Chapter 6: Excretion and Homeostasis Flashcards
Functions of the kidneys
- Excretion: The removal of metabolic waste. Removal of salts, water, urea, uric acid.
- Osmoregulation: The maintenance of body fluids at the correct salt and water concentration. Kidneys keep the blood at the correct concentration.
- Homeostasis: The ability of an organism to maintain a constant internal environment. Kidneys control the pH of the blood.
The Structure of the Kidney
1) Blood enters the kidney through the renal artery.
2) Filtration occurs in the outer cortex.
3) Reabsorption takes place in the cortex and the medulla.
4) Some substances are actively secreted into the cortex: H+ an K+ ions.
5) Unwanted waste and toxins are left in solution in the kidneys as urine.
6) Urine flows from the pyramids of the medulla into the pelvis of each kidney.
7) The two ureters then carry it to the bladder.
8) Urine is expelled from the bladder to the outside through the urethra.
Functions of the Nephron
1) Filtration
2) Selective Reabsorption
Filtration
1) Blood enters the glomerulus from the afferent arteriole.
2) The pressurised filtration that occurs in the glomerulus is called ultrafiltration.
3) Blood pressure forces fluid into the Bowman’s capsule.
4) Large proteins and blood cells are too big to pass through the pores so they stay in the blood.
5) Filtrate contains: salts, water, glucose, amino acids, urea, uric acid.
6) 20% of the plasma passes into the kidneys.
Selective Reabsorption
Most or all useful substances are returned to the blood: glucose and amino acids.
The Proximal Convoluted Tube
- All glucose and amino acids are reabsorbed by active transport.
- Most salts are reabsorbed - some by active transport and some by diffusion.
- ATP is required for active transport, so there are many mitochondria in the cells of the lining of the PCT.
- Most of the water is reabsorbed by osmosis.
Why is the PCT effective at reabsorption?
It is thin walled, long and contains many microvilli (which give it a larger surface area).
What is the function of the Loop of Henle?
Reabsorption of water.
The descending limb is…
Permeable to water.
The ascending limb is..
Permeable to salts. They pass from the nephron into the fluid of the medulla. The salts leave by active transport at the top of the ascending limb.
The Distal Convoluted Tube
More water is reabsorbed in the DCT by osmosis, due to the high salt concentration in the medulla.
The DCT fine tunes the salt concentration in the blood.
The Antidiuretic Hormone controls…
Whether the DCT and the CD reabsorb water or not.
If the blood is too dilute?
The hypothalamus signals to the pituitary gland to reduce the release of the ADH.
If the blood is too concentrated?
The hypothalamus signals to the pituitary gland to secrete the ADH. More ADH cause the DCT and the CD to become more permeable to water and therefore more water is reabsorbed into the blood.
