Kidney Function & Anatomy PDF

 Overview of Renal Function  the kidneys have several major regulatory functions, which include: 1.Regulation of water and electrolyte balance. 2. Excretion of metabolic waste products. 3. Excretion of foreign chemicals, e.g. drugs, food additives. 4. Endocrine functions...

 Overview of Renal Function  the kidneys have several major regulatory functions, which include: 1.Regulation of water and electrolyte balance. 2. Excretion of metabolic waste products. 3. Excretion of foreign chemicals, e.g. drugs, food additives. 4. Endocrine functions of the kidney: a) Regulation of erythrocyte production : The kidneys secrete erythropoietin hormone, which stimulates the production of R.B.Cs. (Severe anemia develops in people with severe kidney disease). b) Regulation of vit. D 3 production which plays an important role in calcium and phosphate homeostasis. c) Renin secretion. 5. Regulation of arterial blood pressure: I. Short-term regulation: renin-angiotensin aldosterone system. II. Long-term regulation: through excreting variable amounts of sodium and water. 6. Regulation of acid-base balance by: Elimination of acids produced from the metabolism of proteins.  ❖ Physiologic Anatomy of the Kidneys The two kidneys lie on the posterior wall of the abdomen, outside the peritoneal cavity. The renal artery, vein, lymphatics, nerve supply and ureter enter the kidney at the hilum on its medial side. Nephron as a functional Unit: The functional unit of the kidney is the nephron. There are approximately 1.3 million nephrons in each human kidney. Each nephron is capable of forming urine, i.e. acts as a functional unit. Each nephron is composed of: 1. Glomerulus:  It is formed of a tuft of capillaries (glomerular capillaries) contained within the dilated blind end of the renal tubule known as (Bowman's capsule).  The capillaries are supplied by an afferent arteriole and drained by a smaller efferent arteriole.  The glomrulus is a high pressure capillary bed, the hydrosatic pressure in glomerular capillaries'is 60 mmHg. 2. Renal tubule:  It is a thin tube, which is subdivided into: a. Proximal Convoluted Tubule. b. Loop of Henle. c. The Distal Convoluted Tubule. d. Collecting Ducts. Renal blood flow In a resting adult, the kidneys receive 1.2 -1.3 liter of blood per minute, i.e. 21% of the cardiac output. There are two capillary beds associated with each nephron: 1) The glomerular capillary bed: "High pressure bed" It receives its blood from the afferent arteriole. The hydrostatic pressure in the glomerular capillaries is about 60 mmHg which cause rapid filtration of fluid. 2) The peritubular capillary bed "Low pressure bed" The hydrostatic pressure is about 13 mmHg. The low pressure in these capillaries permits fluid reabsorption from the interstitium into the blood.  ❖ Formation of Urine Each nephron is capable of forming urine by three processes: 1) Glomerular filtration: Filtration from the glomerular capillaries into Bowman's capsule of a fluid that is nearly free of proteins. 2) Tubular reabsorption: It is the transfer of water and solutes from the filtrate back into the blood of the peritubular capillaries. 3) Tubular secretion: It is the transfer of solutes from the peritubular capillaries into the tubular lumen. - The term excretion refers to what finally comes out in urine. - The rate at which different substances are excreted in urine represents the sum of the three processes. Urinary excretion rate = Filtration rate - reabsorption rate + secretion rate.  Handling of Certain important Solutes by the Renal Tubules ❖ Na + Handling by the Renal Tubules Na + is filtered in large amounts through the glomeruli, but Na + is reabsorbed out of all portions of the tubule except the thin descending segment of the loop of Henle. 96% to well over 99% of the filtered Na + is reabsorbed. Hormonal Control (Hormones that increases Na + reabsorption): I. Mineral corticoids: - Aldosterone increases Na + reabsorption in exchange with K + or H + II. Glucocorticoids: Cortisol has weak mineralocorticoid activity. III. Angiotensin II: It is the most powerful sodium-retaining hormone. IV. Sex Hormones: Estrogen increases Na + reabsorption by renal tubule.  ❖ Glucose reabsorption by the renal tubules Normally all of the filtered glucose is reabsorbed in the early portion of the proximal convoluted tubule. Only few milligrams appear in urine per 24 hours. * Renal threshold for glucose: The plasma level at which glucose first appears in the urine than the normal minute amounts. * Value: Arterial blood: 200 mg / dl Venous blood: 180 mg / dl ❖ Glycosuria It is excretion of glucose in urine in considerable amounts. * Causes: 1) Diabetes Mellitus: Glycosuria occurs when the blood glucose level is elevated and exceeds renal threshold. 2) Renal glycosuria: Glycosuria occurs at normal plasma glucose level. The renal threshold for glucose is lowered below 180 mg % due to congenital defect in the glucose transport mechanism in the renal tubule. Excretion of the osmotically active glucose molecule entails the loss of large amounts of water (osmotic diuresis).  ❖ Water Reabsorption & Excretion of water * Water excretion: Normally 180 L of fluid is filtered through the glomeruli each day, while the average daily urine volume is about 1 L. * Water reabsorption: I. Obligatory water reabsorption: It comprises 87% of the filtered water which is reabsorbed independent of ADH II. Facultative water reabsorption: It comprises about 13% of the filtered water which is controlled by ADH.