A&p Ii - Urinary System

Front 1

The urinary system is composed of:

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* 2 kidneys which form urine
• 2 ureters—tubes carrying urine from the kidneys to the bladder
• urinary bladder—where the urine is stored
• urethra—which takes the urine from the bladder out of the body

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Functions of the urinary system:

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--to remove excess salts and nitrogenous wastes
--maintains water and electrolyte balance
--helps maintain acid/base balance

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Gross Anatomy of the Kidneys:

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The kidneys are reddish-brown organs with a concave medial portion. They are retroperitoneal, located fairly high in the lumbar region. Left kidney is slightly higher (2-3 cm.) than the right.

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renal hilus

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The indented area on the medial side of each kidney. . It is where the renal artery, renal vein, and ureter come out.

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renal capsule

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The outermost covering of the kidney.

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renal pelvis

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There is an expanded funnel shape portion where the urethra first goes into the kidney.

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major calyxes

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There are 2 or 3 branches from the renal pelvis.

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minor calyces

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Each major calyx will have several of these.

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renal pyramids

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The cone shaped masses of tissue at the end of the minor calyx.

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renal medulla

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Combine all the renal pyramids together to form this.

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renal cortex

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outside the medulla

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renal columns

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. Between the pyramids are cortex-like tissues, but these can’t be called cortex because they are deep between the pyramids.

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renal papilla

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The tip of each pyramid fits into a minor calyx. Pierced by a number of holes or openings, and what comes out of these openings is fully formed urine.

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renal artery

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Most kidneys are fed by a single one of these.

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interlobar arteries

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Renal artery comes into the hilus and branches into these.

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arcuate arteries

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Interlobar arteries travel through the renal columns and go to the base of the pyramids, and branch to form these.

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interlobular arteries (also called cortical radiate arteries)

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Arising from the arcuates arteries are these, which travel into the cortex.

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afferent arterioles

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Interlobular arteries give rise to these.

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nephrons

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Afferent arterioles lead to the these. They are microscopic, and are the actual urine-forming structures. Each kidney has ~ 1,000,000.

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glomerulus

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The afferent arteriole give rise to an extensive network of capillaries called this. From them blood enters an efferent arteriole (vs. a venule.) What is unusual about them is that, unlike other capillaries which are between an arteriole and a venule, they are between two arterioles. Efferent arterioles are smaller than the afferents.

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Bowman’s capsule

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Each glomerulus is surrounded by a cup-like structure, which is formed of simple squamous epithelium.

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proximal convoluted tubule (PCT)

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Bowman’s capsule opens into this. Composed of simple cuboidal epithelium with extensive microvilli. This is the only place in nephron with microvilli.

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loop of Henle

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From the proximal tubule, we go into this, which is divided into 2 portions—there is the descending limb (then a hairpin turn) and the ascending limb. The thin segment is a change of epithelium—simple squamous. The thick parts are simple cuboidal without microvilli. There is variation in the stop-point of the thin segment, but always some part of the descending limb will be thin. Once past the thin segment, the epithelium of the nephron is simple cuboidal without significant microvilli.

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distal convoluted tubule (DCT)

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From the ascending limb, go into this and then goes into a collecting duct.

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single collecting duct

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Several nephrons open into this.

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Where do the nephrons fit into the kidney?

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The loop of Henle is in the medulla. The collecting duct travels through the medulla, terminating at the renal papilla. and everything else (glomerulus, Bowman’s capsule, PCT, DCT) is in the cortex. The holes in the papilla are ends of collecting ducts.

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peritubular capillary system

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The efferent arteriole gives rise to this. It is an extensive network of capillaries surrounding all parts of the nephron (except the glomerulus).

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Vasa recta

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Refers to the peritubular capillaries in the medulla (those around the loop of Henle and collecting duct).

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juxtoglomerular complex

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There is a place where part of the distal tubule lies against the afferent arteriole, and where they touch forms this. It has cells, which monitor renal blood pressure and release renin.

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Urine formation in the kidney (nephrons) requires 3 processes:

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1. glomerular filtration—the movement of fluid out of the capillaries of the glomerulus and into Bowman’s capsule. This is similar to filtration of fluid out of other capillaries. So the fluid leaks out and goes into Bowman’s capsule. This fluid is called glomerular filtrate—just like what becomes interstitial fluid anywhere else. No RBCs, few plasma proteins, but has all the other stuff. So as move fluid through nephron system, it has to be changed significantly.
2. Reabsorption—is the movement of substances out of the nephron and into the peritubular capillaries (to save the good stuff). Anything that moved out of nephron stays in the body—reabsorb water, glucose, and some salts. But there are some substances we want to eliminate (e.g.—hydrogen ions {acids}). This is where…
3. Secretion is important. This is the movement of substances from peritubular capillaries and interstitial fluid into the nephron (concentrating the bad stuff) for elimination. Some will get absorbed as move through the nephron, but more occurs with secretion.

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Glomerular Filtration Rates (GFR) in ml/min or liters/day

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How much fluid leaves glomerulus and enters Bowman’s capsule and how fast. Body can regulate diameter of the arterioles. GFR will be directly proportional to the pressure inside the glomerulus. Increased fluid  increased pressure. Pressure in glomerulus tends to be higher than that of other capillaries. Glomerular pressure and this tend to remain fairly stable even over a wide range of overall blood pressure.

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Glomerular Filtration Rates (GFR) would go up a little but not significantly with increased BP. How is this working?

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If normal and BP drops a little, can detect and regulate right at the glomerulus and will then constrict the efferent arteries and dilate the afferent. This will regulate GFR. It works around systemic pressures. Auto-regulation if there is an increase in BP. Constrict afferents and control. What kidneys can’t compensate for is a significant decrease in BP as in that occurring with cardiovascular shock. No supplying pressure at afferent and GFR drops dramatically. This is why kidney failure is a big issue with cardiovascular shock.

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Average Adult Glomerular Filtration Rates (GFR)

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125ml/min or 180 liters/day (~45 gallons). Kidneys receive ~20% of cardiac output. All of the body’s plasma filters through the kidney ~60x/day. So there is significant reabsorption.

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hypertonic envirnoment

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We need to have a hypertonic urine. Get rid of waste while preserving water. This is tricky in terms of the body and osmotic balance. Nephron is a nifty device for this because usually to balance hypertonic situation, water from body will rush to it.

So, the interstitial fluid of the renal medulla is extremely hypertonic. Most hypertonic of the body. Solutes here are about 400% more concentrated than in other extracellular fluids. How this occurs will be seen in the process of urine formation.

So recall that as the glomerular filtrate enters Bowman’s capsule from the glomerulus, this fluid is essentially identical to fluid filtering out of capillaries elsewhere. There should not be any RBCs and very little plasma protein in the filtrate, but the filtrate contains other plasma components. As the filtrate passes through the nephron and collecting duct, we need to reabsorb and save the “good stuff” (water, glucose, bicarb ions, some electrolytes, etc) and concentrate (by renal secretion) and eliminate the “bad stuff” (urea, hydrogen ions, excess salts, etc). It is essential that the kidneys save water and produce a hypertonic urine, as approx 180 liters of fluid pass through the kidney each day.

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formation of a hypertonic urine

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1. Proximal convoluted tubule—Na+ ions are actively pumped out and reabsorbed. Cl- follows passively. H2O will follow because of [ ] gradient—osmosis. So the volume of filtrate is greatly reduced (by 75%). Microvilli add to the surface area. Glucose is also actively reabsorbed here. Did not change the overall [ ] gradient which is still ~ 300 milliosmoles/liter because of the water going out.
2. Descending limb of the loop of Henle—Here we are going deeper into the nephron and so the interstitial fluid of the medulla is more concentrated. More hypertonic. Na+ , Cl- and urea diffuse into the descending limb because the interstitial fluid is more concentrated and water moves out by osmosis. The volume further decreases and the concentration of the filtrate greatly increases. So by the bottom of the loop of Henle, filtrate is very concentrated.
3. Ascending loop of Henle—is completely impermeable to H2O (by virtue of lipid basal membrane). So volume won’t change. NaCl and urea are actively pumped out. So now the filtrate will decrease in [ ] and so the filtrate is back to isotonic (or even hypotonic) by time it reaches the distal tubules. These ions will diffuse across the loop of Henle—through the interstitial space—to enter the descending loop.
4. Distal convoluted tubule—back to an isotonic state. here there is a fine-tuning of the solutes in the filtrate depending on the body’s needs. Usually (under influence of aldosterone) NaCl is reabsorbed to the body and more H2O follows. As Na moves out, K+ moves in (is secreted). Bicarbonate is reabsorbed, and hydrogen ions are secretes. Many organic substances (e.g histamine, penicillin, etc) are secreted. The pH is dropping. Thus, volume is further decreased and the overall chemical composition of the filtrate is altered, but the [ ] remains the same.
5. Collecting Duct. Here is where the final concentration of urine occurs. Recall that the collecting ducts are passing through the increasingly hypertonic interstitial fluid of the medulla. The permeability of the collecting ducts to water is highly variable and is under direct control of the neurohypophyseal hormone ADH. The higher the ADH, the more permeable are the collecting ducts to water. Water leaves the collecting duct, enters the vasa recta and is saved. This is why the loops of Henle cycle ions in order to keep the surrounding interstitial fluid hypertonic. By the time the filtrate leaves the collecting ducts at the renal papillae and enters a minor calyx, it is fully formed urine.

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Effect of Diuretics

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Decrease the activity of the sodium pump in the tubules.

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normal pH of the body

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pH 7.35 to 7.45

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acidosis

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If pH drops to 7.0, which will result in death if not immediately corrected.

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alkalosis

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If the pH reaches 7.8, which will result in death if not immediately corrected.

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H+ in solutions

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HCl → H+ + Cl-
H2CO3 →← H+ + CO3-

Similarly, strong bases have great affinity for H+, strongly binding them while weak bases have a lesser attraction to H+. Thus, ultimately, maintaining acid/base balance requires the control of hydrogen ion concentrations.

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Sources of H+

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1. the primary source of H+ in the body is an indirect outcome of cellular respiration and the resulting production of CO2. Recall the following equation discussed in the transport of blood gases: CO2 + H2O → H2CO3 → H+ + HCO3-

therefore, the more CO2 added to the overall system, the greater the [H+] and vice versa. This is why interrupted breathing will result in acidosis (respiratory acidosis) whereas hyperventilation will cause alkalosis.

2. Another source of H+ is from the incomplete oxidation of fatty acids and the resulting formation of acidic ketones. while some ketones are produced normally all the time, conditions of starvation, crash diets, and/or lack of insulin will cause significant increasing amounts of ketones, and increasing acidosis (metabolic acidosis). There are also some other minor sources of H+ (mainly diet related).

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Control of pH:

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there are 3 mechanisms by which pH is controlled: 1. Buffer systems, 2. Respiratory control, and 3. Renal control.

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1. buffer systems:

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buffers act to stabilize pH and prevent wide swings in pH over relatively short periods of time. Most buffer systems are composed of a combination of a weak acid and a weak base. When a strong acid is added, it interacts with the weak base, and the strong acid is converted into a weak acid. Similarly strong bases are converted into weak bases. Let’s look more closely at one of the 3 buffers acting in the body ( and the one whose components we are most familiar with), the bicarbonate buffer system. This buffer is composed of the weak acid—carbonic acid (H2CO3) and the weak base—sodium bicarbonate (NaHCO3). Remember that these will each partially dissociate, so that we will have the following ions present:

weak acid: H2CO3 H+ + HCO3-
weak base: NaHCO3 Na+ + HCO3-

If HCl is added, it interacts with weak base as follows:

{HCl  H+ + Cl-} + {NaHCO3 Na+ + HCO3}  H2CO3 + NaCl

The effect is to turn the strong HCl acid into weak carbonic acid. Buffers act immediately to resist pH changes.

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2. Respiratory control:

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Breathing rates affect pH by controlling P CO2. Refer back to the section regarding cellular respiration (under sources of H+) on the previous page for explanation. Altered breathing rates affect pH in a few minutes.

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3. Renal control:

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The kidneys help control pH by secreting hydrogen ions and reabsorbing bicarbonate ions (primarily at the distal tubules). It takes the kidneys several hours up to a few days to effectively alter pH.

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Electrolytes

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Charged particles (ions). Among the most important are sodium (Na+), potassium (K+), calcium (Ca++), magnesium (Mg++), chloride (Cl-), phosphates (PO4---), bicarbonate (HCO3-), and sulfate (SO3---). These solutes are dissolved in water. Most are obtained in food, and are lost via urine, sweat, and feces. Please note the close relationship between water balance and this balance; for example, if one was dehydrated, the effective concentration of the these is increased. This explains why diarrhea can be lethal, and world-wide is the leading proximate cause of death (there are many reasons or causes of diarrhea).

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Water Intake

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Controlled by a thirst center in the hypothalamus. Water is lost through urine , feces, sweat, and evaporation from the lungs. The hormone ADH is the primary regulator of water balance.

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2 Types of Body Fluids

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About 2/3 of all the fluid is found within the cells. This is the intracellular fluid (ICF). The remaining 1/3 is found outside the cells, this is the extracellular fluid (ECF).

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extracellular fluid (ECF)

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ECF has several components including plasma, interstitial fluid, cerebrospinal fluid, lymph serous (pericardial, etc), synovial fluid, and the humors of the eye. All of the ECFs are quite similar. Plasma is the most distinct, in that it contains more protein than the other ECFs. All ECFs contain rather high concentrations of Na+, Cl-, and HCO3-, and are low in K+, Ca++, and Mg++. ICF has high concentrations of K+, PO4---, Mg++, and protein (much more than even plasma) and is low in Na+, Cl-, and HCO3-.

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Control of sodium and potassium:

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sodium is the most abundant positive extracellular electrolyte. Concentration is controlled primarily by the levels of the hormone aldosterone, which stimulates renal reabsorption of Na+ and renal secretion of K+. thus this hormone also controls potassium levels. Atrial natriuretic factor (ANF) also helps regulate Na and K; recall that its effects are the opposite of aldosterone.

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Control of Ca:

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the hormone PTH is the primary regulator of Ca++. PTH promotes osteoclast activity, increases Ca absorption from the small intestine, and increase renal Ca reabsorption (& renal secretion of phosphates). The effect is to increase [Ca] and decrease [PO4---]. Calcitonin also controls Ca and phosphates by stimulating osteoblast activity, decreasing serum Ca and phosphate.

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Control of chlorides:

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chlorides are passively controlled. They primarily follow sodium due to the attraction of opposite electrical charges.