Lesson # 21The Urinary System 2

Chapter 26Objectives:

1- Describe the basic processes responsible for urine formation.2- Discuss the factors that influence glomerular filtration pressure.3- Describe the hormone influence in the volume and concentration of urine.

Fenestrated endothelium of capillaries

Renal tubule

Peritubular fluid

Blood

Basic Processes of Urine FormationDifferent segments of the nephron form the urine by three different processes:1- Filtration

2- Reabsorption

3- Secretion

Small solute molecules pass through pores and larger solutes and suspended materials are retained.

It is produced in the renal corpuscle when the blood pressure (hydrostatic pressure) forces water through the filtration membrane.

It is the removal of water and useful solutes from the filtrate. It is a selective process.

Most of the reabsorbed materials are nutrients that the body can use.

It occurs after filtrate has left the renal corpuscle.

It is the transport of solutes from the peritubular fluid to the filtrate.

Secretion

Reabsorption

Blood pressure Filtration

Waste that did not pass by filtration.

Almost any molecule smaller than 3 nm can pass freely through the filtration membrane:

2- Some substances of low molecular weight are bound to the plasma proteins and cannot get through the membrane (most calcium, iron, and thyroid hormone).

Kidney infections and trauma can damage the filtration membrane and allow albumin or blood cells to filter.Proteinuria (albuminuria):

Hematuria:

It is the presence of protein in the urine.

It is the presence of blood in the urine.

1- Water, electrolytes, glucose, fatty acids, amino acids, nitrogenous wastes, and vitamins.

Turned back:- Blood cells.- Plasma proteins.- Protein-bound minerals and hormones.- Most molecules > 8 nm in diameter.

Passed through filter:- Water- Electrolytes- Glucose- Aminoacids- Fatty acids- Vitamins- Urea- Acid uric- Creatinine

BLOOD

Renal corpuscle

FILTRATIONSmall solute molecules pass through pores and larger solutes and suspended materials are retained.

PCT

REABSORPTIONIt is the removal of water and useful solutes from the filtrate. It is a selective process.

NEPHRON LOOP

REABSORPTIONFurther reabsorption of water, sodium and chloride ions.

Water, ions and organic nutrients.

WaterWater

Sodium, Chloride Sodium, Chloride

DCT

SECRETIONSecretion of ions, acids, drugs, toxins.

COLLECTING DUCT

Variable reabsorption of water and reabsorption or secretion of sodium and potassium. Under control of Aldosterone and ADH.

Reabsorption or secretion of hydrogen, and bicarbonate. It is an important site for the control of body fluid pH.

PAPILLARY DUCTDelivery of urine to the minor calyx.

REABSORPTION and SECRETION

Peritubular fluid

Water

Na+, K+

H+, CO3H-

Na+, K+

H+, CO3H-

Blood in glomerular capillaries

Plasma proteins

Afferent arteriole

Efferent arteriole PCT

50 mm Hg

15 mm Hg

Net Hydrostatic Pressure:50 – 15 = 35 mm Hg

25 mm HgNet Filtration Pressure:35 – 25 = 10 mm Hg

Filtration Pressures

Capsular Space

Hydrostatic PressureColloid Osmotic Pressure

10 mm Hg

H2O

The Distal Convoluted Tubes and the Collecting Ducts are relatively impermeable to water when ADH is not present.

Large volume of dilute urine

The concentration of sodium and chloride in the peritubular fluid produce an osmotic flow of water from the Distal Convoluted Tubes and Collecting Ducts.

H2O

H2O

H2O

H2O

H2O

H2O

H2O

H2O

The presence of ADH causes the appearance of special water channels (aquaporins) in the membranes of the DCT and collecting ducts.

Small volume of concentrated urine

1- Aldosterone

It stimulates the reabsorption of more Na+ and secretion of K+. Water and Cl- follow the Na+. The net effect is that the body retains NaCl and water and blood pressure rises.

It is a steroid secreted by the adrenal cortex.

2- Natriuretic Peptides (ANP and BNP) They are secreted by the myocardium of the heart in response to high blood pressure.They reduce thirst and block the release of Aldosterone and ADH, thus increasing the diuresis and decreasing blood volume and pressure.

The Control of Urine Volume and Osmotic Concentration

3- Antidiuretic Hormone (ADH) It is secreted by posterior lobe of pituitary in response to dehydration and rising blood osmolarity.It makes the collecting duct more permeable to water. Water in the tubular fluid reenters the tissue fluid and bloodstream rather than being lost in urine. Consequently, less volume of more concentrated urine is produced.

4- Parathyroid Hormone (PTH) It is secreted from parathyroid glands in response to calcium deficiencyIt acts on PCT to increase phosphate excretion, and acts on the thick segment of the ascending limb of the nephron loop, and on the DCT to increase calcium reabsorption.

ADH stimulates the hypothalamic thirst center.