15 - The Kidneys

Main Functions of the Kidneys

1. Regulation of ECF volume and blood pressure

2. Regulation of osmolarity

3. Maintenance of ion balance

4. Homeostatic regulations of pH

5. Excretion of wastes

6. Production of hormones

1) Regulation of ECF volume and blood pressure

• If ECF volume decreases, blood pressure decreases, and blood flow to brain and organs decreases

• Kidneys work with cardiovascular system to maintain BP and tissue perfusion

2) Regulation of Osmolarity

• Through thirst and drinking

• Reabsorption of water (i.e. producing more concentrated urine)

• Sensors for osmolarity found in hypothalamus and the kidney (macula densa)

3) Maintenance of Ion Balance

Balance dietary intake with urinary loss (Na⁺, Ca²⁺, K⁺, etc.)

4) Homeostatic Regulation

• Kidneys remove H⁺ and conserve HCO₃^- when pH decreases (perform the opposite when pH increases)

• Not as big of a role as the lungs in pH balance

5) Excretion of wastes

Removal of metabolic waste products and foreign substances

6) Production of Hormones

• Erythropoietin for RBC synthesis

• Renin for Na⁺ balance

• Vitamin D for Ca²⁺ balance

Characteristics of the Kidneys

• Have a renal artery and vein

• Receives 20-25% of cardiac output (but only represents 0.4% of body weight)

• Acts on plasma flowing through it to produce urine

• Outer surface - the renal cortex

• Inner surface - the renal medulla

Characteristics of the Ureters

• Hollow tube leading from kidney to bladder

• One for each kidney

Characteristics of the Bladder

• Temporarily stores urine

• Hollow, distensible, smooth muscle-walled sac

• Periodically empties to the outside of the body through the urethra

Characteristics of the Urethra

• Conveys urine to the outside of the body

• Females = straight and short

• Males = longer and curved

  • Dual functions:

    • Route for eliminating urine from the bladder

    • Passageway for semen from reproductive organs

Characteristics of the Nephron

Functional unit of the kidney

• Smallest unit that can perform all the functions of the kidney

• The arrangement of the nephrons produces two distinct regions:

  • Renal cortex (granular in appearance)

  • Renal medulla (made up of striated triangles called renal pyramids)

• Two types of nephrons are distinguished by location and length of their structures

  • Juxtamedullary nephrons

  • Cortical nephrons (80% of all nephrons)

Components of the Nephron (2)

• Vascular component

• Tubular component

  • Both components have countercurrent flow - essential to function

The Glomerulus - Vascular Component

Dominant part of the vascular component

• Water and solutes are filtered as blood passes through the glomerulus

  • From the renal artery, in-flowing blood passes through afferent arterioles which deliver blood to glomerulus

  • Filtered fluid then passes through nephron’s tubular component

Vascular Component of the Nephron

• Efferent arteriole transports blood from glomerulus

• Efferent arteriole breaks down into peritubular capillaries which surround tubular part of nephron

• Peritubular capillaries join into venules which transport blood into the renal vein

Tubular Component of the Nephron

Hollow, fluid-filled tube formed by a single layer of epithelial cells

The 3 Basic Renal Processes

1. Glomerular filtriation - movement of fluid from the blood into the lumen of the nephron

2. Tubular reabsorption - substances from filtrate moved back into the blood (peritubular capillaries)

3. Tubular secretion - removes molecules from blood and adds them to filtrate in the lumen

   1. Items in the lumen are destined to be removed, if the body wants to keep it must be removed/reabsorbed

1) Glomerular Filtration

First step in urine formation

• Filtrate similar to plasma in composition (but has no plasma proteins)

• 20% of plasma that enters glomerulus is filtered (180L/day of filtered fluid formed)

• Transfer between kidney tubules and pertubular capillaries ensures that enough fluid is kept in the system

Renal Corpuscle

Glomerular capillaries surrounded by Bowman’s capsule

3 Filtration barriers components had to pass through - Glomerular Filtration

1. Glomerular capillary epithelium (fenestrated)

• The pores in the glomerular capillaries are fairly large (to allow for bulk fluid to move out of the vessel)

1. Basal lamina (aka. basement membrane)

2. Epithelium of Bowman’s capsule

1) Glomerular Capillary Wall

• Single layer of flattened endothelial cells

• Large pores

• 100x more permeable to water and solute than other capillaries

  • HIGHLY permeable

2) Basal Lamina

Basement membrane

• Acellular gelatinous layer

• Composed of collagen & glycoproteins

• Negatively charged glycoproteins repel negatively charged plasma proteins

3) Epithelium of Bowman’s Capsule

• Consists of podocytes (special endothelial cells)

• Podocyte “feet” mingle with neightbouring podocyte “feet”

• Sits between the feet used for filtration

Physical Forces Involved in Glomerular Filtration (3)

1. Glomerular capillary blood (hydrostatic) pressure

2. Plasma-colloid osmotic pressure

3. Bowman’s capsule hydrostatic pressure

1) Hydrostatic pressure of blood

Fluid pressure exerted by blood within glomerular capillaries

• Depends on:

  • Contraction of the heart

  • Resistance to blood flow offered by afferent and efferent arterioles

• Major force producing glomerular filtration

• ~55 mmHg

2) Plasma-Colloid Osmotic Pressure

Causes by unequal distribution of plasma proteins across glomerular membrane

• Opposes filtration (favours movement back into the capillaries)

• ~30 mmHg

3) Bowman’s Capsule Hydrostatic Pressure

Pressure exerted by fluid in initial part of tubule

• Tends to push fluid out of Bowman’s capsule

• Opposes filtration

• ~15 mmHg

Glomerular Filtration Rate (GFR)

Volume of fluid that filters into Bowman’s capsule per unit time

• Influenced by net filtration pressure and filtration coefficient

• Filtration coefficient has two components:

1. Surface area of glomerular capillaries available for filtration (mesangial cells & constriction)

2. Permeability of interface between the capillary and Bowman’s capsule (podocytes)

Controlled Adjustments in GFR

• GFR too high = an excess of water and solutes is lost due to high urine output

• GFR too low = waste builds up

• Glomerular capillary BP can be controlled to adjust GFR to suite the body’s need

• Two major control mechanisms:

  • Autoregulation

  • Extrinsic sympathetic control

Autoregulation

Prevents spontaneous changes in GRF

• Myogenic mechanism (contraction in response to stretch — muscle)

• Tubuloglomerular feedback (TGF)

Tubuloglomerular Feedback

Involves juxtaglomerular apparatus (where tubule passes through angle formed by afferent and efferent arterioles)

• Specialized tubular cells (macula densa) detected changes in salt level of fluid flowing by

  • Increased GFR = increased salt delivery

  • Release of ATP from macula densa cells results in increased adenosine levels

  • Adenosine causes vasoconstriction in adjacent afferent arteriole and decreased GFR

Extrinsic Sympathetic Control

Overrides autoregulatory responses

• Aimed at long-term regulation of arterial blood pressure

  • Mediated by smpathetic nervous system input to afferent arterioles (increase peripheral resistance)

  • Baroreceptor reflex causes vasoconstriction of afferent arteriole when BP is low

    • Decreased GFR = decreased urine output; conservation of plasma volume

Tubular Reabsorption