Functions of the Kidneys and Reproductive Physiology

This set of flashcards encompasses key terms related to kidney functions, hormonal regulation, and reproductive physiology, focusing on the processes of filtration, gametogenesis, and hormonal cycles.

Glomerulus

A high-pressure capillary bed located within the renal corpuscle, specifically nested inside Bowman's capsule. It serves as the primary filtration station of the nephron, utilizing a fenestrate endothelium and a basement membrane to filter blood plasma. This selective process allows water and small solutes (such as glucose, amino acids, and ions like Na^{+} and K^{+}) to pass into the capsular space as filtrate, while retaining blood cells and large plasma proteins.

Net Glomerular Filtration Pressure (NGFP)

The total pressure gradient that determines the movement of fluid across the glomerular membrane, calculated as: NGFP = P{g} - (P{c} + \pi{g}). In this equation, P{g} (Glomerular Hydrostatic Pressure, $\approx 55\text{ mmHg}$) is the driving force favoring filtration, while it is opposed by P{c} (Capsular Hydrostatic Pressure, $\approx 15\text{ mmHg}$) and \pi{g} (Blood Colloid Osmotic Pressure, $\approx 30\text{ mmHg}$), resulting in a net outward pressure of about 10\text{ mmHg}.

Glomerular Filtration Rate (GFR)

The total volume of filtrate formed by both kidneys per minute, typically averaging 125\text{ ml/min} (approx. 180\text{ L/day}) in a healthy adult. GFR is a critical indicator of renal health and is influenced by the Net Glomerular Filtration Pressure (NGFP), the total surface area available for filtration, and the permeability of the filtration membrane.

Tubular Reabsorption

The selective transepithelial process through which water and solutes are recovered from the renal filtrate and returned to the blood via peritubular capillaries. It occurs throughout the tubule, most significantly in the Proximal Convoluted Tubule (PCT), where nearly 100\% of glucose and amino acids and 65\% of water and sodium are regained. Reabsorption involves both active transport (using ATP) and passive transport (diffusion and osmosis).

Counter Current Multiplier System

A specialized mechanism in the nephron loops (Loops of Henle) of juxtamedullary nephrons that establishes a medullary osmotic gradient (ranging from 300\text{ mOsm} to 1200\text{ mOsm}). The descending limb is permeable to water but not solutes, causing filtrate concentration, while the ascending limb is impermeable to water but actively pumps out Na^{+} and Cl^{-}. This gradient is essential for the concentration of urine under the influence of Antidiuretic Hormone (ADH).

Spermatogenesis

The sequence of events in the seminiferous tubules that produces male gametes. It involves mitosis of spermatogonia (diploid stem cells), meiosis (I and II) to reduce the chromosome count to haploid (1n), and spermiogenesis—the physical transformation of spermatids into functional, motile spermatozoa with heads, midpieces, and tails.

Follicular Phase

The first phase of the ovarian cycle (days 1-14), characterized by the growth of primary follicles stimulated by FSH. As follicles mature, they secrete increasing levels of estrogen. This phase ends with a surge in Luteinizing Hormone (LH), which triggers ovulation—the release of a secondary oocyte into the fallopian tube.

Menstrual Phase

The initial stage of the uterine cycle (days 1-5) where the functional layer of the endometrium (stratum functionalis) is shed. This process is triggered by the degeneration of the corpus luteum, which causes a sharp decline in progesterone and estrogen levels, leading to the constriction of spiral arteries and subsequent tissue ischemia.

Penile Erection

A hemodynamic process regulated by the parasympathetic nervous system. Sexual stimulation leads to the release of Nitric Oxide (NO), which relaxes smooth muscle in the deep arteries of the penis. This vasodilation allows the corpora cavernosa to fill with blood, compressing venous drainage and resulting in the rigidity of the organ.

Corpus Luteum

A temporary yellow endocrine structure formed from remnants of a ruptured follicle after ovulation. It is responsible for secreting high levels of progesterone and some estrogen to prepare and maintain the endometrium for potential embryo implantation. If fertilization does not occur, it degenerates into a corpus albicans, causing hormone levels to drop.

Bowman's Capsule

A double-walled, cup-shaped structure at the beginning of the nephron that collects the filtrate from the glomerulus. Its visceral layer contains specialized cells called podocytes that form 'filtration slits,' which allow small molecules to pass into the capsular space while blocking large proteins and cells.

Proximal Convoluted Tubule (PCT)

The first segment of the renal tubule following Bowman's capsule and the most active site for reabsorption. Its cells feature a dense 'brush border' (microvilli) to maximize surface area and abundant mitochondria to provide energy for active transport. It is responsible for reclaiming nearly all nutrients and the majority of water and electrolytes from the filtrate.

Antidiuretic Hormone (ADH)

A peptide hormone produced by the hypothalamus and released by the posterior pituitary. It regulates water balance by increasing the number of aquaporins (water channels) in the apical membranes of the collecting ducts. This increases water reabsorption, resulting in more concentrated urine and an increase in blood volume/pressure.

Leydig Cells

Also known as interstitial endocrine cells, these are situated between the seminiferous tubules in the testes. Their primary function is the production and secretion of testosterone in response to Luteinizing Hormone (LH), which is essential for stimulating spermatogenesis and maintaining male secondary sex characteristics.

Sertoli Cells

Large supportive cells found within the seminiferous tubules that 'nurse' developing sperm cells. They form the blood-testis barrier via tight junctions, provide nourishment to germ cells, facilitate the movement of cells toward the lumen, and secrete inhibin to regulate FSH levels via negative feedback.

Luteal Phase

The phase of the ovarian cycle following ovulation (days 15-28). During this period, the corpus luteum is active and secretes progesterone. These hormone levels inhibit the release of GnRH, FSH, and LH from the pituitary, preventing the growth of additional follicles during the remainder of the cycle.

Secretory Phase

The uterine phase that coincides with the ovarian luteal phase. Under the influence of progesterone from the corpus luteum, the endometrium continues to thicken, and the uterine glands enlarge and secrete a glycogen-rich fluid to support a potential embryo. The spiral arteries also become more coiled and developed to ensure a rich blood supply.