The kidney

Kidneys...

Kidneys excrete waste products and regulate the water potential of the blood.

VIA  ULTRAFILTRATION

Useful substances are selectively reabsorbed back into the blood via CAPILARIES surrounding various TUBULES in the NEPHRON. Water is also reabsorbed so as to maintain the correct WATER POTENTIAL in the blood.

In short the kidneys are all about maintaining an ideal WATER POTENTIAL for the blood.

1. Arterioles split into a GLOMERULUS (site of ultrafiltration). The GLOMERULUS is surrounded by a structure called the BOWMANS CAPSULE.

AFFERENT ARTERIOLE = before the GLOMERULUS - Bigger

EFFERENT ARTERIOLE = after the GLOMERULUS - Smaller

2. The higher pressure in the efferent arteriole forces solutes +       water out of the blood into the BOWMANS CAPSULE.

3. Large moleculs cannot pass through, so stay in the blood.

4. GLOMERULAR FILTRATE passes through into the rest of the NEPHRON.

Contains:

• Glucose
• Water
• Salts - Sodium ions
• Urea

Doesn't contain:

• Proteins
• Blood cells
• Anything that is too big to fit through the gaps in the glomerulus.

Usually a 1 mark question...

Occurs in the:

• PROXIMAL CONVOLUTED TUBULE
• LOOP OF HENLE
• DISTAL CONVOLUTED TUBULE

1. The useful substances are absorbed into the capillary network surrounding the nephron.

2. PCT has micro-villi to provide a larger SA for selective reabsorption.

3. Selective reabsorption occurs via ACTIVE TRANSPORT + FACILITATED DIFFUSION.

4. The reabsorption of water occurs via OSMOSIS, when the water potential of the blood is LOWER than the filtrate. 

The reabsorption of water occurs in:

• PCT + DCT
• LOOP OF HENLE
• COLLECTING DUCT

Low WATER POTENTIAL = More water is reabsorbed via osmosis form the NEPHRON. Leads to a more concentrated urine.

High WATER POTENTIAL = Less water is reabsorbed by OSMOSIS into the blood from the nephron. Leads to a more dilute urine.

Water can be reabsorbed throughout the nephron but the regulation of WATER POTENTIAL happens mostly in the LOH, DCT + CD.

The permeability of the DISTAL CONVOLUTED TUBULE AND THE COLLECTING DUCT are controlled by HORMONES (e.g. ADH) 

DEHYDRATED = low water potential (results in a more concentrated urine)

• OSMORECEPTORS in the HYPOTHALAMUS begin the lose water by OSMOSIS.
• This stimulates the PITUITARY GLAND to release ADH into blood.
• Increased ADH levels means that the COLLECTING DUCT + THE DISTAL CONVOLUTED TUBULE are made more permeable to water.
• Results in more water being reabsorbed from the nephron via OSMOSIS.

OVER-HYDRATED = higher water potential of the blood (results in a more dilute urine)

• There is now a need to reduce the ammount of water in the body.
• OSMORECEPTORS in the HYPOTHALAMUS detect the water levels.
• The pituitary gland is instructed to release less ADH.
• The DISTAL CONVOLUTED TUBULE + COLLECTING DUCT become less permeable.

* Consists of a DESCENDING LIMB and an ASCENDING LIMB 

The limbs control the movement of SODIUM IONS to create the necessary concentration gradient for the reabsorbtion of water into the MEDULLA.

1. DESCENDING LIMB ---> is selectively permeable ---> Is water permable, but impermeable to    sodium ions. 

2. ASCENDING LIMB ---> Is impermeable to water, but permeable to sodium ions.

The aim of the processes in the LOOP OF HENLE is to lower the water potential of the medulla so that water moves out of the collecting duct and can be reabsorbed back into the blood.

Process 

1. Water moves out of the DESCENDING LIMB into the MEDULLA via osmosis, as the   MEDULLA  has a lower water potential than the descending limb.

2. Filtrate in the LOH becomes more concentrated, as water is moving out but sodium ions remain in as descending limb isn't Na permeable.

3. At the start of the ASCENDING LIMB Na ions diffuse out into the medulla. This lowers the water potential of the medulla.

4. Near the end of the ascending limb Na ions are pumped out against the gradient via ACTIVE TRANSPORT. An even lower water potential is created in the medulla as no water moves out with it.

REMEMBER! THE PURPOSE OF THE LOOP OF HENLE IS TO CREATE A LOW WATER POTENTIAL IN THE MEDULLA SO WATER WILL MOVE OUT OF THE COLLECTING DUCT.

Much of the water in the kidney filtrate is reabsorbed from the collecting duct.

a) Name the part of the nephron which provides the osmotic gradient for reabsorbtion. (1)

Loop of Henle

b) Suggest one way in which this part of the nephron might be modified in desert animals. (1)

May be longer to provide a larger surface area for reabsorption 

c) Apart from water and glucose, name 2 substances that will be present in glomerular filtrate (1)

Salts and Urea

d) Name the process that separates these molecules from the blood plasma (1)

Ultrafiltration

Describe the mechanisms that are triggered in the mamillian body when water intake is reduced. (6)

• The low water potential of the blood is detected by osmorecteptors in the hypothalamus. Water begins to move out of the osmoreceptors via osmosis. 
• This stimulates it to signal for the pituitary gland to release more ADH.
• More ADH in the blood makes the collecting duct and the distal convoluted tubule more permeable to water.
• Therefore more water moves out of these via osmosis, into the medulla which has a lower water potential, which is then reabsorbed into a network of capillaries.