Urinary System

What is the urinary system? (2)

• Aka renal system

• Responsible for excretion and osmoregulation

What is excretion?

The process of removing metabolic waste from the cells, tissue fluid, and blood of living organisms

What is osmoregulation?

The control of water balance of the blood, tissue, or cytoplasm of a living organism

What are the structures in the urinary system?

Kidneys → ureters → urinary bladder → urethra

What is the main organ of the urinary system?

Kidney

What are the 3 tissues that surround the kidney?

Renal fascia,

What are the 11 parts of the kidney?

• Renal artery

• Renal vein

• Renal fascia

• Adipose capsole

• Renal capsule

• Renal cortex

• Renal medulla/ pyramids

• Renal pelvis

• Ureter

• Minor calyx

• Major calyx

Note: You don’t really need to know the last 2

What is the renal artery? (2)

• Carries high-pressure arterial blood to the kidney to be filtered (cleaned)

• This blood is high in nitrogenous wastes and oxygen

What is the renal vein?

• Takes filtered venous blood away from the kidney

• This blood is low in nitrogenous wastes and oxygen (which is used by renal cells to fuel ATP production)

What does the renal fascia do? (3)

• Outermost layer

• Thin layer of dense connective tissue

• Anchors the kidney to the surroundings

What does the adipose capsule do?

• Middle layer

• Fatty layer

• Prevents the kidney from trauma

What does the renal capsule do?

• Deepest layer

• Smooth, semitransparent sheet of dense connective tissue

• Adheres tightly to the outer surface of the kidney and gives it its distinctive bean shape

What is the renal cortex?

• The outer region of the kidney just below the capsule

• This should be a different shade than the rest of the kidney

• This is where the glomeruli and Bowman’s capsules are located

What is the renal medulla?

• Inside part of the kidney with loops of Henle to control salt and water balance by pumping of Na⁺

• ADH (antidiuretic hormone) works here

• The region below the cortex that is segregated into triangular and columnar regions.

  • The triangular regions are called the renal pyramids, which should be striated (or striped) in appearance due to the collecting ducts running through them.

What is the renal pelvis?

• Innermost region located below the renal pyramids

• A hollow, sac-like cavity within the kidney that is connected to the ureter.

• This is where urine is collected before draining into the ureter.

What is the ureter?

Tubes that carry urine from kidney to bladder for temporary storage until it can be released by urethra

What are the nephrons?

• The functional unit in the kidneys

• Responsible for blood filtration and formation of urine

Note: There are more than 1 million nephrons in a human kidney

What are the 2 structural components of the nephrons?

Renal corpuscle and renal tubule

What are the 2 components in the renal corpuscle?

• Glomerulus

  • A ball of capillaries that have pores

  • Surrounded by a membrane that filters the blood

• Bowman’s capsule

  • A cup-shaped structure at the end of the nephron

  • Surrounds the glomerulus

What is the afferent arteriole?

• Takes blood to the glomerular capillaries from the renal artery

  • The blood is filtered through tiny pores where small solutes and water combine in what is now called the filtrate

  • The filtrate is captured by the Bowman’s capsule

What is the efferent arteriole?

• Takes blood away from the glomerular capillaries into the peritubular capillaries and then into the renal vein

Note: Unlike other capillaries, the glomerulus does not transfer blood to a venule but rather to the efferent arterioles

What are the peritubular capillaries?

Part of the circulatory system (capillary network) surrounding the nephrons

What are the 4 components in the renal tubule?

• Proximal convoluted tubule (PCT)

  • Lined with microvilli to increase surface area

  • Has many mitochondria to provide ATP for active transport

• Nephron loop (aka the loop of Henle)

  • Carries the filtrate from the proximal tubule to the distal tubule

  • Descends into the medulla of the kidney

  • The concentration gradient of salt increases as you move down the medulla of the kidney

• Distal convoluted tubule

  • Conducts urine from the loop of Henle to the collecting duct

  • It is the final place where blood pH and ions are balanced

• Collection duct

  • Collects urine and carries it into the renal pelvis to the ureter

  • This is where the final water balance of the blood occurs

How does blood travel in the kidneys?

Aorta →renal artery → afferent arterioles → glomerulus → efferent arterioles → peritubular capillaries → renal veins → proximal tubule → loop of Henle →distal tubule → collecting ducts

What are the 4 main functions of the kidney?

1. Filter the blood and produce urine

2. Balances blood volume and water levels (aka Osmoregulation)

3. Regulates/balances blood pH

4. Endocrine Gland

How do the kidneys filter the blood and produce urine?

In 3 steps: glomerular filtration, reabsorption, and secretion

What happens in glomerular filtration?

Movement of materials from the blood → the Bowman’s capsule

• Unfiltered blood is carried from the renal artery → afferent arteriole → glomerulus

• The glomerulus receives blood under high pressure

• Small molecules (water, salts, urea, glucose, and amino acids) diffuse into the Bowman’s capsule

  • This product is called the filtrate

• Large substances (red and white blood cells, platelets, and plasma proteins) remain in the blood

  • They move through the efferent arteriole into a bed of peritubular capillaries surrounding the tubules, which eventually leave the kidney as the renal vein

What is the filtrate?

What happens in reabsorption?

• Transfer of solutes and water from the nephron back into blood

• Selective reabsorption takes place in the proximal tubule, loop of Henle, distal tubule, and collecting duct

Why does selective reabsorption occur in the reabsorption process?

• Changes in the permeability of cells lining the nephron

• Change in solute concentration of tissue surrounding nephron

What happens in the reabsorption process at the 1st destination?

• Filtrate enters the proximal tubule

• Active transport of Na⁺ (while Cl^- follows passively)

• Passive transport of water via osmosis as a result of the active transport of sodium chloride

• Active transport of glucose, vitamins, and amino acids

  • Reabsorbed into the interstitial fluid (fluid directly surrounding tubule) and eventually the peritubular capillaries

• Reabsorption of glucose occurs until a threshold has been reached

  • Excess will remain in the nephron and be excreted in urine

What happens in the reabsorption process at the 2nd destination?

• Filtrate descends the loop of Henle, which dips deep into the medulla of the kidney

• The descending loop of Henle is impermeable to salts, but water leaves via osmosis

• Results in the filtrate becoming increasingly more concentrated

  • Thus, the loop of Henle is hypotonic (higher salt concentration) to the medullary fluid (interstitial space)

• The ascending loop of Henle is permeable to salt, but not water (reverse of the former)

  • As the filtrate ascends, Na⁺ and Cl^- initially move out passively, then are actively pumped out at the top of loop

What happens in reabsorption at the 3rd destination?

• Filtrate enters the distal tubule

• Na⁺, Cl^-, and HCO₃^- are actively pumped out of the lumen of the loop of Henle into the interstitial space

• Water follows passively (both substances are reabsorbed into the peritubular capillaries that surround it)

• Producing a hypertonic environment

What happens in reabsorption at the 4th (last) destination?

• Filtrate enters the collecting duct

• Na+, Cl– and HCO3– are actively transported out of the collecting duct

• Water and urea diffuse out as well

• Formation of urine occurs at this point.

What happens in secretion?

Transfer of materials from blood into the nephron (reverse of reabsorption)

• Excess H⁺, K⁺, and drugs are moved out of peritubular capillaries (blood) into the proximal and distal tubule lumen (filtrate) to be excreted.

• Protein by-products such as ammonia are secreted into the proximal tubule

• Substances typically move through active transport

• Critical for balancing blood pH (secretion of H+).

What is urine?

A liquid waste product filtered from your blood by the kidneys

What does urine contain?

• 96% water

• 2% urea

• 2% uric acid, creatine, ammonia, sodium, potassium, chloride, phosphate, sulphate, oxalate

What are the 3 nitrogenous wastes?

Ammonia, urea, and uric acid

What is ammonia?

• Extremely toxic

• Requires a lot of water for biological excretion

• Animals release ammonia across the whole body surface or through gills/ aquatic animals

What is urea?

• Less toxic version of ammonia

• Ammonia is converted into urea in the liver of mammals and most adult amphibians

• The circulatory system carries urea to the kidneys, where it is excreted.

• Conversion of ammonia to urea is energetically expensive, but uses less water than ammonia

What is uric acid?

• Excreted by Insects, land snails, and many reptiles (including birds)

• Largely insoluble in water

• Can be secreted as a paste with little water loss

• More energetically expensive to produce than urea

How do the kidneys balance blood volume and water levels?

• Includes regulating extracellular fluid (ECF) levels

Regulating Blood Volume

• Blood volume is directly related to water balance/osmoregulation and urine production

• Increased water/fluid intake will result in greater blood volume which the kidney will counter by increasing urine production

• Decreased water/fluid intake will result in lower blood volume which the kidneys will counter by decreasing urine production

How does the kidney regulate blood (osmotic) pressure?

• When blood pressure (aka osmotic pressure) is high (due to excess sweating, dehydration)

• Sensed by osmoreceptors in the hypothamus

• Signals thirst → Increased fluid intake to lower osmotic pressure

• Shruken hypothalamus cells signal the pituitary gland to sevrete the hormone ADH (antidiuretic hormone, aka vasopressin)

• ADH acts on the collecting duct to increase water reabsorption (decrease urine production)

• When blood pressure (aka osmotic pressure) is low (due to bloo lass, dehydration)

• Specialized cells within the juxtaglomerular apparatus (part of the kidney) relase renin

• Renin converts angiotensinogen to angiotensin

• Angiotensin:

  • Causes constriction of blood vessels

  • Stimulates the adrenal gland to produce the hormone aldosterone → increases salt and water reabsorption by the kidneys/nephrons

How does the kidney regulate blood pH?

• Blood pH must stay within a range of 7.3 – 7.5.

• Regulated by the active transport of hydrogen ions (H+ ) into the urine.

• As blood pH fluctuates, the secretion of H+ either increases or decreases until it returns to normal.

How do the lungs help in the regulation of blood pH?

• The bicarbonate-carbonic acid buffer system

  • Via the selective secretion of [H⁺] ions

  • Bicarbonate-Carbonic acid buffer

• If the blood is too acidic, excess H+ combine with HCO3 - (bicarbonate ion) to form H2CO3 (carbonic acid), which results in an increased production of CO2 . This process will increase the pH of the blood (become more basic).

  • Enzyme: Carbonic anhydrase

What is the secondary function of the kidneys in the endocrine system?

The kidneys produce and secrete 3 major hormones:

• Renin

  • Regulates angiotensin and aldosterone

  • Helps regulate blood pressure

• Calcitriol

  • The active form of vitamin D

  • Increases absorption of calcium in the intestines

  • And reabsorption of phosphate in the kidney (proximal convoluted tubule)

• Erythropoietin

  • Released in response to hypoxia (low levels of oxygen)

  • Stimulates erythropoiesis (production of red blood cells) in the bone marrow

  • Makes red blood cells

What are 3 kidney diseases?

Diabetes Mellitus, Diabetes Insipidus, and Bright’s Disease

What is diabetes mellitus?

• Caused by inadequate secretion of insulin from islet cells in the pancreas

• Blood sugar levels tend to rise without insulin

• Excess sugar remains in the nephron and provides an osmotic pressure that opposes the osmotic pressure created by other solutes that have been actively transported out of the nephron

• Excretion of large volumes of urine

• Often thirsty

What is diabetes insipidus?

• Destruction of the ADH-producing cells of the hypothalamus or the destruction of n

What are diseases of the kidney?

Nephrology - internal medicine, deals with kidney function and disease related to renal malfunction – dialysis and organ transplantation

Urology - surgery, deals with kidney structural abnormalities – cancer, cysts, urinary tract (bladder) issu