Chapter 10: Urinary System and Excretion

The Urinary System

The major roles of the urinary system are:

1. Maintaining homeostasis in blood of salt, water, and pH.

2. Removal of metabolic wastes from the body.

This process is called excretion.

Four Functions of the Urinary System

 Excretion of Metabolic WastesThe most notable metabolic wastes kidneys excrete are nitrogenous wastes. The primary one is urea and three others are ammonium, uric acid, and creatinine.

 Maintain Water-Salt BalanceKidneys maintain appropriate water-salt balance of blood to cause osmosis, the diffusion of water. This has the kidneys involved with regulating blood pressure.

Four Functions of the Urinary System

  Maintain Acid-Base Balance Kidneys monitor and help control blood pH by mainly excreting hydrogen ions to keep a 7.4 blood pH.

  Secretion of HormonesKidneys assist the endocrine system by secreting the hormones: Erythropoietin, Calcitriol, and Renin.

Organs of the Urinary System

The urinary system organs consists of the: kidneys, ureters, urinary bladder, and urethra.

Kidney Location

The bean-shaped kidneys are located near the small of the back, on either side of the vertebral column.

Organs of the Urinary System

The kidneys are covered by a tough tissue called the renal capsule.

The concave side of a kidney has a depression where the renal artery enters and the renal vein and ureter exit the kidney.

Two small muscular tubes called ureters conduct the urine from the kidneys to the urinary bladder for storage. Two sphincter muscles are located at the bottom opening of the bladder, allowing it to retain urine.

Urination – Figure 10.2

Urination

The inner sphincter is involuntarily controlled and the outer external sphincter can be voluntarily controlled.

With maturation, the brain controls this reflex and delays urination, until a suitable time.

Urination

As the bladder fills with urine, stretch receptors in its muscle layers send sensory nerve signals to the spinal cord and impulses return that cause the bladder to contract and sphincters to relax, so that urination can occur.

A small tube called the urethra carries urine from the bladder to an external opening.

Kidney Structure

A kidney has three major regions inside:

The Renal cortex,

Renal medulla, and

Renal pelvis.

The “functional units of excretion” are the microscopic tubules called nephrons.

A kidney is composed of over one-million nephrons that produce urine.

Blood Supply in a Kidney

Anatomy of a Kidney

Kidney Diagram – Figure 10.3

Anatomy of a Nephron

Each nephron has several parts.Glomerular capsule (Bowman’s capsule)

Closed cuplike end, lined with squamous cells.

Proximal convoluted tubuleTwisted part of tube, closest to the capsule.

Loop of the nephron (loop of Henle)Descending narrow tube that makes a U-turn.

Distal convoluted tubuleTwisted part of tube further from the capsule.

Collecting ductLarger tubule several nephrons finally enter.

Parts of a Nephron

Proximal Convoluted Tubule

Distal Convoluted Tubule

CollectingDuct

Nephron Loop

GlomerulusCapsule

Blood Supply of a Nephron

A wider afferent arteriole carries blood into the glomerular capsule, where it divides to become a knot of capillaries called the glomerulus.

Each nephron has its own blood supply.

Blood Supply of a NephronEach nephron has its own blood supply.

An efferent arteriole carries blood out of the glomerular capsule. An important feature here is the narrower efferent arteriole which increases pressure. The efferent arteriole then branches into the peritubular capillary network, surrounding the nephron.

Blood then enters a venulethat carries blood into the renal vein.

Nephron Anatomy

Urine Formation

Urine is 95% water and 5% solids (urea, salts, bicarbonate ions).

Urine formation is divided into three steps.

1.Glomerular filtration

2.Tubular reabsorption

3.Tubular secretion

Urine Formation

Glomerular Filtration

• Small molecules: including water, nutrients, salts, and nitrogenous wastes are forced by pressure out of the blood inside the glomerulus (capillaries) and into the glomerular capsule.

• Blood cells, platelets, and plasma proteins are too large and unable to pass through the capillary walls.

• About 180 liters of water are filtered daily.

Tubular Reabsorption

• Certain nutrients, water and some urea move from the nephron tubules back into the blood of the peritubular capillary network. Almost all glucose and amino acid molecules re-enter the blood from the proximal convoluted tubules.

• Tubular reabsorption is a selective process because

only molecules recognized by carrier protein molecules are actively reabsorbed.

• The number of carriers limits the rate of this process.

Reabsorption from Nephrons

Tubular Secretion

• Specific substances such as hydrogen ions (H+), creatinine, and drugs such as penicillin move from the blood into the nephron tubules.

• In the end, urine contains substances that have undergone glomerular filtration, but have not been reabsorbed and substances added by tubular secretion.

Steps in Urine

Formation

Figure 10.7

Kidneys and Homeostasis

The kidneys play a major role by: 1.Removing nitrogenous waste such as

urea, which is essential to maintaining homeostasis.

2.Maintaining the salt-water balance of the blood for proper blood volume and blood pressure.

3.Maintaining the 7.4 blood pH.

Water Reabsorption

The antidiuretic hormone (ADH) plays an important role of conserving body water by closing water channels in distal nephron tubules, thus reducing the loss of water in urine.

This hormone is released by the pituitary gland when water intake is low.

Kidneys and Homeostasis

Diuretics are chemicals that increase the flow of urine.

– Alcohol inhibits secretion of ADH; dehydration after drinking may contribute to the effects of a hangover.

– Caffeine increases the glomerular filtration rate and decreases tubular reabsorption of sodium.

Kidney Function Disorders

Many types of illness cause progressive renal disease and ultimate renal failure.

•Diabetes•Hypertension•Inherited conditions

Urinary tract Infections:•Nephritis (Pyelonephritis)•Cystitis•Urethritis

Kidney stones are hard granules of calcium, uric acid, and proteins that form in the renal pelvis.

Artificial Kidney Machine

Kidney dialysis requires patients to sit relatively still during a treatment, which often lasts over three hours and is required three to four times a week.

Replacing a Kidney

Patients with renal failure sometimes undergo a kidney transplant operation, during which a compatible donor kidney is received.