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Production of urine by the mammalian kidney. Overview of the urinary system What urine is How it is formed Control of urine formation. nephron. Blood enters and leaves here. Urine leaves here. Why do kidneys produce urine? To regulate blood composition and volume.

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Production of urine by the mammalian


Overview of the urinary system

What urine is

How it is formed

Control of urine formation



Blood enters

and leaves here

Urine leaves here

why do kidneys produce urine to regulate blood composition and volume
Why do kidneys produce urine?To regulate blood composition and volume
  • Disposal of metabolic waste
    • Urea, creatinine, ammonium, uric acid
  • Maintenance of water-salt balance
    • Sodium, potassium, bicarbonate, etc.
  • Maintenance of acid-base balance
    • Blood pH must be approx. 7.4

Urine is formed in nephrons

About 1 million nephrons per kidney

Each DAY:

Approx 1000-2000 liters of blood flow though


Approx 180 L of filtrate processed

Approx 1.2 L of urine actually excreted

the rest is reabsorbed



Steps in urine formation

Filtration- water and small molecules

removed from blood

Reabsorption- water and essential

molecules returned to blood

Secretion- wastes and excess salts

added from body fluids

to filtrate (urine)




(loop of Henle)


Control of filtration rate

Hydrostatic pressure of blood forces it out

of capillary and into glomerulus

Osmotic pressure of plasma in glomerulus

and hydrostatic pressure in glomerulus

resists it

(Net filtration pressure: the difference)


Filtration rate is affected by:

Constriction of afferent or efferent arteriole

Colloid pressure (of blood or filtrate)

Generally about 180L of urine is formed

during the day

Kidney damage can occur if blood pressure

drops too low


Control of urine production

High solute concentration in tissue- helps draw

water out of tissue

Hormones- regulate water reabsorption, ion

secretion or reabsorption

affect urine composition and volume

Hormones that control blood pressure act

on kidney- to retain or lose water


Regulation of filtration rate

Rises when body fluid levels are high; decreases

when fluids must be conserved

Sympathetic nervous system- conserve fluid

by constricting afferent arterioles

Renin-angiotensin system




All small molecules enter tubule; most are

put back

Tends to be localized to certain parts of the

nephron- differences in permeability

Most reabsorption takes place in proximal

convoluted tubule

amino acids


various ions; vitamins; acids


Efficiency of urine formation

Component Urine Blood plasma

Sodium (mEq/l) 147.5 138.4

Potassium 47.5 4.4

Glucose (mg/dL) 0.009 90

Proteins 0.000 7500

Urea (mg/dL) 1800.00 305

Ammonia 60.00 0.2


Regulation of urine concentration and volume

Aldosterone- additional reabsorption of sodium

(and therefore water) in distal tubule

secretion of potassium

ADH acts on distal convoluted tubule and

collecting duct, to reabsorb water

(if body fluid levels are high, ADH secretion drops)


Tubular secretion

Substances are removed from capillaries to

renal tubule

Waste products

Excess ions


Micturition (urination)

Bladder is distended as it fills; stretch receptors

trigger the micturition reflex

Parasympathetic nerves stimulates the detrusor

muscle to contract

External urethral sphincter is under voluntary

control; neurons originate in brain stem

and cerebral cortex


Kidney failure is fatal if untreated



first successful organ transplant in

humans: kidney, 1954

What about transplants?