The kidneys (Renal function) Role of the kidneys Nephron and its blood supply Formation of Urine Some of the common renal disease

The Kidneys There are two kidneys. The working unit of the kidney is Nephron, each kidney contain one million of nephron. The structure of nephron is shown below

Role of the kidneys  Eliminate soluble waste products of metabolism  Preserve the internal environment of the cells (maintain water balance, pH, ionic equilibrium, and fluid osmotic pressure)  Synthesis of erythropoietin (hormone stimulating red blood cell production)  Synthesis of 1-hydroxylation of 25-hydroxy- vitaminD 3 to form the biological active vitamin D –hormone

Representation of nephron and its blood supply

Formation of Urine A large volume of arterial blood flows through the glomerulus, which consider to be a very specialized blood filter. The red and white blood cells, platelet, and molecules with molecular weight > 50,000 Dalton are retained in the capillaries, while ions and small molecules like water and glucose pass through the capillary membrane into the surrounding capsule. About 20% of the plasma is filtered in one single pass of the blood flowing through the kidney. This the first step in the formation of urine. The filtration process is passive does not require energy input for it occur, water and small molecules pass easily through the pores to form an ultrafiltration of plasma. The ultrafiltration is large in volume (about 200L/day for average adult). A bout 99% of ultrafiltrate water and a large percentage of its constituent must be reabsorbed before the urine leaves the collecting tubules on its way to the bladder. The second step in the formation of urine is the passage of the ultrafiltrate down to the proximal tubule where the various selective processes of absorption begin.

 The filtrate starts with the same specific gravity as plasma and same pH 7.4  About 70% of water, Na +, and Cl - and all of glucose, amino acids and K+ are reabsorbed in proximal tubules. Some substances such as urea, phosphate, and Ca++ are incompletely reabsorbed. H + is exchange for Na+ throughout the tubule, while K+ is exchanged for Na+ only in distal tubule.  Na + - K + the exchange is regulated hormone aldosterone. The filtrate is reduced to about 1/3 of its original volume in the proximal tubule.  Water is always reabsorbed passively when the osmotic pressure outside of semipermeable membrane is higher than inside, it follows an osmotic gradient toward restoration of equilibrium. The osmotic gradient is usually produced by active Na + transport (the sodium pump).  From the following table it can be seen that the kidney perform a tremendous amount of work in forming normal urine.

Average filtration, Reabsorption and exchange of certain substances of plasma

Renal hydrogen ion excretion. Reabsorption of bicarbonate in the renal tubule

ADH and aldosterone in the control of the reabsorption of water and Na +, respectively, by kidney. ADH secreted by the posterior pituitary is stimulated by increased plasma osmolality, aldosterone secreted by the adrenal cortex is triggered by a decreased blood volume.

Some of the common renal disease  Glomerulonephritis -Inflammatory disease (caused by a group A, β- hemolytic streptococcus ). -Hematuria -Proteinuria -Varying degree of hypertension -Renal insufficiency and edema  Nephrotic syndrome -Heavy proteinuria -Hypoalbuminemia -Edema -Hyperlipidemia -The syndrome is associated other disease like syphilis -May caused by the treatment with adrenocortical steroids

 Pyelonephritis -Inflammatory disease (caused by infectious organisms that have ascended the urinary tract and invaded kidney tissues). -Some loss in renal function.  Arteriolar nephrosclerosis -A thickening of the inner lining of the renal arterioles, resulting in a decrease lumen and increase blood pressure. -The blood vessels become necrotic, the kidney becomes contracted. -Uncontrolled hypertension -Fetal impairment of renal function.