Anatomy and physiology of kidney

Anatomy and physiology of kidney

1. ANATOMY AND PHYSIOLOGY OF KIDNEY DR PRATEEK LADDHA DNB UROLOGY (SENIOR RESIDENT) CMC LUDHIANA

2. INTRODUCTION: The kidneys excrete the end products of metabolism and excess water. Both of these actions are essential to the control of concentrations of various substances in the body fluids, e.g. maintaining electrolyte and water balance approximately constant in the tissue fluids. The kidneys also have endocrine functions producing and releasing erythropoietin which affects red blood cell formation, renin which influences blood pressure, 1,25-hydroxycholecalciferol, which is involved in the control of calcium metabolism and is a derivative of vitamin D, and perhaps modifies the action of the parathyroid hormone, and various other soluble factors with metabolic actions.

3. Anatomy of Kidney Reddish-brown IN COLOR. They are situated posteriorly behind the peritoneum on each side of the vertebral column and are surrounded by adipose tissue. Superiorly - upper border of the twelfth thoracic vertebra, Inferiorly - third lumbar vertebra. The right is usually slightly inferior to the left, probably reflecting its relationship to the liver. The long axis of each kidney is directed inferolaterally and the transverse axis posteromedially . The transpyloric plane passes through the superior part of the right renal hilum and the inferior part of the left KIDNEY.

4. Gerotas fascia: The kidneys and adrenal glands, including the perirenal fat surrounding them, are enclosed by a condensed, membranous layer of renal ( Gerota ) fascia , which continues medially to fuse with the contralateral side. This fascia extends inferomedially along the abdominal ureter as a periureteral fascia. The Gerota fascia encasing the kidneys, adrenal glands, and abdominal ureters is closed superiorly and laterally and serves as an anatomic barrier to the spread of malignancy and a means of containing perinephric fluid collections. Because it is open inferiorly, perinephric fluid collections can track inferiorly into the pelvis without violating the Gerota fascia . The Gerota fascia is further surrounded by a layer of condensed fat called the paranephric fat, which is most obvious posteriorly and represents the extraperitoneal fat of the lumbar region. Superiorly, the Gerota fascia is continuous with the diaphragmatic fascia on the inferior surface of the diaphragm, and, inferiorly, the anterior and posterior layers of the Gerota fascia are loosely attached .

5. The Gerota fascia is attached with the paranephric fat by collagen bundles. Therefore the kidneys are relatively kept fixed in position by these collagen bundles, the Gerota fascia, and paranephric fat. The relationships of the kidneys have important surgical implications. To access the kidneys, adrenals, or abdominal ureters, the Gerota fascia must be opened. To access the kidneys transperitoneally , the colon needs to be mobilized from the white line of Toldt , which is the lateral reflection of posterior parietal peritoneum over the ascending and descending colon. To access the right renal hilum, the second stage of the duodenum and head of pancreas need to be carefully mobilized using the Kocher maneuver . To access the left renal hilum, the tail of the pancreas together with the spleen and splenic vessels need to be mobilized medially.

6. Relations of Kidney

7. Relations of Kidneys Anterior relations Posterior relations

8. Vasculature of Kidney The kidneys receive approximately 20% of the cardiac output even though it accounts for only 1% of TBW . The blood supply to the kidneys arises from the paired renal arteries at the level of L2. They enter into the renal hilum, the passageway into the kidney, with the renal vein anteriorly; the renal artery; and the renal pelvis posteriorly. The first branch off of the renal artery is the inferior suprarenal artery. The renal artery then branches off into 5 segmental branches. Posterior segmental artery supplies most of the posterior kidney, with the exception of the lower pole. Anterior branches are the superior segmental artery, anterior superior segmental artery, anterior inferior segmental artery, and inferior segmental artery. These arteries branch into interlobar arteries, which travel in a parallel fashion in between the major calyces and then branch further into arcuate arteries that run within the cortex across the bases of the renal pyramids. They then radiate into interlobular arteries, which extend into the cortex of the kidney to finally become afferent arterioles, then peritubular capillaries to efferent arterioles. Some of the terminal branches of the interlobular arteries become perforating radiate arteries, which supply the renal capsule. Renal pelvic and superior ureteric branches also originate from the renal artery and supply the upper portion of the collecting system (see the image below).

9. Innervation of the Kidney The kidney can function well without neurologic control, as evidenced by the successful function of transplanted kidneys on the Expert Consult website). Sympathetic preganglionic nerves originate from the 8th thoracic through 1st lumbar spinal segments , with contributions mainly from the celiac plexus and a lesser contribution from the greater splanchnic, intermesenteric , and superior hypogastric plexuses. Postganglionic sympathetic nerve fiber distribution generally follows the arterial vessels throughout the cortex and the outer medulla. These postganglionic fibers travel to the kidney via the autonomic plexus surrounding the renal artery. In addition, parasympathetic fibers from the vagus nerve travel with the sympathetic fibers to the autonomic plexus along the renal artery. The renal sympathetics cause vasoconstriction, and the parasympathetics cause vasodilatation.

10. Microanatomy of kidney

11. Nephron Functional unit of the kidney Filtration, tubular reabsorption, tubular secretion Renal corpuscle: Glomerulus – capillaries Glomerular or Bowman’s capsule

12. Functions of Kidney Excretion of metabolic waste products and foreign chemicals Regulation of water and electrolyte balances Regulation of body fluid osmolality and electrolyte concentrations Regulation of arterial pressure Regulation of acid-base balance Regulation of erythrocyte production Secretion , metabolism, and excretion of hormones Gluconeogenesis

13. Excretion of Metabolic Waste Products, Foreign Chemicals, Drugs, and Hormone Metabolites P rimary means for eliminating waste products of metabolism that include urea (from the metabolism of amino acids), creatinine (from muscle creatine ), uric acid (from nucleic acids), end products of hemoglobin breakdown (such as bilirubin ) metabolites of various hormones . The kidneys also eliminate most toxins and other foreign substances that are either produced by the body or ingested, such as pesticides, drugs, and food additives.

14. Regulation of Water and Electrolyte Balances. For maintenance of homeostasis, excretion of water and electrolytes must precisely match intake. If intake exceeds excretion, the amount of that substance in the body will increase. If intake is less than excretion, the amount of that substance in the body will decrease.

15. Regulation of Arterial Pressure. the kidneys play a dominant role in long-term regulation of arterial pressure by excreting variable amounts of sodium and water. The kidneys also contribute to short-term arterial pressure regulation by secreting hormones and vasoactive factors or substances (e.g., renin ) that lead to the formation of vasoactive products (e.g., angiotensin II)

16. PHYSIOLOGY OF KIDNEY

17. Functions of Different parts of nephron

18. Steps in Urine formation Bowman’s capsule Receives filtrate Proximal convoluted tubule Reabsorption of water and solutes Nephron loop or Loop of Henle Regulates concentration of urine Distal convoluted tubule and Collecting duct Reabsorption of water and electrolytes ADH, aldosterone, ANP Tubular secretion 18

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22. 22 Filtration Renal corpuscle Filtration membrane Fenestrated endothelium of capillaries Basement membrane of glomerulus Slit membrane between pedicels of podocytes

23. 23 Forces that influence filtration Glomerular blood hydrostatic pressure Opposing forces: Plasma colloid osmotic pressure Capsular hydrostatic pressure

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25. 25 Glomerular Filtration Rate Volume of plasma filtered / unit time Approx. 180 L /day Urine output is about 1- 2 L /day About 99% of filtrate is reabsorbed

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27. 27 GFR influenced by: Blood pressure and blood flow Obstruction to urine outflow Loss of protein-free fluid Hormonal regulation Renin – angiotensin Aldosterone ADH ANP

28. 28 Juxtaglomerular apparatus Juxtaglomerular cells lie in the wall of afferent arteriole Macula densa in final portion of loop of Henle – monitor Na + and Cl - conc. and water Control blood flow into the glomerulus Control glomerular filtration

29. 29 Tubular reabsorption Water, glucose, amino acids, urea, ions Sodium diffuses into cell; actively pumped out – drawing water with it By end of proximal tubule have reabsorbed: 60- 70% of water and sodium about 100% of glucose and amino acids 90 % of K+, bicarb , Ca++, uric acid Transport maximum – maximum amount of a substance that can be absorbed per unit time Renal threshold – plasma conc. of a substance at which it exceeds Tm.

30. 30 Tubular secretion In addition to reabsorption, also have tubular secretion – substances move from peritubular capillaries into tubules – a second chance to remove substances from blood.

31. 31 Loop of Henle Responsible for producing a concentrated urine by forming a concentration gradient within the medulla of kidney . When ADH is present, water is reabsorbed and urine is concentrated. Counter-current multiplier

32. 32 Distal convoluted tubule and collecting ducts What happens here depends on ADH Aldosterone affects Na+ and K+ ADH – facultative water reabsorption Parathyroid hormone – increases Ca++ reabsorption

33. 33 Distal convoluted tubule and collecting ducts Tubular secretion to rid body of substances: K+, H+, urea, ammonia, creatinine and certain drugs Secretion of H+ helps maintain blood pH (can also reabsorb bicarb and generate new bicarb )

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35. 35 Substances not normally present in urine Acetone Bile, bilirubin Glucose Protein – albumin Renal disease involving glomerulus

36. 36 Blood Urea Nitrogen BUN Urea produced by breakdown of amino acids - influenced by diet, dehydration, and hemolysis Normal range 10-20 mg/ dL If the GFR decreases due to renal disease or blockage, or decreased blood flow to kidney - BUN increases General screen for abnormal renal function

37. 37 Creatinine clearance Creatinine is an end product of muscle metabolism Muscle mass is constant; creatinine is constant Normal 0.7 – 1.5 mg/ dL in plasma Can then be compared to creatinine in urine over 24 hour period to determine clearance Creatinine clearance is an indirect measure of GFR and renal blood flow Creatinine is neither reabsorbed nor secreted, just freely filtered. Amount excreted = amount filtered Useful to monitor changes in chronic renal function Increases with trauma with massive muscle breakdown

38. 38 Diagnostic testing Inulin clearance - not absorbed or secreted = GFR PAH – para- aminohippuric acid – not absorbed ; actively secreted = renal plasma flow