Excretory Products and their Elimination

Nitrogen-containing wastes arise mainly from protein and nucleic acid catabolism. Different animals excrete different nitrogenous end products depending on habitat and water availability:

Urea is produced in the liver by the ornithine cycle (urea cycle). Uric acid is produced by purine catabolism.

The human excretory system consists of:

• Kidneys (2) — primary excretory organs; filter blood and produce urine; located in the abdominal cavity, retroperitoneal.
• Ureters (2) — carry urine from kidneys to bladder.
• Urinary bladder — stores urine; transitional epithelium.
• Urethra — carries urine out; longer in males.

Kidney structure:

• Outer cortex — contains renal corpuscles and convoluted tubules.
• Inner medulla — contains renal pyramids with loops of Henle and collecting ducts; papillae open into minor calyces → major calyces → renal pelvis → ureter.
• Hilum — concave medial surface where renal artery enters and renal vein, ureter, and nerves leave.

The nephron is the structural and functional unit of the kidney. Each kidney has ~1 million nephrons.

Nephron parts: Malpighian corpuscle (Bowman's capsule + glomerulus) → Proximal Convoluted Tubule (PCT) → Loop of Henle (descending + ascending) → Distal Convoluted Tubule (DCT) → Collecting Duct.

Urine formation in three steps:

• Glomerular filtration (ultrafiltration) — hydrostatic pressure forces water, glucose, amino acids, urea, ions out of blood into Bowman's capsule. GFR (glomerular filtration rate) ~125 mL/min (~180 L/day). Large proteins and blood cells are NOT filtered.
• Tubular reabsorption — PCT reabsorbs ~70–80% of filtered Na⁺, glucose, amino acids, and water. Loop of Henle concentrates urine via counter-current mechanism. DCT reabsorbs Na⁺ (under aldosterone) and water (under ADH).
• Tubular secretion — H⁺, K⁺, creatinine, drugs, and metabolic wastes actively secreted into tubular filtrate; fine-tunes urine pH and composition.

ADH (antidiuretic hormone / vasopressin) — released from posterior pituitary when blood osmolarity rises or blood volume falls:

• Increases water permeability of DCT and collecting duct → more water reabsorbed → concentrated urine.
• Deficiency → diabetes insipidus (large volumes of dilute urine).

RAAS (Renin-Angiotensin-Aldosterone System) — activated when blood pressure falls:

Low BP → Juxtaglomerular (JG) cells release Renin → Angiotensinogen → Angiotensin I → Angiotensin II (lungs, ACE) → Adrenal cortex releases Aldosterone → Kidneys retain Na⁺ and water → Blood volume and pressure rise.

ANF (Atrial Natriuretic Factor) — released from atria when blood volume is high; opposes RAAS; promotes Na⁺ and water excretion.

Counter-current mechanism: Maintained by the loop of Henle (creates a medullary osmotic gradient of 300–1200 mOsm/L) and vasa recta (blood supply to medulla; preserves gradient without washing it away).

Common kidney disorders:

• Uremia (Uraemia) — accumulation of urea in blood due to kidney failure; leads to toxicity.
• Renal calculi (kidney stones) — crystallisation of salts (oxalates, urates, phosphates) in the pelvis or calyces; cause excruciating pain (renal colic).
• Glomerulonephritis — inflammation of glomeruli; reduced GFR; proteins and blood cells appear in urine (haematuria, proteinuria).
• Nephritis — inflammation of nephrons; caused by infections, immune reactions, or toxins.

Haemodialysis (artificial kidney):

• Blood is drawn out, passed through a dialysis machine (semipermeable membrane) against a dialysate (blood-like fluid without waste products).
• Wastes diffuse out; cleaned blood is returned to the patient.
• Used when kidneys fail to maintain homeostasis.

Kidney transplant is the permanent solution; requires donor organ compatibility.