Lecture 13: Overview of the Urinary System

Reabsorption

The process by which substances are returned from the filtrate in the nephron back into the bloodstream.

Filtration

The process where blood pressure forces fluid and solutes from the glomerular capillaries into the Bowman's capsule.

Excretion

The process of removing waste products from the body via urine.

Secretion

The active transport of substances from the blood into the renal tubule.

Interstitial

Refers to the space between cells, often filled with extracellular fluid.

Hydrostatic Pressure

The force exerted by a fluid against the walls of its container (e.g., blood pressure in the glomerular capillaries).

Osmotic Pressure

The pressure required to prevent the flow of water across a semipermeable membrane due to differences in solute concentration.

Aquaporin

Membrane proteins that facilitate the movement of water across cell membranes.

Saturation

The point at which a solution can no longer dissolve more solute.

Acid

A substance that donates H+ ions in solution.

Alkaline

A solution with a pH above 7, often called basic.

Electrolyte

Substances that dissociate into ions in solution, crucial for electrical conduction in the body.

Buffer

A substance that resists changes in pH by absorbing or releasing hydrogen ions.

Nephron

The functional unit of the kidney, composed of a renal corpuscle and renal tubule.

Renal Corpuscle

Includes the glomerulus and Bowman's capsule, involved in filtration.

Renal Tubule

Composed of the proximal convoluted tubule, nephron loop, and distal convoluted tubule, involved in reabsorption and secretion.

Collecting Duct

Final site of reabsorption and secretion of water and electrolytes, influenced by hormones like ADH and aldosterone.

Glomerular Filtration

Hydrostatic pressure forces water and solutes from the blood into the glomerular capsule, forming the filtrate.

Tubular Reabsorption

Substances like glucose, amino acids, and water are reabsorbed into the bloodstream from the nephron tubules.

Tubular Secretion

Waste products and excess ions, such as H+ and K+, are secreted into the tubule to form urine.

Water Balance

Fluid intake and output must be balanced to maintain homeostasis.

Electrolyte Balance

The kidneys regulate ions like Na+, K+, and Ca2+ to maintain proper cell function.

Acid-Base Balance

Blood pH must remain between 7.35 and 7.45.

Blood pH regulation

The kidneys and respiratory system work together to regulate blood pH by excreting or reabsorbing H+ and HCO3-.

Major functions of the urinary system

Filtration, reabsorption, secretion, and excretion to maintain homeostasis (fluid balance, electrolyte regulation, pH regulation, waste elimination).

Impacts of kidney failure

Impaired filtration, fluid imbalance, electrolyte disturbances (e.g., hyperkalemia), acid-base imbalance, inability to remove waste products.

Plasma proteins and capsular space

Plasma proteins are too large to pass through the filtration membrane, which consists of endothelial cells, a basement membrane, and podocytes with filtration slits.

Substance appearance in urine

If the substance exceeds the transport maximum (Tm) for reabsorption, it cannot be fully reabsorbed and thus appears in the urine.

Effect of aldosterone on K+ concentration

Aldosterone increases sodium reabsorption and potassium secretion in the distal tubules, leading to a higher concentration of K+ in the urine.

Effect of decreased blood pressure on GFR

A decrease in blood pressure lowers the glomerular filtration rate (GFR) as hydrostatic pressure in the glomerulus drops, reducing the filtration of water and solutes.

Primary mechanism for thirst

The primary mechanism is the activation of osmoreceptors in the hypothalamus, which detect increased plasma osmolarity and trigger the sensation of thirst.

Difference between filtrate and urine

Filtrate is the fluid that initially enters the nephron, containing water, electrolytes, and waste products. Urine is the final product after reabsorption and secretion, containing waste products like urea, creatinine, and excess ions.

Regulation of Na+, ECF volume, and blood pressure

Na+ levels influence ECF volume and blood pressure. The renin-angiotensin-aldosterone system regulates Na+ reabsorption, which in turn helps maintain blood pressure and ECF volume.

Respiratory system and acid-base balance

The respiratory system regulates pH by controlling CO2 levels (which convert to H+ in the blood).

Urinary system and acid-base balance

The urinary system regulates pH by excreting or reabsorbing H+ and HCO3-.

Definition of an acid

An acid donates H+ ions.

Definition of a base

A base accepts H+ ions.

Definition of a buffer

A buffer resists pH changes.

Consequences of abnormal pH

If pH falls outside the normal range (7.35-7.45), enzymes and cellular functions are impaired.

Importance of regulating electrolytes

Electrolytes are critical for proper muscle function, nerve transmission, and fluid balance. Imbalances can lead to muscle weakness, arrhythmias, and other serious health issues.