Kidney Physiology and Function

This set of flashcards covers kidney functions, nephron anatomy, urine formation processes, blood pressure regulation, and micturition based on lecture notes.

Acid-base balance (Kidneys)

The kidneys regulate blood pH by excreting $H^+$ ions and reabsorbing $HCO_3^-$; in acidosis, they stimulate $H^+$ transport into urine, and in alkalosis, they decrease $H^+$ excretion and increase $HCO_3^-$ excretion.

Urea

A waste product from amino acid metabolism; its concentration in the blood rises when kidney function is abnormal.

Creatinine

A metabolite from muscle creatine that is only excreted through the kidneys; it is used as a marker for kidney failure when blood levels rise.

Uric acid

A nucleonic acid metabolite excreted through the kidneys; its accumulation can trigger a goat attack with joint pain.

Erythropoietin

A hormone produced in peritubular cells (mostly endothelial cells) during hypoxia or sympathetic stimulation that is necessary for erythrocyte development.

Renin

An enzyme produced by juxtaglomerular cells in response to low blood pressure, low $Na^+$ levels, or high $K^+$ levels; it activates the renin-angiotensin-aldosterone system to increase blood pressure.

Vitamin D3 activation

A process taking place in tubular epithelial cells necessary for regulating calcium and phosphate equilibrium in the bloodstream.

Gluconeogenesis (Kidneys)

The production of glucose from $\text{α-ketoglutarate}$ (derived from glutamine) in tubular epithelial cells, occurring with high intensity during fasting states.

Nephron

The functional unit of the kidney consisting of a tubular part and a vascular part; dead nephrons are not substituted and their number decreases over life.

Renal corpuscle

The component of the nephron consisting of the glomerular capillary network where blood plasma is filtered into the Bowman´s capsule.

Cortical nephrons

Accounting for $85\%$ of nephrons, these are small, located in the superficial cortex, and have short loops extending only into the outer medulla.

Juxtamedullary nephrons

Accounting for $15\%$ of nephrons, these have big corpuscles in the deep cortex and long loops extending into the inner medulla to regulate the osmolarity gradient.

Macula densa cells

Cells in the distal convoluted tubule that monitor $Na^+$ and $Cl^-$ concentrations and signal juxtaglomerular cells to adjust renin production.

Filtration (F)

The process occurring down a pressure gradient from the glomerular capillary network into Bowman´s capsule, producing about $180\,L$ of primary urine per day.

Reabsorption (R)

The transport of substances like $Na^+$, $Cl^-$, $K^+$, $HCO_3^-$, and $H_2O$ from the tubular fluid back into the peritubular capillaries.

Secretion (S)

The transport of substances such as $K^+$ and $H^+$ from the peritubular capillaries into the tubular system to be excreted in urine.

Excretion formula (E)

The calculation for secondary urine production: $E = F - R + S$, where $F$ is filtration, $R$ is reabsorption, and $S$ is secretion.

Dialysis

A procedure to remove waste products and excess fluid from the blood using a semipermeable membrane when kidneys fail.

Dialysate solution

A solution used in dialysis with lower $K^+$ and higher $HCO_3^-$ and dextrose concentrations compared to blood plasma to facilitate waste removal and acid-base balance.

Hydrostatic pressure in glomerular capillaries ($P_h$)

The blood pressure within the glomerular capillaries ($50-70\,mmHg$) that drives filtration; it is regulated by the afferent and efferent arterioles.

Colloid osmotic pressure in glomerular capillaries ($\pi_c$)

The pressure ($20-30\,mmHg$) created by plasma proteins that attracts water back into the blood and inhibits filtration.

Capsule pressure ($P_c$)

The hydrostatic pressure in the Bowman´s capsule ($10-20\,mmHg$) determined by the ultrafiltrate volume, which inhibits further filtration.

Effective filtration pressure ($P_{ef}$)

The net pressure causing filtration, calculated as $P_{ef} = P_h - (\pi + P_c)$; a value of approximately $10\,mmHg$ is needed for normal function.

Primary urine (Ultrafiltrate)

The fluid formed in Bowman's capsule ($125\,ml/min$ in males), composed of blood plasma excluding substances with a molecular weight over $70,000\,Daltons$ or blood cells.

Renal clearance test

A test measuring the volume of blood plasma completely cleared of a substance by the kidneys per unit of time to determine filtration intensity.

Inulin

A polysaccharide that is artificially infused to serve as the gold standard for renal clearance because it is freely filtered but not reabsorbed or secreted.

Vasa recta

Specialized capillary loops in juxtamedullary nephrons that extend into the medulla to provide gas exchange and facilitate reabsorption and secretion.

Renal plasma flow (RPF)

The volume of blood plasma flowing through the kidneys per minute, often determined using the para-aminohippuric acid (PAH) clearance test.

Hematocrit index

The percentage of blood volume made up of red blood cells, used to calculate Renal Blood Flow ($RBF$) from Renal Plasma Flow ($RPF$) using $RBF = RPF / (1 - Ηct)$.

Autoregulation

The mechanism that maintains normal renal blood flow and GFR when arterial blood pressure reflects changes between $80-180\,mmHg$. balance.

Myogenic regulation

An autoregulation mechanism where vascular smooth muscle cells contract in response to stretch (high BP) via $Ca^{2+}$ influx and relax when pressure decreases.

Tubuloglomerular feedback

A mechanism where macula densa cells detect $Na^+/Cl^-$ levels to adjust afferent arteriole diameter and efferent resistance via renin to regulate glomerular pressure.

Reabsorption threshold

The blood plasma concentration of a substance at which kidneys can no longer reabsorb it because transport proteins are saturated, causing it to appear in urine.

Glucose reabsorption threshold

The specific plasma concentration ($11\,mmol/L$ or $200\,mg\%$) at which glucose appears in the urine; normal fasting levels are $3.33-5.55\,mmol/L$.

Definitive urine (Secondary urine)

The final processed urine ($1-2\,L/day$) with a pH of $4.4-8.5$, normally containing no proteins or glucose.

Micturition reflex

An autonomic reflex activated when bladder volume exceeds $200-300\,ml$, causing detrusor muscle contraction and internal sphincter relaxation via parasympathetic nerves ($S2-4$).

Pontine micturition center (PMC)

A center in the reticular activating system of the pons that coordinates voluntary urination by activating spinal parasympathetic neurons while inhibiting somatic motor neurons to the external sphincter.