Renal System Study Notes

Surface markings related to kidneys include the scapular line with landmarks at various vertebral levels:
- T10 near the liver and diaphragm
- T11 near the spleen
- T12 at the level of the kidney and the 12th rib
- Ureter markings
- L4 at the crest of the ilium
- L5 at the dimple indicating posterior superior iliac spine
Additional reference points mentioned: Ang Cen 2 (context suggests an anatomical cross-reference image)

Right kidney relations:
- Proximity to the liver
- Adrenal gland
- Tenth rib landmark
- Ureters and renal vessels (renal artery and renal vein)
- Inferior vena cava in close relation
Left kidney relations:
- Proximity to the spleen
- Renal vessels (renal artery and renal vein) near the abdominal aorta
Major vascular pathways:
- Abdominal aorta
- Inferior vena cava
Urinary tract connections:
- Ureters leading to the urinary bladder
Vascular drainage specifics include common iliac vessels (vein and artery) in certain references

Excretory (urine formation):
- Excretion of urea
- Excretion of other metabolic wastes
- Excretion of exogenous substances
Regulatory:
- Control of body fluids and osmotic pressure
- Control of body fluid volume
- Regulation of electrolytes in body fluids
- Long-term control of blood volume and blood pressure
- Regulation of acid-base balance
Endocrine:
- Erythropoietin production
- Calcitriol synthesis (1,25-dihydroxycholecalciferol; 1,25 Vitamin D3)
Secretory:
- Renin (enzyme) production
- Prostaglandins
- Bradykinin and other kinins

The kidney consists of two regions:
- Cortex
- Medulla
Functional unit: Nephron
Humans have approximately:
- - $ext{Nephrons per kidney} \approx 1.2 \times 10^6$
- $ext{Nephrons per person} \approx 2.4 \times 10^6$

Two main types:
- Cortical nephrons (≈ 85%)
- Juxtamedullary nephrons (≈ 15%)
Cortical nephrons:
- Capsule located in the outer cortex
- Loop of Henle is short
- Peritubular capillaries
- Not primarily involved in the countercurrent mechanism for concentration of urine
- Glomeruli and capsules located in the cortex
Juxtamedullary nephrons:
- Loop of Henle is long and extends deep into the medulla
- Peritubular capillaries + vasa rectae
- Involved in the countercurrent mechanism for urine concentration

Hierarchy of renal blood supply:
- Renal artery → Interlobar arteries → Arcuate arteries → Interlobular arteries → Afferent arterioles → Glomerulus
- Efferent arterioles drain the glomerulus into the peritubular capillaries or vasa rectae
- Interlobular veins → Arcuate veins → Interlobar veins → Renal vein
Additional structures involved:
- Medulla and Cortex separation via renal pyramids and renal columns
- Collecting ducts and their connections to the ureter

Renal blood flow (RBF):
- $ext{RBF} \approx 1.2 \text{L/min}$
Renal plasma flow (RPF):
- $ext{RPF} \approx 660 \text{mL/min}$
Haematocrit: $\text{Hct} \approx 45\%$
Glomerular filtration rate (GFR):
- $\text{GFR} \approx 125 \text{mL/min}$
GFR/RPF relationship:
- $\frac{\text{GFR}}{\text{RPF}} \approx \frac{125}{660} \approx 0.19$
Filtration fraction (FF):
- $\text{FF} \approx 0.20$

Major nephron segments and their functions:
- Glomerulus & Capsule: Filtration
- Proximal Convoluted Tubule (PCT): Tubular transport including reabsorption and secretion; both active and passive processes; generally not under hormonal control
- Loop of Henle: Counter-current system; thin descending limb (DL), thin ascending limb (AL); thick ascending limb (TAL)
- Distal Convoluted Tubule (DCT) & Collecting Ducts (CCD) & Medullary Collecting Duct (MCD): Hormonal regulation (e.g., aldosterone and ADH)
Renal blood flow influences final urine volume and concentration; regulation of Na+ and H2O reabsorption; regulation of K+ and H+ secretion; hormonal control mentioned

Glomerular filtration
Tubular reabsorption
Tubular secretion
Schematic flow:
- Afferent arteriole → Glomerulus → Glomerular capsule (Bowman’s capsule) → Tubular reabsorption (proximal tubule, loop of Henle, distal tubule, collecting duct) via peritubular capillaries → Excretion into renal pelvis and collecting ducts

Concept: Filtration across the glomerular capillary membrane into Bowman's capsule
Process overview:
- Plasma is filtered
- Water and dissolved substances pass into Bowman's space
- Proteins and RBCs are not filtered

Definition: Rate of filtration from all nephrons in both kidneys
Typical values:
- $\text{GFR} \approx 125 \text{mL/min} \ (M)$
- $\text{GFR} \approx 110 \text{mL/min} \ (F)$
More precisely: about $125 \text{mL/min} \ / \ 1.73\ \text{m}^2$
Clinically: ~ $100 \text{mL/min}$

Effective filtration pressure (net driving force)
Starling forces:
- Hydrostatic pressure (from mean arterial pressure, MAP)
- Oncotic pressure (determined by plasma protein concentration)
Glomerular membrane properties:
- Surface area of glomerular membrane
- Membrane permeability

Net ultrafiltration pressure (APUF) is driven by the balance of:
- Filtration pressure from glomerular capillaries
- Opposing pressures from oncotic pressure and capsule hydrostatic pressure
Key terms often shown in diagrams:
- P_GC: Glomerular capillary hydrostatic pressure
- π_GC: Glomerular capillary oncotic pressure
- P_B: Bowman's capsule hydrostatic pressure
- Net ultrafiltration pressure (APUF) drives GFR

Blood flow: ↑ flow → ↑ GFR
Capillary hydrostatic pressure: MAP range approximately $80 \text{mmHg} - 180 \text{mmHg}$
Renal autoregulation:
- If MAP < 80 mmHg, autoregulation maintains GFR within a range
- If MAP < 40–50 mmHg, GFR may drop to 0
GFR remains relatively constant despite moderate MAP changes within autoregulatory range

Hydrostatic pressure in Bowman's capsule (capsular pressure)
Oncotic pressure in the glomerular capillary
Oncotic pressure in Bowman's capsule (typically negligible under normal conditions)
Membrane permeability
Surface area of the filtration membrane

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