Urinary SystemUrinary SystemChapter 18 2011

Introduction

• The function of the urinary system include– Eliminating organic waste products– Regulating plasma concentration of ions– Regulating blood volume and pressure by

adjusting the volume of water lost and releasing hormones

– Stabilizing blood pH and– Conserving nutrients.

Organization

• The urinary system includes the kidneys, the ureters, the urinary bladder and the urethra.

• The kidneys produce urine (a fluid containing water, ions, and soluble compounds), during urination urine is forced out of the body.

The Kidneys

• Both kidneys lie in a retroperitoneal position.• A fibrous renal capsule surrounds each kidney.• The hilus provides entry for the renal artery and exit

for the renal vein and ureter.• The ureter is connected to the renal pelvis. This

chamber branches into two major calyces, each connected to four or five minor calyces which enclose the renal papillae.

• Urine production begins in nephrons.

Anatomy of the Kidney

The Nephron

• The nephron is the basic functional unit in the kidney• It includes the renal corpuscle and a renal tubule,

which empties into the collecting system through a collecting duct.

• From the renal corpuscle, the filtrate travels through the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule.

• As the filtrate travels along the tubule, its composition gradually changes. The changes that occur and the urine that results depends on the specialized activities under way in each segment fo the nephron.

• Each nephron empties into a collecting duct which eventually delivers the fluid, now called urine, into the renal pelvis.

Functions of the Nephron

• Each part of the nephron has a role in converting the filtrate into urine.

• The renal corpuscle is the site of filtration, but this passive process which permits of passage of organic wastes also permits the passage of glucose, fatty acids and amino acids and water. These must be reclaimed.

The Nephron

Blood Supply to the Kidneys

• In healthy individuals, about 1200ml of blood flows through the kidneys each minute – 25% of the cardiac output.

• Each kidney receives blood from a renal artery that originates from the abdominal aorta.

The Functions of the Nephrons and Collecting Tubules of the Kidney

Region Primary Function

Renal Corpuscle Filtration of plasma to initiate urine formation

PCT Reabsorption of ions, organic molecules vitamins, water

Loop of Henle Descending: reabsorption of H2O from filtrateAscending: reabsorption of ions creating a concentration gradient– allows kidney to make concentrated urine.

DCT Reabsorption of sodium ions, secretion of acids ammonia, drugs

Collecting duct Reabsorption of water and sodium and bicarbonate ions.

Principles of Urine Production

• Primary purpose is to maintain homeostasis by regulating the volume and composition of the blood.

• Involves the excretion and elimination of dissolved solutes, specifically the following:– Urea: produced by the breakdown of amino acids.– Creatinine: produced through the breakdown of

creatinephosphate by skeletal muscles.– Uric acid: produced by the breakdown of RNA

• These products must be secreted in solution, so there is water loss.

• Kidneys minimize this by producing a urine that is 4 to 5 times more concentrated than normal body fluids.

• Kidneys also ensure that urine does not contain useful organic substances such as sugars and amino acids

Kidneys use three processes

• Filtration: blood pressure forces water and solutes across filtration membrane.

• Reabsorption: selectively removing water and solute molecules from the filtrate through osmosis and carrier proteins.

• Secretion: Active transport of solutes across the membrane into the filtrate.

• All three processes must be carefully balanced. If both kidneys fail, death will occur within a few days.

Summary of Urine Formation

Control of Kidney Function

• Regulated in three ways.– 1. by local, automatic adjustments in blood

pressure.– 2. through activities of the sympathetic division of

the autonomic nervous system.– 3. through the effects of hormones.

• Aldosterone: a hormone that regulates level of sodium in the body. The higher the aldosterone levels, the more sodium ions are reclaimed and the more potassium ions are lost.

• Antidiuretic hormone (ADH): regulates how much water is reabsorbed in the kidneys. If levels are low, little water reabsorption will occur. If levels are high, very little urine is produced but the concentration of solutes will be 4 to 5 times that of extracellular fluids.

Transport, Storage and Elimination

• Filtrate modification and urine production end when the fluid enters the renal pelvis. The urinary tract ( the ureters, urinary bladder, and urethra) is responsible for the transport, storage and elimination of the urine.

• Ureters: a pair of muscular tubes that carry urine from the kidneys to the urinary bladder.

• Urinary Bladder: a hollow muscular organ that stores urine prior to urination. Can hold about 1 liter of urine.

• Urethra: : extends from the neck of the urinary bladder to the exterior. In females it is short – about 3 cm. In males it extends from the bladder to the tip of the penis – about 20 cm.

Micturition Reflex and Urination

• Urine reaches the bladder by peristaltic contractions of the ureters.

• Stretch receptors in the wall of the bladder make us aware of the fluid pressure in the bladder when about 200 ml of urine are present.

• The external sphincter muscle is under voluntary control. This control continues until the bladder contains about 500 ml. of urine. At that point, urination may happen even if you don’t want it to.

The Nephron

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