolism and maintains the optimal water potential of body fluids.

What the two functions of the kidney?

1. Excretion - producing urine

2. Osmoregulation - controlling water balance in body

How does the Kidney work?

Ultrafiltration followed by selective reabsorption.

What do the Kidneys consist of?

Millions of nephrons

what is each kidney surrounded by?

a layer of adipose (fat) tissue and a layer of connective tissue.

How do the layers of tissue surrounding the kidney serve it?

Keep the kidneys in position and protect them from mechanical damage.

Fibrous Capsule

An outer membrane that protects the kidney

Cortex

A lighter coloured outer region made up of bowman’s capsules, convoluted tubules and blood vessels

Medulla

A darker region made up of loops of Henle, collecting ducts and blood vessels

Renal Pelvis

A funnel shaped cavity that collects urine into the ureter

Renal Artery

A funnel shaped cavity that collects urine into the ureter

Renal artery

Supplies the kidneys with blood from the heart via tha aorta

Renal vein

Returns blood to heart via vena cava

What’s the functional unit of the kidney?

Nephron

Hoe long is a nephron?

14mm

What is each nephron made up from/originates as?

Bowman’s Capsule which is cup-shaped.

what does the bowman’s capsule contain/made up of?

Supplied with blood from the afferent arteriole A mass of blood capillaries known as the glomerulus. Its inner layer is made up of specialised cells called podocytes.

Whats the proximal convoluted tubule?

a series of loops surrounded by blood capillaries. Its walls are made up of cuboidal epithelial cells with microville.

Whats the loop of Henle?

A long hairpin loop that extends from the cortex into the medulla of the kidney and back again. Blood capillaries surround it.

Whats the distal convoluted tubule?

A series of loops surrounded by blood capillaries. Its walls aew made from cuboidal epithelial cells but it is surrounded by fewer capillaries than the proximal tubule.

Whats the collecting duct?

A tube into which a number of distal convoluted tubules empty. It is lined by cuboidal epithelial cells and becomes increasingly wide as it empties into the renal pelvis. Bundles of collecting ducts form pyramids.

Whats the afferent arteriole?

A tiny vessel that ultimately arises from the renal artery and supplies the nephron with blood. The afferent arteriole enters the renal capsule of the nephron where it forms the glomerulus.

Whats the glomerulus?

A many branched tuft of capillaries from which fluid is forced out of the blood. The glomerular capillaries recombine to form the efferent arteriole.

Whats the efferent arteriole?

A tiny vessel that leaves the renal capsule. It has a smaller diameter than the afferent arteriole and so causes an increase in hydrostatic pressure within the glomerulus. The efferent arteriole carries blood away from the renal capsule and later branches from the peritubular capillaries.

Whats the peritubular capillaries?

a concentrated network of capillaries that surrounds the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule and from where they reabsorb mineral salts, glucose and water. The peritubular capillaries merge together into venules that in turn merge together to form the renal vein.

What is Ultrafiltration?

The filtration of plasma and substances below a certain size into the bowman’s capsule due to high hydrostatic pressure and the presence of small molecules.

Where does ultrafiltration take place?

The bowman’s capsule

How many layers are in the bowmans capsule/renal capsule?

three layers

What is the first layer of the bowman’s capsule?

Capillary endothelium - squamous endothelium of the capillary which in the glomerulus this single layer of cells has thousands of pores.

Whats the second layer of the bowman’s capsule?

The basement membrane between the two layers composed of glycoproteins and collagen fibres - extracellular matrix. Mesh like structure acts as the filter.

Whats the third layer of the renal capsule?

Podocytes which are lifted off the surface on little ‘feet’ and so allows the filtrate to pass beneath them and through the gaps in their branches.

How does the basement membrane act as an effective filter?

Stops large molecules from getting through. As a result red blood cells, white blood cells and plasma proteins can’t pass across. Other substances up to a relative molecular mass are squeezed out of the capillary into the filtrate, e.g. glucose, amino acids, salts, water, urea.

What are the factors that affect glomerulus filtration rate?

1. High hydrostatic Pressure

2. Water potential on each side of the membrane

How is High hydrostatic pressure in the glomerulus created?

• The short distance from the heart that the blood travels down the aorta and into the artery before branching into the kidney arterioles

• The afferent arteriole of each glomerulus is wider than the efferent arteriole

• The coiling of the capillaries in the glomerulus further restricts blood flow therefore increasing pressure.

What does the high hydrostatic pressure result in?

Forces the smaller components from the blood into the filtrate e.g. glucose, salts, water and urea

What. is the filtered fluid in the capsule called?

The glomerular filtrate

Explain the water potential within the glomerular capillaries for filtration to occur.

The water potential within the glomerulus capillaries (blood plasma) must exceed/be greater than the water potential within the Bowman’s capsule (glomerular filtrate) so the glomerular filtrate must have a more negative water potential.

What are the two components that make up water potential?

Solute potential and pressure potential

How do you work out water potential?

pressure potential - Solute potential

Why does the filtrate have a less negative solute potential than the blood in the glomerulus?

presence of Plasma proteins which are only found in the blood not the filtrate. These proteins cause solute potential hence filtrate less negative.

Define osmosis

The net movement of water through a selectively permeable membrane, from a solution of less negative water potential to a solution of more negative water potential.

What is the solute potential represented by?

The plasma proteins in the blood

Where is the solute potential higher?

In the blood of the glomerular capillary.

Is there plasma proteins in the filtrate?

No as the plasma proteins are too large to fit through.

Is the filtrate more negative or less negative solute potential?

it is less negative so opposes filtration

The difference in solute potential opposes filtration but…

it is insignificant to the differences of hydrostatic pressure across the basement membrane

What does the net filtration pressure result in?

Fluid to move from the glomerular capillaries into the Bowman's capsule.

What is selective reabsorption?

the process by which useful substances (water, glucose and amino acids are taken back into the blood as the filtrate flows along the nephron.

Where does Selective reabsorption take place?

Proximal Convoluted Tubule

Adaptations the proximal convoluted tubule has for reabsorption?

• Microvilli to increase surface area

• Many mitochondria to provide ATP for active transport

• Carrier proteins for facilitated diffusion and active transport

• Infolding to increase surface area

• Proximity of blood capillaries for short diffusion distance

What is selectively absorbed in the proximal convoluted tubule?

Glucose, amino acids, salts, vitamins and hormones

How does glucose, amino acids and ions move into the PCT?

Diffusion into the cuboidal epithelium cells of the PCT

How are the substances transported to the intracellular spaces?

Active uptake

How do substances move from intracellular spaces into the blood?

Diffusion/ facilitated diffusion

How does this affect water potential in the blood?

The blood becomes more negative creating an osmotic gradient

How does this affect the reabsorption of water into the blood?

Water moves into the blood via osmosis from a high water potential to a low water potential down water potential gradient. Massive reduction in volume of fluid in filtrate.

How are small proteins reabsorbed?

Endocytosis/ pinocytosis

Is urea reabsorbed?

Some urea will diffuse back into the blood along the length of the PCT. The Urea concentration increases in the filtrate due to the reabsorption of water.

What the role and function of the Loop of Henle?

Responsible for the reabsorption of water from the collecting duct and so creates hypertonic urine. Main function to conserve water.

How does the loop of Henle conserve water?

By creating a higher salt concentration in the medulla

Describe the loop of Henle in mammals that live in or close to water?

It is short as they don’t need to absorb much water back

Describe the loop of Henle in mammals that live dry conditions?

Longer as they can’t consume much water so need to retain/reabsorb as much as possible.

descending limb of loop of henle

is highly permeable to water. Any water that leaves the descending limb by osmosis enters the capillary network via the vasa recta and is removed from the medulla, so has little effect on the solute potential.

ascending limb of the loop of henle

thick and is impermeable to water. It has cuboidal epithelium and mitochondria providing ATP necessary to pump sodium and chloride ions into the medulla.

The loop of Henle is an example of:

a concurrent system

Salt is pumped out of the ascending limb into:

the medulla causing the filtrate to become more dilute

The ascending limb is impermeable to water so

the water in the filtrate cannot follow by osmosis.

Salt concentration in the medulla is high which…

Causes water to leave the descending limb by osmosis

The loop of Henle creates a region of higher salt concentration…

as it descend deeper into the medulla

The longer the loop of hence…

The higher the salt concentration in the medulla
The more water can be reabsorbed into the blood by osmosis
So more concentrated/hypertonic urine produced

What does the addition of sodium and chloride ions to the interstitial space between the two limbs of the loop of henle create?

A more negative solute potential in the medulla therefore a water potential gradient is created.

What are the walls of the ascending limb of?

Cuboidal cells which are rich in mitochondria as atp is needed for active transport of ions into the medulla.

Explain the term countercurrent multiplier effect?

The osmotic differences between the descending and ascending limbs at any one level are small but the cumulative effect over the length of the limbs (depth of medulla) is significant. This together with the filtrates in the limbs travelling in opposite directions, is why the process is described as countercurrent multi-lier effect.

The distal convoluted tubule function

is a fine control of the water and salt reabsorption to fit in with precise needs of the body at this time

Structure of DCT

Similar to PCT
Permeablility of membrane is determined by hormone ADH

collecting duct function

The medulla draws water out by osmosis from the collecting duct due to the high water potential within the collecting duct. This allows urine to be more concentrated as it passes down the duct to the bladder.

Define osmoregulation

The maintenance of optimal water potential of blood fluids. Does this through controlling water balance in blood, therefore content of tissue fluids and cells also controlled.

Receptor cells in osmoregulation?

Osmoreceptors in hypothalamus

Effectors in osmoregulation?

Pituitary gland and walls of collecting duct

Where is the Antidiuretic Hormone (ADH) produced?

The hypothalamus

Where is ADH stored?

posterior lobe of the pituitary gland

Function of the Antidiuretic Hormone (ADH)?

Retain water in blood and constrict blood vessels. Controls the degree of permeability of collecting duct walls by increasing the number of water permeable channels called aquaporins.

What happens when there is low ADH?

• Collecting duct less permeable to water

• increase in conc. of NaCl and urea in tissue fluid

• Large volumes of dilute urine produced (hypotonic)

What happens when there is high levels of ADH?

• Collecting duct walls more permeable to water

• water moves from the collecting duct and into conc. Tissue fluid → blood capillary

• Small vols of concentrated urine is produced (hypertonic)

What happens when the blood becomes too concentrated/removal of water?

1. Reduction in water, increased sweating and increased salt intake

2. blood solute potential more negative

3. Detected by osmoreceptors in hypothalamus

4. Pituitary gland releases more ADH

5. Walls of collecting duct (DCT) becomes more permeable to water due to more aquaporins

6. More water reabsorbed back into the blood and less leaves body as urine

7. Solute potential of blood returns back to normal (set point)

What happens when the blood becomes too dilute?

1. Intake of water and reduced salt intake

2. blood solute potential less negative

3. Detected by osmoreceptors in hypothalamus

4. Pituitary gland releases less ADH

5. Walls of collecting duct (DCT) becomes less permeable to water due to less aquaporins

6. less water reabsorbed back into the blood and more leaves body as urine

7. Solute potential of blood returns back to normal (set point)

Why do diabetics have glucose in their urine?

Little to no insulin to convert glucose to glycogen. This results in too much glucose in the PCT. Not enough channel proteins to reabsorb glucose into the blood.