Physiology د. نصير جواد المختار

Lecture II: Function of Kidney, Functional Anatomy of the Kidney

Function of Kidney:

  1. Excretion of metabolic waste products: the kidney eliminate waste product of metabolism that are not needed by the body. These products include urea (from the metabolism of amino acids), creatinine (from muscle creatine), uric acid (from nucleic acids), bilirubin (from hemoglobin breakdown) and metabolites of various hormone the kidneys eliminate most toxins and other substances such as drugs.
  2. Regulation of water and electrolyte balance: The capacity of the kidneys to alter sodium excretion in response to change in sodium intake is 10 fold than normal. This also true for water and other electrolytes such as potassium, calcium and hydrogen.
  3. Regulation arterial pressure: By excreting variable amount of sodium and water, and secretary vasoactive factors such as rennin, that lead to formation of vasoactive products such as Angiotensin II.
  4. Regulation acid-base balance: By excreting acid and by regulating the body fluid buffer stores. Kidneys eliminate certain types of acid such as sulfuric acid and phosphoric acid by metabolism of proteins.
  5. Regulation of erythrocytes production: The kidneys secrete erythropoietin which stimulates the production of red blood cells; one important stimulus for erythropoietin secretion is hypoxia. The people with severe kidney disease, severe anemia develops as result of decreased erythropoietin production.
  6. Regulation of 1, 25-Dihyroxy vitamin D3 production: the kidneys produce the active form of D3 (1, 25-Dihydroxy vitamin D3) calcitriol, which is essential for normal calcium deposition in bone and calcium reabsorption by gastro-intestinal tract. It play role in calcium and phosphate regulation.
  7. Glucose synthesis: The kidneys synthesis glucose from amino acid during prolonged fasting (gluconeogenesis).

Functional anatomy of the kidney:

General organization of the kidney and urinary tract:

Two kidneys lie on the posterior wall of the abdomen, outside the peritoneal cavity. Each kidney of adult human weight about 150 grams. Through hilum pass the renal artery, vein, lymphatic, nerve supply and ureter that carry the urine from kidney to the bladder, where it is stored urine until it is emptied by micturation. Kidney contain outer cortex and inner medulla. The medulla is divided into multiple cone shaped masses of tissue called renal pyramid, terminate into renal pelvis.

Renal blood flow:

Blood flow to the kidneys is normally about 22% of COP (1100 ml/minute). The renal artery enters kidney through the hilum from interlobular arteries, arcuate arteries, interlobular arteries, afferent arterioles which lead to glomerular capillaries where large amount of fluid and solutes (except the plasma proteins) are filtered to formation of urine. Distal end of capillaries of glomerulus coalesce to form efferent arteriols then peritubular capillaries. Hydrostatic pressure in the glomerular capillaries (60mmHG) causes rapid fluid filtration, where as a much lower hydrostatic pressure in the peritubular capillaries (13 mmHg) permit rapid fluid reabsorption. Peritubular capillaries empty into the vessels of venous systems interlobular vein > arcuate vein > interlobular vein > renal vein (Fig. 1).

Nerve supply:

The kidney has a rich adrenergic sympathetic nerve supply distributed to the:

  1. Vascular smooth muscle to cause vasoconstriction.
  2. Juxtaglomerular cells to cause rennin sercretion.
  3. Tubular cells to stimulate Na and water reabsorption.

Juxtaglomerular complex:

The juxtaglomerulus complex consists of macula densa cells in the initial portion of the distal tubule and juxtaglomerular cells in the walls of the afferent and efferent arterioles. The macula densa is a specialized group of epithelial cells in the distal tubules that comes in close contact with afferent and efferent arterioles. The macula densa cells contain Golgi apparatus, which are intracellular secretary organelles, directed toward the arterioles, suggesting that these cells may be secreting a substance (rennin) toward the arterioles (Fig. 2).

Nephron:

It is basic functional unit of the kidney and capable of forming urine. There are about million nephrons in each kidney in human body. Kidneys cannot regenerate new nephron and their number decrease with aging. Each nephron consist of:

  1. Bowman’s capsule: It is the invaginated end of the tubule that encased to glomerulus (branching capillaries). The pressure in the glomerular capillaries is higher than that in other capillary beds. This membrane is different from other capillary membrane by having three layers instead of two. These three layers are endothelial layer, a basement membrane and a layer of epithelial cells. Despite the number of layers, the permeability of the glomerular membrane is from 100-500 times as great as that the usual capillary due to presence of thousands of small holes which are called fenestrate in the endothelial cells by the presence of large spaces un the basement membrane and by incontinuity of cells that form the epithelial layer.
  2. Proximal convoluted tubule: They lie in the renal cortex along with the glomerulus. The epithelial cells of them are highly metabolic cells, with a large number of mitochondria to support rapid active transport process. It contains a brush border due to the presence of microvillus. Reabsorption in the proximal tubule us usotonic; osmolarity of fluid in all parts of the proximal tubule is that of plasma. Over all 65% of the filtered Na, and water, almost all the filtered glucose, amino acids, organic acids, small amount of protein, much of K, Ca, phosphate and urea are reabsorped in the proximate tubule.
  3. Loops of Henle: The epithelial cells of the thin descending segment of the loop of Henle are very thin with no brush border and very few mitochondria. They are highly permeable to water but much less permeable to urea, sodium and most other ions. The epithelial cells of ascending thin segment are less permeable to water but more permeable to urea than descending portion. The epithelial cells of the ascending thick segment are similar to those of proximal tubules except that they have a rudimentary brush border and much tighter tight junction. The cells adapted for strong active transport of sodium, potassium and possibly chloride ions, impermeable to both water and urea. The active reabsorption of Cl ions creates interluminal positivity which enhances the movement of NA and other ions passively down the electrical gradient from the lumen of the tubule to inside of the tubular epithelial cell. This active transport of CL ions can be inhibited by drugs such as diuretic (frusemide) (Fig. 3).

The thick ascending segment ascends back to the same glomerulus through angle between the afferent and efferent arterioles. The cells of this portion of thick ascending segment which are in complete attachment with the epithelial cells of the afferent and efferent arterioles that come in contact with the macula densa. The specialized smooth muscle cells of afferent arterioles that come in contact with the macula densa are called juxtaglomerular cells which contain rennin granules. Macula densa and juxtaglomerular cells + few granulated cells between them are known as juxtaglomerular comlex or apparatus which has a dense adrenergic neural innervations (Fig. 4).

  1. Distal convoluted tubule: they lie in the real cortex. The first half of the distal tubule as the thick segment of ascending limb of the loop of Henle. It absorbs most of ions but impermeable to both water and urea. The second half of distal tubule and the cortical collecting tubule are similar to each other and both impermeable to urea reabsorb sodium ions in an exchange with K ions under the effect of aldosterone, permeable to water only in the presence of antidiuretic hormone. Its epithelium is similar to that of the proximal tubule but they have distinct brush border.
  2. Collecting tubules and ducts: About eight distal tubules coalesce to form the collecting tubule which passes from cortex downward into medulla,where it becomes the collecting ducts. The epithelium of the collecting ducts made up of P cells which are involved in NA ions reabsorption and vasopressin stimulated water reabsorption and intercalated cells which are concerned with acid secretion and bicarbonate ions transport.

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