Kidney: Functional Unit Nephrons and Urinary Systems | BIOL 2020, Study notes of Physiology

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Kidney

Ch 26

The urinary system

• Filters your blood at an amazing rate; about 180

liters/day!

• You only excrete ~1-2 liters/day (urine)

• Also removes a type of waste called urea from your blood.

• Urea is produced when foods containing protein, such as

meat, poultry, and certain vegetables, are broken down in

the body.

• Urea is carried in the bloodstream to the kidneys.

Nephrons

• (^) Renal corpuscle and both convoluted tubules in cortex of kidney
• (^) loop of Henle extends into medulla
• (^) Distal convoluted tubule of several nephrons empty into single

collecting duct

• (^) Cortical nephrons – 80-85% of nephrons
  • (^) Renal corpuscle in outer portion of cortex and short loops of Henle

extend only into outer region of medulla

• (^) Juxtamedullary nephrons – other 15-20%
  • (^) Renal corpuscle deep in cortex and long loops of Henle extend

deep into medulla

• (^) Ascending limb has thick and thin regions
• (^) Enable kidney to secrete very dilute or very concentrated urine

The structure of nephrons and associated blood vessels

Overview of renal physiology

1. Glomerular filtration

• (^) Water and most solutes in blood

plasma move across the wall of the

glomerular capillaries into glomerular

capsule and then renal tubule

2. Tubular reabsorption

• (^) As filtered fluid moves along tubule

and through collecting duct, about

99% of water and many useful

solutes reabsorbed = returned to

blood

3. Tubular secretion

• (^) As filtered fluid moves along tubule

and through collecting duct, other

material secreted into fluid such as

wastes, drugs, and excess ions –

removes substances from blood

• (^) Solutes in the fluid that drains into the

renal pelvis remain in the fluid and are

excreted

• (^) Excretion of any solute = glomerular

filtration + secretion - reabsorption

Structures and functions of a nephron Renal corpuscle Renal tubule and collecting duct Peritubular capillaries Urine (contains excreted substances) Blood (contains reabsorbed substances) Fluid in renal tubule Afferent arteriole Filtration from blood plasma into nephron Efferent arteriole Glomerular capsule 1 Renal corpuscle Renal tubule and collecting duct Peritubular capillaries Urine (contains excreted substances) Blood (contains reabsorbed substances) Tubular reabsorption from fluid into blood Fluid in renal tubule Afferent arteriole Filtration from blood plasma into nephron Efferent arteriole Glomerular capsule 1 2 Renal corpuscle Renal tubule and collecting duct Peritubular capillaries Urine (contains excreted substances) Blood (contains reabsorbed substances) Tubular secretion from blood into fluid Tubular reabsorption from fluid into blood Fluid in renal tubule Afferent arteriole Filtration from blood plasma into nephron Efferent arteriole Glomerular capsule 1 2 3

The filtration membrane

Pedicel of podocyte Filtration slit Fenestration (pore) of glomerular endothelial cell Basal lamina Lumen of glomerulus (b) Filtration membrane TEM 78,000x (a) Details of filtration membrane Filtration slit Pedicel Fenestration (pore) of glomerular endothelial cell: prevents filtration of blood cells but allows all components of blood plasma to pass through Podocyte of visceral layer of glomerular (Bowman’s) capsule 1 Pedicel of podocyte Filtration slit Fenestration (pore) of glomerular endothelial cell Basal lamina Lumen of glomerulus (b) Filtration membrane TEM 78,000x (a) Details of filtration membrane Filtration slit Pedicel Fenestration (pore) of glomerular endothelial cell: prevents filtration of blood cells but allows all components of blood plasma to pass through Basal lamina of glomerulus: prevents filtration of larger proteins Podocyte of visceral layer of glomerular (Bowman’s) capsule 1 2 Pedicel of podocyte Filtration slit Fenestration (pore) of glomerular endothelial cell Basal lamina Lumen of glomerulus (b) Filtration membrane TEM 78,000x (a) Details of filtration membrane Filtration slit Pedicel Fenestration (pore) of glomerular endothelial cell: prevents filtration of blood cells but allows all components of blood plasma to pass through Basal lamina of glomerulus: prevents filtration of larger proteins Slit membrane between pedicels: prevents filtration of medium-sized proteins Podocyte of visceral layer of glomerular (Bowman’s) capsule 1 2 3

NET FILTRATION PRESSURE (NFP)

=GBHP – CHP – BCOP

= 55 mmHg 15 mmHg 30 mmHg = 10 mmHg

GLOMERULAR BLOOD

HYDROSTATIC PRESSURE

(GBHP) = 55 mmHg Capsular space Glomerular (Bowman's) capsule Efferent arteriole Afferent arteriole

Proximal convoluted tubule NET FILTRATION PRESSURE (NFP) =GBHP – CHP – BCOP = 55 mmHg 15 mmHg 30 mmHg = 10 mmHg

CAPSULAR HYDROSTATIC

PRESSURE (CHP) = 15 mmHg

GLOMERULAR BLOOD

HYDROSTATIC PRESSURE

(GBHP) = 55 mmHg Capsular space Glomerular (Bowman's) capsule Efferent arteriole Afferent arteriole

Proximal convoluted tubule NET FILTRATION PRESSURE (NFP) =GBHP – CHP – BCOP = 55 mmHg 15 mmHg 30 mmHg = 10 mmHg

BLOOD COLLOID

OSMOTIC PRESSURE

(BCOP) = 30 mmHg

CAPSULAR HYDROSTATIC

PRESSURE (CHP) = 15 mmHg

GLOMERULAR BLOOD

HYDROSTATIC PRESSURE

(GBHP) = 55 mmHg Capsular space Glomerular (Bowman's) capsule Efferent arteriole Afferent arteriole

Proximal convoluted tubule

Glomerular filtration

• Glomerular filtration rate = amount of filtrate formed in all

the renal corpuscles of both kidneys each minute (~

ml/min; you will filter more than 11 liters during class today)

• Homeostasis requires kidneys maintain a relatively constant

GFR

• Too high – substances pass too quickly and are not

reabsorbed

• Too low – nearly all filtrate is reabsorbed and some

waste products not adequately excreted

– GFR is directly related to pressures that determine net

filtration pressure

GFR

• GFR can be regulated by:

1. Adjusting blood flow in and out of

glomerulus

2. Altering the capillary surface area of the

glomerulus (open or close afferent arteriole)

3 Mechanisms regulating GFR

1. Renal autoregulation
   • (^) Kidneys themselves maintain constant renal blood flow and GFR despite normal

fluctuations in blood pressure using:

• (^) Myogenic mechanism –when blood pressure increases it increases GFR; increased BP also stretches the afferent arteriole which triggers contraction of smooth muscle cells in afferent arterioles – this reduces GFR
• The opposite happens when blood pressure decreases
• (^) Tubuloglomerular mechanism – part of the renal tubules (macula densa) provides feedback to glomerulus
• (^) When GFR is above normal due to increased BP, filtered fluid flows more rapidly, leading to lower reabsorption of Na+, Cl-, and water
• (^) Macula densa cells sense increased Na+ & Cl- and inhibit release of NO (NO causes vasodilation) from cells near the glomerulus (juxtaglomerular cells) causing afferent arterioles to constrict resulting in less blood flow to the glomerulus and therefore decreasing GFR http://www.youtube.com/watch?v=4wp3m1vg06Q

Mechanisms regulating GFR

2. Neural regulation  (^) Kidney blood vessels supplied by sympathetic ANS fibers that release norepinephrine causing vasoconstriction  (^) Moderate stimulation – both afferent and efferent arterioles constrict to same degree and GFR decreases only slightly  (^) Greater stimulation constricts afferent arterioles more and GFR drops (occurs during exercise or hemorrhage)

Mechanisms regulating GFR

3. Hormonal regulation

• (^) Angiotensin II reduces GFR – potent vasoconstrictor of both afferent and efferent arterioles---reduces filtration rate
• (^) Atrial natriuretic peptide is released from the atria when they are stretched (due to increased blood volume)
• (^) ANP causes an increase in GFR by increasing capillary surface area for filtration (dilate capillaries)