Excretory and reproductive systems

Excretory system

The excretory system helps maintain homeostasis in multiple ways, particularly by regulating blood pressure, osmoregulation, ion balance, acid-base balance, and the removal of nitrogenous waste:

• Blood pressure:
  If pressure is too low, the kidney’s juxtaglomerular apparatus releases renin, initiating the renin-angiotensin pathway, ultimately producing angiotensin II.
  This stimulates the adrenal glands to secrete aldosterone, prompting the kidney distal tubules to reabsorb more Na⁺ (and thus water), while ADH from the hypothalamus/pituitary can further increase water reabsorption. High levels of ADH also promote vasoconstriction, both raising blood pressure.
  Conversely, if blood pressure is too high, these hormones cease production, and the heart secretes ANP (Atrial natriuretic peptide), which antagonizes aldosterone to excrete more Na⁺ and water, and can also induce vasodilation. \

• Osmoregulation:
  The blood osmolarity depends mainly on $N{a}^{+}$ and $C{l}^{-}$. When osmolarity is low, aldosterone acts to reabsorb $N{a}^{+}$, with $C{l}^{-}$ often following. The kidney tubules modulate fluid composition by selectively secreting and reabsorbing ions, thereby maintaining osmotic balance.

• Ion balance:
  $K^{+}$ excretion is regulated by aldosterone (as $N{a}^{+}$ is reabsorbed, $K^{+}$ is lost). Meanwhile, PTH (Parathyroid hormone) regulates calcium and phosphate levels by** increasing $C{a}^2+$ reabsorption** in the kidney and mobilizing it from bone.

• Acid-base balance:
  The body’s primary buffer system uses bicarbonate ($HC{O}_3^{-}$). Exhaling $C{O}_2$ via respiration reduces acidity, while the kidney tubules can excrete or reabsorb bicarbonate ion and secrete $H^{+}$ as needed to stabilize blood pH.\

• Nitrogenous waste:
  The kidneys eliminate soluble waste—particularly urea, which is a less toxic product of ammonia metabolism. This waste, along with water and salt, forms urine that is excreted to preserve the body’s internal chemical stability.

Kidney structure

• The kidney is divided into an outer region called the cortex, where most of the convoluted tubules reside, and an inner region called the medulla, which contains the loop of Henle. The kidney’s functional unit is the nephron, comprising the glomerulus, Bowman’s capsule, the proximal tubule, the loop of Henle, the distal tubule, and the collecting duct.

Nephrons

• Within each nephron, the glomerulus forms a ball of fenestrated capillaries where filtration begins. Surrounding it is Bowman’s capsule, a cup-like structure that collects the filtrate.
• The adjacent proximal tubule serves as the primary site of nutrient and salt reabsorption, as well as secretion of various substances (except potassium, which is mainly handled by the distal tubule under aldosterone regulation).
  From there, the filtrate flows into the loop of Henle, a U‐shaped loop that dips into the renal medulla and carries out the countercurrent multiplier mechanism. Its descending limb is permeable to water but not solutes, causing water to leave by osmosis and concentrate the filtrate at the bottom.
  In contrast, the ascending limb allows salt to diffuse or be actively transported out but is impermeable to water, thus diluting the filtrate as it moves upward.
• The fluid then enters the distal tubule, which fine‐tunes electrolyte and water reabsorption (and secretes potassium under aldosterone influence). Multiple distal tubules drain into a common collecting duct, where ADH controls water permeability and additional hormone‐regulated salt transport helps finalize urine composition.

Formation of urine

• The glomerular filtration process uses hydrostatic pressure to push fluid and small solutes, including nutrients and waste products, from the blood into Bowman’s capsule. Both “good” substances (like glucose and amino acids) and “bad” substances (like urea, creatinine, and uric acid) enter the filtrate. As the filtrate travels through the proximal tubule, valuable nutrients and the majority of ions are selectively reabsorbed back into the bloodstream, while waste products are secreted for eventual excretion.
• The filtrate then passes into the loop of Henle, which implements the countercurrent multiplier mechanism to create an osmotic gradient in the renal medulla. Its descending limb is permeable to water but not solutes, allowing water to exit by osmosis; at the loop’s bottom, the filtrate becomes highly concentrated.
• The ascending limb, by contrast, is impermeable to water but actively transports salts out, driven by a $NaCl$ pump, diluting the filtrate.
• Within the distal tubule, hormone regulation (particularly via aldosterone) fine-tunes ion and fluid balance by reabsorbing sodium and secreting excess potassium.
• The final adjustments occur in the collecting duct, where ADH (antidiuretic hormone) controls water reabsorption through capillaries based on the osmotic gradient—largely established by salt pumping and urea recycling—thus concentrating the urine if needed.
• Simultaneously, the kidneys help maintain blood pH by secreting $H^{+}$ when blood is too acidic or excreting bicarbonate when blood is too alkaline. The overall process yields a urine rich in nitrogenous waste, primarily ammonia converted to safer urea.
• This fluid drains from nephrons into the ureter, which leads to the bladder—a muscular reservoir lined with transitional epithelium that can expand as it fills. Once the bladder is sufficiently full, urine exits through the urethra, completing the excretory pathway. Sphincter muscles exert muscular control over urine release by contracting to retain urine and relaxing to permit its expulsion.

Reproductive system

Male and female reproductive structures and their functions

• The gonads represent the primary reproductive organs: in males, the testes produce sperm within the seminiferous tubules and secrete testosterone, while in females, the ovaries house immature oocytes and produce estrogen. Male reproductive structures are largely external, whereas female structures are primarily internal.

In the male, sperm travel from the seminiferous tubules to the epididymis for storage, then pass via the vas deferens and ejaculatory duct into the urethra, exiting through the penis.

In the female, each month an oocyte matures in a follicle inside the ovary. The fallopian tubes receive the released egg, potentially allowing fertilization en route to the uterus, where a thickened endometrium either supports embryo implantation or is shed during menses (if fertilization does not occur). The vagina serves as the canal connecting the uterus to the external environment.

Hormonal control of reproduction

• Control of the female cycle depends on GnRH from the hypothalamus, which triggers release of FSH (follicle-stimulating hormone) and LH (luteinizing hormone).

FSH promotes follicular growth and estrogen production, while estrogen typically inhibits FSH and LH until it reaches a threshold—there, it induces an LH surge, causing the follicle to rupture and the primary oocyte to become a secondary oocyte. The ruptured follicle transforms into the corpus luteum, which releases estrogen and progesterone to sustain the endometrium.

If fertilization fails, the corpus luteum atrophies, hormone levels drop, and menstruation begins. With fertilization, the embryo secretes hCG, mimicking LH to preserve the corpus luteum until the placenta can take over hormone production.

Male hormonal control of reproduction starts at puberty, when the hypothalamus releases GnRH, which prompts the anterior pituitary to secrete FSH and LH. FSH activates Sertoli cells in the testes to drive spermatogenesis, while LH stimulates Leydig cells to produce testosterone. This testosterone promotes secondary sexual characteristics—such as a deepened voice, increased body hair, and sex drive—and further enhances spermatogenesis.

A negative feedback mechanism regulates this system: high testosterone levels signal the hypothalamus and anterior pituitary to decrease the release of GnRH, FSH, and LH, moderating sperm production. Additionally, Sertoli cells secrete inhibin when the sperm count is high to further suppress these hormones, while reduced inhibin secretion at lower sperm counts allows hormone levels to rise and stimulate more spermatogenesis.

Comparing the sexes, the male reproductive tract merges with the urinary tract and is largely external, facilitating spermatogenesis in the testes and frequent ability for fertilization, while the female tract is mostly internal and separated from the urinary system. Oogenesis in the ovaries proceeds cyclically, culminating in a monthly opportunity for fertilization.

Pregnancy, parturition, lactation

• Hormonal changes during pregnancy
  • During pregnancy, hCG maintains the corpus luteum, which secretes progesterone until the placenta assumes that role. As pregnancy progresses, estrogen levels rise alongside progesterone, but near childbirth, they begin to shift—progesterone drops to allow uterine contractions, while estrogen supports growth of the myometrium and upregulates oxytocin receptors to drive labor.
• Parturition
  • This term refers to childbirth. Throughout gestation, high progesterone suppresses contractions. As levels fall before birth, the uterus can contract more forcefully, aided by estrogen-induced oxytocin receptors. Rising oxytocin then drives uterine contractions, with cervical dilation and uterine distention further increasing oxytocin release in a positive feedback loop, ultimately leading to delivery.
• Lactation
  • After birth, progesterone and estrogen levels drop sharply, removing their inhibition on the newly developed milk duct system (the lactiferous ducts). Meanwhile, prolactin remains elevated, promoting milk production (galactopoiesis).
  • In contrast, oxytocin triggers the contraction of smooth muscle surrounding alveoli, enabling the “milk letdown” reflex when the nipples are stimulated. Prolactin is essential for milk production, whereas oxytocin is vital for milk secretion.