Premedical Biology: Excretory System

• The excretory system is a key system in premedical biology

Structures of the Excretory System

• A pair of kidneys are part of the system
• A pair of ureters transport urine
• The urinary bladder stores urine temporarily
• The urethra is a tube that carries urine out of the body

Ureters

• The ureters are tubes that transport urine from the kidneys' pelvis to the urinary bladder

Urinary Bladder

• The urinary bladder serves as a temporary storage site for urine before elimination

Urethra

• The urethra is a tube responsible for transporting urine from the urinary bladder to the outside
• A circular muscle called a sphincter controls the outer end of the urethra

Kidney Anatomy

• Each kidney consists of the cortex, medulla, and pelvis

Cortex

• The cortex is the location where blood filtration occurs

Medulla

• The medulla contains collecting ducts that transport filtrate, or filtered substances, to the renal pelvis

Pelvis

• The pelvis a hollow cavity where urine accumulates before draining into the ureter

Nephron: The Functional Unit

• Nephrons are the structural and functional filtration units within the kidney
• A normal kidney contains 800,000 to 1 million nephrons

Renal Corpuscle

• The renal corpuscle is a filtering component that consists of a glomerulus and Bowman's capsule
• An afferent arteriole enters the glomerulus, and an efferent arteriole exits it
• The glomerulus comprises a capillary tuft receiving blood from an afferent arteriole
• Bowman's capsule surrounds the tuft, featuring visceral and parietal layers
• Primary urine is produced between the two layers
• The parietal outer layer consists of simple squamous epithelium
• The visceral layer comprises podocytes, facilitating ultrafiltration of blood plasma
• The filtration barrier includes the endothelium, podocytes, and their basement membranes
• Glomerular blood pressure provides the driving force for filtering blood plasma into Bowman's capsule
• An adult human produces 125 mL of primary urine per minute or 1.5-2 liters per day

Juxtaglomerular Apparatus

• The juxtaglomerular apparatus consists of the macula densa, attaching to the afferent arteriole
• The juxtaglomerular apparatus cells affect blood pressure by renin production, stimulating the Renin-Angiotensin-Aldosterone axis

Renal Blood Circulation

• The renal artery, originating from the aorta, supplies each kidney
• The renal artery divides into 2-3 branches before entering the renal cortex
• One branch supplies the upper, another the middle, and the last one the lower portion of the kidney
• Interlobular arteries branch from these, giving rise to afferent arterioles
• Blood then flows to the glomerulus through the afferent arterioles
• Interlobular veins collect blood and then pass it to the arcuate veins
• The arcuate veins continue to the interlobar veins, which merge into the renal vein

Renal Tubule

• The proximal tubule leads from Bowman's capsule to the Loop of Henle
• It is lined by simple cuboidal epithelium
• The proximal tubule is responsible for absorption of amino acids, proteins, glucose, lactate, and urea
• The proximal tubule is also responsible for ion transport, bicarbonate, sodium, phosphate transport including water
• The loop of Henle, a long loop extending into the medulla, has a descending limb that is squamous and permeable to water
• The ascending limb of the loop of Henle is cuboidal and transports ions
• Connecting the loop of Henle to the collecting duct, the distal tubule is lined with simple cuboidal cells
• The distal tubule's functions involve ion transport, absorption and secretion under hormonal control

Tubule Functions

• Functions involve reabsorption and secretion of solutes like ions (sodium), carbohydrates (glucose), and amino acids (glutamate)
• Each nephron segment has specialized functions, including reabsorption of glucose, water, salts, and urea

Transports

• Passive diffusion occurs in the direction of the concentration or electrical gradient
• Primary active transport occurs against the gradient and requires energy (ATP)
• Secondary active transport uses a transport protein that harnesses the concentration gradient established by primary active transport through another transport protein

Loop of Henle

• A counter current multiplier system promotes high osmotic pressure

Collecting Duct System

• Intercalated cells with mitochondria and principal cells interact with antidiuretic hormone (ADH)
• Electrolyte and fluid balance is maintained through reabsorption and excretion
• The system regulates various electrolytes: chloride, potassium, hydrogen ions, and bicarbonate

Functions of The Renal System

• Homeostatic functions include regulating electrolytes and reabsorbing water, proteins, glucose, and amino acids
• Normal diuresis is 1.5-2 l/day, polyuria is higher than 2 l/day, oliguria is lower than 0.5 l/day, and anuria is lower than 0.1 l/day
• Excretion of wastes includes nitrogenous wastes like urea from protein catabolism and uric acid from nucleic acid metabolism
• Produces hormones like calcitriol, renin, and erythropoietin secreted by the kidney
• Antidiuretic hormone, aldosterone, parathyroid hormone, epinephrine, and natriuretic peptides (ANP and BNP) regulate kidney functions
• Aldosterone increases reabsorption of sodium and water while releasing potassium
• Antidiuretic hormone (Vasopressin) affects tissue permeability to control water reabsorption in tubules, and maintains homeostasis through water, glucose, and salt regulation

Osmolality Regulation

• The hypothalamus detects significant changes in plasma osmolality and results in the secretion of antidiuretic hormone (ADH)
• ADH increases water reabsorption by the kidneys

Acido-Base Homeostasis

• Maintaining pH is achieved by the kidneys and lungs
• Kidneys regulate bicarbonate (HCO3-) concentration: increasing reabsorption for increased pH, and increasing production (excretion of H+) to decrease pH

Blood Pressure Regulation

• Blood pressure is regulated by the juxtaglomerular apparatus
• Delivery changes of sodium and chloride to the distal nephron part, alters kidney secretion of the enzyme renin
• Renin, angiotensin, and aldosterone regulate blood pressure

Renin-Angiotensin-Aldosterone System (RAAS)

• When blood volume decreases, kidney juxtaglomerular cells secrete renin
• Renin converts angiotensinogen made in the liver, to angiotensin I
• Angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II
• Angiotensin II acts on receptors in the adrenal cortex, blood vessels, and brain
• Angiotensin II causes blood vessels to constrict, increasing blood pressure
• Aldosterone secretion is stimulated by angiotensin, which causes kidney tubules to reabsorb sodium and water, leading to increased fluid volume and blood pressure