BIOL 318: CH. 23 - URINARY SYSTEM: REVIEW OF ORGANS, NEPHRONS

What is one function of the kidneys related to blood filtration?

Excretion of waste products

How do the kidneys help regulate blood volume, pressure, and osmolarity?

By controlling water reabsorption and urine output

How do the kidneys maintain blood electrolyte balance?

Regulate ionic concentration and acid-base balance

What role do the kidneys play in red blood cell production?

Detect hypoxemia and produce erythropoietin (EPO), increasing hematocrit

How do the kidneys support bone metabolism?

Help form calcitriol

How are hormones and drugs cleared from the blood?

Proximal convoluted tubule (PCT) removes them

How do the kidneys handle free radicals?

Detoxify them

What do the kidneys produce from amino acids during extreme starvation?

Glucose via gluconeogenesis

What is one way the kidneys participate in vitamin D metabolism?

By helping synthesize calcitriol

What function do the kidneys perform related to drug metabolism?

Clear drugs from the blood

How do the kidneys assist in managing oxidative stress?

Detoxify free radicals

What metabolic process involving amino acids occurs in the kidneys during low glucose availability?

Synthesize glucose from amino acids to support blood glucose levels

What is a waste in biological terms?

Any substance that is useless to the body or present in excess of the body’s needs

What is a metabolic waste?

Waste substance produced by the body

What is nitrogenous waste?

Small nitrogen-containing substance produced as metabolic waste and excreted in urine

What makes up about 50% of nitrogenous waste in the body?

Urea, a by-product of protein catabolism

How is ammonia produced in the body?

Deamination of amino acids

How is urea synthesized?

From ammonia and carbon dioxide

How is uric acid produced?

From nucleic acids

How is creatinine produced?

From creatine phosphate

What is blood urea nitrogen (BUN)?

Level of nitrogenous waste in the blood

What does an elevated BUN indicate?

Azotemia, which may suggest renal insufficiency

What can azotemia progress to?

Uremia, a syndrome involving diarrhea, vomiting, dyspnea, and cardiac arrhythmia due to nitrogenous waste toxicity

What is excretion?

Process of separating wastes from body fluids and eliminating them from the body

Which 4 organ systems are involved in excretion?

Respiratory, integumentary, digestive, urinary

What does the respiratory system excrete?

CO₂, small amounts of other gases, water

What does the integumentary system excrete?

Water, inorganic salts, lactate, urea through sweat

What does the digestive system excrete?

Water, salts, CO₂, lipids, bile pigments, cholesterol, other metabolic wastes

What does the urinary system excrete?

Metabolic wastes, toxins, drugs, hormones, salts, hydrogen ions, water

State at least 4 functions of the kidneys other than forming urine.

Regulate blood volume and pressure, regulate osmolarity, secrete erythropoietin, help regulate calcium and bone metabolism

List 4 nitrogenous wastes and their metabolic sources.

Urea – from protein catabolism, uric acid – from nucleic acid breakdown, creatinine – from creatine phosphate, ammonia – from amino acid deamination

Name some wastes eliminated by 3 systems other than the urinary system.

Respiratory: CO₂, water
Integumentary: water, salts, urea
Digestive: bile pigments, cholesterol, lipids

What are some wastes eliminated by systems other than the urinary system?

CO₂ by respiratory system, bile pigments by digestive system, salts/water/urea by integumentary system

Where are the kidneys located?

Against back of abdominal wall, below diaphragm, right kidney slightly lower than left

How many layers of connective tissue surround each kidney?

Three layers

What is the fibrous capsule?

Innermost layer that encloses kidney, protects from trauma and infection, shiny appearance

What is the perirenal fat capsule?

Middle layer of adipose tissue that cushions kidney and holds it in place

What is the renal fascia?

Outermost fibrous connective tissue layer that binds kidney to abdominal wall

What is the renal parenchyma?

Glandular tissue of kidney that forms urine

What is the renal sinus?

Medial cavity in kidney containing blood/lymphatic vessels, nerves, urine-collecting structures

How much of the cardiac output does the kidney receive?

21% of cardiac output (renal fraction)

What is the main purpose of the high blood flow to the kidney?

Waste removal, not to meet metabolic demands of kidney tissue

What does the renal tubule do with most of the water and solutes filtered at the glomerulus?

Reabsorbs them and returns them to the bloodstream via peritubular capillaries

How does reabsorbed fluid from the renal tubule return to the bloodstream?

Through peritubular capillaries

How much of the total renal blood flow does the renal medulla receive?

Only 1–2%

What supplies blood to the renal medulla?

A network of vessels called the vasa recta

How many nephrons are in each kidney?

1.2 million nephrons

What is the outer layer of Bowman’s capsule called, and what type of tissue is it made of?

Parietal layer; simple squamous epithelium

What is the inner layer of Bowman’s capsule called, and what is it made of?

Visceral layer; podocytes wrapped around glomerular capillaries

What are the two poles of the renal corpuscle?

Vascular pole and urinary pole

What happens at the vascular pole?

Afferent arteriole enters capsule (brings blood to glomerulus), efferent arteriole exits (carries blood away)

What happens at the urinary pole?

Parietal wall turns into renal tubule; epithelium changes from simple squamous to simple cuboidal

What fills the spaces among capillaries in the glomerulus?

Mesangial cells

What are the functions of mesangial cells?

Support glomerular capillaries, regulate blood flow, phagocytize debris to prevent clogging of filtration membrane

What is the renal tubule?

A duct that leads away from the glomerular capsule and ends at the tip of a medullary pyramid.

What are the four regions of the renal tubule?

PCT, LOH, DCT, CD.

Where does the proximal convoluted tubule (PCT) arise from?

It arises from the glomerular capsule.

What are the characteristics of the PCT?

Longest and most coiled part of the renal tubule; lined with simple cuboidal epithelium with microvilli (brush border) to allow more reabsorption.

Where is the loop of Henle (LOH) mostly located?

Mostly in the medulla.

What are the two main limbs of the loop of Henle?

Ascending limb and descending limb.

What type of epithelium is found in the thick segments of the LOH?

Simple cuboidal epithelium.

What part of the LOH do the thick segments form?

Initial part of the descending limb and part or all of the ascending limb.

What is the function of cells in the thick segments?

Engaged in active transport of salts; have high metabolic activity and many mitochondria.

What type of epithelium is found in the thin segments of the LOH?

Simple squamous epithelium.

What part of the LOH do the thin segments form?

Most of the descending limb.

What are the characteristics of cells in the thin segments?

Low metabolic activity; very permeable to water.

When does the distal convoluted tubule (DCT) begin?

Shortly after the ascending limb reenters the cortex.

How does the DCT compare to the PCT in length and coiling?

Shorter and less coiled.

What type of epithelium lines the DCT?

Simple cuboidal epithelium with smooth-surfaced cells, nearly devoid of microvilli.

What does the DCT mark?

The end of the nephron.

Where does the collecting duct (CD) receive fluid from?

From the DCTs of several nephrons.

What happens as the CD passes back into the medulla?

Numerous CDs converge toward the tip of a medullary pyramid and merge to form a larger papillary duct.

Where does urine drain after the papillary duct?

Into the minor calyx that encloses the papilla.

What type of epithelium lines the CD and papillary ducts?

Simple cuboidal epithelium.

What is the flow of fluid from the formation of glomerular filtrate to the exit of urine?

Glomerular capsule → PCT → LOH → DCT → CD → papillary duct → minor calyx → major calyx → renal pelvis → ureter → urinary bladder → urethra.

What are juxtamedullary nephrons?

Nephrons located close to the medulla; make up 15% of nephrons and solely responsible for maintaining the osmotic gradient.

What are cortical nephrons?

Nephrons located farther from the medulla.

What is the function of nephron loops (LOH)?

Maintain an osmotic gradient in the medulla to help conserve water.

What is the renal plexus?

A network of nerves and ganglia wrapped around each renal artery.

What type of innervation does the renal plexus carry?

Sympathetic innervation from the abdominal aortic plexus and afferent pain fibers from kidneys to spinal cord.

What is the effect of sympathetic stimulation on glomerular blood flow?

Reduces glomerular blood flow, which decreases urine production.

How do sympathetic fibers respond to falling blood pressure?

Stimulate kidneys to secrete renin.

What is renin?

An enzyme that activates hormonal mechanisms for restoring blood pressure.

What parasympathetic innervation do kidneys receive?

Branches from the vagus nerve, though the function is unknown.

What is the renal fraction of cardiac output?

About 21%, or 1.2 L of blood per minute.

Why do the kidneys receive such a high volume of blood?

Primarily for waste removal rather than to meet metabolic demands.

Arrange the following in order from most to least numerous in the kidney.

Glomeruli, major calyces, minor calyces, cortical radiate arteries, interlobar arteries.

Describe the path taken by one red blood cell from the renal artery to the renal vein.

Renal artery → segmental artery → interlobar artery → arcuate artery → cortical radiate artery → afferent arteriole → glomerulus (stays in capillaries) → efferent arteriole → peritubular capillaries or vasa recta → cortical radiate vein → arcuate vein → interlobar vein → renal vein.

Describe the path taken by one molecule of urea in urine, from leaving the bloodstream to exiting the body.

Bloodstream → glomerulus (filtered) → capsular space → PCT → nephron loop → DCT → collecting duct → papillary duct → minor calyx → major calyx → renal pelvis → ureter → urinary bladder → urethra → out of body.

What is the functional unit of the kidney?

Nephron

How many nephrons are in each kidney?

About 1.2 million

What is the role of the renal corpuscle?

Filters the blood plasma

What is formed during filtration in the renal corpuscle?

Filtrate

What surrounds the glomerulus inside the renal corpuscle?

Bowman’s capsule

What type of epithelium makes up the parietal layer of Bowman’s capsule?

Simple squamous epithelium

What cells make up the visceral layer of Bowman’s capsule?

Podocytes

What do podocytes wrap around?

Capillaries of the glomerulus

What is the glomerulus?

Ball of fenestrated capillaries

What is the renal tubule?

Long, coiled tube that converts filtrate into urine

What happens in the renal tubule?

Reabsorption of most water and solutes back into the bloodstream via peritubular capillaries