Lecture 21: Metabolism & Excretion & Reproduction

What are the general steps of digestion?

Mouth → pharynx & esophagus → small intestine → colon → large intestine → rectum & anus.

What is aspiration?

When food or water goes down the wrong pipe (trachea instead of esophagus).

What is the esophagus?

The muscular tube that carries food and drink down to the stomach.

What happens during the fight-or-flight response?

Fight-or-flight response triggers an outpouring of adrenaline, the hormone associated with preparing for dangerous situations.

Your heart rate and blood pressure spike.

How does the human excretory system work?

Maintains homeostasis by regulating water balance, ion concentrations (Na+, K+, Ca2+), blood pH, and removal of metabolic waste.

Filters ~180 liters of plasma/day through the kidneys but produces only ~1-2L of urine → massive selective reabsorption.

Eliminates nitrogenous waste (urea), drug metabolites, and toxins through highly regulated nephron (kidney) processes.

Disruptions in filtration (diabetes) can alter urine protein and glucose levels. Chronic kidney disease often progresses silently due to kidneys’ high compensatory ability.

What do the cortex and medulla do?

Cortex vs medulla: filtration begins in the cortex (glomerulus) while concentration gradients in the medulla allow water reabsorption and urine concentration.

What to nephrons do?

There are ~1 million nephrons in each human kidney.

• Regulation of extracellular fluid (ECF) volume, which directly influences blood pressure.

• Control of plasma osmolarity, primarily by adjusting water reabsorption.

• Maintenance of ion balance, especially Na+, K+, Ca2+, Cl-, HCO3-.

• Regulation of blood pH through selective secretion and reabsorption of H+ and HCO3-.

• Excretion of metabolic wastes (urea, creatinine), toxins, and foreign substances (drugs, xenobiotics).

What is the glomerular filtration apparatus?

Structure in the renal corpsucle responsible for the intitial step of urine formation, allowing plasma (minus proteins/cells) to enter the nephron.

Functions as a highly selective, pressure-driven filter that determines what components of blood become “filtrate”, setting the stage for all downstream reabsorption and secretion.

What are the filtration layers in the glomerula filtration apparatus?

1. Fenestrated capillary endothelium.

2. Basement membrane.

3. Podocyte slit diaphragms.

These work together to restrict cells and most proteins.

How does filtration in the kidneys work?

Only 20% of the plasma that enters each glomerulus is filtered.

Total plasma volume = ~3L.

Kidneys filter entire plasma volume ~60 times/day or 2.5 times/hour.

What is molarity vs osmolarity?

Molarity (M) = moles of solute/liter of solution.

Osmolarity (OsM) = molarity x number of particles of dissociation (= total number of solute particles per 1L of solution).

Why is osmolarity a better measure of solute concentration than molarity in physiological systems?

Physiological fluids contain many different types of dissolved solutes. This makes it difficult to determine the concentration of each. Osmolarity provides an integrated measure of total number of solutes.

What does vasopressin (ADH) do?

Adjusts how much water the nephron reabsorbs in the collecting duct.

ADH is released when the body needs to conserve water – either because blood osmolarity increased or blood pressure dropped.

Baroreceptors in the carotic and aortic bodies signal ADH release when blood volume/pressure decreases.

ADH increases the permeability of the collecting duct by inserting AQP2 channels, allowing the nephron to rapidly adjust water reabsorption in response to changes in blood pressure or osmolarity.

What is the evolutionary significance of the ability to produce concentrated urine?

The ability to produce concentrated urine – driven by vasopressin and a deep renal medulla – is a key adaptation that allowed mammals (especially terrestrial) to conserve water and thrive in variable or dry environments.

What is asexual reproduction?

Produces genetically identical offspring (clones) via mitosis, common in plants, fungi, protists, and some animals (budding, fission, parthenogenesis).

Fast and energy-efficient, allowing rapid population growth when environmental conditions are stable and predictable.

Offers no genetic mixing, making populations more vulnerable to disease, environmental change, and evolutionary pressures.

What is sexual reproduction?

Involves fusion of gametes (sperm + egg), creating offspring with genetic variation, which increases adaptability to changing environments.

Can occur through internal fertilization (land vertebrates; protects gametes, supports embryo) or external fertilization (aquatic species; requires water for gamete dispersal).

Energetically costly – requires finding mates, producing gametes – but enhances long-term evolutionary success through diversity.

How does reproduction work in males?

The hypthalamus releases GnRH, which stimulates the anterior pituitary to secrete LH and FSH.

LH acts on Leydig cells → stimulates testoterone production, which supports sperm development & produces negative feedback to the brain.

FSH acts on Sertole cells → stimulates spermatogenesis and secretion of inhibin, which specifically inhibits FSH release.

Testosterone + FSH are both required for full, healthy sperm production.

Why do anabolic steroids lower sperm count?

External steroids mimic high testoterone, causing strong negative feedback on the hypothalamus and pituitary.

→ Reduced testoterone production inside the testis, smaller testicular size, and reduced sperm production.

How does reproduction work in females?

Ovulation, fertilization, development and implantation of a human egg in the fallopian tube and uterus.

Follicular development begins with primordial follicles and presses towards a single dominant follicle that will ovulate an oocyte.

After ovulation the ruptured follicle transforms into the corpus luteum, which produces progesterone to support early pregnancy; if fertilzation does not occur, it regresses into the corpus albicans.

What is the evolutionary perspective of female reproduction?

The ability to mature one high-quality oocyte per cycle (instead of many) is an adaptation that conserves maternal energy while maximizing offspring survival in mammals with large parental investment.

What is salpingitis?

Inflammation of the fallopian tubes, often caused by sexually transmitted infections like chlamydia and gonorrhea, and is a common part of Pelvic Inflammatory Disease (PID).

What is Pelvic Inflammatory Disease?

An infection of the female reproductive organs that can travel to the fallopian tubes and cause scarring that leads to infertility or increases the risk of ectopic pregnancy.

What is an ectopic pregnancy?

Occurs when a fertilized egg implants in the fallopian tube instead of the uterus. It is a life-threatening emergency.

What are the stages of the menstrual cycle?

Follicular phase: as follicle develops, it secretes estrogen. This leads to a burst of LH and FSH which triggers ovulation.

Luteal phase: corpus luteum (derived from follicle) secretes estrogen and progesterone.

These hormones a) prevent growth of a new follicle until the corpus luteum has degenerated, b) promote implantation of the fertilized egg by thickening the endometrium.

What happens if fertilization occurs or not?

Does occur: the developing placenta secretes human chorionic gonadotropic hormone (hCG). This causes the corpus luteum to keep producing progesterone and thereby maintain the endometrium and the pregnancy. After ~10 weeks the placenta assumes this role.

Does not occur: the corpus luteum gradually degenerates, resulting in menses.

How is genetic diversity an evolutionary engine?

Sexual reproduction increases genetic variation, which speeds adaptation and enables long-term evolutionary flexibility.

Why does sex still exist?

Sexual reproduction has been strongly favored because it enhances survival under changing or unpredictable environments.

Red Queen Hypothesis: hosts must continually generate novel genotypes to keep pace with evolving parasites.

What are the evolutionary limits of asexual reproduction?

Asexual species often dominate short-term ecological opportunities but lineages tend to be evolutionarily short-lived due to mutation accumulation and low adaptability.

What is the difference between endotherms and ectotherms?

Endotherms (birds, mammals) have very high metabolic rates to maintain constant body temperature → they produce more metabolic waste, especially nitrogenous waste, and require kidneys that can filter blood rapidly and efficiently.

Ectotherms (reptiles, amphibians, fish) have lower energy demands, producing less nitrogenous waste and often relying on simpler or less energetically expensive excretory pathways.