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• Homeostasis is the maintenance of internal balance.

Definition and Importance

• In homeostasis, humans aims to maintain constant internal environment despite external changes
• Humans exhibit homeostasis for physical and chemical properties like body temperature at 37°C.
• Blood pH levels are maintained at 7.4.
• Blood glucose concentration is in range of 70-110 mg of glucose per 100 mL of blood.
• Homeostatic control systems maintain variables at or near a set point.
• Fluctuations above or below the set point stimulates a sensor.
• Sensors signal a control center, which triggers a response to return a variable to its set point.

Negative Feedback Mechanism

• Negative feedback mechanisms reduce the stimulus.
• Negative means acts opposite of stimulus.
• Feedback means Response towards stimulus.
• An increase in substances or activity inhibits the process by decreasing the substance or activity.
• Examples include thermoregulation and blood glucose regulation.

Thermoregulation

• Process to maintain body temperature within normal range.
• The thermostat in the hypothalamus activates mechanisms for heat loss or gain.
• If body temperature decreases, the thermostat inhibits heat loss and activates heat gain through shivering or vessel constriction in the skin.
• If body temperature increases, the thermostat shuts down heat retention and promotes cooling via vessel dilation in the skin, sweating, or panting.

Blood Glucose Regulation

• After a meal when high blood glucose levels, the beta cells in the pancreas secrete insulin.
• Insulin triggers glucose transport into body cells and stimulates the liver to store glucose as glycogen, which decreases blood glucose.
• Between meals, when there is low blood glucose, alpha cells in the pancreas secrete glucagon.
• Glucagon promotes glycogen breakdown into glucose in the liver and releases it into the blood, increasing blood glucose.

Osmoregulation

• Process to control solute concentration and water balance in the internal environment.
• It balances water and solutes in the internal environment.
• Ex: sodium, calcium must be maintained for normal activities of muscles, neurons, and body cells.

The Human Urinary System

• It consists of kidneys and organs for transporting and storing urine.
• Urine from each kidney exits through the ureter, draining into the urinary bladder.
• During urination, urine is expelled from the bladder through the urethra.
• The outer part of the kidney is the cortex and the inner is the medulla.
• Kidneys are supplied with blood via the renal artery and drained by the renal vein.
• Renal tubules carry filtrate within the cortex and medulla.
• Fluid in the filtrate is reabsorbed into blood vessels and exit kidneys in the renal vein.
• Urine is collected in the renal pelvis.
• It exits through ureter and drains to urinary bladder.

Structure and Function of the Nephron

• Nephron is the kidney’s functional unit within the cortex and medulla.
• Components include the glomerulus (capillaries) and the renal tubule (Bowman's capsule, proximal tubule, loop of Henle, and distal tubule).
• Blood is supplied by the afferent arteriole and it branches to form the glomerulus.
• Capillaries leaving the glomerulus form the efferent arteriole.
• The efferent arteriole branches forming the peritubular capillary (surrounding the proximal and distal tubules).
• The vasa recta serves the medulla.
• Filtrate forms when blood pressure forces fluid from the glomerulus into the lumen of Bowman's capsule.
• Filtrate is processed as it goes through: the proximal tubule, the loop of Henle, and the distal tubule.
• A collecting duct gets processed filtrate and transports it to the renal pelvis.

Urine Formation

• Excretory system function includes: filtration, reabsorption, secretion, and excretion
• Filtration is where the renal/excretory tubule collects a filtrate from the blood
• Water and solutes are forced by blood pressure across the membranes into the excretory tubule.
• Reabsorption is where the transport epithelium reclaims valuable substances from filtrate and returns them to body fluids.
• Secretion is where substances like toxins and excess ions are extracted from body fluids and added to the contents of the excretory tubule.
• Excretion is where the altered/processed filtrate (urine) leaves the system of the body.

Bowman's Capsule

• Ultrafiltration occurs at the Bowman's capsule because of high pressure in the glomerulus.
• Pressure is due to the difference in diameter between afferent and efferent arterioles because the diameter of afferent > efferent arteriole.
• Fluid and small molecules move through the fenestrations of glomerular endothelium, glomerular basement membrane, and the filtration slits of podocytes.
• Filtrate produced has salts, glucose, amino acids, vitamins, nitrogenous wastes, and molecules
• Blood cells and large molecules/ proteins, are retained in blood vessels.

Proximal Tubule

• Reabsorption of ions, water, and nutrients in the proximal tubule.
• Molecules are transported into the interstitial fluid and then capillaries.

Secretion

• As the filtrate passes through, excrete materials becoming concentrated.
• Some toxic materials are secreted into the filtrate.

Descending Limb of the Loop of Henle

• Water reabsorption through aquaporin protein channels.
• Movement driven by high osmolarity of the interstitial fluid.
• The filtrate becomes more concentrated.

Ascending Limb of the Loop of Henle

• Salt diffuses into the interstitial fluid.
• Filtrate is increasingly dilute here.

Distal Tubule - Reabsorption and Secretion

• It manages the concentrations of body fluids
• Ion movement contributes to pH regulation.

Collecting Duct

• It carries filtrate through the medulla to the renal pelvis.
• It reabsorbs solutes and water.
• Urine is hyperosmotic to body fluids.

Antidiuretic Hormone

• Osmoreceptor cells regulates release of ADH from the posterior pituitary.
• When osmolarity rises above set point, ADH release increases, leading to more concentrated urine.
• When osmolarity drops below the set point, there is less ADH secretion and more dilute urine.
• On hot days, ADH is released from the posterior pituitary to receptors on collection ducts increasing aquaporin, water diffuses by osmosis, and osmolarity drops
• Drinking a lot of fluid inhibits ADH release, decreasing aquaporin, leading to water retention and osmolarity increase.

Renin-Angiotensin-Aldosterone System (RAAS)

• It responds to the drop in blood pressure and blood volume (without changing osmolarity).
• Reduced pressure near the glomerulus causes the juxtaglomerular apparatus (JGA) to release renin.
• Renin triggers angiotensin II formation.
• Angiotensin II raises blood pressure by vasoconstriction and reduces blood flow to the kidneys.
• Aldosterone is released to increase blood volume and pressure, which increases permeability of Na+ at the distal tubule.
• More Na+ reabosorption occurs and water by osmosis into blood vessel.
• Volume and blood pressure increase to normal again.