Mastering Renal Anatomy, Nephron Function, and Fluid Balance

Drew Bennett

Welcome to VNSG 1320 Anatomy and Physiology, the lectures condensed into podcast form. I'm Drew Bennett. Today we're covering the body's maintenance systems: the urinary system, fluids and electrolytes, the digestive system, and how nutrition ties into everything else. Now, here's the deal. Your kidneys filter about 180 liters of fluid out of your blood every day. That's roughly 60 times your total blood volume. And you pee out one to two liters.

Lizzie Stevens

So about 99 percent of what gets filtered goes right back into the blood. I'm Lizzie Stevens, and that number changed how I think about kidneys. They don't just make urine. They constantly decide what the body keeps and what it throws away. On the floor, that decision shows up as a number on a chart. If urine output drops below 30 mL an hour, that's a warning sign for poor kidney perfusion or acute kidney injury, and you need to speak up.

Drew Bennett

Let's build the machine first, starting with the layout. The kidneys are retroperitoneal, which means they sit behind the parietal peritoneum, against the back wall of the abdomen. They run from about T12 down to L3. The ribs and a thick cushion of fat protect them. The right kidney sits a little lower than the left because the liver takes up the space above it. Each one is about four inches long. The doorway is the hilum, a notch on the inner side where the renal artery, renal vein, nerves, and ureter come and go.

Lizzie Stevens

And inside, think crust and core. The outer region is the renal cortex, like a crust. The inner region is the medulla, which is divided into cone shaped renal pyramids. The tip of each pyramid is a renal papilla, and that's where urine drips out. Follow the urine with me: papilla, minor calyx, major calyx, renal pelvis, ureter. Then it goes down the ureter to the bladder.

Drew Bennett

The blood supply matters just as much. The renal artery branches straight off the abdominal aorta, and the renal vein drains into the inferior vena cava. More than 20 percent of the blood the heart pumps each minute goes to the kidneys. Yet the kidneys use only about 8 percent of the body's energy at rest. Compare that to skeletal muscle at 22 percent, the liver at 21, and the brain at 20. The kidneys get far more blood than they need for their own metabolism, because the blood isn't there to feed them. It's there to be cleaned.

Lizzie Stevens

So the working unit is the nephron. Each kidney has around 1.2 million of them. The blood arrives through an afferent arteriole. Afferent arrives. It branches into a tuft of capillaries called the glomerulus, and the blood leaves through the efferent arteriole. Efferent exits. Here's the neat part. The afferent vessel is wider than the efferent one, so blood comes in faster than it can leave. That creates pressure inside the glomerulus, and the pressure is what pushes fluid out.

Drew Bennett

The glomerulus sits inside Bowman's capsule. Your textbook gives an analogy I like. Push your fist into an inflated balloon. The balloon folds around your fist in two layers. Your fist is the glomerulus, and the balloon is Bowman's capsule. Fluid squeezes out of the capillary and collects in the space between the layers. Another way to picture it: the glomerulus is a coffee filter and Bowman's capsule is the cup catching the filtrate.

Lizzie Stevens

What gets through is water, electrolytes, glucose, amino acids, vitamins, and nitrogenous wastes. What stays behind is blood cells and most plasma proteins, because they're too big for the pores. Which gives us a nursing hook. If a urinalysis shows significant protein, or blood, that isn't normal. It tells you the filtration membrane has been compromised, because protein belongs in the bloodstream.

Drew Bennett

One more structural detail before we move on. About 85 percent of nephrons are cortical nephrons, sitting mostly in the cortex with short loops. The other 15 percent are juxtamedullary nephrons, with long loops that reach deep into the medulla. Those long loops are the ones that make concentrated urine possible. Hold onto that, because it explains how you can lose water and still hold onto what you need.

Lizzie Stevens

Urine formation has three steps. Your deck's memory hook is perfect: filter it, take back what we need, throw in anything else we want gone. So filtration, reabsorption, secretion. Filtration sends blood into the tubule. Reabsorption sends material from the tubule back into the blood. Secretion sends material from the blood into the tubule.

Drew Bennett

Filtration first. The normal glomerular filtration rate, or GFR, is about 125 mL a minute, which works out to roughly 180 liters a day. Your body wants that rate steady even when blood pressure wobbles. If the rate runs too high, you lose water and nutrients. If it runs too low, the tubules can reabsorb wastes you wanted gone. That steadiness is what the next system is protecting.

Lizzie Stevens

Then comes reabsorption, and the star is the proximal convoluted tubule. It's lined with microvilli, and it reclaims about 65 to 70 percent of the filtrate volume. Under normal conditions, 100 percent of the glucose and amino acids come back here, along with sodium, potassium, and bicarbonate. Sodium gets actively pumped out into the peritubular capillaries. And this is where our earlier foundations episode pays off. Active transport, the pump burning ATP, creates the gradient. Water follows the solute.

Drew Bennett

Water follows sodium, right. The loop of Henle uses that same rule in a clever way. In the descending limb, water moves out, so the filtrate gets more concentrated. In the ascending limb, sodium and chloride are pumped out, but the wall is thick and water can't follow. Descending, water disappears. Ascending, salt leaves and water stays put. That's how the loop helps the kidneys concentrate urine.

Lizzie Stevens

After that, the distal convoluted tubule and collecting duct fine tune things. They reabsorb variable amounts of water and salts, depending on hormones. Meanwhile secretion is going on. Hydrogen ions, ammonia, uric acid, and drugs like penicillin and aspirin get moved out of the blood and into the tubule. The kidney is also how you clear excess potassium. So when a patient's kidneys aren't working well, drug levels and potassium can both climb.

Drew Bennett

Now the control system, and I want you to not overcomplicate it. Low blood pressure reduces blood flow to the kidneys. Juxtaglomerular cells in the afferent arteriole release renin. Renin converts angiotensinogen, made by the liver, into angiotensin I. In the lungs, angiotensin converting enzyme, ACE, turns that into angiotensin II, a strong vasoconstrictor. Angiotensin II tells the adrenal cortex to release aldosterone.

Lizzie Stevens

And aldosterone saves salt. It acts on the distal tubule and collecting duct to reabsorb sodium, and water follows. Blood volume goes up, blood pressure goes up. Potassium gets excreted in exchange. Picture a patient with severe blood loss. Kidney perfusion drops, renin goes up, aldosterone goes up, sodium and water retention goes up, and blood pressure moves back toward normal. That's the chain your instructors will ask you to predict. And it's why medications that affect RAAS can change blood pressure, volume status, potassium, and kidney function.

Drew Bennett

There are three more hormones to know. ADH, from the posterior pituitary, tells the collecting duct to keep water. Less water leaves as urine, and the urine gets more concentrated. ANP comes from the atria of the heart when they're stretched by extra volume. It inhibits aldosterone and ADH, so you lose sodium and water. Away with sodium and pressure. And PTH, from the parathyroid glands, makes the tubules keep calcium while dumping phosphate.

Lizzie Stevens

Quick drill, since your deck asks it. A severely dehydrated patient. ADH goes up, water reabsorption goes up, urine volume goes down, urine concentration goes up. The body is conserving water. And while we're talking about what kidneys do beyond urine, they also make erythropoietin to drive red blood cell production, and they help activate vitamin D. So kidney trouble can show up as anemia or bone problems, not just urine changes.

Drew Bennett

Let's finish the plumbing. Ureters are smooth muscle tubes that push urine along with peristaltic waves from the renal pelvis to the bladder. The bladder wall is the detrusor muscle. Its lining has folds called rugae that flatten as it fills. On the floor is the trigone, a smooth triangle formed by the two ureter openings and the urethral opening. Infections commonly start there.

Lizzie Stevens

And the urethra is the big difference for UTI risk. In females it's only about 3 to 4 centimeters, and it exits just in front of the vagina. In males it's about 20 centimeters and passes through the prostate. A short tube means bacteria have a shorter trip to the bladder. Normal urine is about 95 percent water, pale yellow from urochrome, with urea, uric acid, ammonia, creatinine, and electrolytes. Output is one to two liters a day. Under 400 to 500 mL a day is oliguria. Under 50 is anuria.

Drew Bennett

Last piece is the micturition reflex, and it has a built in safety valve. At around 200 mL or more, stretch receptors send signals to the sacral spinal cord. The cord tells the detrusor to contract and the internal sphincter to relax. The internal sphincter is smooth muscle, involuntary. Internal is automatic. But the stretch signals also reach the pons, which weighs whether now is a good time. The external sphincter is skeletal muscle, voluntary, so it stays clamped until you decide otherwise. External is under your control.

Lizzie Stevens

Now let's put the kidney's work in context. Roughly 65 percent of your body water is intracellular fluid, inside the cells. About 35 percent is extracellular fluid. Most of that is interstitial fluid between cells, with plasma making up about a fifth of it. There's also transcellular fluid, like cerebrospinal fluid and synovial fluid. Total body water is around 55 to 60 percent of adult body weight.

Drew Bennett

And remember the rule from our foundations episode. Water follows solute. If the solute outside the cell rises, water leaves the cell and it shrinks. If the solute outside falls, water enters and the cell swells. Hypertonic shrinks, hypotonic swells. It happens within seconds. So when we say sodium problems are really water problems, this is why.

Lizzie Stevens

That's where intake and output earns its place. An adult takes in about 2,500 mL a day: roughly 1,600 from drinking, 700 from food, and 200 as metabolic water. Output is about 1,500 through urine, 500 through skin, 300 through breathing, and 200 through stool. When I&O is more than writing down numbers, it can reveal a fluid deficit, a fluid overload, kidney dysfunction, or a change in perfusion. And elderly adults have a diminished thirst sensation, so they can slide into trouble without feeling thirsty.

Drew Bennett

Two terms that students mix up. Volume depletion is losing water and sodium together, like blood loss. Osmolarity stays about the same, but circulating volume drops. Dehydration is losing more water than sodium, so the remaining extracellular fluid is more concentrated and water pulls out of cells. Check skin turgor. Tenting suggests fluid loss. And edema is a different thing: fluid collecting in the interstitial spaces. A patient can have edema and still have poor circulating volume.

Lizzie Stevens

Sodium first. Normal is 135 to 145 mEq per liter, and it's the major extracellular cation. Hyponatremia, under 135, means too much water relative to sodium, and water rushes into brain cells. That means cerebral edema, confusion, and seizures. Hypernatremia, over 145, dehydrates cells and brings neurological agitation. Aldosterone and ADH keep it in range.

Drew Bennett

Potassium is the major intracellular cation, and your deck gives a normal range of 3.5 to 5.3. Some references use 5.0 as the upper limit, so always go by your facility's reference range. The rule on the slide is simple: when potassium is significantly abnormal, think heart. Hyperkalemia can come from crush injuries, severe burns, kidney failure, or potassium sparing diuretics. Hypokalemia often comes from potassium wasting diuretics, vomiting, or chronic diarrhea. Either one can cause life threatening arrhythmias.

Lizzie Stevens

So a patient with three days of severe vomiting and diarrhea. Potassium should be on your radar right away. Calcium too, with a normal of 8.2 to 10.2 mg per dL. Low calcium makes nerves and muscles twitchy, with spasm and tetany, and Chvostek's and Trousseau's signs. High calcium gives weakness, lethargy, and kidney stones. Remember that muscles need calcium to contract and bones store it.

Drew Bennett

Now the pH. The normal window is 7.35 to 7.45. Below 7.35 is acidosis, above 7.45 alkalosis. Lizzie, remind them what happens when it slips.

Lizzie Stevens

I'll never forget that NICU baby whose pH fell to 7.21. The cellular enzymes stopped working properly, because proteins and enzymes are built to function in that narrow window. That's why the body has three lines of defense. First, chemical buffers, working in seconds. The bicarbonate system is the main one, with phosphate and protein buffers too. Second, the lungs, working in minutes. Third, the kidneys, working over hours to days.

Drew Bennett

Here's the money rule for the lungs. Carbon dioxide acts like an acid. It combines with water to make carbonic acid, which releases hydrogen ions. So carbon dioxide up means pH down, and carbon dioxide down means pH up. A hypoventilating patient retains carbon dioxide and drifts toward respiratory acidosis. Hyperventilation blows it off and gives respiratory alkalosis. Respiratory problems are about carbon dioxide. Metabolic problems are about bicarbonate or non respiratory acids.

Lizzie Stevens

And the kidneys are the slow but powerful finishers. They're the only buffer system that actually expels hydrogen ions from the body, and they reabsorb bicarbonate. That's the long term fix. Metabolic acidosis can come from ketones in diabetes, lactic acid, kidneys that can't excrete hydrogen, or bicarbonate lost with chronic diarrhea. Metabolic alkalosis comes from vomiting, gastric suction, or excess antacids. Test friendly version: diarrhea gives acidosis, vomiting gives alkalosis.

Drew Bennett

One more link back to potassium. In acidosis, hydrogen moves into cells and potassium shifts out, so acidosis can raise serum potassium. In alkalosis, potassium moves into cells and you can get hypokalemia. Acid up, potassium out. Acid down, potassium in. So a sick patient's acid base status and potassium level are tied together.

Drew Bennett

Now the digestive system, which affects nutrition, hydration, electrolytes, medication absorption, blood glucose, bowel function, and healing. Big picture: it breaks food down, moves it along, absorbs what the body needs, and eliminates the rest. The GI tract, or alimentary canal, is one continuous tube: mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus. The accessory organs are teeth, tongue, salivary glands, liver, gallbladder, and pancreas. Food travels through the tract. The accessory organs help digestion happen.

Lizzie Stevens

The wall of that tube has four layers from the inside out. Mucosa meets the meal: it's the innermost layer, in direct contact with the contents, and it secretes mucus, enzymes, and hormones, absorbs nutrients, and holds lymphoid tissue. Submucosa supports it, with blood vessels, lymphatics, glands, and nerves. Muscularis moves it. Serosa surrounds it, and in the esophagus and rectum that outer covering is called adventitia.

Drew Bennett

Here's the exam trap. If the question asks which layer propels food, it's the muscularis. It has an inner circular layer and an outer longitudinal layer of smooth muscle. Not mucosa, not submucosa, not serosa. Tucked between the muscle layers is the myenteric nerve plexus, and the submucosal plexus sits in the submucosa. Together they form the enteric nervous system, with some 500 million neurons and more than 30 neurotransmitters, many identical to the brain's.

Lizzie Stevens

People call it the second brain, and it can run motility, secretions, and blood flow even if it's disconnected from the central nervous system. But it talks both ways with the brain, and that conversation may influence mood and behavior. So when someone says their stomach has opinions, there's real neurology behind it.

Drew Bennett

Wrapping around all this is the peritoneum. Folds called mesenteries suspend the digestive organs and carry their blood vessels, nerves, and lymphatics. The greater omentum hangs from the greater curvature of the stomach over the small intestines like a fatty apron. The lesser omentum runs from the lesser curvature to the liver. The mesocolon fixes the colon to the back wall.

Lizzie Stevens

Digestion starts in the mouth, before the stomach. Mechanical digestion is chewing: 32 adult teeth, with incisors cutting, canines tearing, and premolars and molars crushing and grinding. Chemical digestion is saliva, from three pairs of glands. The parotid is in front of the ear, and the mumps virus swells it. The submandibular empties beside the lingual frenulum, and the sublingual drains onto the floor of the mouth. Saliva carries amylase, which starts breaking down starch, and lipase, which starts on fat.

Drew Bennett

Swallowing is a coordinated safety event. The tongue pushes the bolus to the back of the mouth. The soft palate lifts and closes off the nasopharynx. The larynx rises, which tips the epiglottis over the entrance to the trachea, and food goes toward the esophagus instead of the airway. When swallowing goes wrong, that's aspiration risk. Then the esophagus moves the bolus by peristalsis, and the lower esophageal sphincter acts like a gate. If it doesn't stay closed, acid backs up. That's reflux, the anatomy behind GERD.

Lizzie Stevens

The stomach stores, mixes, and digests. It has four regions: cardia, fundus, body, and pylorus, and the pyloric sphincter controls the exit into the duodenum. Its muscularis has an extra oblique layer between the circular and longitudinal ones, so it can churn vigorously. And rugae, the folds in its lining, flatten as food arrives. Rugae equal room to grow.

Drew Bennett

The stomach has to digest food without digesting itself, and the gastric pits are how it manages. Mucous cells make protective mucus. Parietal cells make hydrochloric acid, which kills microbes in swallowed food, and intrinsic factor. Chief cells make pepsinogen, which acid converts to pepsin for protein breakdown. Enteroendocrine cells release gastrin, which influences digestive function, and ghrelin, which stimulates appetite.

Lizzie Stevens

And intrinsic factor is the clinical gold. You need it to absorb vitamin B12, which is taken up in the terminal ileum. Damage the parietal cells and you lose intrinsic factor, B12 absorption fails, and red blood cell production suffers. That's pernicious anemia. So the health of those stomach cells ends up in the red blood cell count.

Drew Bennett

Gastric secretion happens in three phases. Cephalic: the sight, smell, taste, or even thought of food sends signals through the brainstem, and the parasympathetic system starts gastric juice and gastrin. Gastric: food stretches the stomach, and secretion ramps up. Intestinal: chyme enters the duodenum, and nerve signals and hormones inhibit gastric secretion. After the stomach mixes food into semifluid chyme, about 30 mL squirts through the pyloric sphincter with each contraction. Complete emptying takes about four hours after a typical meal.

Lizzie Stevens

Now the helpers. The liver is the body's largest gland, with more than 250 tasks: storing and releasing glucose, processing vitamins and minerals, filtering toxins, recycling old blood cells. It gets two blood supplies. The portal vein brings oxygen poor but nutrient rich blood from the digestive organs. The hepatic artery brings oxygenated blood from the aorta. In each lobule the blood flows through sinusoids, the filtering spaces, where Kupffer cells, the cleanup crew, remove bacteria, worn out red cells, and debris. The central vein carries processed blood out, and canaliculi carry bile toward the hepatic ducts.

Drew Bennett

And the line that gets tested constantly: the liver makes bile, the gallbladder stores it. The gallbladder holds only 30 to 50 mL, but it concentrates it. Bile helps with fat through emulsification, breaking big fat clumps into small droplets so lipase can work. The pancreas supplies the rest. Its acinar cells make digestive enzymes in inactive form, which activate in the duodenum, and its duct cells secrete sodium bicarbonate to buffer the acidic chyme. About 1.5 liters of pancreatic juice a day.

Lizzie Stevens

So the small intestine is where the action is. The duodenum, the first 10 inches, takes chyme, bile, and pancreatic juice. D is for digest. The jejunum, about 8 feet, is for just absorbing. The ileum, about 12 feet, has Peyer's patches for immune surveillance and absorbs B12 and bile salts. Circular folds, villi, and microvilli multiply the surface area enormously. More surface area, more chances to absorb.

Drew Bennett

Two kinds of contractions work here. Segmentation stirs: ring like constrictions that knead and mix food with digestive juices. Peristalsis pushes: wavelike ripples that move residue along. If the stem says mixes, think segmentation. If it says propels, think peristalsis. Inside each villus are blood vessels and a lymph vessel called a lacteal, because nutrients take different routes.

Lizzie Stevens

Carbs start in the mouth with amylase, resume with pancreatic amylase, and finish at the villi with sucrase, lactase, and maltase, producing glucose that's absorbed right away. Protein starts in the stomach with pepsin, continues in the duodenum with trypsin and chymotrypsin, and finishes with peptidases making amino acids. Fat needs bile first, then pancreatic lipase. Glycerol and short chain fatty acids go into the villus blood. Long chain fatty acids get rebuilt into triglycerides and enter the lacteal, travelling through lymph to the bloodstream at the left subclavian vein.

Drew Bennett

Then the large intestine, which recovers water and forms feces. The path is ascending, transverse, descending, sigmoid, into the rectum and anal canal. The ileocecal valve keeps material moving forward. The appendix hangs off the cecum and holds lymphoid tissue. The anus has an internal sphincter, involuntary, and an external one, voluntary. Gut bacteria here ferment fiber, and the brief for this episode adds they help make vitamin K for clotting and some B vitamins.

Lizzie Stevens

Which brings us to the nursing consequence. When digestion fails, absorption fails. You see weight loss, weakness, vitamin deficiencies, poor wound healing, and stool changes. And persistent diarrhea costs potassium and bicarbonate, giving hypokalemia and metabolic acidosis. Persistent vomiting costs stomach acid, and you get metabolic alkalosis. Follow the food, know who makes what, know where absorption happens.

Drew Bennett

Nutrition and metabolism connect every system we've covered. Nutrition affects energy, healing, muscle mass, immunity, weight, blood glucose, hydration, and recovery. Start with basal metabolic rate, the energy the body needs at rest. What does it cost just to keep you alive? Heartbeat, breathing, temperature, brain, kidneys, cell maintenance. A calorie is the heat needed to raise one gram of water by one degree Celsius.

Lizzie Stevens

More muscle means more energy used, and metabolism generally falls with age. Fever, anxiety, eating itself, and thyroid hormones all push the rate up. Appetite is hormonal. Ghrelin from the stomach says go eat. Leptin from fat tissue says leave the food alone, and people with obesity can build resistance to it. PYY, insulin, and CCK also signal satiety, and your deck's quiz says only ghrelin stimulates appetite.

Drew Bennett

Macronutrients first. Carbohydrates are the primary energy source, 40 to 50 percent of calories, and all of them end up as glucose. Fiber resists digestion, adds stool bulk, and gut bacteria ferment it into short chain fatty acids. Those nourish the bowel lining, lower bowel pH, and calm inflammation. Most Americans don't get enough.

Lizzie Stevens

Lipids store energy, insulate, help absorb certain vitamins, and build cell structure. Fat gives about nine kilocalories per gram, versus four for carbohydrate and protein. But rapid fat breakdown makes too many acetyl CoA units, and the liver packages them as ketone bodies. Pile up enough and you get ketoacidosis, a pH imbalance. That's the diabetic emergency tying metabolism back to acid base and the kidneys.

Drew Bennett

Protein is construction material. Over 100,000 different proteins are built from just 20 amino acids. Complete proteins have all the essential amino acids, mostly from animal sources. Incomplete ones lack one or more, mostly from plants, but the body can combine complementary proteins over a day. A nonessential amino acid is one the body can make. It doesn't mean the body doesn't need it. And protein breakdown makes ammonia, which the liver converts to urea, and the kidneys have to clear.

Lizzie Stevens

Vitamins split by solubility. Water soluble ones, the B vitamins and vitamin C, are absorbed with water, excreted by the kidneys, and needed daily. Fat soluble ones, A, D, E, and K, are absorbed with dietary fat and stored in liver and fat tissue, so megadosing can cause toxicity. Minerals matter too: calcium and phosphorus are the most abundant, iron builds hemoglobin, and iodine feeds the thyroid.

Drew Bennett

Metabolism is catabolism, cutting apart, and anabolism, assembling. For glucose, the story is simple. Glucose plus oxygen gives lots of energy. Glycolysis splits glucose into two pyruvic acid molecules without oxygen. If oxygen is short, pyruvic acid becomes lactic acid, which travels to the liver, giving less energy. With oxygen, pyruvic acid enters the mitochondria and becomes acetyl CoA, feeding the citric acid cycle and electron transport chain, ending in carbon dioxide, water, and a lot of ATP.

Lizzie Stevens

Last piece is heat. Most body heat is a byproduct of cell chemistry, and the hypothalamus is the thermostat. Too hot: vessels dilate and you sweat. Too cold: vessels constrict and you shiver. Heat leaves by radiation, conduction, and evaporation. Hot equals dilate and sweat. Cold equals constrict and shiver.

Drew Bennett

So pull it together. Nutrition supplies the electrolytes, amino acids, and water. The gut absorbs them. The cardiovascular system circulates that volume. And the kidneys filter it, balancing volume and electrolytes to protect preload and perfusion. Metabolism then hands the kidneys urea from protein and ketones from fat, and the buffers have to cope.

Lizzie Stevens

If you remember nothing else, remember the mantra from your deck: follow the water, watch the potassium, track the carbon dioxide. Watch output under 30 mL an hour. Watch what a patient's vomiting or diarrhea is costing them. The body is constantly balancing, and you're there to notice when it can't.