6.9: Control of blood water potential

What is osmoregulation?

The regulation of the water content of the blood

What ie excretion?

The removal of metabolic waste and excess substances from the blood

What is the role of the hypothalamus in osmoregulation?

• Contains osmoreceptors which can detect an increase or decrease in blood water concentration

• Produces more ADH when water potential is low, or less ADH when water potential is high

What is the role of the posterior pituitary gland in osmoregulation?

Secretes more/less ADH due to signals from the hypothalamus

What is the role of ADH in osmoregulation?

• ADH attaches to receptors on the collecting duct and distal convoluted tubule

• This stimulates the addition of aquaporins into the cell-surface membranes

• So cell membrane permeability to water of the collecting duct and distal convoluted tubule is increased

• So water reabsorption from the collecting duct and distal convoluted tubule back into the blood via osmosis is increased

• So urine volume is decreased, and urine concentration is increased

Label the structure of the kidney

• Cortex

• Renal pelvis

• Ureter

• Nephron

• Medulla

Label the structure of the nephron

• Proximal convoluted tubule

• Renal (Bowman’s) capsule

• Glomerulus

• From renal artery

• To renal vein

• Loop of henle: descending limb, ascending limb

• Collecting duct

• Medulla

• Cortex

• Distal convoluted tubule

What are the functions of the different parts of the nephron (Bowman’s capsule, proximal convoluted tubule, loop of henle, distal convoluted tubule, collecting duct)?

• Bowman’s capsule: formation of glomerular filtrate

• Proximal convoluted tubule: reabsorption of water and glucose

• Loop of henle: maintenance of a gradient of sodium ions in the medulla

• Distal convoluted tubule/collecting duct: reabsorption of water

What is the process of the nephron producing urine?

• Ultrafiltration: small molecules are filtered out the blood and into the Bowman’s capsule of the nephron, forming glomerular filtrate

• Selective reabsorption: useful molecules are taken back from the filtrate and returned to the blood, the remaining filtrate forms urine

• Urine then flows out the kidneys along the ureters and into the bladder where it is temporarily stored

How is glomerular filtrate formed?

• There is high hydrostatic pressure in the glomerulus, as the diameter of the afferent arteriole (that carries blood into the glomerulus) is wider than the efferent arteriole (that carries blood out of the glomerulus)

• Small substances such as water, glucose and urea are forced into glomerular filtrate

• This is filtered by pores between capillary endothelial cells, the capillary basement membrane and podocytes

• Large proteins and blood cells remain in the blood

How is water reabsorbed by the proximal convoluted tubule?

• Active transport of Na+ from proximal convoluted tubule into capillary lowers the blood water potential of the capillaries

• So water can move by osmosis from a high water potential in the proximal convoluted tubule to a lower water potential in the capillary down a water potential gradient 

How is glucose reabsorbed by the proximal convoluted tubule?

• Na+ is actively transported out of the epithelial cells to the capillary

• Na+ moves by facilitated diffusion (from lumen) into the epithelial cells down a concentration gradient, bringing glucose against its concentration gradient via co-transport

• Glucose moves into the capillary by facilitated diffusion down its concentration gradient

How is the proximal convoluted tubule adapted for the rapid reabsorption of glucose into blood?

• Microvilli: large surface area

• Many channel/carrier proteins: facilitated diffusion/co-transport

• Many carrier proteins: active transport

• Many mitochondria: produces ATP for active transport

• Many ribosomes: produces carrier/channel proteins

Why is glucose found in the urine of an untreated diabetic person?

• Blood glucose concentration is too high so not all glucose is reabsorbed by the proximal convoluted tubule

• As glucose carrier/cotransporter proteins are fully saturated

What is the role of the loop of Henle in maintaining a gradient of sodium ions in the medulla?

In the ascending limb

• Na+ is actively transported out

• Causing concentration of filtrate to decrease

• This increases concentration of Na+ in the medulla, lowering water potential

In the descending limb

• Water moves out by osmosis and is then absorbed by capillaries 

• Causing filtrate concentration to increase

• Na+ is recycled so it diffuses back into the ascending limb

Why do animals that need to conserve water have long loops of Henle?

• More Na+ moves out, so Na+ gradient is maintained for longer in the medulla

• So water potential gradient is maintained for longer

• So more water can be reabsorbed from collecting ducts by osmosis

How is water reabsorbed by the distal convoluted tubule and collecting ducts?

• Water moves out the distal convoluted tubule and collecting duct by osmosis down a water potential gradient

• This is controlled by ADH which increases their permeability