Chapter 18: Urinary system and acid base balance

What are the functions of the urinary system?

Elimination of wastes, water balance, regulation of PH

What is true metabolic waste?

Substances produced from metabolic processes that are toxic to the body and need to be eliminated, such as urea, creatine, and uric acids.

What organ is the most powerful long-term regulator of blood pH

The kidneys

What is the normal range of urine pH

4.5 to 8.0

How do the kidneys respond to excess hydrogen ions (acidosis

By actively secreting H⁺ ions into the urine, lowering urine pH

How do the kidneys respond to excess base (alkalosis

By excreting excess bicarbonate, raising urine pH

What is the role of the lungs in acid-base balance?

They regulate blood pH by controlling the amount of CO₂ exhaled.

What chemical equation summarizes the lungs' role in pH regulation?

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃

What happens to the reaction above during acidosis (too much H⁺)?

It shifts left, forming CO₂ and H₂O, which are then exhaled to reduce acidity

How does breathing faster help correct acidosis

It removes more CO₂, shifting the equation left and reducing H⁺ in the blood

What happens if a person breathes too slowly (hypoventilates

CO₂ accumulates, shifting the reaction right, increasing H⁺ and causing acidosis

What condition results from too much CO₂ due to impaired respiration

Respiratory acidosis.

What condition results from excessive loss of CO₂ due to hyperventilation

Respiratory alkalosis

What type of acidosis is commonly seen in people with renal failure?

Metabolic acidosis.

What effect does eating acidic or basic foods (e.g., lemons, TUMS) have on the urinary system

The kidneys adjust acid/base excretion to maintain homeostasis regardless of diet

What is the normal pH of arterial blood

Around 7.4, which is slightly basic

What are the three major contributors to blood pH

Hydrogen ions, bicarbonate ions, and carbon dioxide

How does CO₂ affect blood pH

CO₂ combines with water to form carbonic acid, which increases H⁺ and lowers pH

What is the equation representing the acid-base buffer system in blood

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

What role does bicarbonate (HCO₃⁻) play in pH regulation

It acts as a base, binding excess H⁺ to decrease acidity

What is an acid in terms of hydrogen ions?

A substance that donates H⁺ ions to a solution

What is a base in terms of hydrogen ions

A substance that binds or removes H⁺ ions from a solution.

What does pH measure?

The concentration of free hydrogen ions (H⁺) in a solution

What is the pH scale range

From 0 (very acidic) to 14 (very basic); 7 is neutral

What pH range is optimal for most biological enzymes

Between 6–8 (except stomach enzymes like pepsin, which prefer pH 2)

What is a buffer

A substance or system that minimizes changes in pH by neutralizing excess acids or bases

What is GFR (glomerular filtration rate)?

The amount of filtrate produced per minute by the kidneys

What happens if GFR is too high or too low?

High GFR: essential substances pass without reabsorption.
Low GFR: wastes may be retained due to excessive reabsorption

How does the body regulate GFR and reabsorption

Through mechanical (arteriole size) and chemical (hormonal) controls.

What triggers the release of ADH?

Dehydration or high blood solute concentration

What does ADH do

Increases water reabsorption in the kidneys, decreasing urine volume

What substances inhibit ADH

Caffeine and alcohol.

What does aldosterone do

Increases Na⁺ reabsorption and K⁺ secretion in the kidneys

What is the effect of aldosterone on urine and blood

Decreases Na⁺ in urine, increases K⁺ in urine; raises blood Na⁺ and blood pressure

What is the relationship between Na⁺ and K⁺ under aldosterone

Saving Na⁺ causes the body to lose K

What is tubular secretion?

The process by which substances that were not filtered out initially are added to the filtrate from the blood, mainly at the distal convoluted tubule

Where does tubular secretion primarily occur

In the distal convoluted tubule (DCT), but it can occur along the entire renal tubule

What types of substances are commonly secreted into the renal tubule

Potassium ions (K⁺), hydrogen ions (H⁺), and organic substances like drugs (e.g., penicillin).

What is the main purpose of tubular secretion

To eliminate waste and maintain pH balance and proper ion concentrations

What transport mechanism is primarily used in tubular secretion

Active transport

What is tubular reabsorption

The process of reclaiming water and useful solutes from the filtrate back into the blood

Where does the majority of reabsorption occur?

: In the proximal convoluted tubule (PCT

What are the two limbs of the loop of Henle, and what do they do?

The descending limb allows water reabsorption, and the ascending limb reabsorbs solutes like Na⁺

What happens if glucose is found in the urine

It indicates a condition like Diabetes Mellitus, where glucose levels exceed the renal threshold.

What is the renal threshold (tubular maximum

The maximum amount of a substance that can be reabsorbed by the kidneys

How much filtrate is reabsorbed during urine formation?

About 99% is reabsorbed; only about 1% leaves the body as urine

What processes are involved in tubular reabsorption

Diffusion, osmosis, and active transport

What are the peritubular capillaries

Capillaries that surround the renal tubules and collect reabsorbed substances to return them to the bloodstream

What are the three main steps in urine formation?

Filtration, reabsorption, and secretion.

Where does filtration occur in the nephron

In the glomerulus

What is the functional unit of the kidney?

The nephron

What creates the pressure that drives filtration in the glomerulus

The afferent arteriole is larger than the efferent arteriole, creating back pressure

What substances are filtered out of the blood during glomerular filtration?

Water, glucose, sodium ions, waste metabolites—anything smaller than proteins or blood cells

What substances do not get filtered during glomerular filtration

Proteins, red blood cells, white blood cells, and platelets

What is the fluid called immediately after filtration occurs

Filtrate

Approximately how much filtrate is produced daily

About 48 gallons

How much urine is typically excreted per day

About 1 liter or 1/4 gallon.

What are the two parts of the nephron’s anatomy?

Renal corpuscle (glomerulus and Bowman's capsule) and renal tubular system

What structure surrounds the glomerulus like a koozie around a can

Bowman's capsule.

What are the parts of the renal tubular system

Proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct

What feature of the PCT increases its reabsorption capacity

Apical microvilli forming a brush border

What are the two limbs of the loop of Henle

Descending and ascending limbs

What is the main function of the loop of Henle in juxtamedullary nephrons

To create concentrated urine

How does the DCT differ from the PCT in appearance

The DCT has fewer and more spaced-out microvilli, lacking a brush border

What do the collecting ducts drain into

Minor calyces at the renal papillae

What is the pathway of urine after the collecting ducts?

Minor calyces → Major calyces → Renal pelvis → Ureter → Urinary bladder → Urethra

What are the two urethral sphincters and what are they made of?

Internal urethral sphincter (smooth muscle, involuntary) and external urethral sphincter (skeletal muscle, voluntary).

What arteries supply blood to the kidneys, starting from the aorta

Renal artery → Segmental arteries → Interlobar arteries → Arcuate arteries → Interlobular arteries → Afferent arteriole

What capillary networks follow the efferent arteriole

Peritubular capillaries and vasa recta.

What is #1?

Urethra

What is #2?

Bladder

What is #3?

Ureter

What is #4?

Kidneys

What is #1?

Glomerulus

What is #2?

Bowmans capsule

What is #3?

Afferent arteriole

What is #4?

Efferent arteriole