Lecture 2 : Renal Physiology

Kidneys, ureters, urinary bladder, and urethra

What organs make up the Urinary system?

Nephron

What is the Functional unit of the kidney?

About 1-1.5 million

How many nephrons are in each kidney?

• Cortical nephron

• Juxtamedullary nephron

Two types of Nephrons

Cortical nephron

• Removal of waste products and reabsorption of nutrients

• Compromise about 85 % of total nephrons of the kidneys.

Juxtamedullary nephron

Responsible for the Concentration of urine

• Collecting Duct

• Loops of Henle

Primarily in the medullary part of your kidneys

• Proximal Convoluted Tubule

• Distal Convoluted Tubule

• Glomerulus

Mostly found within the cortex portion of your kidneys.

• Glomerular filtration

• Tubular reabsorption

• Tubular secretion

Renal Functions

Renal artey —> Afferent arterioles —> Glomerulus —> Efferent arterioles —> Peritubular capillaries —> Vasa recta —> Renal vein

Trace the Renal Blood Flow

25 %

Amount of the overall cardiac output the kidney recieve through the renal artery

Maintains osmotic gradient by exchanging salt and water around the Loops of Henle.

Role of the vasa recta

Reabsorption of essential substances from the proximal and distal tubules.

Purpose of peritubular capillaries

About 1,200 mL/min of blood flows to the glomerulus

The approximate total renal blood flow in an average-sized adult (1.73 m² bodysurface)

600-700 mL/min

Normal range for total renal plasma flow

Overall kidney function

What does the GFR measure?

Glomerulus

• Has eight capillary lobes with a glomerular filtration barrier.

• Located in the Bowman capsule

• The structure of Capillary Walls

• Bowman Capsule

• Hydrostatic Pressure'

• Oncotic Pressure

• RAAS system

Factors influencing filtration process

• Fenestrated endothelium

• Basement membrane

• Podocyte foot processes

Three layers of the glomerular filtration barrier

Shield of negativity

A property of the glomerular membrane that repels negatively charged molecules like albumin

It contains pores or fenestrations, making it a fenestrated endothelium

What special features does the endothelial lining of glomerular capillaries have?

Substance with a molecular weight less than 70,000

What type of substance can pass through the fenestrated endothelium of the glomerulus?

Red blood cells

Not normally found in urine because they are too large to pass through the fenestrated endothelium of the glomerulus.

Albumin

It is the primary protein associated with renal disease. 

Also termed as the kryptonite of the kidneys.

Hydrostatic Pressure

found both within the capillaries and Bowman’s Capsule.

Pushes fluids out of capillaries.

Oncotic pressure

Pulls fluid back into capillaries due to albumin

When there is a constant push and pull between hydrostatic and oncotic pressure

When does Filtration happens?

It creates greater hydrostatic pressure, which enhances filtration in the glomerulus

What happens when the efferent arteriole is smaller in size than the afferent arteriole?

It helps push unfiltered plasma through the filtration barrier into Bowman’s capsule

What is the function of hydrostatic pressure in the glomerulus?

Juxtaglomerular apparatus

Maintains glomerular blood pressure and ensures constant filtration despite fluctuations in systemic blood pressure.

• Juxtaglomerular cells

• Macula densa

components of the Juxtaglomerular apparatus

Juxtaglomerular cells

Release renin and regulate arteriole constriction or dilation to control blood ressure and filtration rate.

The afferent arteriole dilates, and the efferent arteriole constricts to preserve glomerular filtration pressure.

What happens during acute blood loss (low BP) to maintain kidney function?

The afferent arteriole constricts to reduce blood flow into the glomerulus, preventing over-filtration.

What happens when a blood pressure increases?

The flow of blood to and within the glomerulus

What does the Renin-Angiotensin-Aldosterone System regulate?

Sodium content of the body

What is the RAAS system dependent on?

Changes in Blood pressure and Plasma sodium levels, detected by the juxtaglomerular apparatus

What triggers the activation of RAAS system?

Both blood volume and blood pressure decrease

What happens to blood volume and pressure when sodium levels decrease?

Renin

An enzyme produced by the Juxtaglomerular cells

Angiotensinogen

Renin is secreted and they react with _________, they produce Angiotensin I

Angiotensin I

An inert hormone (inactive) not usable.

Angiotensin-converting enzymes

This enzyme will convert Angiotensin I into Angiotensin II

Angiotensin II

The active form of hormone in RAAS

• Dilates the afferent arteriole (increase blood flow)

• Constricts the efferent arteriole (to maintain filtration pressure)

What are the effects of Angiotensin II on renal blood flow?

Sodium reabsorption

What does Angiotensin II stimulate in the proximal concoluted tubule?

Aldosterone

This hormone is released due to Angiotensin II. It reabsorbs sodium in the distal convoluted tubule.

Antidiuretic Hormone (ADH)

It is released by Angiotensin II and it helps Reabsorb Water in the Collecting Duct

120 mL per minute

How much water-containing low-molecular-weight substance do the glomeruli filter per minute?

Tubular reabsorption

The process by which the kidney reabsorb essential substances (like water, glucose, and ions) from the filtrate back into the blood

Active transport and passive transport

What are the two types of transport involved in tubular reabsorption?

The filtrate concentration exceeds the maximal absorptive capacity (Tm) , so excess substances appear in the urine.

What happens when plasma concentration of a normally reabsorbed substance becomes abnormally high?

It’s the maximum rate at which a substance can be actively reabsorbed by the renal tubules.

What is meant by “maximum absorptive capacity (Tm)”?

Reabsorption stops, and the excess substance is excreted in the urine.

What happens when the kidneys reach their maximal absorptive capacity (Tm) for a substance?

Renal threshold

What is the term for the plasma concentration at which active transport of a substance stops?

160-180 mg/dL

Glucose Renal Threshold

Starts at the Proximal Convoluted Tubule (PCT) and continues up to the collecting duct.

Where does tubular reabsorption begin and end?

Loops of Henle (descending and ascending limbs)

Where does the actual concentration of urine begin?

Medullary portion

In which part of the kidney are the Loops of Henle primarily located?

Countercurrent mechanism

Is is a process in the Loop of Henle where the descending limb reabsorbs water and ascending limb reabsorbs sodium and chloride, creating a concentration gradient in the renal medulla that allows the kidney to concentrate or dilute urine. 

300 mOsm/L

What is the osmolality of the filtrate as it leaves the glomerulus?

Water is reabsorbed by osmosis, increasing osmolality from 300 —> 600 —> 900 —> 1,200 mOsm/L

What happens to osmolality as the filtrate moves down the descending loop of Henle?

Descending loop of Henle

It is permeable to water, allowing osmosis to occur

Ascending loop of Henle

It is impermeable to water, but reabsorbs sodium and chloride, lowering osmolality.

It maintains the osmotic gradient of the medulla, essential for urine concentration in the collecting duct.

Why is the countercurrent mechanism important?

Presence or absence of Antidiuretic hormone (ADH)

What determines the final concentration of urine?

The collecting duct becomes permeable to water, leading to low-volume, concentrated urine.

What happens when ADH is present?

The collecting duct becomes impermeable to water, resulting in large-volume, dilute urine.

What happens when ADH is absent?

Tubular secretion

The transfer of substances from the blood in the peritubular capillaries to the tubular filtrate.

• Eliminating waste products not filtered by the glomerulus

• Regulating acid-base balance

What are the two major functions of tubular secretion?

Vasopressin

Other term for Antidiuretic Hormone

Because they cannot be filtered by the glomerulus and would become toxic if retained.

Why must certain medications be secreted by the kidneys?

7.4 (slightly alkaline)

What is the normal blood pH?

By buffering the blood and excreting excess acids formed from metabolism and diet.

How do the kidneys help maintain normal blood pH?

It reabsorbs filtered bicarbonate to conserve this important alkaline buffer.

What does the kidney do with bicarbonate?

By combining with filtered phosphate ions or ammonium ions to form H2PO4 - or NH4+, which are excreted in urine. 

How are Hydrogen ions eliminated?

To prevent acidosis and maintain the blood’s acid-base balance.

Why is hydrogen ion excretion important?

Proximal Convoluted Tubule

Where is bicarbonate reabsorbed?

To maintain blood pH and prevent acidosis

Why is bicarbonate reabsorbed?

Secretion of Hydrogen ions by renal tubular cells

What prevents bicarboonate loss in urine?bicarbonate

Carbonic acid (H2CO3)

What forms when Hydrogen combines with bicarbonate?

Carbonic anhydrase

What enzyme forms carbonic acid from CO3 and H2O?

It breaks into H2O and CO2, which are reabsorbed.

What happens to H2CO3 in tubular cells?

Bind with phosphate —> H2PO4-, excreted

How are hydrogen ions excreted via phosphate ?

NH3 + NH4, excreted

How are Hydrogen ions excreted via ammonium?

No, they are secreted

Are H2PO4- and NH4 + reabsorbed?

To prevent acidosis and balance pH

Why are hydrogen ions secreted?

• HCO3 is reabsorbed.

• PO4³ is excreted.

• NH4 is excreted.

What happens when hydrogen binds with different ions?