Urinary System

Module 15

Function of Urinary System

1. Filter blood to form urine.
2. pH control
3. Blood pressure regulation
4. Stimulation of red blood cell formation by the bone marrow
5. Vitamin D activation
6. Transport of urine
7. Storage and release of urine

Gross Anatomy of Urinary System

Kidneys - filter the blood

Ureters - carry urine to bladder

Urinary bladder - stores urine until it can be released

Urethra - releases urine from bladder to the outside

Kidneys are Retroperitoneal

Retroperitoneal means behind the peritoneum.

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Kidneys are surrounded by perirenal fat (adipose tissue).

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Movement of kidneys can be serious especially if the ureters get pinched or blocked.

Renal Capsule - Composed of

Fibrous connective tissue.

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Cortex - outer region

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Medulla - inner region

Medulla -

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1. Renal pyramids

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• Renal columns

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Anatomy of the Kidney

Medulla

Renal Pyramid

Renal Papilla

Minor Calyx

Major Calyx

Renal pelvis

Renal Pyramid

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Renal Papilla

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Minor Calyx

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Major Calyx

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Renal Pelvis

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Ureter

Function of Urinary System

• Filter blood to form urine
• pH control
• Blood pressure regulation
• Stimulation of red blood cell formation by the bone marrow (Erythropoiesis)
• Vitamin D activation
• Transport of urine
• Storage and release of urine

Nephron - Functional Unit of Kidney

1. A microscopic filter composed of precisely folded tubules
2. Anatomy of the nephron is crucial to its function

1. Renal Corpuscle
1. Bowman’s capsule
2. Glomerulus
2. Proximal Convoluted Tubule
3. Loop of Henle
4. Distal Convoluted Tubule
5. Collecting Tubule

Anatomy of the Renal Corpuscle

1. Bowman’s Capsule
1. Two layers
1. Parietal layer - outer layer
1. Composed of simple squamous epithelium
2. Visceral layer
• Porous
• Composed of podocytes
2. Allows a lot of filtrate to get pass through the filtration membrane
2. Glomerulus

Anatomy of the Renal Corpuscle

• Bowman’s Capsule
• Glomerulus
• A network of capillaries
• Very porous
• Held together by the podocytes

Filtration Membrane

Composed of

1. Porous wall of the glomerular capillaries
2. Basement membrane of the glomerular capillaries
3. Podocytes

Nephron

• Where is it located in the kidney?
• How does its unique anatomy enable filtration and formation of blood?

Filtration of blood to form urine

1. Glomerular Filtration
2. Reabsorption
3. Secretion
4. Reabsorption of water

1. Filtration = interstitial fluid or plasma minus the proteins. Need to get “good” stuff back.
2. Reabsorption - reabsorb “good” or rather needed stuff and water.
3. Secrete any big stuff or molecules that must be regulated by the kidney.
4. Urine (excretion) is the filtrate minus what was reabsorbed back into the blood and plus the molecules that were secreted into it.

Step 1:Glomerular Filtration

• The process of glomerular filtration results in the formation of filtrate NOT urine!
• Filtrate is blood plasma minus the proteins, filtered by the nephrons of the kidneys
• Renal blood flow rate - the rate at which blood flows through the kidneys (1 liter/minute)

Step 1:Glomerular Filtration

• The porous capillaries, and porous visceral layer of Bowman’s capsule allows most of the plasma and components (not proteins) to leave the capillaries and enter the capsular space.
• The filtrate then enters the proximal convoluted tubule.
• But what it left behind and how does this all work?!

Step 1:Glomerular Filtration

• Occurs very fast!
• Glomerular filtration rate - the rate at which filtrate is produced in glomerular filtration (125mL/minute)
1. Kidneys filter about 180 liters every day
2. All blood is filtered every 25 minutes

How?

Step 1:Glomerular Filtration

Why does the plasma leave the capillary?

1. Highly permeable filtration membrane
2. High blood pressure within the glomerular capillaries
1. What causes the high blood pressure in these capillaries?

Step 1:Glomerular Filtration

1. Highly permeable filtration membrane
2. High blood pressure within the glomerular capillaries
• What causes the high blood pressure in these capillaries?
1. Anatomy of the capillaries

Efferent arteriole is narrower than the afferent arteriole.

Step 1:Glomerular Filtration

• Highly permeable filtration membrane
• High blood pressure within the glomerular capillaries
• What causes the high blood pressure in these capillaries?
• Anatomy of the capillaries - Efferent arterioles are narrower than the afferent

Step 1:Glomerular Filtration

High blood pressure within the glomerular capillaries

Anatomy of the capillaries - Efferent arterioles are narrower than the afferent

Glomerular Capillary Pressure (GCP) - High GCP pushes filtrate out of glomerular capillaries

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Step 1:Glomerular Filtration

What factors oppose the Glomerular Capillary Pressure (GCP)?

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1. Capsular Pressure
2. Colloid osmotic pressure

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If systemic blood pressure drops 10 mm Hg,renal failure and death can occur.

Step 2:Reabsorption

1. Active reabsorption
1. Requires ATP & a carrier
2. Nutrients: glucose, amino acids, minerals, vitamins C & B
3. Proteins don’t have a carrier but are reabsorbed via pinocytosis
2. Passive reabsorption
• Does NOT require ATP

Step 2:Reabsorption

• Active reabsorption
• Tubular maximum - the maximum rate of reabsorption by active transport through the nephron tubules
1. Limits how much of it can be reabsorbed
• Example - glucose

Step 2:Reabsorption

• Passive Reabsorption
• Water
• Chloride
• urea

Step 3:Secretion

• Sending waste that wasn’t removed during glomerular filtration out of the blood and into the nephron tubules
• Occurs in distal tubules
• Examples: drugs, potassium ions (K⁺), hydrogen ions (H⁺)

Step 4:Reabsorption of Water

• Water is reabsorbed into the blood via osmosis in the descending loop of Henle.
• Solutes diffuse into the tubules
• As the filtrate reaches the bottom of the loop, homeostasis is achieved and the filtrate is concentrated as compared to the filtrate in the proximal tube

Step 4:Reabsorption of Water

• Water is reabsorbed into the blood via osmosis in the descending loop of Henle.
• Solutes diffuse into the tubules
• As the filtrate reaches the bottom of the loop, homeostasis is achieved and the filtrate is concentrated as compared to the filtrate in the proximal tube

Step 4:Reabsorption of Water

• Descending loop of Henle is permeable to both water and solutes.
• Ascending loop of Henle is not permeable to water and solutes.
• Active transport of sodium, potassium and chloride ions only back to the blood
• This further concentrates the filtrate.

Step 4:Reabsorption of Water

• Descending loop of Henle is permeable to both water and solutes.
• Ascending loop of Henle is not permeable to water and solutes.
• Active transport of sodium, potassium and chloride ions only back to the blood
• This further concentrates the filtrate.

Impact of ADH

• ADH - Antidiuretic Hormone
• Secreted by posterior pituitary
• ADH makes the membrane of the collecting duct permeable to water so that it can be reabsorbed in the blood.
• Without AG, the membrane is less permeable and water will be secreted in urine

Storage & Release of Urine

• Stored in Bladder
• Composed of transitional stratified epithelium
• Designed to stretch
• Parasympathetic Nervous System is stimulated as the bladder is stretched, triggering contractions of the smooth muscle of the bladder to expel urine.

Storage & Release of Urine

• External Urethra Sphincter
• Composed of skeletal muscle so is under conscious brain control
• Normally contracted so urine is not expelled randomly
• Consciously relax to expel urine

Blood Pressure Control by Kidndeys

Volume of blood determines blood pressure.

1. Increase blood volume - increase blood pressure.
2. Decrease blood volume - decrease blood pressure.

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Blood Pressure Control by Kidndeys

• Juxtaglomerular cells located in afferent arteriole detect decreases in blood pressure or sodium content.
• These cells release renin, an enzyme that converts an inactive protein called angiotensinogen to angiotensin I which is converted to angiontensis II in the lungs.

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Blood Pressure Control by Kidneys

Angiotensin II does 4 things:

1. Increases vasoconstriction
2. Increases thirst
3. Increases salt appetite
4. Causes release of aldosterone, a hormone produced by adrenal cortex that increases sodium ion reabsorption

Increase in Blood Volume

• Detected in heart cells which produce a hormone called atrial natriuretic.
• Atrial natriuretic inhibits nephrons from reabsorbing sodium ions
1. Since water follows solutes, less water is reabsorbed and blood volume decreases which lowers blood pressure.

Acid - Base Balance

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