Renal Structure and Function

The kidneys control the volume, composition and pressure of body fluids by regulating the excreation of water and solutes. Kidneys also influence red cell production and blood pressure by hormonal mechanisms. Urine is formed in the kidneys as an aqueous solution containing metabolic waste products, foreign substances and water soluble constituents of the body in quantities depending upon homeostatic needs. The kidneys are bilateral, retroperitoneal structures, each consisting of an outer cortex and an inner medulla. The medulla is arranged into several cone-shaped or pyramidal projections separated from each other by sections of cortex called renal columns and the cortex contains glomeruli and tubules.

The kidneys possess numerous blood vessels and because of their low vascular resistance receive approximately 1200ml of blood or 25% of cardiac output each minute. The major resistance to blood flow occurs in the glomerular capillary bed and is produced by a relatively high resistance in the efferent arterioles.

The basic functional unit of the kidney is the neprhon, a long tubular structure made up of successive segments of diverse structure and transport functions. It includes renal corpuscle, proximal tubule, hairpin loop, distal tubule, and a collecting duct system. There are approximately one million nephrons in each human kidney.

The glomerulus acts as an ultrafilter, allowing passage of water, electrilytes, and small organic molecules such as glucose, but not blood cells and large protein melecules. The ultrafilter produced by the glomeruli of both kidneys amounts to about 70 ml/min/sq m or 150 L/day/sq m. A principal function of the kidney is to remove various solutes from the blood which are not essential to the body, and to conserve those that the body requires. A solute is never totally removed from the blood in any one passage through the kidneys.

Tubular Function

Transport or movement of solutes and water across tubular cells is one of the principal activities of the renal tubules. Transport is termed resorption when it proceeds from the tubular lumen to the interstitial fluid and secretion when it proceeds in the reverse direction. Many solutes are transported in both directions simultaneously, but one direction usually dominates and the resultant is termed.

Granular cells of juxtaglomerular apparatus secret renin  in response to; low [Na+] in plasma, low e.c.f. volume or low BP (all stimuli that require aldosterone ­ to ­ plasma volume). Note renin is stored as prorenin). Renin acts enzymically, activating angiotensinogen (plasma protein produced by liver, present in [high]) into angiotensin I (AT I), a 10 amino acid polypeptide with no known physiological function. ACE is present mainly on pulmonary vasculature (also renal). As AT I passes these sites it is converted by ACE ® angiotensin II (8 amino acid polypeptide). If [K+] is high, levels of AT II ­ (\ ­ aldosterone \ ­ Na+/K+ exchange in late DCT \ lose excess K+ ). AT 2 has several important functions related to the low fluid level;

• Vasoconstrictor, affected ­ Blood Preassure
• Dipsogenic stimulus (makes you thirsty, \ ­ fluid volume).
• The 1o stimulus for aldosterone release by adrenal cortex. Leads to ­ Na+ reabsorption in exchange for K+ in DCT. Aldosterone increases the number of Na+ pumps (by ­ gene expression) in the membrane hence ­ Na+ reabsorption, and ­ K+ (or H+) secretion into the tubule. Atrial natriuretic peptide is released in response to high BP, [NaCl] or e.c.f. volume; it increases Na+ loss & \ water loss & inhibits RAA system.

Describe the factors influencing the Renal Artery Aneurysm (RAA) system. Comprehend the renal baroreceptor & macula densa mechanisms for renin release, the influence of renal nerve activity, catecholamines, Angiotensin II & ADH; the action of K+ & ACTH on aldosterone release. Describe the actions of the RAA system. Be aware of the influence of renin on the generation of angiotensin I, & all the subsequent events including renal Na+ & K+ handling & fluid intake.