KIDNEYS-Location and Structure Although many believe that the kidneys are located in the lower back, this is not their location. These small, dark red organs with a kidney bean shape lie against the dorsal body wall in a retroperitoneal position (beneath the parietal peritoneum) in the superior lumbar region. The kidneys extend from the T12 to the L3 vertebra; thus they receive some protection from the lower part of the rib cage. Because it is crowded by the liver, the right kidney is positioned slightly lower than the left. It is convex laterally and has a medial indentation called the renal hilus. Atop each kidney is an adrenal gland, which is part of the endocrine system and is a distinctly separate organ functionally. A fibrous, transparent renal capsule encloses each kidney and gives a fresh kidney a glistening appearance. The adipose capsule, surrounds each kidney and helps hold it in place against the muscles of the trunk wall.
When a kidney is cut lengthwise, three distinct regions become apparent, as can be seen in this picture. The outer region, which is light in color, is the renal cortex. Deep to the cortex is a darker reddish-brown area, the renal medulla. The broader base of each pyramid faces toward the cortex; its tip, the apex, points toward the inner region of the kidney. The pyramids are separated by extensions of cortex like tissue, the renal columns.
Medial to the hilus is a flat, basinklike cavity, the renal pelvis Pelvis is continuous with the ureter leaving the hilus. Extension of the pelvis, calyces (calyx), form cup-shaped areas that enclose the tips of the pyramids. The calyces collect urine, which continuously drains from the tips of the pyramids into the renal pelvis. Urine then flows from the pelvis into the ureter, which transport it to the bladder for temporary storage.
Blood supply The kidneys continuously cleanse the blood and adjust its composition, so it is not surprising that they have a very rich blood supply One-quarter of the total blood supply of the body passes through the kidneys each minute.
The arterial supply of each kidney is the renal artery As the renal artery approaches the hilus, it divides into Segmental arteries. Once in side the pelvis, the segmental arteries break up into lobar arteries Each of which gives off several branches called interlobar arteries then branch off the arcuate arteries and run outward to supply the cortical tissue. The venous blood draining from the kidney flows through veins that trace the pathway of the arterial supply but in a reverse direction- interlobular veins to arcuate veins to interlobar veins to the renal vein, which emerges from the kidney hilus
Nephrons and Urine Formation Each kidney contains over a million tiny structures called nephrons. Nephrons are the structural and functional units of the kidneys and, as such, are responsible for forming urine. Each nephron consists of two main structures: a glomerulus, which is a knot of capillaries, and a renal tubule. The cup- shaped of the renal tubule is called the glomerular, or Bowman’s, capsule. The inner layer of the capsule is made up of highly modified octopus- like cells called podocytes
Extends from the glomerular capsule, it coils and twists before forming a hairpin loops and then again becomes coiled and twisted before entering a collecting tubule called the collecting duct. (these different regions of the tubule have specific names) These different regions of the tubule have specific names. Most nephrons are called cortical nephrons because they are located almost entirely within the cortex. The collecting ducts, each of which receives urine from many nephrons, run downward through the medullary pyramids, giving them their striped appearance.
The afferent arteriole, which arises from an interlobular artery, is the “feeder vessel,” and the efferent arteriole receives blood that has passed through the glomerulus. The glomerulus, specialized for filtration, is unlike any other capillary bed in the entire body. The second capillary bed, the peritubular capillaries, arises from the efferent arteriole that drain the glomerulus. Unlike the high-pressure glomerulus, these capillaries are low- pressure, porous vessels that are adapted for absorption instead of filtration. The peritubular capillaries drain into interlobular veins leaving the cortex.
Urine Formation It is a result of three processes: FILTRATION TUBULAR REABSORPTION TUBULAR SECRETION
Filtration Glomerulus Acts as a Filter Water and solutes smaller than proteins are forced through the capillary walls and pores of the glomerular capsule into the renal tubule. Both proteins and blood cells normally too large to pass through the filtration membrane and when either one of these appear in urine it is evident there is a problem with the glomerular filters
Con’t Also, systemic blood pressure has to be normal in order for filtration to happen If the arterial blood pressure falls too low, the glomerular pressure becomes inadequate to force substances out of the blood and into the tubules, and filtrate formation stops
Homeostatic Imbalance Oliguria: an abnormal low urinary output if it is between 100 and 400 ml/day Anuria if it is less than 100ml/day Low urinary output indicates that glomerural blood pressure is too low to cause filtration However, Anuria may also result from transfusion reactions and acute inflammation or from crush injuries of the kidneys
Con’t Blood from afferent arteriole flows into the glomerulus (capillaries) Due to blood pressure in the glomerulus, filtration occurs Water and small molecules (such as salts, amino acids, urea, uric acid, glucose) move from the blood plasma into the capsule Small molecules that escape being filtered and the nonfilterable components leave the glomerulus by the Efferent arteriole This produces a filtrate of blood, called glomerular filtrate
Filterable Blood Nonfilterable BloodComponents Components Water Formed elements (blood cells and Nutrients platelets) Salts Plasma Proteins Ions Nitrogenous Waste
Tubular Reabsorption As the filtrate moves along the tubule some of the molecules and ions are actively and passively (by diffusion) reabsorbed into the capillary bed from the tubule Active transport: transport of molecules against a concentration gradient (from regions of low concentration to regions of high concentrations) with the aid of proteins in the cell membrane and energy from ATP
Con’t About 99% of filtered water and many useful molecules (such as salts, urea, nutrients, glucose, amino acids, sodium Ion Na+, chloride ion Cl-) returned to the blood Reabsorption of water is by osmosis Most of the reabsorption occurs in the proximal convoluted tubules, but the distal and the collecting duct are also active
Tubular Secretion More substances such as ions (hydrogen ion, creatinine, some drugs (penicillin), toxic substances, are actively secreted from the capillary network to tubules The fluid (urine), from filtration that was not reabsorbed and from tubular secretion, then flows into the collecting duct, then renal pelvis Substances found in urine are water, salts, urea, uric acid, ammonia, creatinine (NOT large molecules (proteins, blood cells), glucose Also, if all those substances werent reabsorbed by tubules (glucose, water, salts, urea) than the body would continually lose water, salt and nutrients
Characteristics of Urine Nephrons filter 125 ml of body fluid per minute; filtering the entire body fluid component 16 times each day In a 24 hour period nephrons produce 180 liters of filtrate, of which 178.5 liters are reabsorbed. The remaining 1.5 liters forms urine
Con’t Freshly voided urine is generally clear and pale to deep yellow The more solutes are in a urine, the deeper yellow its color; whereas dilute urine is a pale, straw color When formed, urine is sterile, and its odor is slightly aromatic Ph is slightly acid (around 6) Urine weight more than distilled water (because it has water plus solutes)
Ureters•It is a slender tube each 25-30 cm longand 6mm in diameter•Each tube descends beneath theperitoneum, from the hilum of a kidney,to enter the bladder at its dorsal surface
Con’t The ureters is a passageway that carry urine from the kidneys to the bladder Although it may seem like urine may drain to the bladder by gravity, but the ureters do play an active role in urine transport Smooth muscle layers in their walls contract to propel urine into the bladder by peristalsis (even if a person is laying down) Once urine has entered the bladder, it is prevented from flowing back into the ureters by small valvelike folds of bladder mucosa that flap over the ureter openings
Homeostatic Imbalance When urine becomes extremely concentrated, solutes such as uric acid salts form crystals that precipitate in the renal pelvis These crystals are called renal calculi, or kidney stones The crystals may grow into a stone ranging in size from a grain of sand to a golf ball. Most stones form in the kidneys. Very small stones can pass through the urinary system without causing problems. However, larger stones, when traveling from the kidney through the ureter to the bladder, can cause severe pain called colic. Most stones (70 to 80 percent) are made of calcium oxalate. A smaller number are made of uric acid or cystine
Con’t For treatment, surgery is a choice However, a newer noninvasive procedure (lithotripsy) may be used Uses ultrasound waves to break the stones into small fragments (about the size of grain of sand) They then can be eliminated painlessly in the urine
Urinary Bladder The urinary bladder stores urine until it is expelled from the body The bladder is located in the pelvic cavity, behind the public symphysis and beneath the peritoneum The bladder has three openings---two for the ureters and one for the urethra, which drains the bladder
Con’t The smooth triangular region of the bladder base outlined by these three openings is called the tridone The trigone is important clinically because infections tend to persist in this region In males the prostate gland surrounds the neck of the bladder were it empties into the urethra The bladder wall contains three layers of smooth muscle called the detrusor muscle and its mucosa is a special type of epithelium: transitional epithelium When the bladder is empty it is collapsed, 5-7.5 cm long at most and its walls are thick and thrown into folds
Con’t As urine accumulates, the bladder expands and rises superiorly in the abdominal cavity Fig 15.7 Its muscle wall stretches and the transitional epithelial layer thins, allowing the balder to store more urine without substantially increasing its internal pressure A full bladder is about 12.5 cm long and hold about 500 ml of urine, but it is capable of holding more than twice that amount When the bladder is really distended, or stretched by urine, it becomes firm and pear shaped and may be felt just above the public symphysis Although urine is formed continuously by the kidneys, it is usually stored in the bladder until its release is convenient
The anatomy of the urethra The epithelium of the urethra starts off as transitional cells as it exits the bladder. Further along the urethra there are stratified columnar cells, then stratified squamous cells near the external meatus (exit hole). There are small mucus- secreting urethral glands, that help protect the epithelium from the corrosive urine
The female urethra Female urethra In the human female, the urethra is about 1 1/2-2 inches (3-5 cm) long and opens in the vulva between the clitoris and the vaginal opening. Because of the short length of the urethra, women tend to be more susceptible to infections of the bladder (cystitis) and the urinary tract.
The female urethra is a narrow membranous canal, extending from the internal to the external urethral orifice. It is placed behind the symphysis pubis, imbedded in the anterior wall of the vagina, and its direction is obliquely downward and forward; it is slightly curved with the concavity directed forward. Its lining is composed of stratified squamous epithelium, which becomes transitional near the bladder. The urethra consists of three coats: muscular, erectile, and mucous, the muscular layer being a continuation of that of the bladder. The release of urine is controlled by two sphincters. Internal urethral sphincter External urethral sphincter
Male urethra The male urethra extends from the internal urethral orifice in the urinary bladder to the external urethral orifice at the end of the penis. It presents a double curve in the ordinary relaxed state of the penis. Its length varies from 17.5 to 20 cm.; and it is divided into three portions, the prostatic, membranous, and cavernous, the structure and relations of which are essentially different. Except during the passage of the urine or semen, the greater part of the urethral canal is a mere transverse cleft or slit, with its upper and under surfaces in contact; at the external orifice the slit is vertical, in the membranous portion irregular or stellate, and in the prostatic portion somewhat arched.
1. The prostatic portion (pars prostatica), the widest and most dilatable part of the canal, is about 3 cm. long. 2. The membranous portion (pars membranacea) is the shortest, least dilatable, and, with the exception of the external orifice, the narrowest part of the canal It extends downward and forward, with a slight anterior 3. The cavernous portion concavity, between the apex of the (pars cavernosa; penile or prostate and the bulb of the spongy portion) is the longest urethra, perforating the urogenital part of the urethra, and is diaphragm about 2.5 cm. below contained in the corpus and behind the pubic symphysis. cavernosum urethræ. It is about 15 cm. long, and extends from the termination of the membranous portion to the external urethral orifice.
The structure of the male urethra The structure of the urethra (tube) These muscle fibres are arranged itself is a continuous mucous in a circular configuration that membrane supported by separates the mucous membrane submucous tissue connecting it and submucous tissue from the to the other structures through surrounding structure - which is which it passes. the tissue of the corpus spongiosum (labeled simply "penis" The mucous coat is continuous in the diagram above). with the mucous membrane of the bladder, ureters and kidney. In the Unlike the female urethra, the male membranous and spongy sections urethra has a reproductive function (2. and 3. above), the mucous in addition to its urinary function - membrane is arranged in it conveys semen out of the body longitudinal folds when the tube is at ejaculation. For further empty. information about this function red the section about the male The submucous tissue consists reproductive system. of a vascular (i.e. containing many blood vessels) erectile layer surrounded by a layer of smooth (involuntary) muscle fibres.
The Function of the Urethra Gender differences: The females only carries urine. The males carries urine and is a passageway for sperm cells.
Micturition of the urethra Male and female Both sphincter muscles must open to allow voiding. The internal urethral sphincter is relaxed after stretching of the bladder Activation is from an impulse sent to the spinal cord and then back via the pelvic splanchnic nerves. The external urethral sphincter must be voluntarily relaxed.
Fluid, Electrolyte, and Acid-BaseBalanceBlood composition depends on three major factors:2. Diet3. Cellular metabolism4. Urine outputIn general, the kidneys have four major roles to play, which help keep the blood composition relatively constant.7. Excretion of nitrogen containing wastes8. Maintaining water in the blood9. Maintaining electrolyte balance in the blood, and10. Ensuring proper blood pH
Maintaining Water and Electrolyte Balance of BloodBody Fluids and Fluid Compartments:Of the hundreds of compounds present in your body, the most abundant is water.Males weighing 154 pounds will have an average of 60% of their body weight, nearly 40L, as water. Females about 50%. (based on nonobese individuals).The more fat present in the body, the less total water content per kg of body weight .Female body contains slightly less water per kg of weight because it contains slightly more fat than the male body.
In a newborn, water may account for up to 80% of body weight. That percentage increases if the infant is born premature.The percentage of body water decreases rapidly during the first 10 years of life.In elderly individuals, the amount of water per kg of body weight increases (because old ages is often accompanied by a decrease in muscle mass -65% water- and in increase in fat -20% water-)Water is the universal body solvent within which all solutes (including the very important electrolytes) are dissolved.*picture pg 619 body weight
Body Fluid Compartments:Total body water can be subdivided into two major fluid compartments called “extracellular” and “intracellular” fluid compartments.Extracellular: consists mainly of the liquid fraction of whole blood called the plasma, found in the blood vessels and the interstitial fluid that surrounds the cell. In addition, lymph, cerebrospinal fluid, humors of the eye, and the specialized joint fluids are also considered extracellular fluid.Intracellular: largest volume of water by far. Located inside of the cells.+diagram page 618
Mechanisms that maintain fluid balance3 sources of fluid intake: the liquids we drink, the water in the food we eat, and the water formed by catabolism of foods.Fluid output from the body occurs through four organs: the kidneys, lungs, skin, and intestines. The fluid output that changes the most is that from the kidneys.The body maintains fluid balance mainly by changing the volume of urine excreted to match changes in the volume of fluid intake
Regulation of Fluid IntakeWhen fluid loss from the body exceeds fluid intake, salivary excretion decreases, producing a “dry mouth” feeling, and the sensation of thirst. The individual then drinks water, thereby increasing fluid intake and compensating for previous fluid losses. This tends to restore fluid balance.Water is continually lost from the body through expired air and diffusion through the skin.Although the body adjusts fluid intake, factors that adjust fluid output, such as electrolytes and blood proteins, are far more important. (chart from yellow text!!!)
Balance betweentypical fluidintake and outputin a 70 kg adult.(Values are ml per24 hours.)
What are electrolytes?Electrolyte: substance that dissociates into ions in solution, rendering the solution capable of conducting an electric current.Electrolyte balance: homeostasis of electrolytes
The types and amounts of solutes in the body, especially electrolytes such as sodium, potassium, and calcium ions, are vitally important to overall homeostasis.Very small changes in electrolyte balance (solute concentrations in various fluid compartments) cause water to move from one fluid compartment to another. This alters blood volume and blood pressure, but it can also severely impair the activity of irritable cells like the nerve and muscle cell.Chart from text book!
Importance of Electrolytes in Body FluidsCompounds such as ordinary table salt, or sodium chloride (NaCl) that have molecular bonds that permit them to break up, or dissociate, in water solution to separate particles (Na+ and Cl-) are electrolytes. The dissociated particles of an electrolyte are ions and carry an electrical charge.Important positively charged ions include sodium (Na+), Calcium (Ca++), potassium (K+), and magnesium (Mg++). Important negatively charged ions include chloride (Cl-), bicarbonate (HCO3-), phosphate (HPO4-), and many proteins. Although blood plasma contains a number of important electrolytes, by far the most abundant one is sodium chloride (table salt).
A variety of electrolytes have important nutrient or regulatory roles in the body.For example, Iron required for hemoglobin production. Iodine must be available for synthesis of thyroid hormones.Electrolytes are also needed for many cellular activities such as nerve conduction and muscle contraction.
Electrolytes influence the movement of water among the three fluid compartments of the body.To remember how ECF electrolyte concentration affects fluid volumes, remember this one short sentence: “Where sodium goes, water soon follows”For example, concentration of sodium in interstitial fluid spaces rises above normal, the volume of IF soon reaches abnormal levels too (edema) which results in tissue swelling.
Reabsorption of water and electrolytes by the kidney is regulated primarily by hormones.When blood volume drops for any reason, (ie due to hemorrhage or excessive water loss sweating or diarrhea), arterial blood pressure drops, which in turn decreases amount of filtrate formed by kidneys. In addition, highty sensitive cells in the hypothalamus called somoreceptions react to the change in blood composition. (That is. Less water and more solutes.)
Sodium imbalance, potassium imbalance, calcium imbalance p 627 yellow book
Maintaining Fluid Homeostasis1. Overall fluid balance requires that fluid output equal fluid intake.2. The type of fluid output that changes most is urine volume.3. Renal tubule regulation of salt and water is the most important factor in determining urine volume.4. Aldosterone controls sodium reabsorption in the kidney.5. The present of sodium forces water to move (Where sodium goes, water soon follows).The aldosterone mechanism helps restore normal ECF volume when it decreases below normal.
The kidney acts as the chief regulator of sodium levels in body fluids.Many electrolytes such as sodium not only pass into and out of the body but also move back and forth between a number of body fluids during each 24 hour period.During this 24 hour period, more than 8 liters of fluid containing 1000 to 1300 mEq of sodium are poured into the digestive system as part of saliva, gastric secretions, bile, pancreatic juice, and IF secretions.This sodium is almost completely reabsorbed in the large intestine. Very little sodium is lost in the feces. Precise regulation and control of sodium levels are required for survival.Chart in yellow text
Capillary Blood Pressure and Blood ProteinsCapillary blood pressure = “water pushing” forceIf capillary blood pressure increases, more fluid is pushed (filtered) out of blood into the IF.This effect transfers fluid from blood to IF. This fluid shift changes blood and IF volumes.IT DECREASES BLOOD VOLUME BY INCREASING IF VOLUME.If, on the other hand, capillary blood pressure decreases, less fluid filters out of blood into IF.
Plasma proteins act as a water-pulling or water-holding force. They hold water in the blood and pull it into the blood from IF.e.g. if the concentration of proteins in blood decrease appreciably, less water moves into blood from IF. As a result, blood volume decreases and IF volume increases.Of the 3 main body fluids, IF volume varies the most.Plasma volume usually fluctuates only slightly and briefly. If a pronounced change in its volume occurs, adequate circulation cannot be maintained.
Fluid ImbalancesDehydration: seen most often. In this potentially dangerous condition, IF volume decreases first, but eventually, if treatment has not been given, ICF and plasma volumes also decrease below normal levels.Prolonged diarrhea or vomiting may result in dehydration due to the loss of body fluids. Loss of skin elasticity is a clinical sign of dehydration.Overhydration: less common than dehydration; giving intravenous fluids too rapidly or in too large of an amount can put too heavy a burden on the heart.
Sources Mader, S.S (2006) Inquiry into Life Marieb, E.N (2006) Essentials of Human Anatomy & Physiology http://www.kidney.ca/page.asp?intNodeID=22132