Unit 2: Excretion and Cardiovascular System

Excretion

Excretion is the process of removing metabolic waste products and excess substances from the body. The urinary system, with the kidneys as its primary organs, plays a crucial role in maintaining homeostasis by filtering blood and forming urine.

Structure of Nephron

The nephron is the structural and functional unit of the kidney, responsible for filtering blood and producing urine. Each human kidney contains approximately one million nephrons.

• Renal Corpuscle (Malpighian Body):
  • Glomerulus: A tuft of capillaries formed by the afferent arteriole and drained by the efferent arteriole. It is the site of blood filtration.
  • Bowman's Capsule: A double-walled cup-like structure that encloses the glomerulus. It collects the glomerular filtrate.
• Renal Tubule: A long, convoluted tube extending from Bowman's capsule.
  • Proximal Convoluted Tubule (PCT): Highly coiled segment immediately after Bowman's capsule. Major site of reabsorption of essential nutrients, water, and electrolytes.
  • Henle's Loop: A U-shaped segment crucial for maintaining the medullary osmotic gradient. It has a descending limb and an ascending limb.
    • Descending Limb: Permeable to water, impermeable to electrolytes.
    • Ascending Limb: Impermeable to water, permeable to electrolytes (active transport of NaCl).
  • Distal Convoluted Tubule (DCT): Highly coiled segment after Henle's loop. Involved in conditional reabsorption of Na⁺ and water, and secretion of K⁺ and H⁺.
  • Collecting Duct (CD): A straight tube that receives filtrate from many nephrons. It extends from the cortex to the inner medulla, playing a role in final water reabsorption under ADH influence.
• Associated Blood Vessels:
  • Afferent Arteriole: Brings blood to the glomerulus.
  • Efferent Arteriole: Carries blood away from the glomerulus.
  • Peritubular Capillaries: A network of capillaries surrounding the renal tubules, involved in reabsorption and secretion.
  • Vasa Recta: Specialized capillary network parallel to Henle's loop in juxtamedullary nephrons, crucial for maintaining the medullary osmotic gradient.

Mechanism of Urine Formation

Urine formation involves three main processes: glomerular filtration, tubular reabsorption, and tubular secretion.

1. Glomerular Filtration (Ultrafiltration):
   • Occurs in the renal corpuscle. Blood is filtered from the glomerulus into Bowman's capsule.
   • The filtration membrane consists of the endothelium of glomerular blood vessels, the basement membrane, and the podocytes of Bowman's capsule.
   • Only small molecules (water, salts, glucose, amino acids, urea) pass through; large proteins and blood cells are retained.
   • Glomerular Filtration Rate (GFR): The volume of filtrate formed by all nephrons per minute (approx. 125 mL/min or 180 L/day).
   • The net filtration pressure (NFP) drives filtration, resulting from glomerular hydrostatic pressure minus Bowman's capsule hydrostatic pressure and colloid osmotic pressure.
2. Tubular Reabsorption:
   • The process by which useful substances (water, glucose, amino acids, Na⁺, etc.) are reabsorbed from the filtrate in the renal tubule back into the peritubular capillaries.
   • Mostly occurs in the PCT (approx. 70-80% of electrolytes and water, all glucose and amino acids).
   • Also occurs in Henle's loop, DCT, and collecting duct.
   • Can be active (e.g., glucose, amino acids, Na⁺) or passive (e.g., water, urea, Cl⁻).
3. Tubular Secretion:
   • The process by which unwanted substances (K⁺, H⁺, creatinine, drugs) are secreted from the peritubular capillaries into the filtrate in the renal tubule.
   • Primarily occurs in the PCT and DCT.
   • Helps in maintaining ionic and acid-base balance of body fluids.
   • Eliminates waste products not completely filtered.

Summary of Urine Formation: Blood enters glomerulus → Filtration forms glomerular filtrate → Filtrate enters renal tubule → Useful substances reabsorbed → Waste substances secreted → Remaining fluid is urine.

Counter-current Mechanism

The counter-current mechanism is a process that uses energy to create an osmotic gradient within the renal medulla, enabling the production of concentrated urine.

This mechanism involves Henle's loop and the vasa recta, working in a counter-current (opposite direction) flow system.

• Components:
  • Henle's Loop: Acts as a counter-current multiplier, actively pumping salts out of the ascending limb, making the interstitial fluid hypertonic.
  • Vasa Recta: Acts as a counter-current exchanger, maintaining the medullary osmotic gradient by minimizing solute washout.
  • Collecting Duct: Utilizes the medullary gradient to reabsorb water, concentrating the urine, especially in the presence of ADH.
• How it Works:
  1. Ascending Limb of Henle's Loop: Actively transports NaCl into the medullary interstitial fluid. It is impermeable to water. This increases the osmolarity of the interstitial fluid.
  2. Descending Limb of Henle's Loop: Permeable to water but not to electrolytes. As filtrate moves down, water moves out into the hypertonic interstitial fluid, concentrating the filtrate.
  3. Vasa Recta: Blood flows slowly in the opposite direction to the filtrate in Henle's loop. It picks up reabsorbed water and returns it to circulation, while salts diffuse into the vasa recta on its descending limb and out on its ascending limb, thus maintaining the osmotic gradient without dissipating it.
  4. Collecting Duct: As filtrate passes through the increasingly hypertonic renal medulla, water is reabsorbed from the collecting duct into the interstitial fluid, especially when Antidiuretic Hormone (ADH) is present, leading to the formation of concentrated urine. Urea also cycles between the collecting duct and Henle's loop, contributing to the medullary hypertonicity.
• Importance: Ensures the kidney can produce urine with varying concentrations, from very dilute (when excess water needs to be excreted) to highly concentrated (when water needs to be conserved).

Cardiovascular System

The cardiovascular system is responsible for transporting blood throughout the body, delivering oxygen and nutrients to cells, and removing waste products. It consists of the heart, blood vessels, and blood.

Composition of Blood

Blood is a specialized connective tissue composed of a fluid matrix (plasma) and formed elements (blood cells).

Blood makes up approximately 7-8% of total body weight.

• Plasma (approx. 55% of blood volume): The straw-colored, viscous fluid matrix.
  • Water (90-92%): Solvent for transporting substances.
  • Plasma Proteins (6-8%):
    • Albumins: Maintain osmotic balance, transport substances.
    • Globulins: Involved in defense mechanisms (antibodies) and transport.
    • Fibrinogen: Essential for blood clotting.
  • Other Components: Glucose, amino acids, lipids, vitamins, hormones, electrolytes (Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻), metabolic wastes (urea, uric acid, creatinine).
• Formed Elements (approx. 45% of blood volume): Produced in the bone marrow.
  • Erythrocytes (Red Blood Cells - RBCs):
    • Most abundant cells (5-5.5 million/mm³).
    • Biconcave, anucleated (in mammals), contain hemoglobin.
    • Primary function: Oxygen transport.
    • Life span: ~120 days.
  • Leukocytes (White Blood Cells - WBCs):
    • Fewer in number (6,000-8,000/mm³).
    • Nucleated, generally larger than RBCs.
    • Primary function: Immune defense.
    • Categorized into:
      • Granulocytes: Neutrophils (phagocytic, most abundant), Eosinophils (allergic reactions, parasitic infections), Basophils (inflammatory responses, secrete histamine/serotonin/heparin).
      • Agranulocytes: Lymphocytes (T and B cells, specific immunity), Monocytes (phagocytic, differentiate into macrophages).
  • Thrombocytes (Platelets):
    • Cell fragments produced from megakaryocytes (1.5-3.5 lakh/mm³).
    • Lack nucleus.
    • Primary function: Blood clotting (hemostasis).

Haemostasis

Haemostasis is the process that stops bleeding from an injured blood vessel, maintaining the integrity of the circulatory system.

It involves a rapid and localized response, consisting of three main stages:

1. Vascular Spasm (Vasoconstriction):
   • Immediately after injury, the smooth muscle in the wall of the damaged blood vessel contracts.
   • This reduces blood flow to the injured area, minimizing blood loss.
2. Platelet Plug Formation:
   • Platelets adhere to exposed collagen fibers in the damaged vessel wall (platelet adhesion).
   • Adhered platelets become activated, changing shape and releasing chemical messengers (e.g., ADP, serotonin, thromboxane A2).
   • These chemicals attract more platelets to the site, which aggregate and form a temporary plug (platelet aggregation) that seals the small break.
3. Coagulation (Blood Clotting):
   • A complex cascade of enzymatic reactions involving various clotting factors (proteins, Ca²⁺, vitamin K).
   • The cascade can be initiated by extrinsic (tissue injury) or intrinsic (blood vessel damage) pathways, both converging to a common pathway.
   • Key steps in common pathway:
     1. Damaged tissue/platelets release factors that activate prothrombin activator.
     2. Prothrombin activator (with Ca²⁺) converts inactive prothrombin to active thrombin.
     3. Thrombin then converts soluble fibrinogen into insoluble fibrin monomers.
     4. Fibrin monomers polymerize to form long fibrin threads, which entrap blood cells and form a stable blood clot.
   • Clot retraction then occurs, pulling the edges of the damaged vessel together.

Structure of Heart

The heart is a muscular, hollow organ located in the mediastinum (between the lungs), responsible for pumping blood throughout the circulatory system.

It is roughly the size of a clenched fist and protected by a double-walled membranous sac called the pericardium, which encloses the pericardial fluid.

• Chambers: The human heart is four-chambered.
  • Atria (Upper Chambers):
    • Right Atrium: Receives deoxygenated blood from the body via the superior and inferior vena cava.
    • Left Atrium: Receives oxygenated blood from the lungs via the pulmonary veins.
  • Ventricles (Lower Chambers):
    • Right Ventricle: Pumps deoxygenated blood to the lungs via the pulmonary artery.
    • Left Ventricle: Pumps oxygenated blood to the rest of the body via the aorta. It has the thickest muscular walls.
• Septa: Internal walls separating the chambers.
  • Interatrial Septum: Separates the two atria.
  • Interventricular Septum: Separates the two ventricles (thick and strong).
  • Atrioventricular Septum: Separates atrium and ventricle on the same side.
• Valves: Ensure unidirectional blood flow and prevent backflow.
  • Atrioventricular (AV) Valves: Located between atria and ventricles.
    • Tricuspid Valve: Between right atrium and right ventricle (3 cusps).
    • Bicuspid (Mitral) Valve: Between left atrium and left ventricle (2 cusps).
  • Semilunar (SL) Valves: Located at the exits of the ventricles.
    • Pulmonary Semilunar Valve: At the opening of the right ventricle into the pulmonary artery.
    • Aortic Semilunar Valve: At the opening of the left ventricle into the aorta.
• Major Blood Vessels Associated with the Heart:
  • Superior & Inferior Vena Cava: Bring deoxygenated blood from the body to the right atrium.
  • Pulmonary Arteries: Carry deoxygenated blood from the right ventricle to the lungs.
  • Pulmonary Veins: Bring oxygenated blood from the lungs to the left atrium.
  • Aorta: Carries oxygenated blood from the left ventricle to the rest of the body.

Origin and Conduction of the Cardiac Impulse

The heart is myogenic, meaning it generates its own electrical impulses, which then propagate to stimulate rhythmic contractions.

The specialized cardiac musculature of the heart forms the nodal tissue, which is responsible for initiating and conducting the cardiac impulse.

1. Sinoatrial (SA) Node (Pacemaker):
   • Located in the upper right corner of the right atrium.
   • Generates the maximum number of action potentials per minute (70-75/min in a healthy individual), setting the pace of the heart.
   • This impulse causes both atria to contract simultaneously.
2. Atrioventricular (AV) Node:
   • Located in the lower left corner of the right atrium, close to the interventricular septum.
   • Receives the impulse from the SA node.
   • Briefly delays the impulse, allowing the atria to fully empty their blood into the ventricles before ventricular contraction begins.
3. Bundle of His (AV Bundle):
   • A bundle of fibers originating from the AV node.
   • Passes through the atrioventricular septa to enter the interventricular septum.
   • Divides into right and left bundle branches.
4. Purkinje Fibers:
   • Fine fibers that arise from the bundle branches and spread throughout the ventricular musculature.
   • Rapidly conduct the impulse to the entire ventricular walls, causing synchronous ventricular contraction.

Conduction Pathway: SA Node → Atrial muscles → AV Node → Bundle of His → Right and Left Bundle Branches → Purkinje Fibers → Ventricular muscles.

Cardiac Cycle

The cardiac cycle is the sequence of events that occurs from the beginning of one heartbeat to the beginning of the next, involving rhythmic contraction (systole) and relaxation (diastole) of the heart chambers.

It takes approximately 0.8 seconds to complete one cardiac cycle at a normal heart rate of 72 beats/minute.

1. Joint Diastole (0.4 seconds):
   • All four chambers are in a relaxed state.
   • Blood flows from the vena cava and pulmonary veins into the right and left atria, respectively.
   • From the atria, blood passively flows into the ventricles as the AV valves are open and semilunar valves are closed.
2. Atrial Systole (0.1 seconds):
   • The SA node generates an action potential, causing atrial contraction.
   • This forces the remaining blood (about 30%) from the atria into the ventricles.
   • The end of atrial systole marks the end-diastolic volume (EDV) in the ventricles.
3. Ventricular Systole (0.3 seconds):
   • The impulse reaches the ventricles, causing them to contract.
   • Isovolumetric Contraction: Initially, both AV and semilunar valves are closed, and ventricular pressure rises rapidly without a change in volume.
   • As ventricular pressure exceeds arterial pressure (aortic and pulmonary artery), the semilunar valves open, and blood is ejected from the ventricles into the aorta and pulmonary artery (ventricular ejection).
   • The AV valves close due to rising ventricular pressure, producing the first heart sound ("lub").
   • The amount of blood pumped out by each ventricle during one beat is called the stroke volume (approx. 70 mL).
4. Ventricular Diastole (part of Joint Diastole):
   • As the ventricles relax, ventricular pressure falls.
   • Blood attempts to flow back from the aorta and pulmonary arteries into the ventricles, causing the semilunar valves to close. This closure produces the second heart sound ("dub").
   • The ventricles continue to relax (isovolumetric relaxation), and as ventricular pressure falls below atrial pressure, the AV valves open, and blood starts flowing passively from the atria into the ventricles, restarting joint diastole.

Heart Sounds:

• First Heart Sound ("lub"): Caused by the closure of the tricuspid and bicuspid (AV) valves at the beginning of ventricular systole.
• Second Heart Sound ("dub"): Caused by the closure of the semilunar valves (aortic and pulmonary) at the beginning of ventricular diastole.

Cardiac Output: The volume of blood pumped by each ventricle per minute.

Cardiac Output = Stroke Volume x Heart Rate

Typically, 70 mL/beat x 72 beats/min = approx. 5040 mL/min or 5 liters/min.