Anatomy and Function of the Kidney

Study Notes

The kidney is a key organ in homeostasis, responsible for maintaining the volume and composition of extracellular fluid (ECF).
Kidneys regulate the volume and composition of plasma, which in turn regulates the volume and composition of ECF.

Kidneys are bilateral retroperitoneal organs, located on either side of the spine under the ribcage.
Each kidney has a renal artery and vein as its blood supply.
Filtrate forms in Bowman's capsule in the cortex of the kidney.
Urine flows from collecting ducts into calyces.

Regulate the volume and composition of ECF:
- Regulate plasma volume and composition.
- Maintain blood pressure.
- Maintain osmolarity (solute concentration) of body fluids.
Eliminate potentially toxic metabolic wastes and foreign compounds.
Regulate the quantity and concentration of extracellular fluid ions (Na+, K+, Cl-, Ca2+, PO43-).
Maintain acid-base balance (pH).
Excrete waste products of metabolism (urea, uric acid).
Excrete foreign compounds (drugs, food additives).
Convert vitamin D to its active form (by proximal tubule cells).
Produce erythropoietin (by cells in the interstitium of the cortex/outer medulla).

Water accounts for 60% of body mass (2/3 intracellular, 1/3 extracellular).
Extracellular fluid is the fluid outside cells, consisting of plasma and interstitial fluid.
There is free exchange of water and solutes (except proteins) between plasma and interstitial fluid across capillary walls.
Distribution of fluid depends on hydrostatic and osmotic forces.

Plasma osmolarity is determined mainly by NaCl content relative to water content.
A decrease in water content increases NaCl concentration, increasing osmolarity.
An increase in water content lowers NaCl concentration, lowering osmolarity.
Kidneys maintain water balance to maintain plasma/ECF osmolarity constant.

The nephron is the functional unit of the kidney, with approximately 1-1.2 million nephrons in each kidney.
Each nephron consists of two parts: the renal corpuscle (glomerulus and Bowman's capsule) and the tubular system.
The tubular system includes the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.

Filtration of blood to form tubular filtrate (mainly salt and water).
Reabsorption of most of the filtrate back into the blood, adjusting salt and water content to match intake.
Secretion from blood into filtrate of certain substances.
Final urine formation occurs in the collecting duct.

Renal blood flow (RBF) is approximately 1.2 L/min, with a high blood supply to the kidneys (1/5 of cardiac output).
Blood supply to the nephron comes from the renal artery, which subdivides into many afferent arterioles.
Glomerular capillaries recombine to leave Bowman's capsule as efferent arterioles, which give rise to peritubular capillaries investing the tubular system.