Cardiovascular, Respiratory & Renal Systems

• The cardiovascular system, respiratory system, and renal system are vital for maintaining homeostasis in the body.

Cardiac Cycle & Blood Flow

• Blood flows through the heart in the following sequence: right atrium (RA), tricuspid valve, right ventricle (RV), pulmonary valve, lungs, left atrium (LA), bicuspid (mitral) valve, left ventricle (LV), aortic valve, and finally to the body.
• The phases of the cardiac cycle are atrial systole, ventricular systole (including isovolumetric contraction and ejection), and diastole (including isovolumetric relaxation and filling).

Conduction & Cardiac Myocyte Depolarization

• Autorhythmic cells, such as those in the sinoatrial (SA) node, depolarize through a slow influx of Na⁺ (via funny channels) followed by Ca²⁺ influx. Repolarization occurs with K⁺ efflux.
• Contractile cells depolarize rapidly with Na⁺ influx, followed by a brief K⁺ efflux, a Ca²⁺ influx that creates a plateau, and then repolarization via K⁺ efflux.
• The conduction pathway in the heart is SA node, atrioventricular (AV) node, Bundle of His, bundle branches, and Purkinje fibers.

Blood Vessels

• Arteries have thick, muscular walls, handle high pressure, and carry blood away from the heart.
• Arterioles have smooth muscle, experience medium pressure, and regulate resistance via vasoactivity.
• Capillaries, with their single-cell-thick walls, experience low pressure and facilitate gas and nutrient exchange.
• Venules are thin, experience low pressure, and collect blood from capillaries.
• Veins are thin-walled, contain valves, experience very low pressure, and return blood to the heart, also acting as a blood reservoir.

Flow Rule

• Flow is directly proportional to the pressure gradient (ΔP) and inversely proportional to resistance (R).
• Poiseuille’s Law: Resistance is proportional to 1/r⁴ (r being the radius of the blood vessel).

Cardiac Output & Blood Pressure

• Cardiac output (CO) is the product of heart rate (HR) and stroke volume (SV): CO = HR × SV.
• Blood pressure (BP) is the product of cardiac output and total peripheral resistance (TPR): BP = CO × TPR.
• Systolic blood pressure (SBP) is the peak pressure during ventricular contraction.
• Diastolic blood pressure (DBP) is the minimum pressure during relaxation.
• Pulse pressure is the difference between systolic and diastolic blood pressure: Pulse pressure = SBP − DBP.

Blood Components

• Hematocrit is the percentage of blood volume composed of red blood cells (RBCs).
• Hemoglobin (Hb) is the O₂-carrying protein found in RBCs.
• Red blood cells are produced in the red bone marrow.
• Erythropoietin (EPO), a hormone from the kidneys, controls RBC production.

Alveoli & Respiratory Membrane

• The respiratory membrane, consisting of a Type I alveolar cell, capillary endothelium, and basement membrane, is thin and moist, facilitating gas exchange.
• The alveoli are the primary site of gas exchange in the lungs.

Mechanics of Breathing

• Inspiration occurs when the diaphragm contracts, increasing thoracic volume, decreasing pressure, and allowing air to flow into the lungs.
• Expiration is passive at rest, becoming active with the use of abdominal and internal intercostal muscles during exercise or forced breathing.
• Boyle’s Law underlies the mechanics of breathing: Pressure (P) is inversely proportional to Volume (V).

Gas Exchange (Partial Pressures)

• In the lungs, O₂ diffuses into the blood because the partial pressure of O₂ in the alveoli (~100 mmHg) is higher than in the blood (~40 mmHg), and CO₂ diffuses out of the blood.
• In the tissues, O₂ diffuses into cells, and CO₂ diffuses into the blood.

O₂ and CO₂ Transport

• O₂ is mostly transported bound to hemoglobin (Hb), with a small amount dissolved in plasma.
• CO₂ is transported in three forms: 70% as bicarbonate (HCO₃⁻), 20% bound to Hb, and 10% dissolved in plasma.
• Factors such as pH, temperature, and CO₂ levels affect O₂ and CO₂ transport (Bohr effect).

Control of Ventilation

• Central chemoreceptors in the medulla respond to increased CO₂ levels by detecting changes in pH.
• Peripheral chemoreceptors in the aortic and carotid bodies respond to decreased O₂, increased CO₂, and decreased pH.

Functional Unit: The Nephron

• The nephron consists of the glomerulus, Bowman’s capsule, proximal convoluted tubule (PCT), Loop of Henle, distal convoluted tubule (DCT), and collecting duct.

Steps in Urine Formation

• The three steps in urine formation are filtration (in the glomerulus), reabsorption (mostly in the PCT), and secretion (mainly in the DCT and collecting duct).

What is Filtration?

• Blood pressure forces plasma (minus proteins) across the filtration membrane into Bowman’s space.

GFR Control

• Intrinsic controls:
  • Myogenic mechanism: Afferent arteriole constricts when stretched.
  • Tubuloglomerular feedback: Macula densa senses flow and releases paracrines.
  • Mesangial cells: Change the surface area of glomerular capillaries.
• Extrinsic controls:
  • RAAS (Renin-Angiotensin-Aldosterone System) increases blood pressure via renin, angiotensin II, vasoconstriction, and aldosterone secretion.
  • Sympathetic stimulation decreases GFR during low blood pressure.

Reabsorption & Secretion

• Reabsorption occurs in the PCT, Loop of Henle, DCT, and collecting duct, involving the recovery of glucose, Na⁺, water, and Cl⁻ from the filtrate back into the blood.
• Secretion occurs in the DCT and collecting duct, involving the transport of H⁺, K⁺, drugs, and creatinine from the blood into the filtrate.

Osmolarity & BP Regulation (ADH Role)

• ADH (antidiuretic hormone) from the posterior pituitary increases water reabsorption in the collecting duct and urine concentration, which helps increase blood volume and blood pressure.