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School of Pharmacy

Excretion System

FA2141 – Human Anatomy and Physiology II

Dhyan Kusuma Ayuningtyas

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Background

The urinary system removes most metabolic wastes generated by body cells. The digestive and integumentary systems assist; however, their contributions are minor compared to that of the urinary system.

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Teaser

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Regulation

GOSTOP

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Functions

• Conserving valuable nutrients by preventing their excretion in the urine.

• Eliminating metabolic wastes, especially urea, uric acid, toxic substances, and drugs.

• Regulating blood plasma concentrations of sodium, potassium, chloride, calcium, and other ions by controlling the quantities excreted in the urine.

• Regulating blood volume and blood pressure by (a) adjusting the volume of water lost in the urine, (b) releasing erythropoietin, and (c) releasing renin.

• Working with the respiratory system to regulate blood pH

• Synthesizing calcitriol, a hormone derivative of vitamin D3 that stimulates calcium ion absorption by the intestinal epithelium.

• Helping the liver detoxify poisons; and during starvation, deaminating amino acids to be used by other tissues.

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Overview

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Overview

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Overview

KUBU

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Kidney: Anatomy Position

• The kidneys lie lateral to the vertebral column between the last thoracic and third lumbar vertebrae on each side.

• Due to the presence of the liver on the right side of the abdominal cavity, the right kidney sits slightly lower than the left kidney.

• The anterior surface of the right kidney is covered by the liver, the right colic (hepatic) flexure, and the duodenum.

• The anterior surface of the left kidney is covered by the spleen, stomach, pancreas, jejunum, and left colic (splenic) flexure.

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Kidney: Protective Barrier

1. The fibrous capsule covers the outer surface of the entire organ. This layer of collagen fibersmaintains the shape of the kidney and provides physical protection.

2. A layer of adipose tissue, the perinephric fat or perirenal fat capsule surrounds the fibrous capsule.

3. The renal fascia anchors the kidney to surrounding structures and attaches to the deep fascia surrounding the muscles of the posterior body wall.

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Kidney Structure (Including Nephron) 3D

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Kidney Structure 2D

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Kidney Structure 2D

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Kidney: Blood Supply

• Twenty to twenty-five percent of the total cardiac output, or about 1200 mL of blood, flows through the kidneys each minute.

• Each kidney receives blood from a renal artery. The renal artery branches from the lateral surface of the abdominal aorta near the superior mesenteric artery. It enters the renal sinus where it branches into the segmental arteries.

• These vessels supply portions of the adjacent kidney lobe. Numerous afferent arterioles branch from each cortical radiate artery to supply individual nephrons.

• A network of venules and small veins converges on the cortical radiate (interlobular) veins. The cortical radiate veins deliver blood to arcuate veins that empty into interlobar veins. The interlobar veins merge to form the renal vein; there are no segmental veins.

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Kidney: Nephron and Collecting System

• The nephron is the structural and functional unit of the kidney.

• Each nephron consists of a renal corpuscle and a renal tubule.

• The tubular segments of the nephron are named according to their location (proximal or distal), thickness (thick or thin), and shape (convoluted or straight).

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Kidney: Nephron and Collecting System

• Each nephron consists of a renal corpuscle and a renal tubule.

• The renal corpuscle, a spherical structure consisting of the glomerular capsule, a cup-shaped chamber, and a capillary network called the glomerulus.

• Each renal tubule empties into the collecting system, a series of tubes that carry tubular fluid away from the nephron.

• From the renal corpuscle, the filtrate enters a long tubular passageway that has three major subdivisions: • the proximal convoluted tubule.• the nephron loop, or loop of Henle. • The distal convoluted tubule.

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Kidney: Glomerulus

• Blood arrives at the glomerulus through the afferent arteriole and leaves by the efferent arteriole.

• Filtration across the walls of the glomerulus produces a protein-free solution known as the glomerular filtrate, or simply filtrate.

• The filtration process at the renal corpuscle involves five filtration barriers.

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Kidney: Glomerulus Filtration Barrier

• The endothelial surface layer. This glycocalyx limits filtration of large plasma proteins out of the glomerular capillaries.

• The capillary endothelium. The glomerular capillaries are fenestrated capillaries with pores 60–100 nm (0.06–0.1 mm) in diameter. These openings are small enough to prevent the passage of blood cells, but they are too large to prevent the diffusion of solutes, even those the size of smaller plasma proteins.

• The basement membrane. This prevents the passage of larger plasma proteins, but permits the movement of small plasma proteins, amino acids, glucose, and ions.

• The glomerular epithelium: the filtrate entering the capsular space consists of water with dissolved ions, small molecules, and few if any plasma proteins.

• The subpodocyte space: The subpodocyte space occupies approximately 60 percent of the filtration space of the glomerulus. It assists the filtration slits of the glomerular epithelium in the filtration process.

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Kidney: Proximal Convoluted Tubule (PCT)

• The proximal convoluted tubule (PCT) is the first part of the renal tubule.

• The primary function of the proximal convoluted tubule is reabsorption.

• Its cells actively reabsorb organic nutrients, ions, and plasma proteins (if any) from the filtrate.

• In addition, as the tubular fluid passes through the tubule, the epithelial cells reabsorb 60 percent of the sodium ions, chloride ions, and water.

• The tubule also actively reabsorbs potassium, calcium, magnesium, bicarbonate, phosphate, and sulfate ions.

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Kidney: Nephron Loop

• Nephron loop is divided into a descending limb and an ascending limb.

• Fluid in the ascending limb flows toward the renal cortex. The ascending limb contains active transport mechanisms that pump sodium and chloride ions out of the tubular fluid.

• The descending thin and ascending thick limbs are freely permeable to water but are impermeable to ions and other solutes.

• The net effect is that the nephron loop reabsorbs an additional 25 percent of the water from the tubular fluid and an even higher percentage of the sodium and chloride ions.

• Reabsorption in the PCT and nephron loop reclaims all of the organic nutrients, 85 percent of the water, and more than 90 percent of the sodium and chloride ions. The remaining water, ions, and all the organic wastes filtered at the glomerulus remain in the nephron loop and enter the distal convoluted tubule.

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Kidney: Distal Convoluted Tubule (DCT)

• The PCT is involved in reabsorption, while the DCT is involved in secretion.

• The DCT is an important site for • active secretion of ions, acids, and other

materials

• reabsorption of sodium and calcium ions from the tubular fluid

• reabsorption of water, which helps concentrate the tubular fluid.

• The sodium transport activities of the distal tubule are controlled by circulating levels of aldosterone secreted by the adrenal cortex.

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Kidney: The Collecting System

• The collecting system transports tubular fluid from the nephron to the renal pelvis and makes final adjustments to its osmotic concentration and volume.

• Antidiuretic hormone (ADH) controls the permeability of the collecting system. This permeability change is significant because the collecting ducts pass through the medulla, where the nephron loop has established very high solute concentrations in the interstitial fluids.

• If collecting duct permeability is low, most of the tubular fluid reaching the collecting duct will flow into the renal pelvis and the urine will be dilute.

• However, if collecting duct permeability is high, this promotes the osmotic flow of water out of the duct into the medulla. This results in a small amount of highly concentrated urine.

• The higher the levels of circulating ADH, the greater the amount of water reabsorbed, and the more concentrated the urine.

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Kidney: Review of Nephron Function

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Urine Transport, Storage, and Elimination

The ureters, urinary bladder, and urethra are responsible for the transport, storage, and elimination of urine.

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Ureter

• The ureters are a pair of retroperitoneal muscular tubes extending inferiorly from the kidneys to the urinary bladder.

• Each ureter is approximately 30 cm long and begins as a funnel-shaped continuation of the renal pelvis.

• The ureters penetrate the posterior wall of the urinary bladder. They pass through the bladder wall at an oblique angle, and the ureteral orifices are slit-like rather than rounded.

• This shape prevents backflow of urine into the ureter and toward the kidneys when the urinary bladder contracts

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Urinary Bladder

• The urinary bladder is a hollow, muscular organ that serves as temporary storage for urine.

• The dimensions of the urinary bladder vary, depending on the state of distension, but a full urinary bladder can contain about a literof urine.

• The urethral entrance lies at the most inferior point in the bladder.

• The region surrounding the urethral opening, known as the neck of the urinary bladder, contains a muscular internal urethral sphincter.

• The smooth muscle of the internal urethral sphincter provides involuntary control over the discharge of urine from the bladder.

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Urethra

• The urethra extends from the urinary bladder to the exterior.

• The male urethra is subdivided into three segments, while the female urethra is shorter and has only one segment.

• In the female, the urethra is very short, extending 3–5 cm from the bladder to the vestibule. The external urethral opening, or external urethral orifice, is situated near the anterior wall of the vagina.

• In the male, the urethra extends from the neck of the urinary bladder to the tip of the penis, a distance that may be 18–20 cm. The male urethra is subdivided into three portions: (1) the prostatic urethra, (2) the membranous urethra, and (3) the spongy urethra

• Only the external urethral sphincter is under voluntary control, through the perineal branch of the pudendal nerve. The sphincter has a resting muscle tone and must be voluntarily relaxed to permit urination. The autonomic innervation of the external sphincter becomes important only if voluntary control is lacking, as in infants or in adults after spinal cord injuries

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Urine Reflex: Storage and Voiding

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

• The urge to urinate generally appears when your urinary bladder contains about 200 mL of urine.

• Urine storage occurs by spinal reflexes and the pontine storage center. When urine is being stored, low-frequency afferent impulses from stretch receptors in the urinary bladder act to (1) increase sympathetic activity (inhibit detrusor contraction and stimulate internal urethral sphincter contraction) and (2) stimulate contraction of the external urethral sphincter

• Urine voiding occurs by spinal reflexes and the pontine micturition center. The voiding reflex begins when high-frequency afferent impulses from stretch receptors in the urinary bladder stimulate interneurons that relay sensations to the pontine micturition center. This center initiates sacral spinal reflexes that (1) stimulate increased parasympathetic activity (detrusor contracts and internal urethral sphincter relaxes), (2) decrease sympathetic activity (internal urethral sphincter relaxes), and (3) decrease somatic motor nerve activity (external urethral sphincter relaxes).

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