Kidney Functional Anatomy and Nephron Structure

Flashcards covering the functional anatomy of the kidneys, including gross and microscopic structures, and the basic components and functions of the nephron.

Kidneys

Excretory organs that process and condition blood plasma, remove waste and unwanted substances, and maintain homeostasis of the body fluid environment.

Retroperitoneal organs

A classification for organs like the kidneys, meaning they are located on the posterior abdominal wall behind the peritoneal membrane.

Hilum

The medial concave border of the kidney where blood vessels and nerves enter.

Renal Cortex

The outer layer of the kidney.

Renal Medulla

The inner layer of the kidney.

Ureter

A tube that carries urine from each kidney to the bladder.

Nephron

The basic functional unit of the kidney, responsible for filtering blood plasma and forming urine.

Bowman's capsule

A hollow, double-walled epithelial sphere that surrounds the glomerulus and marks the beginning of the proximal tubule.

Glomerulus

A dense tuft of fenestrated capillaries within Bowman's capsule, highly permeable to plasma constituents for filtration.

Renal corpuscle

The collective term for the glomerulus and its surrounding double-walled Bowman's capsule.

Glomerular filtration membrane

The structure formed by the visceral layer of Bowman's capsule and fenestrated glomerular capillary endothelium, where the first step of urine formation occurs.

Proximal convoluted tubule

The curved, twisted tubule draining filtrate from Bowman's capsule, lined with microvilli for reabsorption.

Loop of Henle

A part of the nephron consisting of a straight descending limb, a sharp hairpin turn, and an ascending limb.

Cortical nephrons

Nephrons located in the outer renal cortex with short loops of Henle that do not extend deep into the medulla.

Juxtamedullary nephrons

Nephrons located deeper in the cortex near the medullary border, possessing long loops of Henle that penetrate deeply into the medulla.

Distal convoluted tubule

The tubule section located beyond or distal to the loop of Henle, residing in the renal cortex.

Collecting duct

A long, straight tubule that connects with the distal tubules of several nephrons and extends deep into the medulla.

Peritubular capillaries

A capillary network branching from the efferent arteriole that closely surrounds the proximal and distal convoluted tubules, functioning primarily in reabsorption.

Vasa recta

A branch of the peritubular circulation that surrounds the loop of Henle, receiving only a small percentage of renal blood flow.

Waste substances eliminated by kidneys

Urea, creatinine, uric acids, bilirubin, metabolites of assorted hormones, various toxins, and foreign substances.

Macula densa

Densely packed distal tubular cells located where the distal convoluted tubule touches the afferent and efferent arterioles.

Juxtaglomerular cells

Smooth muscle cuffs of the afferent and efferent arterioles, which are part of the juxtaglomerular apparatus.

Juxtaglomerular apparatus

A collective term for the macula densa and juxtaglomerular cells, which secretes renin when systemic blood pressure decreases.

Renin

An enzyme secreted by the juxtaglomerular apparatus when systemic blood pressure decreases. It activates angiotensin, leading to widespread systemic arteriole constriction.

Substances cleared by nephrons in excess quantities

Sodium, potassium, chloride, and hydrogen ions.

Filtrate (renal)

The renamed blood plasma that passes through the glomerular membrane filter into the proximal tubules, representing about 20\% of renal blood plasma.

Basic functions of the nephron

1. Filtration: Blood plasma passes through the glomerular membrane filter into the proximal tubules.2. Reabsorption: Most of the filtrate's water, electrolytes, and all glucose are returned to the bloodstream from the tubules by peritubular capillaries.3. Secretion: Certain substances are actively transported directly from the nephron tubule walls into the filtrate for excretion.4. Excretion: The remaining filtrate (water and waste products) is released from the body as urine.

Renal filtration

The initial step of nephron function where approximately 20\% of renal blood plasma passes through the glomerular membrane filter into the proximal tubules, forming filtrate.

Renal reabsorption

The process by which the peritubular capillaries surrounding the proximal tubules reabsorb most of the proximal tubule filtrate’s water and electrolytes, and all of the glucose, returning them to the bloodstream.

Renal secretion

The active process where nephron tubule walls actively secrete certain substances directly into the filtrate for excretion, meaning urine contains both filtered and secreted substances.

Renal Excretion

The final step of nephron function where the remaining filtrate (water and waste products) is released from the body as urine.

Homeostatic function of kidneys

The kidneys' role in maintaining the balance of the body's internal fluid environment through processing blood plasma and removing waste.

Filtration (renal definition)

The movement of water and solutes from the plasma in the glomerulus, across the glomerular membrane, and into Bowman’s capsule.

What causes renal filtration?

A pressure gradient between glomerular capillary blood and the capsular filtrate.

Main factor establishing filtration pressure

The hydrostatic pressure of the glomerular blood, which is about 60\ mm\ Hg.

Major cause of high glomerular hydrostatic pressure

The high outflow resistance of the efferent arteriole.

Forces opposing renal filtration

Blood osmotic pressure and capsular hydrostatic pressure.

Net Filtration Pressure (NFP) equation

60\ mm\ Hg\ -\ (18\ mm\ Hg\ +\ 32\ mm\ Hg)\ =\ 10\ mm\ Hg

Where:

• 60\ mm\ Hg = Glomerular hydrostatic pressure
• 18\ mm\ Hg = Capsular hydrostatic pressure
• 32\ mm\ Hg = Glomerular osmotic pressure

Reason for relatively high osmotic pressure in glomerular blood (32\ mm\ Hg)

The process of filtration concentrates the blood, as water is filtered out, leaving plasma proteins behind.

Why does glomerular filtration occur much more rapidly than filtration in other tissue capillaries?

1. The glomerular membrane is about 100 to 500 times more permeable than other capillaries.
2. Glomerular hydrostatic pressure is much higher than in other capillaries.

Glomerular Filtration Rate (GFR)

The rate at which filtrate is produced, typically about 125\ mL/min with a normal filtration pressure of 10\ mm\ Hg.

Daily filtrate production

About 180\ L of filtrate is produced daily at a GFR of 125\ mL/min.

Filtration fraction

About 20\% of the glomerular blood flow, representing the percentage of plasma filtered.

Percentage of filtrate reabsorbed into the blood daily

About 99\% of the 180\ L of filtrate produced daily.

Total daily urine output

Slightly

Nonthreshold substances

Substances that pass through the glomerular membrane into the filtrate and are not reabsorbed, regardless of their plasma concentration.

Example of a nonthreshold substance

Creatinine

Electrolytes almost totally reabsorbed from the tubules

Sodium, potassium, chloride, and bicarbonate.

Determinants of Glomerular Filtration Rate (GFR)

Factors affecting GFR include glomerular pressure, plasma osmotic pressure, and Bowman’s capsular pressure, all of which are influenced by renal blood flow and the vessel diameter of afferent and efferent arterioles.

Effect of increased renal blood flow on GFR

Increases glomerular pressure and enhances filtration. It also reduces the rise in plasma osmotic pressure by decreasing the time blood spends in the glomerulus, leading to a less severe inhibitory effect on filtration.

Effect of afferent arteriole constriction on GFR

Decreases glomerular blood flow and glomerular filtration pressure, resulting in a decreased GFR. Dilation has the opposite effect.

Effect of efferent arteriole constriction on GFR

Increases glomerular outflow resistance and glomerular capillary pressure, which initially increases the filtration rate. However, severe constriction can greatly reduce blood flow, leading to a significant rise in plasma osmotic pressure that may counteract filtration pressure and slow the GFR.

Kidney's filtration and reabsorption efficiency

Kidneys continually filter about 20\% of circulating blood plasma into tubules and return 99\% of it to circulation after removing waste and excess substances.

Difference between Filtrate and Urine

Filtrate is water and solutes filtered out of blood at the glomerulus; urine is about 1\% of filtrate excreted from the body.

Reabsorption vs. Secretion (renal)

Reabsorption is reclaiming substances from filtrate back to blood; secretion is active transport of substances from renal tubule cells into filtrate.

Mechanisms of filtrate reabsorption in nephron tubules

Nephron tubules reabsorb most filtrate into the blood via active transport or passive diffusion.

Kidney autoregulation of GFR

Kidneys use autoregulatory feedback to maintain a constant Glomerular Filtration Rate (GFR) despite large systemic blood pressure changes.

Regulation of Renal Blood Flow vs. GFR

Renal blood flow is less precisely regulated than GFR; GFR autoregulation often prioritizes itself over renal blood flow regulation.

Urine output as an indicator

Urine output is directly related to blood pressure and cardiac output and indicates their adequacy.

Importance of fluid volume regulation

Regulation of the body’s fluid volume is crucial for sustaining adequate blood flow and preventing edema.

Sodium channel blockers (e.g., amiloride)

Diuretics that act on the luminal membrane of the collecting duct to block the reabsorption of Na^+ ions.

Mechanism of sodium channel blockers

Directly block sodium channels in the luminal membrane of the collecting duct, preventing Na^+ reabsorption from the filtrate.

Osmotic diuretic effect of sodium channel blockers

Unreabsorbed Na^+ ions remain in the filtrate, creating an osmotic gradient that pulls water with them, increasing urine output.

Potassium-sparing action of sodium channel blockers

By reducing Na^+ reabsorption, they decrease the activity of the Na^+-K^+ pump in the nonluminal membrane, causing less K^+ to be pumped into tubule cells and thus retaining K^+ in the blood.

Effect of increased blood pressure on urine output

Slightly raises the Glomerular Filtration Rate (GFR) despite autoregulatory mechanisms, leading to more solutes in the tubules than can be reabsorbed. These solutes create an osmotic gradient, 'pulling' water into the urine and increasing output.

Impact of doubled systemic arterial blood pressure on urine output

Increases urine output seven to eight times greater than normal.

Diuretic drugs

Drugs that increase the rate of urine output, reducing the body’s fluid volume, often used to treat conditions like cardiogenic pulmonary edema and systemic hypertension.

Mechanism of diuretic drugs

Diuretic drugs primarily increase the concentrations of osmotically active substances in the filtrate of renal tubules, causing water to remain in the tubules and be excreted in the urine.

Osmotic diuretics

Substances that freely pass through the glomerular membrane into the tubular filtrate but are not easily reabsorbed by the peritubular capillaries.

Mannitol

An example of an osmotic diuretic, acting mainly in the proximal tubules, which elevates the osmotic pressure of the filtrate to retain water in the tubules for excretion.

Therapeutic uses of osmotic diuretics

In addition to diuretic activity, these drugs are effective in treating cerebral edema in patients with head injuries by causing fluid to move out of cerebral tissues into the blood, decreasing brain swelling and intracranial pressure.

Primary purpose of the afferent vasodilator mechanism in kidney regulation

To regulate Glomerular Filtration Rate (GFR); autoregulation of renal blood flow is merely incidental to this mechanism.

Effect of severe decreased blood pressure on efferent arterioles and renal blood flow

Causes efferent arteriole constriction, which helps maintain GFR but reduces renal blood flow.

Comparison of renal blood flow and GFR regulation

Renal blood flow is more poorly regulated than GFR.

Myogenic reflex mechanism (renal)

A phenomenon where arterioles in the body contract their smooth muscle to resist stretching from increased blood pressure and vascular wall tension, bringing renal blood flow and GFR back to normal when blood pressure is too high.

Mechanism by which the myogenic reflex causes