7.2 - the nephron

FILTRATION

  • afferent (incoming) arteriole goes into the bowman’s capsule and branches into the glomerulus

  • blood is filtered between the glomerulus and bowman’s capsule

  • filtration occurs at 4x the pressure (pressure is higher so solutes/wastes can be pushed out) than in a normal capillary

A.) FILTERABLE BLOOD COMPONENTS

  • dissolved substances (water, nitrogenous wastes, nutrients, ions) move from areas of high pressure to low

  • fluid in the bowman’s capsule is called the glomerular filtrate

(glomerular filtrate forms when high pressure in the glomerulus pushes dissolved substances into the bowman’s capsule forming fluid with the substances / glomerular filtrate)

B.) NON-FILTERABLE BLOOD COMPONENTS

  • the components (blood cells, proteins) of the blood that do not leave the capillaries, carry on away from the glomerulus through the efferent arteriole

BLOOD SUPPLY

  • the efferent arteriole forms a capillary network around the nephron called the peritubular capillary network

  • as filtrate flows through the nephron, many substances are reabsorbed actively (with energy - against the concentration gradient - low → high) or passively (without energy - along the concentration gradient - high → low) into the peritubular capillary network

REABSORPTION

  • the movement of ‘good’ substances (from filtrate) from the nephron back into the blood stream

  • primarily occurs in the proximal convoluted tubule and loop of henle

PROXIMAL CONVOLUTED TUBULE

  • is where active transport of nutrients from the glomerulus filtrate moves back into the blood stream

  • by the time the glomerulus filtrate reaches the loop of henle there should be no nutrients in it should be reabsorbed

  • glucose, amino acids, and na are all actively reabsorbed at the PCT and moved back into the blood stream in the tissue of the cortex

  • Cl is passively reabsorbed by following Na back into the blood through Cl channels

  • as solute concentration increases in the blood, water follows the concentration gradient and moves out of the filtrate

  • 85% of water is passively reabsorbed as it moves from high water concentration to low water concentration

LOOP OF HENLE

  • the descending loop of henle is impermeable to everything except water

  • the ascending loop of henle is impermeable to everything except Na and Cl

(additional water / na and cl are reabsorbed here in the loop of henle back into the blood)

WATER REABSORPTION

  • water is also reabsorbed in the distal convoluted tubule and the collecting duct

  • essentially, water can be reabsorbed anywhere in the nephron except the ascending loop of henle

KIDNEY REGIONAL ENVIRONMENT

  • all nephrons are layered on top of each other as well as besides each other

  • this means that if there is a change in environment in one PCT (like altered blood), there is a change in the environment around all PCT’s

  • this leads to a change in environment throughout the cortex of the kidney

COUNTER CURRENT EXCHANGE

  • NaCl moves out of the thick part of the ascending loop of henle

  • as the fluid moves up, less NaCl becomes available to move out, therefore the outer medulla has less osmotic pressure (less solutes) than the inner medulla (higher solutes)

  • urea which leaks from the lower part of the collecting duct also increases the osmotic pressure in the inner medulla

  • the establishment of this osmotic gradient ensures that water continues to move out of the entire length of both the descending loop of henle and the collecting duct

(ascending loop - impermeable to water so it releases NaCl to ensure that the surroundings are high in solute so that water can be released later on. permeability of the loop of henle regulates the concentrations of water/osmotic gradient as the ascending loop increases the gradient)

TUBULAR SECRETION

  • active transport of molecules (substances the body wants to remove or regulate) from the blood to the nephron

  • occurs in the distal convoluted tubule

  • uric acid, urea, K+ and H+ are secreted

  • penicillin and histamine are also secreted

URINE

  • urine is made up of the substances that have been filtered, secreted, and not absorbed

these include:

  • small amounts of water

  • nitrogenous wastes

  • bicarbonate, sulphates, and phosphates

  • excess ions

URINATION

  • from the renal pelvis urine goes down the ureters and into the bladder

  • as the bladder fills, stretch receptors send impulses to the spinal cord

  • impulses from the spinal cord then cause the bladder to contract and urine is released

  • usually, this reflex is controlled by the brain