8.1: Structure and Function of the Kidney

What fraction of the average individual's cardiac output do the kidneys filter?

Approximately ¼ (25%)

What are the major functions of the kidneys (6)?

Filter blood, process essential ions, reabsorb substances that maintain body fluids, excrete wastes as urine, regulate blood pressure and volume, and stimulate red blood cell production

About what size is an adult kidney?

About the size of a fist

Approximately how much does an adult kidney weigh?

About 5 ounces

Why does the right kidney lie slightly lower than the left kidney?

Because of the liver's location above it

Between which vertebrae are the kidneys located?

T12–L3

What is the kidney's medial surface called?

Hilus

What is the hilus?

A concave cleft where the ureters, blood vessels, and nerves enter the kidney

What are the functions of the fatty connective tissue surrounding the kidney?

Protects from injury and helps hold the kidney in place

What does it mean that the kidneys are retroperitoneal organs?

They are located posterior to the peritoneal cavity

What is the basic structural and functional unit of the kidney?

The nephron

Approximately how many nephrons are found in each kidney?

About 1 million

What are the primary functions of the nephron (3)?

Filter blood, reabsorb needed materials, and excrete waste products as urine

What does the nephron regulate(3)?

Blood volume, pH, blood pressure, and electrolyte levels

What are the 3 major parts of a nephron?

The glomerular capsule (renal corpuscle), renal tubule, collecting duct

What is another name for the glomerular capsule?

Renal corpuscle

What is the function of the glomerular capsule?

Filters the blood

What is the function of the renal tubule?

Reabsorbs needed materials

What is the function of the collecting ducts?

Carry the remaining material away as urine

Which region of the kidney houses the glomeruli, convoluted tubules, and blood vessels?

Outer cortex

Which region contains the Loop of Henle and renal pyramids?

Medulla

What are cortical columns?

Portions of the cortex that project through the medulla to the renal pyramids

What forms a lobe of the kidney?

Each renal pyramid

What is the renal pelvis?

centermost region of the kidney that connects the ureter as it leaves the hilum

What are calyces?

Extensions of the renal pelvis that collect urine

Trace the flow of urine from the calyces to the bladder.

Calyces → Renal pelvis → Ureter → Bladder

What percentage of nephrons are cortical nephrons?

85%

Where do cortical nephrons originate?

Superficially in the cortex

Describe the loops of Henle in cortical nephrons.

Short loops that extend only a short distance into the medulla

What percentage of nephrons are juxtamedullary nephrons?

15%

Where do juxtamedullary nephrons originate?

Deeper in the cortex

Describe the loops of Henle in juxtamedullary nephrons.

Thin loops that extend entirely into the medulla

What two systems supply blood to nephrons?

The glomerulus and the peritubular capillary network

What are arterioles?

high resistance vessels resulting in an extremely high-pressure system which can easily force fluid and solutes out of the blood and into the glomerular capillary along its entire length.

Why is the glomerulus considered a high-pressure system?

Because it lies between two high-resistance arterioles

What are peritubular capillaries?

They are low-pressure vessels that surround the tubules, allowing rapid movement of solutes and water

Are peritubular capillaries better suited for reabsorption or filtration?

reabsorption

What is the function of the vasa recta?

Assists in the exchange of solutes and water flowing in and out of kidney

Where is the vasa recta located?

In the medulla

What structures make up the renal corpuscle?

The glomerulus and Bowman capsule

Trace blood flow through the renal corpuscle.

1. Blood flows through the afferent arteriole into the glomerular capillaries | 2. Blood flows out of the glomerular capillaries into the efferent arteriole to the peritubular capillaries. | 3. Solutes and fluids are filtered out of the blood into the bowman space

What is Bowman space?

The fluid-filled space within Bowman capsule

What is filtrate?

Filtered blood that enters Bowman space

What are the three layers of the glomerular capillary membrane?

Endothelial layer, basement membrane, and capsular epithelial layer

What are fenestrations?

Small pores in endothelial cells. Allow filtration of blood

What are podocytes?

Foot processes of epithelial cells that embed into the basement membrane

What are slit pores?

Small pores formed by podocytes that allow glomerular filtrate to pass

What determines the size-dependent permeability of the glomerulus?

Spaces within the basement membrane

Under normal conditions, what substances are prevented from entering the filtrate?

Red blood cells and plasma proteins

What happens if the basement membrane is compromised?

Red blood cells and proteins leak into the filtrate

What are the four segments of the nephron tubule?

Proximal convoluted tubule, Loop of Henle, distal convoluted tubule, and collecting tubule

Which segment directly drains Bowman capsule?

Proximal convoluted tubule

Trace the flow of filtrate through the nephron.

PCT → Descending limb → Ascending limb → DCT → Collecting tubule

What type of tissue lines the nephron tubule?

A single layer of epithelial cells

Why do proximal tubule epithelial cells contain villi?

To increase surface area for reabsorption

Why do proximal tubule cells contain increased numbers of mitochondria?

To facilitate active transport processes

Why do Loop of Henle cells have fewer mitochondria?

Because they perform less reabsorption and fewer metabolic processes

Approximately how many times per day do the kidneys filter the entire plasma volume?

About 60 times per day

Approximately how many gallons of glomerular filtrate are removed daily?

About 47 gallons

What is found in the glomerular filtrate?

Water, nutrients, and essential ions

Approximately how many gallons of urine are formed daily?

About 0.5 gallons

What percentage of water and nutrients are reabsorbed back into the blood?

Approximately 99%

What three processes are required for urine formation?

Filtration, reabsorption, and secretion

Which process occurs in the renal corpuscle?

Filtration

Which processes occur in the renal tubules?

Reabsorption and secretion

Where does filtration occur?

In the glomerulus

Why is filtration considered mechanical filtration?

Because it does not require energy

What force drives filtration across the glomerular membrane?

High hydrostatic blood pressure

What substances are filtered through the glomerular membrane (4)?

Water, glucose, amino acids, and nitrogenous wastes

What substances are normally prevented from crossing the membrane?

Blood cells and most blood proteins

Why does retaining plasma proteins help prevent water loss?

Because plasma proteins maintain the osmotic pressure of glomerular blood

What is the normal glomerular filtration rate (GFR)?

120–125 mL/min

What is the normal GFR in liters per day?

180 L/day

What three factors allow for a high GFR?

Large glomerular capillary surface area, high membrane permeability, and moderate net filtration pressure

What increases GFR?

Increased arterial and glomerular blood pressure in the kidneys

What decreases GFR?

Increased glomerular osmotic pressure (water moves from areas of low to high solute to dilute it)

What commonly causes increased glomerular osmotic pressure?

Dehydration

Why is maintaining a relatively constant GFR important?

To ensure adequate reabsorption of water and nutrients

What happens if filtrate flow is too rapid?

Needed substances cannot be adequately reabsorbed

What happens if filtrate flow is too slow?

Waste products that would normally be excreted may be reabsorbed

What three mechanisms regulate renal blood flow and GFR?

Renal autoregulation, nervous system control, and hormonal control

How is GFR controlled under normal conditions during renal autoregulation?

By regulating the diameter of the afferent and efferent arterioles, allowing the kidney to determine its own rate of blood flow

How does constriction of the efferent arteriole affect GFR?

Increases glomerular pressure and increases GFR

How does constriction of the afferent arteriole affect GFR?

Decreases renal blood flow, decreases glomerular pressure, and decreases GFR

During sympathetic stimulation, where is blood diverted (3)?

To the heart, brain, and skeletal muscles

What may happen to the renal autoregulation during sympathetic activity?

May be superseded by the nervous system

How does sympathetic stimulation affect renal blood flow and GFR?

It decreases renal blood flow and GFR by narrowing the afferent arteriole and releasing epinephrine from the adrenal medulla

What hormone is released from the adrenal medulla during sympathetic activation?

Epinephrine

What does the renin-angiotensin-aldosterone (RAA) system respond to?

Low blood pressure

Where is angiotensinogen (pre-enzyme) produced?

The liver

Which cells release renin?

Juxtaglomerular (JG) cells

What happens when renin is released from the nephron in response to low blood pressure (2)?

There is constriction of afferent and efferent arterioles. Angiotensinogen is converted to angiotensin I.

Where is angiotensin I converted to angiotensin II?

In the lungs

What enzyme converts angiotensin I to angiotensin II?

Angiotensin-converting enzyme (ACE)

What effect does angiotensin II have on blood vessels?

Vasoconstriction and an increase in peripheral blood pressure

What hormone is released from the adrenal cortex in response to angiotensin II?

Aldosterone

What effect does aldosterone have on sodium and water?

Increases reabsorption

Explain RAAS step by step (6)

1. Renin is released when blood pressure drops | 2. Renin causes constriction of afferent and efferent arterioles, and angiotensinogen is converted to angiotensin I. | 3. Angiotensin I is converted to Angiotensin II in the lungs | 4. Angiotensin II vasoconstricts blood vessels, which increases peripheral blood pressure | 5. Angiotensin II causes aldosterone to be released from the adrenal cortex | 6. Aldosterone increases reabsorption of sodium and water

What is tubular reabsorption?

Movement of needed substances through tubule membranes into the peritubular capillaries

In the peritubular capillaries, how is reabsorption of water and ions regulated?

Hormonally, and may occur through passive or active processes