Block 1 Kidney Quiz

Kidney Functions

• The kidneys filter toxic waste products from the body: urea, creatinine, uric acid, hemoglobin breakdown products
• The central function of the kidneys is to regulate water concentration
• The kidneys do not store fat
• The kidneys perform many functions, including:
  • Excretion of foreign chemicals
  • Gluconeogenesis
  • Releasing hormones and enzymes

Kidney Structures

• The combination of a glomerulus and a Bowman’s capsule constitutes a renal corpuscle
• Only about 20% of plasma filters into Bowman's capsule, the remaining blood leaves the glomerulus via the efferent arteriole
• The nephron is the microscopic unit of the kidney

Glomerular Filtration Rate (GFR)

• GFR is the amount of filtrate formed each minute by the kidneys
• Two ways to increase GFR:
  • Dilate the afferent tubule and constrict the efferent tubule
  • Constriction of the efferent arteriole
• Two ways to decrease GFR:
  • Constriction of the afferent tubule and dilation of the efferent tubule
  • A decrease in the glomerular capillary pressure (GCP)
• Normal GFR is indicated by creatinine levels between 0.6 to 1.2 mg/dL
• An elevated creatinine level indicates a problem with kidney function
• High protein diets are a common cause of elevated creatinine
• The kidneys are responsible for the regulation of fluid balance

Body Fluid Compartments

• Our body is about 60% water
• About 60% of our body water is in the intracellular fluid and about 40% is in the extracellular fluid
• Extracellular fluid is comprised of interstitial fluid (which contributes to edema) and plasma

Nephron Processes

• Anything that stays inside the renal tubule is excreted in urine
• Anything that leaves the renal tubule and goes back into the blood is reabsorbed
• Filtration occurs at the glomerulus
• When compounds are taken from the blood into the renal tubule, this is called secretion

Renal Medulla

• The medulla of the kidney has the highest osmolarity
• The high osmolarity in the medulla causes water reabsorption

Nephron Types

• Juxtamedullary nephrons have longer loops of Henle and produce more concentrated urine than cortical nephrons

Glomerular Filtration

• Podocytes are specialized cells that act like a sift in the glomerulus, preventing large molecules from entering the filtrate
• Glomerular hydrostatic pressure (GHP) is the force that pushes fluid out of the glomerulus
• Capsular hydrostatic pressure (CHP) is the force that pushes back on the glomerulus when Bowman's capsule is full
• GFR = (GHP - (CHP + Col HP))
• An elevated blood colloid osmotic pressure (BCOP) is associated with high levels of proteins in the blood
• Creatinine levels in the blood predict the filtering ability of the kidneys

GFR Regulation

• Constriction of the afferent arterioles is the most common way to lower GFR
• Dilating the afferent arteriole increases hydrostatic pressure
• Decreased plasma protein levels lower GFR
• Constriction of the ureter decreases GFR

Diabetes

• Individuals with diabetes often experience polyuria (increased urine production) due to high glucose levels in the blood, which overwhelms the kidneys' ability to reabsorb all glucose
• Common signs and symptoms of diabetes include:
  • Polyuria (increased urination)
  • Polydipsia (increased thirst)
  • Polyphagia (increased hunger)
  • Polygonal (increased size of cells due to excessive glucose)
• Diabetics excrete glucose in their urine because of a lack of carriers for glucose to be reabsorbed

Tubular Reabsorption and Secretion

• Most reabsorption and secretion occurs in the proximal convoluted tubule
• Toxic substances such as drugs, H+, creatinine, and ammonia are secreted into the proximal tubule for excretion

Loop of Henle

• The descending loop of Henle is permeable to water but impermeable to solutes
• The ascending loop of Henle plays a vital role in the reabsorption of Na+, K+, and Cl-
• Urea is not, however, reabsorbed by the ascending loop of Henle
• The loop of Henle plays a critical role in concentrating urine

Distal Convoluted Tubule (DCT)

• The DCT is regulated by aldosterone and parathyroid hormone (PTH) to control the permeability of specific ions
• Aldosterone increases sodium reabsorption and potassium secretion in the DCT
• Aldosterone agonists lower blood pressure
• Conversely, aldosterone antagonists lower blood pressure by reducing sodium reabsorption
• PTH regulates calcium levels in the blood, increasing calcium reabsorption in the DCT

Collecting Duct

• The collecting duct contains aquaporins which are activated by antidiuretic hormone (ADH), allowing for water reabsorption

Renin-Angiotensin-Aldosterone System (RAA System)

• The juxtaglomerular apparatus releases renin in response to low blood pressure:
  • Renin converts angiotensinogen (from the liver) to angiotensin I
  • ACE (released by the lungs) converts angiotensin I to angiotensin II
• The RAA system is as follows: Angiotensinogen → Renin → Angiotensin I → ACE → Angiotensin II → Aldosterone
• Angiotensin II increases sympathetic nervous system activity
• Angiotensin II functions to:
  • Increase blood pressure
  • Reabsorb Na+ and Cl-
  • Reabsorb water
  • Excrete K+
• Aldosterone is released from the adrenal cortex
• Angiotensin II causes systemic vasoconstriction
• Angiotensin II stimulates the release of ADH from the posterior pituitary gland
• ADH is released from the posterior pituitary gland
• Angiotensinogen is produced by the liver and is an important precursor in the RAA system
• ACE inhibitors are commonly used medications to lower blood pressure