Functions of the Kidneys in Regulating Body Fluids

Kidney Functions

• The kidneys maintain the stable internal environment necessary for cells to perform their activities by controlling the volume and electrolyte composition of body fluids.
• The kidneys filter the plasma and remove substances from the filtrate at variable rates, depending on the body's needs.
• The kidneys clear unwanted substances from the filtrate by excreting them in the urine and returning substances needed back to the blood.

Excretion of Waste Products

• The kidneys are the primary means of eliminating waste products of metabolism, such as urea, creatinine, uric acid, and end products of hemoglobin breakdown.
• The kidneys also eliminate toxins and foreign substances produced by the body or ingested, such as pesticides, drugs, and food additives.

Regulation of Body Fluids

• The kidneys regulate water and electrolyte balances by matching excretion to intake precisely.
• If intake exceeds excretion, the amount of the substance in the body will increase, and if intake is less than excretion, the amount will decrease.
• The kidneys regulate body fluid osmolality and electrolyte concentrations.

Regulation of Other Functions

• The kidneys regulate arterial pressure.
• The kidneys regulate acid-base balance.
• The kidneys secrete erythropoietin, which stimulates production of red blood cells by hematopoietic stem cells in the bone marrow.
• The kidneys produce 1,25-dihydroxyvitamin D3 (calcitriol), the active form of vitamin D, essential for normal calcium deposition in bone and calcium reabsorption by the gastrointestinal tract.
• The kidneys synthesize glucose from amino acids and other precursors during prolonged fasting, a process referred to as gluconeogenesis.

Kidney Structure and Location

• The two kidneys lie on the posterior wall of the abdomen, outside the peritoneal cavity.
• Each kidney weighs about 150 grams and is about the size of a clenched fist.
• The medial side of each kidney contains an indented region called the hilum, which passes through the renal artery and vein, lymphatics, nerve supply, and ureter.
• The kidney is surrounded by a tough fibrous capsule that protects its delicate inner structures.