Functions and Anatomy of the Kidneys

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Questions and Answers

What is the primary function of the kidneys?

Control of arterial blood pressure

Formation of urine (correct)

Metabolism of hormones

Regulation of acid-base balance

Which structure does urine pass through after being formed in the kidneys?

Bladder

Urethra

Renal pelvis (correct)

Ureter (correct)

What percentage of cardiac output is supplied to the kidneys?

20%

10%

15%

22% (correct)

How many nephrons are present in each kidney on average?

1 million Signup and view all the answers

Which component of the nephron is directly involved in collecting glomerular filtrate?

Bowman's capsule Signup and view all the answers

What hormone is secreted by the kidneys that plays a role in red blood cell production?

Erythropoietin Signup and view all the answers

What type of nephron constitutes the majority of nephrons in the kidneys?

Cortical nephrons Signup and view all the answers

What is the renal plasma flow approximately in ml/min?

625 ml/min Signup and view all the answers

What percentage of renal plasma flow is typically filtered during glomerular filtration?

20% Signup and view all the answers

Which of the following substances is fully reabsorbed during the tubular reabsorption process?

Glucose Signup and view all the answers

What is the primary purpose of glomerular filtration in the nephron?

To eliminate waste products Signup and view all the answers

Juxtamedullary nephrons are primarily responsible for which of the following functions?

Formation of concentrated urine Signup and view all the answers

What does a filtration fraction of 20% indicate?

Only 20% of renal plasma flow is filtered Signup and view all the answers

During tubular secretion, which process occurs?

Substances from the peritubular capillaries enter the nephron tubule Signup and view all the answers

What is the glomerular filtration rate (GFR) per minute?

125 ml Signup and view all the answers

Which type of substance is classified under filtration only?

Creatinine Signup and view all the answers

What effect does an increase in glomerular capillary colloid osmotic pressure have on GFR?

Decreases GFR Signup and view all the answers

Which mechanism contributes to autoregulation of GFR by responding to changes in vascular pressure?

Myogenic mechanism Signup and view all the answers

What is the effect of strong activation of the sympathetic nervous system on renal blood flow (RBF) and GFR?

Decreases both RBF and GFR Signup and view all the answers

What happens to GFR if the arterial pressure increases by 25% without autoregulation?

Increases to 225 L/day Signup and view all the answers

Which hormone is secreted by the granular cells of the juxtaglomerular apparatus?

Renin Signup and view all the answers

How does the afferent arteriole respond to a decrease in arterial pressure?

Dilates to increase GFR Signup and view all the answers

What role does macula densa play in the autoregulation of GFR?

Monitors NaCl concentration Signup and view all the answers

Increased Bowman's capsule hydrostatic pressure primarily effects GFR how?

Decreases GFR Signup and view all the answers

What is the primary force that favors glomerular filtration?

Glomerular capillary hydrostatic pressure Signup and view all the answers

Which factor would most likely increase the glomerular capillary pressure?

Afferent vasodilation Signup and view all the answers

What role do plasma proteins play in glomerular filtration?

They contribute to glomerular plasma-colloid osmotic pressure, opposing filtration. Signup and view all the answers

How is the net filtration pressure calculated?

Glomerular capillary pressure - (plasma colloid osmotic pressure + Bowman's capsule hydrostatic pressure) Signup and view all the answers

What would happen to glomerular filtration if the arterial blood pressure is significantly reduced?

GFR would decrease. Signup and view all the answers

Which of the following substances is an example of one that has a plasma clearance greater than the GFR?

Para-amino-hippuric acid (PAH) Signup and view all the answers

What is the glomerular filtration rate (GFR) calculated as if net filtration pressure is 10 mm Hg and the filtration coefficient (Kf) is 12.5?

125 ml/min Signup and view all the answers

What is the main impact of tubular obstruction on Bowman's capsule hydrostatic pressure?

It increases the pressure. Signup and view all the answers

Study Notes

Functions of the Kidneys

Primary function: formation of urine
Remove waste products from the body (ingested or metabolized)
Eliminating unwanted substances and retaining needed substances
Control the composition and volume of body fluids:
- Regulation of water and electrolyte balance (ECF)
- Regulation of fluid osmolality
- Regulation of acid-base balance (strongest & slowest)
Regulation of arterial blood pressure
Metabolism and excretion of hormones
Secretion of hormones (erythropoietin, renin)
Gluconeogenesis and activation of vitamin D

Physiologic anatomy

Size of a clenched fist (150 grams)
Capsule, outer cortex, and inner medulla
Medulla (renal pyramids) → papilla → renal pelvis → ureter → bladder→ urethra
Urine drains into the renal pelvis and is channeled into the ureter
Urine stores temporarily in the urinary bladder and is emptied through the urethra
The urethra serves both the urinary and reproductive systems in males

Renal blood supply

Supplied by the renal artery and renal vein
The kidney processes plasma that flows through it
22% of cardiac output goes to the kidneys
Renal blood flow = 1100 ml/min
Renal plasma flow = 625 ml/min
Renal artery → interlobar arteries → arcuate arteries → interlobar arteries → afferent arterioles → glomerular capillaries → efferent arterioles → peritubular capillaries

Nephron

Functional unit of the kidney
1 million nephrons per kidney (2 million total)
After 40 years of age, the number of nephrons decreases by 1% per year
Composed of:
- Vascular components:
  - Afferent arteriole: carries blood into the glomerulus
  - Glomerulus: a network of capillaries surrounded by Bowman's capsule
  - Efferent arteriole: carries blood away from the glomerulus
  - Peritubular capillaries: supply renal tissue and are involved in reabsorption and secretion
- Tubular components:
  - Bowman's capsule: collects glomerular filtrate
  - Proximal convoluted tubule
  - Loop of Henle (LOH): descending and ascending limbs
  - Distal convoluted tubule
  - Collecting duct (CD) (250 per kidney): each collects from 4000 nephrons

Types of nephrons

Cortical nephrons:
- 70-80% of nephrons
- Glomeruli in the outer cortex
- Short loop of Henle
- Travel a short distance in the medulla
- Surrounded by peritubular capillaries
- Form diluted urine
Juxtamedullary nephrons:
- 20-30% of nephrons
- Glomeruli in the inner cortex
- Long loop of Henle
- Travel through the entire medulla
- Surrounded by Vasa Recta
- Form concentrated urine

Glomerular filtration

A random and passive process that takes place in the glomerulus
Protein-free plasma is filtered from the glomerulus into the Bowman's capsule
Blood cells and proteins are not normally filtered
The size and charge of a substance determine its filterability
20% of renal plasma flow is filtered
The remaining 80% enters the efferent arteriole

Glomerular filtrate

Composition is similar to plasma but lacks proteins and red blood cells
Contains less calcium and fatty acids (bind partially to proteins)
Makes up 20% of the renal plasma flow
Glomerular filtrate production rate is 125 ml per minute (180 liters per day)
Glomerular filtration rate (GFR) = 125 ml per minute (180 liters per day)
Filtration fraction = GFR/renal plasma flow = 125/625 = 20%
GFR is high for the following reasons:
- Removing waste products rapidly
- Filtering body fluids many times each day
- Providing precise and rapid control over body fluids (volume and composition)

Tubular reabsorption

A highly selective and variable process
Filtered substances move from the inside of the tubular part of the nephron into the blood of the peritubular capillaries (tubular lumen → peritubular capillary lumen)
The return of substances to the blood is necessary to maintain the composition of the ECF
High reabsorption rates:
- 124 of 125 ml of filtered fluid per minute
- 100% for glucose
- 99.5% for salts
- 99% for water
- 50% for urea

Tubular secretion

A very selective process to eliminate toxic substances rapidly
Substances from the peritubular capillaries enter the lumen of the nephron tubule

Handling of substances by the nephron

A - Filtration only:
- Plasma clearance = GFR
- Ex: creatinine and inulin
B - Filtration and partial reabsorption:
- Plasma clearance < GFR
- Ex: plasma electrolytes
C - Filtration and complete reabsorption:
- Plasma clearance = zero
- Ex: glucose and amino acids
D - Filtration and secretion:
- Plasma clearance > GFR
- Ex: para-amino-hippuric acid (PAH)

Glomerular filtration layers

The pores (fenestra) in the wall (endothelium) of the glomerular capillaries
- 400 times more permeable than other capillaries
The basement membrane: negatively charged gelatinous layer between glomerulus & Bowman's capsule (collagen and proteoglycans)
The inner epithelial layer of Bowman's capsule (through filtration slits between podocytes that encircle the glomerulus tuft)

Factors influencing glomerular filtration

Glomerular capillary hydrostatic pressure (55 mm Hg):
- Result of blood pressure
- Causes the filtration process
- Pushes plasma from glomerulus into Bowman's capsule
- The main parameter controlling the filtration process
- Increased by:
  - Increasing arterial blood pressure
  - Afferent vasodilation (reduced afferent resistance)
  - Moderate efferent vasoconstriction
- Decreased by:
  - Reducing arterial blood pressure
  - Afferent vasoconstriction (increased afferent resistance)
  - Severe efferent vasoconstriction
Glomerular plasma-colloid osmotic (oncotic) pressure (30 mm Hg):
- Due to the retention of plasma proteins in the glomerular blood
- Opposes the filtration process
- Determined by:
  - Arterial plasma colloid osmotic pressure (directly proportional)
  - Filtration fraction (directly proportional): Increased by ↑ GFR or ↓ RPF
Hydrostatic pressure of the Bowman's capsule (15 mm Hg):
- Moves fluids from Bowman's capsule into the glomerulus
- Opposes the filtration process
- Increases with tubular obstruction (kidney stones)

Net filtration pressure

Glomerular capillary pressure - (plasma colloid osmotic pressure + Bowman's capsule hydrostatic pressure) = 55 - (30 + 15) = 10 mmHg

Glomerular filtration rate (GFR)

GFR = Filtration coefficient (Kf) x Net filtration pressure
GFR = 12.5 x 10 = 125 ml/min:
- 125 ml/min = 180 L/day
- Plasma Volume = 3 liters
- Plasma is filtered 60 times daily

Filtration coefficient

Determined by properties of the glomerular membrane:
- The glomerular surface area for penetration
- The permeability of the glomerular membrane

Factors affecting GFR

Reduction in filtration coefficient (Kf) reduces GFR: chronic uncontrolled HTN & DM
Increased Bowman’s capsule hydrostatic pressure reduces GFR: kidney stones
Increased glomerular capillary colloid osmotic pressure reduces GFR
Increased glomerular capillary hydrostatic pressure increases GFR

Factors affecting renal blood flow

Sympathetic nervous system (SNS):
- Strong activation: reduces RBF and GFR
- Moderate activation: little effect
Hormones and autocoids:
- Afferent and efferent vasoconstrictors (NE, EN, endothelin): reduce RBF and GFR
- Efferent vasoconstrictor (angiotensin II): increase RBF and GFR
- Vasodilators (prostaglandin, bradykinin, nitric oxide): increase RBF and GFR

Autoregulation of GFR and RBF

GFR remains constant as arterial pressure ranges from 75 to 160 mmHg
Normally: filtration= 180 L/day, reabsorption = 178.5 L/day, and urine =1.5 L/day
Without autoregulation:
- If pressure ↑ by 25% → GFR = 225 L/day
- If reabsorption is constant → urine= 46.5 L/day
- 30 folds increase in urine formation depletes the body
Autoregulation mechanisms aren't 100% effective
↑ BP always leads to ↑ GFR (pressure diuresis or pressure natriuresis)
Myogenic mechanism:
- Responds to changes in the nephron's vascular component
- Altering the caliber of the afferent arterioles due to stretch of smooth muscle
- ↑ GFR by ↑ arterial pressure → afferent arterioles constrict to lower the GFR
- ↓ GFR by ↓ arterial pressure → afferent arterioles dilate to increase the GFR
Tubuloglomerular feedback:
- Senses changes in flow in the nephron's tubular parts
- Done by the juxtaglomerular apparatus:
  - Formed as the DCT passes between afferent & efferent arterioles
  - Composed of two components: macula densa and granular cells
    - Macula densa: monitors the NaCl concentration in the DCT
    - Juxtaglomerular (granular) cells: secrete renin
- ↓ GFR due to ↓ arterial pressure → ↑ Na reabsorption & ↓ filtrate [NaCl]:
  - Macula densa will sense the low [NaCl] and activate granular cells
  - Granular cells will secrete renin:
    - Renin causes vasodilation of the afferent arteriole (↑ GFR)
    - Renin converts angiotensinogen into angiotensin I

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Description

Explore the essential functions of the kidneys, including urine formation and regulation of body fluid composition. This quiz also covers the physiological anatomy of the kidneys, detailing their structure and connection to the urinary system.