Functions of the kidney

Waste management

1-Excretion of wastes

Every day, the kidneys process about 200 liters of blood to filter out about 2 liters of waste products and extra water and recycle necessary substances. The waste and extra water become urine, which flows to the bladder through tubes called ureters. The bladders stores urine until you go to the bathroom The ability to concentrate wastes into a volume of urine much smaller than the volume of blood occurs via a countercurrent mechanism.

The kidneys excrete a variety of waste products produced by metabolism into the urine, such as:

1. Excess water
2. The nitrogenous wastes from Urea
3. Protein breakdown metabolites such as creatinine
4. Uric acid from nucleic acid (DNA & RNA metabolism)
5. Some ions such as : Na+,K+, Mg2+,Ca2+
6. The kidneys are the primary means for eliminating waste products of metabolism that are no longer needed by the body. These products include urea (from the metabolism of amino acids), creatinine (from muscle creatine), uric acid (from nucleic acids), end products of hemoglobin breakdown (such as bilirubin), and metabolites of various hormones. These waste products must be eliminated from the body as rapidly as they are produced. The kidneys also eliminate most toxins and other foreign substances that are either produced by the body or ingested, such as pesticides, drugs, and food additives.

2-Reabsorption of vital nutrients

1. Glucose at normal plasma levels is completely reabsorbed in the proximal tubule.
2. Amino acids are reabsorbed by sodium dependent transporters in the proximal tubule.
3. Electrolytes  & H2O are reabsorbed at various stages throughout the Nephron

Production line

(Endocrine functions)

• The kidneys secrete several hormones:
  • Erythropoetin
    • Released in response to hypoxia.
    • Stimulates production of red blood cells in the bone marrow (Erythropoesis)
  • Calcitriol
    • The activated form of vitamin D
    • promotes intestinal absorption of Ca2+ and the renal reabsorption of  PO4-
  • Secretes Renin
    • Secreted by pericytes in the vicinity of afferent arterioles in response to low Na+ or Cl- ions ( ie hypovolemia)
    • Part of the RAAS- a blood pressure regulation system
  • ADH (NOT released by the kidney but acts on Kidney)
    • Synthesized by the hypothalamus but released by the posterior pituitary gland
    • Action is on the Kidneys
    • Allows Water to be reabsorbed back into the body, this forming a more concentrated Urine

Maintenance system

• Homeastatic function
  • At first, the tubules receive a combination of waste materials and chemicals that your body can still use.
  • Kidneys measure out electrolytes such as  sodium,  phosphorus, magnesium  and potassium  and release them back to the blood.
• Osmolarity regulation
  • The Hypothalamus detects change in osmolarity
  • The kidneys have receptors of ADH which regulates water reabsorption
• Blood pressure regulation
  • Kidney regulates Blood pressure via Renin- Angiotensin- Aldosterone system (RAAS)
• Acid Base regulation Regulation occurs by:
  • Reabsorption and regeneration of bicarbonate from urine
  • Excretion  H+ and fixed acids (anions of acids) into urine.