Biology 3.7 - Homeostasis and the Kidney

homeostasis

the maintenance of a constant internal environment by negative feedback

what conditions may change within the body

Core body temperature, pH and water potential

negative feedback

A receptor detects a deviation from the set point in the internal environment.

The receptor sends instructions to a co-ordinator

The co-ordinator communicates with effectors

The factor returns to the set poin, this is monitored by the receptor and information is fed back to the effectors, which stop making the correction.

2 main functions of the kidney

Excretion – the removal of nitrogenous waste from the body

Osmoregulation – the control of water potential of the body’s fluids

urea

Excess amino acids are deaminated in the liver

The amino group is removed and converted into ammonia and then to urea

Urea is removed by the kidneys

ureter

transports urine to the bladder

bladder

stores urine

urethra

carries urine out of the body

renal vein

blood returns to the general circulation

renal artery

blood enters the kidney

ultrafiltration

filtration under high pressure.

Small molecules and ions are forced into the tubule as filtrate.

Large molecules and blood cells cannot pass into the tubule as they are too large to be filtered

which 2 structures are responsible for ultrafiltration

bowmans capsule

capillary knot of the glomerulus

how is high hydrostatic pressure generated in the capillary knot

the blood capillaries narrow

process of ultrafiltration

Blood enters the glomerulus via the afferent arteriole and leaves via the efferent arteriole

The afferent arteriole has a wider diameter than the efferent arteriole

This narrowing generates a high hydrostatic pressure

Small molecules pass through three filtration layers and enter the Bowman’s capsule and tubule as filtrate

Glomerular filtrate contains:

Water

Glucose

Salts

Urea

Amino acids

The blood entering the glomerulus is separated from the Bowman’s space by what three layers:

Capillary walls

Basement membrane

Podocytes

Capillary walls

The wall of the capillaries in the glomerulus is one cell layer thick

Tiny pores between cells, called fenestrations allow solutes to pass to the basement membrane

Basement membrane

a selective molecular filter which only allows small molecules to pass through

Podocytes

epithelial cell layer of the Bowman’s capsule

Podocytes have extensions, called pedicels, which wrap around a capillary, pulling it closer to the basement membrane.

The gaps between the pedicels are called…

filtration slits

Selective reabsorption

the process by which useful substances such as glucose, amino acids and salts are reabsorbed back into the blood plasma

where and how does selective reabsorption take place

the proximal convoluted tubule by facilitated diffusion and active transport

cells lining the wall of the proximal convoluted tubule

highly specialised cuboidal epithelial cells

why does the cuboidal epithelium cell have microvilli protruding into the lumen of the PCT

to increase surface area for selective reabsorption

why are there many mitochondria in the epithelial cells of the PCT

to produce ATP for active transport

Tight junctions

hold neighbouring cells together closely to prevent molecules diffusing between adjacent cells

basal channels

increase surface area of the cell membrane at the basement membrane

how are salts reabsorbed

Mainly active transport, but some by facilitated diffusion

how are Glucose & amino acids reabsorbed

Cotransport with sodium ions

how is water reabsorbed

osmosis

how are Urea and small proteins reabsorbed

Facilitated diffusion

cotransport

transport of molecules or ions together through

the same transport protein

Secondary active transport

the coupling of diffusion, down an electrochemical gradient, providing energy for active transport

the glucose threshold

If the concentration of glucose in the filtrate is too high intrinsic transport proteins may become limiting, which means that not all the glucose will be reabsorbed. Glucose will remain in the filtrate and pass out of the body in urine.

what happens in the descending loop of Henle

The descending limb is permeable to water and water leaves the filtrate and enters the blood by osmosis, down a water potential gradient.

Na+ & Cl- ions diffuse into the descending limb from the medulla

how is low water potential in the medulla maintained

the ascending limb of the loop of Henle expelling Na+ & Cl- by facilitated diffusion and then active transport

where is the filtrate most concentrated in the loop of Henle

at the apex as water leaves the descending limb by osmosis

what happens in the ascending loop

The ascending limb is impermeable to water, but is permeable to Na+ & Cl-

Initially Na+ & Cl- leaves the ascending limb by facilitated diffusion, but later as the concentration of solutes decreases, as Na + & Cl- leave the filtrate, active transport takes over the expulsion of Na+ & Cl- into the tissue fluid of the medulla.

why is the loop of Henle called a counter current multiplier

the filtrate flows in opposite directions and the concentration of solutes in the filtrate increases towards the apex

Animals with long loops of Henle are adapted to…

dry environments, such as the desert e.g. camel

Animals with short loops live in…

fresh water environments e.g. otters

The longer the loop the…

more ions can be pumped into the medulla

This lowers the water potential of the medulla further allowing more water to be reabsorbed into the bloodstream by osmosis

osmoregulation

the control of body fluid water potential by negative feedback

process of osmoregulation

Osmoreceptors in the hypothalamus detect a decrease in blood plasma water potential

A signal is sent to the posterior lobe of the pituitary gland which releases the hormone ADH into the bloodstream

ADH is carried to the kidneys and binds to receptor proteins on the wall of the collecting duct and distal convoluted tubule

Aquaporins are added to the cell membranes of the effectors allowing more water to be reabsorbed by osmosis

This increases the water potential of the blood back towards the set point.

This information is fed back to the hypothalamus and less, or no ADH is produced

role of ADH

increases the permeability of the collecting duct and distal convoluted tubule allowing more water to be reabsorbed

urine produced will be more concentrated and a lower volume.

aquaporins

intrinsic protein channels which transport water across the phospholipid bilayer during osmosis

ways to treat kidney failure

Medication can be taken to control blood potassium and calcium levels

A low protein diet will reduce the need for deamination in the liver and less urea will be produced

Drugs can be used to reduce blood pressure

Dialysis

Kidney transplant

what do fish excrete

ammonia

what do birds and insects excrete

uric acid

very little water is needed for its excretion – this reduces water loss