Human Physiology

Organs of The Urinary System

Organs of The Urinary System The urinary bladder stores urine prior to micturition, the urethra expels urine from the bladder, the ureters bring urine to…

Dr. Shweta Bairagi · 30 May 2011

Organs of The Urinary SystemThe urinary bladder stores urine prior to micturition, the urethra expels urinefrom the bladder, the ureters bring urine to the bladder. But the function of the kidneys is NOT to make urine, it is to maintain homeostasis of the blood: excreting wastes, keeping nutrients, maintaining electrolytes, acid base balance, and other things.Structure of the KidneyThe kidney is composed of several layers and is covered with a fibrouscapsule, the renal capsule . The outer layer of the kidney is the cortex . Itcontains the major (upper) portion of the nephrons. The middle layer of thekidney is the medulla. It is composed of the triangular shaped pyramids andthe renal columns. The pyramids contain the collecting tubules and loops ofHenle, the lower portion of the nephrons.Kidney VascularizationThe blood supply of the kidney is paramount in its function. The two kidneysreceive between 15 and 20% of the body's systemic blood flow at rest. Therenal artery branches into lobar and then interlobar arteries.Nephrons: Functional Units of the KidneysThe Nephron : - The nephrons are the functional units of the kidney, i.e.individually and collectively they perform the functions of the kidney.Each nephron is served with blood by the afferent arteriole . This vesselbrings blood into a capillary tuft called the glomerulus . Blood leaving theglomerulus flows into the efferent arteriole .The Bowman's capsule opens into the proximal convoluted tubule whichleads to the loop of Henle . The loop of Henle has a descending limbwhich passes into the medulla, recurves, and becomes the ascending limbwhich leads back up to the distal convoluted tubule in the cortex. Mosthuman nephrons are termed cortical nephrons because their corpuscles arelocated in the mid to outer cortex and their loops of Henle are very short andpass only into the outer medulla. But a small portion are calledjuxtamedullary nephrons and their loops travel deep into the innermedulla.Step 1 in urine formation, Filtration - Fluid pressure forces water anddissolved substances out of the blood into Bowman's capsule. Filtrationaverages 125 ml/min for your two kidneys. This amounts to about 180 Litersper day. Since we urinate an average of 1500 ml per day, more than 99%must be returned to the blood. Filtration involves the small molecules: water,electrolytes, urea, glucose, amino acids. It does not involve the bloodproteins or cells.1. FiltrationFiltration - Hydrostatic pressure (blood pressure) forceswater and dissolved substances out of the glomerular bloodinto Bowman’s capsule.H2O, glucose, amino acids, electrolytes, wastesAverages 125 ml/min for both kidneys 􀃆 180 liters/dayThe Glomerulus and Bowman’s CapsuleThe capillaries of the glomerulus are surrounded by specialized cells whichform the inner (visceral) layer of Bowman's capsule.These specialized cells are called podocytes (foot cells) because theyhave processes called pedicels which interdigitate or interlace producingopenings called filtration slits . The capillaries are fenestrated in order toallow a large amount of filtration. The outer (parietal) layer of Bowman'scapsule consists of epithelial cells with tight junctions and serves to containthe filtrate in the capsular space .Step 2: ReabsorptionReabsorption - the return of substances fromfiltrate in the nephron tubule to the blood orinterstitial fluid.The return of substances from the filtrate to the blood andinterstitial fluid. The major substances reabsorbed are water, NaCl, glucose, andamino acids. Some of the urea, together with other salts are also reabsorbed.Reabsorption occurs in each of these areas for various substances and tovarious degrees: Most reabsorption occurs in the PCT ( ProximalConvoluted Tubule ), but reabsorption of water also occurs from thedescending limb of the Loop of Henle , reabsorption of salt from theascending limb and the DCT ( Distal Convoluted Tubule ), and more waterfrom the Collecting Duct .The Counter-Current MechanismsThis mechanism works in the loop of Henle to increase water reabsorbed from the descending limb as a result of saltreabsorbed from the ascending limb. The term countercurrent comes fromthe fact that fluid is moving in opposite directions in the two limbs of the loop.This magnifies the effect of transport from one limb on transport from theother limb.The Counter-Current Mechanisms1) The counter-current multiplier - increases theamount of H2O reabsorbed because of oppositemovement in the two limbs of the loop of Henle.2) The countercurrent exchange of salt -increases the reabsorption of H2O by retainingNaCl in the medulla.Step 3: SecretionSecretion is the active release of substances into thenephron tubule by the tubular lining cells. Secretion is the release by active transport of substances into the filtrate. It is accomplished by the tubular lining cells. The substances released are usually derived from the blood in the peritubular capillaries. Actually secretion has already been going on but it is the third process we consider. Itbegins in the proximal convoluted tubule and continues in the distalconvoluted tubule and the collecting tube. Secretion occurs as an active transport by the cells of the nephron tubule inwhich they transport substancesobtained from the peritubular blood or interstitial fluid into the nephrontubule. It occurs in the proximalconvoluted tubule, distal convoluted tubule, and collecting tubule.Normal Constituents of UrineWater (sp. Gravity 1.001 to 1.035),urea,uric acid,creatinine,Na+, K+, PO4-3, SO4-2, Ca+2, Mg+2Abnormal Constituents of Urine (Table 26.2)Glucose - Recent intake of sugary foods, diabetes m.Protein - Physical exertion, high protein;hypertension, glomerulonephritis.Ketone bodies - Starvation, untreated diabetes mellitusHemoglobin - Hemolytic anemia, severe burnsBile pigments - Hepatitis, cirrhosis, bile obstructionErythrocytes - Bleeding due to trauma, kidney stones,infection, cancer.Leucocytes - Urinary tract infection