Urinary System

3 main functions of the kidneys

1. Filter metabolic waste from the blood

2. Regulate electrolyte and fluid balance in the blood

3. Regulate water and blood pressure/blood flow

Inner and outer layers of the kidney

• Inner: renal medulla (cone shaped)

• Outer: renal cortex (located directly beneath the renal capsule)

Nephron (description and basic function)

• Microscopic structural and functional unit of the kidney

• Responsible for filtering blood and excreting urine

Glomerulus (description and function)

• Network of capillaries inside the nephron (in the renal cortex)

• First filtration site, responsible for filtering water, salt, glucose, amino acids, and urea from the blood (driven by glomerular blood pressure)

Bowman’s Capsule (description and function)

• Cup-like sac that surrounds the glomerulus at the start of the nephron (in the renal cortex)

• Works with the glomerulus to filter the blood and separate salt, glucose, and urea

• Collects filtrate

Filtrate

• Fluid filtered out of the blood plasma containing water, glucose, amino acids, urea/waste products

• Filtered by the glomerulus and collected by Bowman’s capsule for transportation

Tubular reabsortion

Process that moves water and solutes out of the filtrate and back into the bloodstream

Proximal convoluted tubule

• First segment of the renal tubule, connected to Bowman’s capsule and located in the renal cortex

• Performs the majority (~65%) of tubular reabsorption

  • Especially glucose and amino acids

  • Large amounts of water, bicarbonate, nutrients like potassium, calcium, magnesium

Loop of Henle

• U-shaped part of nephron that starts in the cortex and dips down to switch back in the medulla

  • Between the proximal tubule and the distal tubule

• Also a site of reabsorption: balances water and sodium chloride (descending = water out and salt kept in, ascending = salt out and water kept in)

Distal convoluted tubule

• Short segment of nephron, between the loop of Henle and emptying into the collecting duct (in the renal cortex)

• Also a site of reabsorption: final balancing of electrolytes and pH

• Salt reabsorption here can be controlled by aldosterone

Collecting ducts

• Final part of the nephron system, descend in a straight line down into the medulla

• Water reabsorption here is controlled by antidiuretic hormone (ADH), creating concentrated or dilute urine

• Urine moves through minor & major calyces and then into the renal pelvis

Renal pelvis

• Center of the kidney (funnel shaped), responsible for collecting urine and then passing it into the ureter

Ureter

• Thin muscular tube responsible for carrying urine from the renal pelvis to the bladder

• Typically have two—one descending from each kidney

Urinary bladder

• Hollow muscular organ that stores urine

• Can hold ~400-800 mL

• Internal sphincter at the base where it meets with the urethra

Internal sphincter

• Ring of smooth muscle where the bladder and urethra meet

• Controlled by the autonomic nervous system (involuntary) and signaled to relax/open when the bladder is full

External sphincter

• Ring of skeletal muscle located farther down the urethra (in/near the pelvic floor)

• Controlled by the somatic nervous system (voluntary) and contracted/relaxed to hold/release urine

Urethra

• Hollow tube that carries urine from the bladder to outside the body

How does blood enter the kidneys?

Via the renal artery

How does blood leave the kidneys?

Via the renal vein

Nitrogenous waste

• Toxic nitrogen-containing substance created as a byproduct of breaking down proteins/nucleic acids

• Primarily urea, uric acid, creatinine in humans

• Filtered and removed by the kidneys for excretion

How do dehydration and over-hydration influence blood pressure?

• Dehydration means less water in the blood, which reduces blood volume and therefore reduces blood pressure

• Over-hydration means there is more water in the blood, increasing blood pressure and volume, and causing the collecting ducts to stop permitting water reabsorption which means more water goes with the urine (diluted urine)

Four functions of angiotensin II

1. Constricts blood vessels

2. Stimulates thirst

3. Stimulates production of aldosterone

4. Stimulates production of antidiuretic hormone

All work to restore/increase blood volume when blood pressure is low (and not due to dehydration)

Where is angiotensinogen produced and what does it combine with to create angiotensin 1?

• Produced by the liver

• Combines with renin enzyme produced by the kidneys

How does angiotensin I convert to angiotensin II?

• Angiotensin I (produced by combining angiotensinogen and renin) combines with the ACE enzyme produced by the lungs and kidneys

Aldosterone

• Hormone secreted by the adrenal gland (triggered by angiotensin II)

• Increases reabsorption of sodium ions, which leads to increased reabsorption of chloride ions and water

• Increases blood volume, decreases amount of water lost to urine

Antidiuretic hormone (ADH)

• (Aka vasopressin) Hormone secreted by the pituitary (triggered by angiotensin II)

• Increases the amount of water reabsorbed by the collecting duct

• Decreases amount of water lost to urine