3.1 Cardiovascular, Respiratory, Renal, Musculoskeletal, and Reproductive Systems
Key Takeaways
Cardiac output equals heart rate times stroke volume, and blood pressure is regulated by baroreceptors, the renin-angiotensin-aldosterone system, antidiuretic hormone, and atrial natriuretic peptide.
Red blood cells live about 120 days, and their formation requires iron, vitamin B12, folate, protein, copper, and vitamin B6, plus erythropoietin from the kidney.
The kidneys filter about 180 L of plasma a day (a GFR of roughly 125 mL/min) and return most of it, while excreting urea, creatinine, uric acid, and excess electrolytes and producing erythropoietin, renin, and calcitriol.
In pregnancy, plasma volume expands more than red cell mass, so hemoglobin concentration falls (physiological anemia), while glomerular filtration rises.
Prolactin stimulates milk production, and oxytocin causes milk ejection (let-down); suckling stimulates both.
The anatomy and physiology area of the Nutritional Biochemistry and Clinical Dietetics paper asks examinees to describe the location and function of the major body systems in health and disease. This section covers the systems not discussed in the digestive, endocrine, and immune sections.
The Cardiovascular System
Structure. The heart has four chambers. The right side pumps blood to the lungs (pulmonary circulation), and the left side pumps oxygenated blood to the body (systemic circulation). Arteries carry blood away from the heart, capillaries exchange nutrients and wastes with tissues, and veins return blood.
Cardiac output and blood pressure.
| Regulator | Effect |
|---|---|
| Baroreceptors (carotid sinus, aortic arch) | Rapid adjustment of heart rate and vessel tone |
| Renin-angiotensin-aldosterone system | Vasoconstriction and sodium and water retention raise pressure |
| Antidiuretic hormone | Water retention |
| Atrial natriuretic peptide | Released when the atria stretch; increases sodium and water excretion |
Excess sodium expands blood volume in salt-sensitive people, while potassium, calcium, and magnesium help relax vessels. These links explain the DASH diet.
Blood. Plasma carries nutrients, hormones, and wastes, bound to proteins such as albumin, transferrin, and lipoproteins. Red blood cells carry oxygen on hemoglobin and survive about 120 days. White cells defend against infection, and platelets start clotting.
| Nutrient | Role in blood formation | Deficiency result |
|---|---|---|
| Iron | Heme synthesis | Microcytic, hypochromic anemia |
| Vitamin B12 and folate | DNA synthesis in red cell precursors | Megaloblastic (macrocytic) anemia |
| Vitamin B6 | Heme synthesis (ALA synthase) | Microcytic (sideroblastic) anemia |
| Copper | Iron transport (ceruloplasmin) | Anemia and neutropenia |
| Protein | Globin and red cell production | Anemia of protein-energy malnutrition |
| Vitamin K | Carboxylation of clotting factors II, VII, IX, X | Bleeding |
The Respiratory System
Air moves through the nose, pharynx, larynx, trachea, bronchi, and bronchioles to the alveoli, where oxygen diffuses into capillary blood and carbon dioxide diffuses out. Most oxygen travels bound to hemoglobin, and most carbon dioxide travels as bicarbonate. The lungs help regulate acid-base balance by changing how much carbon dioxide is exhaled.
Nutrition links:
- The respiratory muscles, including the diaphragm, weaken when protein-energy malnutrition causes muscle loss, which makes weaning from a ventilator harder.
- Overfeeding, especially with carbohydrate, increases carbon dioxide production (respiratory quotient above 1.0) and strains failing lungs.
- People with chronic obstructive pulmonary disease use more energy to breathe and often lose weight.
The Renal (Excretory) System
Each kidney contains about a million nephrons. Blood is filtered at the glomerulus, and the tubules (proximal tubule, loop of Henle, distal tubule, collecting duct) reabsorb water, glucose, amino acids, and electrolytes and secrete wastes.
| Kidney function | Details |
|---|---|
| Filtration | About 180 L/day of filtrate (GFR roughly 125 mL/min); about 1-2 L of urine is excreted |
| Waste excretion | Urea (from protein), creatinine (from muscle creatine), uric acid (from purines) |
| Fluid, electrolyte, and acid-base balance | Adjusts water, sodium, potassium, phosphate, hydrogen ion, and bicarbonate |
| Hormone production | Erythropoietin (red cells), renin (blood pressure), calcitriol (active vitamin D) |
| Gluconeogenesis | The renal cortex makes glucose, especially in prolonged fasting |
Adults must excrete a minimum (obligatory) urine volume of about 500 mL/day to clear the daily solute load. In chronic kidney disease, the loss of these functions explains anemia (low erythropoietin), bone disease (low calcitriol, high phosphate, high PTH), high potassium, fluid retention, and acidosis. Other excretory routes are the skin (sweat), lungs (carbon dioxide and water vapor), and intestine (feces).
The Musculoskeletal System
Bone is living tissue made of collagen and hydroxyapatite (calcium phosphate crystals). Osteoblasts build bone and osteoclasts resorb it. Peak bone mass is reached by about age 30; afterward, resorption gradually exceeds formation, and the loss speeds up after menopause. Bone health needs calcium, phosphorus, vitamin D, magnesium, vitamin K, protein, and weight-bearing activity. About 99% of body calcium is in bones and teeth.
Skeletal muscle contracts through the sliding of actin and myosin filaments, powered by ATP. Immediate energy comes from creatine phosphate, short bursts from anaerobic glycolysis, and endurance work from aerobic metabolism of glucose and fatty acids. Type I (slow-twitch) fibers are oxidative and fatigue-resistant; type II (fast-twitch) fibers are glycolytic and powerful. Muscle is the body's largest protein store, so it is broken down to supply amino acids during starvation and stress. Muscle mass also drives resting energy expenditure and creatinine production.
The Reproductive System
Gonadal hormones (estrogen, progesterone, testosterone) regulate reproduction and affect body composition and bone. Menstruation causes iron losses, which is why women of reproductive age need more iron than men (the PDRI 2015 sets 28 mg/day for women aged 19-49 and 12 mg/day for men).
Physiological changes in pregnancy:
| Change | Nutrition implication |
|---|---|
| Plasma volume rises about 40-50%, red cell mass less | Hemoglobin concentration falls (physiological anemia); iron needs rise |
| Glomerular filtration rate rises | More glucose and water-soluble vitamins appear in urine |
| Slower gastrointestinal motility | Constipation and heartburn; nutrient absorption improves for some minerals |
| Insulin resistance in the second and third trimesters | Risk of gestational diabetes |
| Growth of the fetus, placenta, uterus, and breasts | Higher energy and protein needs in the second and third trimesters |
Lactation. Prolactin from the anterior pituitary stimulates milk production, and oxytocin from the posterior pituitary contracts the cells around the alveoli to eject milk (the let-down reflex). Frequent suckling maintains both reflexes, while stress and pain can inhibit let-down.
A pregnant woman's hemoglobin falls slightly in the second trimester even though her iron intake and stores are adequate. What best explains this finding?
Destruction of red blood cells by the placenta
Plasma volume increases proportionally more than red cell mass
Increased iron loss in the urine caused by a higher glomerular filtration rate
Suppression of erythropoietin by estrogen
A patient with stage 5 chronic kidney disease has a normocytic anemia that does not respond to iron alone. Which kidney function loss best explains it?
Reduced renin secretion
Reduced calcitriol production
Reduced erythropoietin production
Reduced urea excretion
A breastfeeding mother produces milk but reports that it does not flow well when she is anxious. Which hormone governs the reflex that is being inhibited?
Oxytocin, which causes milk ejection (let-down)
Prolactin, which stimulates milk synthesis
Estrogen, which maintains milk volume
Progesterone, which triggers milk ejection
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